AP-T196-11
AUSTROADS TECHNICAL REPORT
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures Published December 2011
© Austroads Ltd 2011 This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads.
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures ISBN 978-1-921991-10-3
Austroads Project No. TS1603 Austroads Publication No. AP–T196-11
Project Manager Dr Ross Pritchard Queensland Department of Transport and Main Roads Prepared by Dr Neal Lake ARRB Group
Published by Austroads Ltd Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email:
[email protected] www.austroads.com.au
Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should rely on their own skill and judgement to apply information to particular issues.
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Sydney 2011
About Austroads Austroads’ purpose is to:
promote improved Australian and New Zealand transport outcomes provide expert technical input to national policy development on road and road transport issues promote improved practice and capability by road agencies. promote consistency in road and road agency operations.
Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure and Transport, the Australian Local Government Association, and NZ Transport Agency. Austroads is governed by a Board consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations:
Roads and Maritime Services New South Wales
Roads Corporation Victoria
Department of Transport and Main Roads Queensland
Main Roads Western Australia
Department of Planning, Transport and Infrastructure South Australia
Department of Infrastructure, Energy and Resources Tasmania
Department of Lands and Planning Northern Territory
Department of Territory and Municipal Services Australian Capital Territory
Commonwealth Department of Infrastructure and Transport
Australian Local Government Association
New Zealand Transport Agency.
The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road transport sector.
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
CONTENTS 1
INTRODUCTION ................................................................................................................... 1
1.1 1.2 1.3 1.4
Background ........................................................................................................................... 1 Aims ...................................................................................................................................... 1 Scope .................................................................................................................................... 2 Outline ................................................................................................................................... 2
2
DESIGN OF BCMS................................................................................................................ 3
2.1 2.2
Behaviour of BCMS ............................................................................................................... 3 Failure Mechanisms ............................................................................................................... 5 2.2.1 Corrosion and Abrasion............................................................................................ 6 2.2.2 Strength-related Failures .......................................................................................... 8 2.2.3 Construction Failures ............................................................................................... 8 Overall Design Methodology .................................................................................................. 9 2.3.1 Overall Design Process ............................................................................................ 9 Preliminary Assessment ...................................................................................................... 11 2.4.1 Consideration of BCMS as Appropriate Culvert Type ............................................. 11 2.4.2 Structure Classification (Importance level) – Intended Use (Design Working Life) ........................................................................................................................ 12 2.4.3 BCMS Configuration and Application ..................................................................... 12 2.4.4 BCMS Fabrication and Material Types ................................................................... 13 2.4.5 Site Investigation .................................................................................................... 15 Structural Analysis Approaches ........................................................................................... 17 2.5.1 Design Loads ......................................................................................................... 17 2.5.2 Ring Compression Method ..................................................................................... 23 2.5.3 Limit State Method ................................................................................................. 26 2.5.4 FE Analysis Method ............................................................................................... 28 2.5.5 Design Method Selection ....................................................................................... 29 Design for Durability............................................................................................................. 31 2.6.1 Material Selection................................................................................................... 32 2.6.2 Corrosion Allowance Methods for Durability Design ............................................... 34 2.6.3 Site Investigations/Tests ........................................................................................ 38 Detailing............................................................................................................................... 40 2.7.1 Footings ................................................................................................................. 40 2.7.2 Longitudinal Stiffeners ............................................................................................ 42 2.7.3 End Treatments...................................................................................................... 42 2.7.4 Invert Lining ........................................................................................................... 45 2.7.5 Spacing .................................................................................................................. 45 2.7.6 Cover ..................................................................................................................... 46 2.7.7 Location and Alignment Considerations ................................................................. 46
2.3 2.4
2.5
2.6
2.7
3
CONSTRUCTION GUIDELINES ......................................................................................... 49
3.1
Material Handling ................................................................................................................. 49 3.1.1 Material Delivery .................................................................................................... 49 3.1.2 Handling Damage .................................................................................................. 50 Site Preparation ................................................................................................................... 51 3.2.1 Installation Type ..................................................................................................... 51 3.2.2 Grade ..................................................................................................................... 52
3.2
Austroads 2011 — i—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
3.5
3.2.3 Camber .................................................................................................................. 52 3.2.4 Foundation Requirements ...................................................................................... 53 3.2.5 Bedding .................................................................................................................. 55 Pipe Assembly ..................................................................................................................... 55 3.3.1 Assembly Instructions ............................................................................................ 55 3.3.2 Shape Tolerances .................................................................................................. 58 Backfilling Specifications ...................................................................................................... 59 3.4.1 Material Selection................................................................................................... 59 3.4.2 Compaction Process and Equipment ..................................................................... 60 Construction Loads .............................................................................................................. 62
4
STRUCTURAL MANAGEMENT AND INSPECTION OF BCMS ......................................... 63
4.1 4.2 4.3 4.4 4.5
Structure Management Planning .......................................................................................... 63 Workplace Health and Safety............................................................................................... 64 Level 2 Structural Inspections: Defect Identification ............................................................. 64 Level 2 Structural Inspections: Condition States .................................................................. 67 Level 3 Structural Inspections: Information Collection .......................................................... 68 4.5.1 Type of BCMS ........................................................................................................ 68 4.5.2 Size and Shape ...................................................................................................... 69 4.5.3 Corrugations – Pitch and Depth ............................................................................. 70 4.5.4 Height of Fill Material.............................................................................................. 70 4.5.5 Material Thickness ................................................................................................. 70 4.5.6 Maximum Outside Diameter ................................................................................... 70 4.5.7 Voids Present in Fill ............................................................................................... 70 4.5.8 Estimated Maximum Sag in Pipe due to Settlement ............................................... 71 4.5.9 Waterway Description ............................................................................................ 71 4.5.10 Environmental Conditions ...................................................................................... 71 4.5.11 Water/Soil Samples ................................................................................................ 71 4.5.12 Other Defects and Cause (Construction/In-service) ............................................... 71 4.5.13 Sketches ................................................................................................................ 71 Risk Assessment Method and Treatment Action .................................................................. 71 4.6.1 Situation 1 .............................................................................................................. 73 4.6.2 Situation 2 .............................................................................................................. 74 4.6.3 Situation 3 .............................................................................................................. 75 4.6.4 Situation 4 .............................................................................................................. 76 4.6.5 Situation 5 .............................................................................................................. 77 4.6.6 Situation 6 .............................................................................................................. 78 4.6.7 Situation 7 .............................................................................................................. 79
3.3
3.4
4.6
5
MAINTENANCE AND REPAIR PROCEDURES ................................................................. 81
5.1 5.2
Emergency Propping ........................................................................................................... 81 Repair Methods ................................................................................................................... 81 5.2.1 Repair and Maintenance Methods .......................................................................... 82 5.2.2 Concrete Lining of Invert ........................................................................................ 82 5.2.3 Painting the Invert .................................................................................................. 84 5.2.4 Joint Repairs .......................................................................................................... 85 5.2.5 Replacement of the Culvert .................................................................................... 85 5.2.6 Shotcrete Lining ..................................................................................................... 85 5.2.7 Slip Lining .............................................................................................................. 86 5.2.8 Pipe Jacking Around the Existing Culvert ............................................................... 89 5.2.9 Filling the Culvert ................................................................................................... 89
Austroads 2011 — ii —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
6
CONCLUSIONS .................................................................................................................. 90
6.1
Future Directions ................................................................................................................. 90
REFERENCES ............................................................................................................................. 92 APPENDIX A APPENDIX B
BCMS MANUFACTURERS AND COMPANIES PROVIDING REHABILITATION SERVICES ................................................................ 95 REVIEW OF STATE ROAD AUTHORITY EXPERIENCE ...................... 100
Austroads 2011 — iii —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
TABLES Table 2.1: Table 2.2: Table 2.3: Table 2.4: Table 2.5: Table 2.6: Table 2.7: Table 2.8: Table 2.9: Table 2.10: Table 2.11: Table 2.12: Table 2.13: Table 2.14: Table 3.1: Table 3.2:
Corrosive level determination ................................................................................... 8 BCMS configuration and application....................................................................... 13 Structures geometrical limits for the limit analysis method ...................................... 30 Selection of design methods .................................................................................. 31 BCMS material suitability ....................................................................................... 33 Selection of durability design method ..................................................................... 34 Average base metal loss rate per side ................................................................... 35 Average galvanising loss rate................................................................................. 36 Average galvanised thickness for steel sheet ......................................................... 36 Expected life and metal loss rates vs. pH and resistivity......................................... 37 Corrosive level determination ................................................................................. 39 Abrasion level determination .................................................................................. 39 Requirements for end stiffening ring beam ............................................................. 44 Minimum spacing for multiple structures................................................................. 46 Bolt torque.............................................................................................................. 58 Select fill requirements ........................................................................................... 59
FIGURES Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 2.5: Figure 2.6: Figure 2.7: Figure 2.8: Figure 2.9: Figure 2.10: Figure 2.11: Figure 2.12: Figure 2.13: Figure 2.14: Figure 2.15: Figure 2.16: Figure 2.17: Figure 2.18: Figure 2.19: Figure 2.20: Figure 2.21: Figure 2.22: Figure 2.23: Figure 2.24: Figure 2.25: Figure 3.1:
Behaviour of buried corrugated metal pipes ............................................................. 3 Ring compression theory whereby overburden and live load stresses are evenly distributed to the surrounding soil.................................................................. 4 Possible deformed shape due to backfill sequence .................................................. 5 Heavy corrosion of a BCMS – considerable loss of metal thickness ......................... 6 BCMS invert corroded away due to loss of granular bedding material in invert ........................................................................................................................ 7 Collapse of a steel culvert during backfilling ............................................................. 9 BCMS design actions ............................................................................................. 10 Typical lock-seam cross-section of helically formed structures ............................... 14 Typical configuration of bolted plate structures ....................................................... 14 Height of fill for calculation of dead load pressure .................................................. 17 Typical heavy construction vehicle load ................................................................. 18 Distribution of vehicle loads through fill .................................................................. 20 Live load pressure vs. depth of fill for MS1600 and HLP loadings .......................... 21 Ring compression design method flow chart .......................................................... 24 Pressure variation around pipe-arches ................................................................... 25 Limit state design method flow chart ...................................................................... 27 AISI chart for estimating average invert life for galvanised BCMS .......................... 38 Arch footing forces ................................................................................................. 41 Typical longitudinal stiffener detail .......................................................................... 42 Typical cut-off wall and apron details...................................................................... 43 End treatment using gabions .................................................................................. 43 Typical end stiffening ring beam/headwall .............................................................. 44 Diagram for indicating skew number ...................................................................... 45 Improved alignments through channel changes ..................................................... 47 Various methods of obtaining correct culvert alignment.......................................... 48 Pipe unloading arrangement .................................................................................. 50
Austroads 2011 — iv —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Figure 3.2: Figure 3.3: Figure 3.4: Figure 3.5: Figure 3.6: Figure 3.7: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Figure 4.6: Figure 4.7: Figure 4.8: Figure 4.9: Figure 4.10: Figure 4.11: Figure 4.12: Figure 4.13: Figure 5.1: Figure 5.2: Figure 5.3: Figure 5.4: Figure 5.5: Figure 5.6:
Trench installation .................................................................................................. 51 Embankment installation ........................................................................................ 52 Camber under a high fill ......................................................................................... 53 Bedding on soft, rock and firm foundations............................................................. 54 Component sub-assembly method for multi-plate structure .................................... 57 Backfilling with plum-bob monitoring ...................................................................... 61 Cracks in metal plate probably caused by excessive side pressures during backfill ......................................................................................................... 65 New culvert damaged at joint during backfill, probably due to construction overload ............................................................................................. 66 Multi-plate culvert ................................................................................................... 69 Helically wound culvert ........................................................................................... 69 Corrugation profile for steel pipes ........................................................................... 70 Flowchart for risk assessment and treatment ......................................................... 72 Standing water in BCMS ........................................................................................ 74 BCMS has significant invert corrosion and will need a reinforced concrete invert in the next 2 years .......................................................................... 75 Heavy corrosion in invert with small perforations to metal structure........................ 76 Heavy corrosion – considerable loss of metal thickness ......................................... 76 BCMS invert corroded away (loss of granular bedding material in invert) ............... 78 BCMS ring movement ............................................................................................ 79 BCMS soil arch failure ............................................................................................ 80 Example of emergency propping ............................................................................ 81 A thin concrete invert lining which has separated from the culvert and washed away in flood ............................................................................................. 83 HDPE lining being installed .................................................................................... 87 Relining process..................................................................................................... 87 Estimating largest liner diameter ............................................................................ 88 Typical pipe jacking set-up ..................................................................................... 89
Austroads 2011 — v—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
SUMMARY Buried corrugated metal structures (BCMS) have been used in Australia as an attractive solution to under road drainage requirements due to the low cost and fast construction times achievable. Several incidents of significant failures of BCMS, however, have been reported in current practice. In most observed failures, corrosion has been a critical issue which resulted in subsequent maintenance, rehabilitation and replacement. In addition, thinner sections have been introduced to the Australian market and included in Australian Standards, potentially increasing future problems with premature corrosion and deterioration. It is critical that road authorities, consultants and contractors use these structures in appropriate locations and use appropriate design procedures, detailing and construction techniques. In addition, it is vital that appropriate structural management plans be developed during the design and planning phase of a project to ensure cost effective and safe management of these higher risk structures. These plans need to include regular inspections and maintenance processes with appropriate feedback loops to enhance the management and future design of BCMS. These guidelines provide essential information regarding BCMS from the design process, installation, in-service monitoring, through to maintenance and repair procedures. The content of the guidelines include the following key topics:
A discussion on the available methods for structural designing of BCMS. In addition, durability design considerations are also included in determining the metal and coating thickness in order to achieve the desired service life.
Methods of installation and construction required to satisfy the design performance. It includes construction procedures such as handling of the BCMS, the necessary site preparation, assembly instructions, backfilling specification and consideration of construction loading.
Guidelines for structural management and inspection of BCMS. The guidelines include two major aspects, being defect identification condition rating system and suitable structural management plans.
A discussion of a number of repair methods for damaged BCMS.
A list of BCMS manufacturers in Australia as well as companies which provide repair and rehabilitation services for pre-existing BCMS is provided in the appendix. A summary of the experiences of state road authorities when dealing with design, construction and maintenance of BCMS is also included as part of the appendix.
Austroads 2011 — vi —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
1
INTRODUCTION
1.1
Background
Buried corrugated metal structures (BCMS) offer an attractive solution to under road drainage requirements due to the low cost and fast construction times achievable. BCMS are particularly suited to deep culvert installations where more traditional material types start to become less appropriate. BCMS work well in these situations due to the flexible nature of the culvert allowing soil structure interaction to develop and thus significantly improving the structural resistance of a culvert. Due to the low cost advantages offered by BCMS, their use has become widespread; however, there have been several incidents of significant failures in Australia. In most observed failures, corrosion has been a critical issue which resulted in subsequent maintenance, rehabilitation and replacement. In addition, thinner sections have been introduced to the Australian market and included in Australian Standards, potentially increasing future problems with premature corrosion and deterioration. It is critical that road authorities, consultants and contractors use these structures in appropriate locations and use appropriate design procedures, detailing and construction techniques. In addition, it is vital that appropriate structural management plans be developed during the design and planning phase of a project to ensure cost effective and safe management of these higher risk structures. These plans need to include regular inspections and maintenance processes with appropriate feedback loops to enhance the management and future design of BCMS. Another key issue related to BCMS is a general lack of expertise and technical resources/reference material. What are needed are guidelines that will provide engineers who have little experience in the application of BCMS, with a comprehensive document that addresses most of the critical issues related to BCMS.
1.2
Aims
The aims of this project are to: 1
Review existing Australian and international literature on buried corrugated metal pipe culverts.
2
Collect and report state road authority experiences with the design, construction inspection, maintenance, repair and failures of buried corrugated metal pipe culverts.
3
Develop Austroads guidelines addressing critical issues in the use of BCMS to ensure appropriate performance of BCMS over the specified design life.
4
Identify specific research and development investigations that will deliver the data relevant to understanding the performance of these structures in the Australian environment.
Austroads 2011 — 1—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
1.3
Scope
The guidelines will consider the following key elements relating to BCMS:
structural design
construction
structural management and inspection
rehabilitation.
Hydraulic performance of BCMS is outside the project scope.
1.4
Outline
These guidelines provide essential information regarding BCMS from the design process, installation, in-service monitoring, through to maintenance and repair procedures. Section 2 presents the method for designing BCMS which include structural and durability considerations. The structural design covers two design methods, the ring compression and the limit state design methods, which are described in the detail in draft AS/NZS 2041.1 (2010). A general description of the Finite Element Method (FEM) is also provided. Durability design includes the calculation of metal and coating thickness in order to achieve the desired service life. Section 3 outlines the method of installation and construction required to satisfy the design performance. It includes construction procedures such as handling of the BCMS, the necessary site preparation, assembly instructions, backfilling specification and consideration of construction loading. Section 4 provides guidelines for structural management and inspection of BCMS. This section considers two major aspects. Firstly it covers a defect identification condition rating system and the necessary information needed to be collected during the inspection in order to assess suitable treatment/repair methods. This aspect of the section is aimed primarily at dealing with structures that have not been managed well in the past and have progressed to various serious levels of deterioration. Secondly it addresses aspects of developing suitable structural management plans to adequately manage structures throughout their life ensuring that serious deterioration levels are not reached. Section 5 includes a discussion of a number of repair methods and outlines the advantages and disadvantages of each method. Section 6 concludes the guidelines and highlights areas which still require further work once the draft AS/NZS 2041.1 (2010) is completed. Appendix A provides the user of these guidelines with the list of BCMS manufacturers in Australia as well as companies which provide repair and rehabilitation services for pre-existing BCMS. The experiences of state road authorities when dealing with design, construction and maintenance of BCMS are summarised in Appendix B.
Austroads 2011 — 2—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
2
DESIGN OF BCMS
2.1
Behaviour of BCMS
BCMS are flexible members that rely on soil-structure interaction to function. This makes the response of BCMS complex. Both the soil and metal structure play a vital part in the structural design and performance of BCMS and proper installation plays a key role in ensuring that the structure performs as per the assumptions of the design. The flexible metal culvert can be considered a composite structure made up of the steel culvert and the surrounding soil. Both the barrel and the soil are vital elements in the structural performance of the culvert. As a load is applied to the culvert it attempts to deflect as illustrated in Figure 2.1 and Figure 2.2. In the case of a round pipe, the vertical diameter decreases and the horizontal diameter increases. When good embankment material is well compacted around the culvert, the increase in horizontal diameter of the culvert is resisted by the lateral soil pressure. With a round pipe the result is a relatively uniform radial pressure around the pipe that creates a compressive thrust in the pipe walls (Connecticut DOT 2000).
Source: Connecticut DOT (2000).
Figure 2.1: Behaviour of buried corrugated metal pipes
Austroads 2011 — 3—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: TMR (2010).
Figure 2.2: Ring compression theory whereby overburden and live load stresses are evenly distributed to the surrounding soil
A number of factors may affect the soil-structure interaction for BCMS including structural parameters (profile, size and stiffness), construction method (trench, embankment or tunnel), the type and placement of the backfill material, and external loading. Typically soil-structure interaction is ensured by the following:
Passive pressure reaction above the crown needs to be developed for stability by adequate depth of overburden. Minimum cover depths must be adhered to.
Soil compaction during installation must be adequate. AS/NZS 2041 requires that a value of 90% compaction be obtained in order to use the ring compression method.
The structural resistance mechanisms are formed during the incremental backfilling process and rely on soil-structure interaction. The backfilling of a culvert normally includes three stages (Pritchard 2008):
Stage 1 – placement of the culvert on a prepared base. A small amount of surcharge is placed on the culvert crown prior to placing backfill to limit the vertical deformation of the culvert.
Stage 2 – progressive placement of the backfill layers, each typically 200 mm thick, from the invert until the mid-plane horizontal axis is reached.
Stage 3 – progressive placement of the backfill layers above the mid-plane horizontal axis until completion.
For large diameter BCMS having spans of up to more than 15 m, the quality and properties of the backfill are important for the proper performance of the structure (Sandford 2000). Pritchard (2008) points out that for helical steel culverts, interaction during backfilling involves high lateral earth pressure on the culvert due to compaction of the backfill. The peak bending effects occur during incremental backfilling instead of when the maximum cover is reached, and these bending effects are not increased due to legal live loads. Thus the incremental nature of backfilling is a critical design consideration and should be a fundamental part of the design process.
Austroads 2011 — 4—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Figure 2.3 shows the possible peak bending locations during construction. The figure also shows the possible effects of rolling which highlights the need for symmetric installation of the fill layers.
Source: CSPI (2007).
Figure 2.3: Possible deformed shape due to backfill sequence
Due to the fact that BCMS are flexible buried structures, they rely on the soil-structure interaction for their strength. Proper compaction all around the culvert is a vital factor in the construction stage. Lack of proper compaction of the foundation can make the culvert deflect up and down as loads pass over it. The culvert can bulge sideways due to live load if the compaction of the soil within the culvert’s height is not sufficient. It can also be crushed due to over-concentrated live loads, if the soil on the top of the culvert is not well compacted (ARTC 2006). In addition, BCMS special features, such as stiffeners and relieving slabs, have effects on soil-structure interaction. Longitudinal stiffeners on long spans can improve compaction and live load distribution. Transverse stiffeners on the top part of the BCMS can resist peaking deformations from compaction and live loads acting on the finished structure. A reliving slab helps reinforce the soil above the crown and distribute live load on a wider area (Sandford 2000). Factors affecting the structural resistance mechanisms include minimum cover and minimum spacing for multiple installations. Detailed discussion on the minimum spacing and cover is presented in Section 2.7.5 and Section 2.7.6, respectively.
2.2
Failure Mechanisms
Failures of BCMS may result from serviceability and/or strength-related problems. General types of culvert problems include (Connecticut DOT 2000): Serviceability-related problems:
scour and erosion of streambed and embankments
inadequate flow capacity
corrosion and abrasion of culvert metal
sedimentation and blockage by debris
separation and/or drop off of sections of modular culverts
inadequate length.
Austroads 2011 — 5—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Strength-related problems:
cracking of culvert due to force-induced effects such as compression, seam failure and global buckling
undermining and loss of structural support
loss of the invert of culverts due to corrosion and abrasion causing failure of ring compression resistance
over-deflection and shape deformation.
The main sources of failure of BCMS are discussed below. 2.2.1
Corrosion and Abrasion
Working permanently in wet areas, BCMS are subjected to corrosion and abrasion due to environmental effects. Corrosion occurs in several locations such as on the surface being in contact with the soil, on the inside face at the invert where flowing water is present, or on the surface exposed to the air. It is due to aggressive agents in the air, water or the fill material such as salts, metals or other corrosive chemicals. Figure 2.4 shows an example of corrosion failure of a BCMS.
Figure 2.4: Heavy corrosion of a BCMS – considerable loss of metal thickness
Abrasion, on the other hand, occurs mainly at the invert of the structure when the flowing water contains a bed load of sand or gravel. Figure 2.5 shows an example.
Austroads 2011 — 6—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Figure 2.5: BCMS invert corroded away due to loss of granular bedding material in invert
Corrosion is the major cause of structural failure of buried metal structures. Determination of the corrosion level for buried metal structures is different to those above the ground. In the atmosphere, metal corrosivity can be predicted based on relative humidity, pollution level and temperature. In-ground corrosion is more difficult to predict since it is dependent on local variables, such as soil chemistry and water content/quality of the soil. The corrosivity of a site can be determined through the following tests of the water and soil: 1
pH condition is the indicator of whether water or soil is acidic (pH less than 7) or alkali (pH more than 7). Most of the coating material used in BCMS is expected to perform well in and around a neutral pH (pH = 7). Determination of soil pH should be in accordance with AS 1289.4.3.1. In addition, California test method 643 details the pH and resistivity test methods for both water and soil (California DOT 2007).
2
Resistivity is an indicator of the inability of water or soil to carry an electrical current and is a function of the concentration of salt ions dissolved in the water. The higher the concentration of salt ions that exist in the water, the easier it is to conduct electrical current (less resistivity), which increases the soil’s potential for corrosion. Resistivity should be determined in accordance with AS 1289.4.4.1 (1997).
3
In addition, the draft AS/NZS 2041.1 (2010) suggests that the measurement of concentrate of chloride and sulphate ions in the fill material should also be considered if the results of the tests approach the limits given in Table 2.1. The acceptable levels of chloride and sulphate ions are less than or equal to 200 ppm and 1000 ppm by weight respectively.
The results of the tests should be used in conjunction with Table 2.1 to determine the site specific potential for corrosion.
Austroads 2011 — 7—
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Table 2.1: Corrosive level determination pH
Resistivity (ohm–cm)
Normal condition
5.8–8
> 2000
Mildly corrosive
5.0–5.8
1500–2000
3 m) or large span structures (> 11 m), buckling should be checked. Where the leg is longer than 1.2 m, buckling of the straight lower section of the box culvert should be checked
serviceability deflection checks.
Suitable load factors, combination and failure criteria can be found in the draft AS/NZS 2041.1 standard. 2.5.5
Design Method Selection
For a specific BCMS, the selection of a suitable design method plays an important role in ensuring that the installation procedure, structural features and behaviour are correctly simulated. Each method has advantages and limitations. Ring compression The ring compression method is the most simplified design method for BCMS. The following parameters are the basis for selecting this method for design of BCMS:
This method is only applied to structures that are symmetrical about the vertical axis (AS/NZS 2041-1998).
This method is only valid if the metal structure has a minimum cover of correctly installed fill and adequate side support (that requires 90% compaction) so that arching of the surrounding material can occur. The purpose is to reduce the bending in the metal wall so that compression governs the design of the finished structures.
Failure of a metal structure designed by the ring compression method is assumed to occur on the horizontal axis.
The assumed modes of failure include crushing or yielding, ring buckling, and the transition zone between crushing and buckling.
Structures with rib stiffening are not recommended for the ring compression method since the rib stiffening will add significant bending stiffness to the structural wall and as a result, bending stresses will occur.
Pritchard (2008) points out that the ring compression method does not consider the incremental backfilling process and the resulting bending effects on the culvert during installation. Thus it does not represent the physical behaviour of a culvert subject to installation loadings. Consequently, it
Austroads 2011 — 29 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
may lead to either a conservative design or a lack of check against failure of the culvert during installation. Limit state method This method covers the BCMS that satisfies the following requirements:
at any point in the structure wall, the radius of curvature is not less than 2rt, where rt is the top radius (centre line of corrugation) of the structure
ratio of the radii of mating pates at a longitudinal connection is not greater than 8, except for pipe-arch structures that comply with the haunch pressure requirements
maximum difference in structural base metal wall thickness of lapping plates at a longitudinal connection is 2 mm for a thinner plate of thickness less than 3.1 mm.
This method can be used if transverse stiffeners, such as steel rib and encased concrete ribs, are present. The section properties for steel stiffeners can be calculated as cumulative, while for concrete stiffeners, as composite. Being developed from the ring compression method, the limit state method is limited by a number of factors such as:
simplified soil-structure modelling/behaviour
a set of specific failure modes
construction loading sequence, which is governed by a minimum cover.
These assumptions may be critical for BCMS requiring a high degree of design accuracy such as large span BCMS under shallow fill. For metal box structures, limit state procedures have been developed; however, these procedures are only valid for structures with the geometrical limits given in Table 2.3. Table 2.3: Structures geometrical limits for the limit analysis method Material
Span range (m)
Structural rise (m)
Crown radius (m)
Haunch radius (m)
Steel or aluminium
2.6 to 7.8
0.75 to 3.2
≤ 7.6
≥ 0.75
Aluminium
7.8 to 11.0
0.75 to 3.2
≤ 7.6
≥ 0.75
Steel
7.8 to 15.0
1.96 to 3.17
8.0 to 8.82
1.02 to 1.14
FEA method This is a rigorous limit state method using FE modelling, which can be applied to almost any BCMS. However, current practice shows that this method normally is applied to structures with special features or complex geometries that are beyond the scope of application of other simplified methods such as the ring compression or limit state method. For instance, this method is suitable for metal box structures beyond the limits given in the simplified method or when railway, aircraft or heavy off-road vehicles are involved. Bolted plate structures greater than 3.0 m in span or any bolted plate structures with transverse stiffeners can also use the FEA method. The design of BCMS with the FEA method is not limited to any shape, size and material and may be analysed to withstand dead weight, incremental soil-layer loading, temporary construction loads and surface loads due to traffic.
Austroads 2011 — 30 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Suggested design methods Table 2.4 presents the scope of application of different design methods for BCMS. Table 2.4: Selection of design methods Structure types
Span dh (mm)
Design method
≤ 3000
Limit state method or ring compression method
> 3000
Limit state method
≤ 3000
Limit state method or ring compression method
> 3000
Limit state method
All single installations ≤ 3000
Limit state method or ring compression method
All other structures
Limit state method or FEA
Bolted plate structures with longitudinal stiffeners
All sizes
Ring compression method
Bolted plate structures with transverse stiffeners
All sizes
Limit state method or FEA
Metal box structures
All sizes
Limit state method or FEA
Helically formed sinusoidal pipes Helically formed ribbed pipes Bolted plate structures
Source: Draft AS/NZS 2041.1 (2010).
2.6
Design for Durability
During a typical service life, the structural integrity of BCMS will be affected mostly by the surrounding conditions. The reduction of a structure’s durability can be credited to corrosion and abrasion. Thin wall steel structures such as BCMS rely on the ability of the structure to withstand the ring compression that occurs within the structure walls under load. It is critical that the ‘ring’ cross-sectional profile is complete and in good condition to be able to withstand the load. A number of buried metal structures within Australia have exhibited significant corrosion within as little as 6 years after installation. Many structures constructed in the 1980s and 90s are showing significant corrosion and in many instance rehabilitation measures have been taken. In other areas of Australia, BCMS have been installed for over 50 years and are showing no significant signs of deterioration. The specific site characteristics have a significant impact on the actual design life of a given BCMS configuration. The process of designing for durability can be summarised in the following steps:
determine site environmental characteristics (corrosion and abrasion levels)
determine suitable materials for a given environment
determine durability allowance —
coating loss rate
—
metal loss rate
—
service life
—
if secondary coating is necessary.
It is critical that the durability allowances/sacrificial thickness calculated in the design of BCMS be incorporated into a structural management plan which will form the basis of inspection and monitoring of the structure. Feedback loops are required to both manage the
Austroads 2011 — 31 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
performance/deterioration of the structure as well as to improve the experience base regarding durability performance for future BCMS designs. 2.6.1
Material Selection
In addition to meeting the required structural capacity, BCMS must also satisfy durability requirements which arise from their service conditions. This mainly relates to preventing the effects of corrosion and abrasion. Considerations that help increase the expected life of BCMS include:
increasing material thickness by a durability allowance
protective coatings that resist corrosion and abrasion
selection of less aggressive backfill materials
concrete lining of invert to resist abrasion
use of open bottom structures that avoid invert abrasion, such as arches or box culverts.
BCMS materials include corrugated galvanised steel and corrugated aluminium. Galvanised steel should not be used in conditions where water or wet silt is in contact for long periods, as loss rates for galvanising may be significantly higher than those for normal conditions. Aluminium has better corrosion resistance than steel for a number of applications, including coastal marine application, but has lower strength and abrasion resistance (Sandford 2000). The selection of structural materials and fill materials should take into consideration the effects of corrosion and abrasion, specifically in the following situations:
permanent water
marine or salt spray locations
aggressive soils such as clay soils, saline and sulphate soils
highly acidic or alkaline environments.
The pH value and resistivity value Rb are key factor which influence the rate of deterioration of the BCMS. Based on these two factors, the environmental conditions can be classified as follows:
non-corrosive: material acceptable when pH 5-12, resistivity ≥ 10 000 ohm-cm
normal conditions: material acceptable when pH = 5-8, resistivity = 2000-10 000 ohm-cm
mildly corrosive: material acceptable when pH = 5-8, resistivity = 1500-2000 ohm-cm
corrosive for galvanised steel: material acceptable when pH = 5–10, resistivity > 1500 ohm–cm
corrosive for aluminium: material acceptable when pH = 4–9, resistivity > 500 ohm–cm, except where structures are exposed to sea water, resistivity > 35 ohm–cm
non-abrasive: no bed load regardless of velocity
low abrasion: minor bed-load of sand and gravel, velocity > 4.6 m/s
moderate abrasion: bed-load of sand, small stone and gravel, 1.5 m/s < velocity < 4.6 m/s
high abrasion: heavy bed-load of gravel and rock, velocity > 4.6 m/s.
Austroads 2011 — 32 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Abrasion velocity should be evaluated on the basis of frequency (average recurrence interval) and duration. Consideration should be given to a frequent storm such as a 2 year event (average recurrence interval) or to the mean annual discharge or less when velocity determination is necessary. Where cementitious material is used in conjunction with aluminium structures, the barrier coating/membrane need not be applied if epoxy paint system is applied at the aluminium/cement interface unless all of the following conditions are met:
Long-term pH of the cementitious material is ≤ 12.
No additives or contaminants are present in the cementitious material. These may result in a more corrosive environment.
The cementitious material usually remains dry throughout the life of the structure (e.g. ring beam) or is relatively impermeable. Good quality well placed/compacted concrete usually satisfies these conditions.
All steel reinforcing and other dissimilar metals should be electrically isolated from the aluminium structure to prevent dissimilar metal corrosion. This may be achieved by protective coating or by physical separation via plastic packers/chairs. Once determined, the site environmental characteristics can be used to select suitable BCMS base material and coating to ensure the design working life is achieved. Table 2.5 can be used as a guide in determining appropriate material and coating for given environmental characteristics. Further information is given in the draft AS/NZS 2041.1. Table 2.5: BCMS material suitability Corrosion level
Abrasion level
Non-corrosive/ normal condition
Mildly corrosive
Corrosive
Non-abrasive/ low abrasion
Moderate abrasion
High abrasion
Galvanised steel
Yes
Yes
No
Yes
No
No
Galvanised steel with concrete invert lining
Yes
Yes
No
Yes
Yes
Yes
Aluminium
Yes
Yes
No
Yes
No
No
Polymer pre-coated galvanised steel
Yes
Yes
Yes
Yes
Yes
No
Polymer pre-coated galvanised steel with concrete invert lining
Yes
Yes
Yes
Yes
Yes
Yes
Material and coating
By providing an invert with a concrete lining, the pipe’s overall abrasion protection can be significantly improved regardless of the coating type. It should be noted, however, that aluminium reacts with cementitious linings. As a result, aluminium pipes arguably should not be combined with concrete lining.
Austroads 2011 — 33 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
2.6.2
Corrosion Allowance Methods for Durability Design
There are simplified and detailed procedures to make allowance for corrosion. The simplified procedure is used where the conditions of installation are known in general but no testing has been carried out. The soil pH may be estimated using simple methods. Where there is any uncertainty about the conditions being met, a detailed analysis should be carried out. Table 2.6 lists the scope of application for each method. For large projects with multiple pipes, the detailed procedure is used. Table 2.6: Selection of durability design method Structure Importance level 1
2
3, 4 and 5
Durability design method
Span, (dh), mm
Steel
Aluminium
< 600
Simplified procedure
Simplified procedure
≥ 600
Simplified procedure or detailed procedure
Simplified procedure and consider salts and abrasion
< 600
Simplified procedure if local experience indicates satisfactory performance
Simplified procedure if local experience indicates satisfactory performance
≥ 600, < 3000
Simplified procedure or detailed procedure
Simplified procedure or detailed procedure and consider salts and abrasion
≥ 3000
Detailed procedure
Detailed procedure
All sizes
Detailed procedure
Detailed procedure
Simplified procedure According to the draft AS/NZS 2041.1 (2010) simplified method for galvanised steel structures, no durability allowance is required if the following set of criteria is met:
the structures are of lesser importance – less than 1.5 m in height or width and not expensive to replace
design working life is ≤ 30 years
no permanent water
not exposed to airborne salts, salt or brackish water within 20 km of the coast and not on an estuary
no corrosive run-off from mines or industry in the area
the fill is usually dry and is free draining
the pH of the local soil and fill material is within the range of 5 to 8
the local soil is not saline nor does it contain sulphates
abrasion is low.
If the design working life is 50 years and the span is up to 3.0 m, a 1 mm durability allowance may be required. For aluminium structures, the above criteria apply, except that the pH of the local soil and fill material is within the range of 4 to 9 instead of 5 to 8.
Austroads 2011 — 34 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Detailed procedure The structure is required to have sufficient wall thickness for the design loadings throughout its entire service life. The base metal starts the corrosion process after a number of years in service when the protective coating is completely lost. When the design service life of the structure is reached, the remaining wall thickness is no less than the minimum thickness determined by the structural design. The detailed durability design procedure according to the draft AS/NZS 2041.1 (2010) includes the following steps:
Step 1: Determine the corrosion loss rate of the structure (steel loss rate and galvanising loss rate) for the pH and resistivity of the fill material under consideration from Table 2.7 and Table 2.8, respectively.
Step 2: Take the average of the loss rate and calculate the loss over the design working life for the base metal and any coating.
Step 3: Determine the life of the coating by dividing the protective coating thickness by the average galvanising loss rate calculated in Step 2. The protective coating thickness is determined from relevant standards based on the thickness of the base metal as shown in Figure 2.9.
Step 4: Determine the durability allowance for the base metal required to attain the design life. The allowance thickness is no less than the total loss of the base metal thickness during the remaining design life of the structure after the protective coating is completely lost.
Step 5: Determine other necessary protection such as invert lining, lining and additional coating. Table 2.7: Average base metal loss rate per side Soil condition pH
Chlorides
Range of metal loss rates (µm/yr) Resistivity (ohm–cm)
Drained soils
Undrained soils
In soil (%)
In water (ppm)
Steel
>5
< 0.5
> 1 000
> 5 000
< 10
< 10
4–5
0.5–2
1 000–10 000
2 000–5 000
< 10
10–20
3–3.9
2–5
10 000–20 000
1 000–2 000
10–20
20–40
5
> 20 000
< 1 000
10–40
40–300 Aluminium
4–9
1.0–1.5
< 20 000
> 500
Source: Draft AS/NZS 2041.1 (2010).
Austroads 2011 — 35 —
5–7
5–13
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Table 2.8: Average galvanising loss rate Soil pH
Range of average galvanised coating loss rate (µm/yr) Drained soils
Undrained soils
6.5
> 20
4–4.9
2.6–5.2
6.7–13.3
5–7.9
2.2–4.3
5.5–11.0
8–9
3.3–6.5
6.1–12.1
>9
> 8.6
> 17.2
Soil resistivity (ohm–cm)
All soils
< 500
> 3.5
500–1000
1.5–3.5
1000–2000
1.3–1.5
2000–5000
0.9–1.5
> 5000
< 0.9
Source: Draft AS/NZS 2041.1 (2010).
Table 2.9: Average galvanised thickness for steel sheet Product type
Sheet thickness (mm)
Galvanised thickness average (local min.) (µm)
Helically formed pipes (AS 1397–2001)
≤2
45.5
>2
47.6
< 1.5
45
1.5–3
55
3–6
70
>6
85
Bolted plate structures (AS/NZS 4680–2006) Source: Draft AS/NZS 2041.1 (2010).
The thickness of galvanised coating may be assumed to be 1 µm for each 7.15 g/m2 coating weight. The draft AS/NZS 2041.1 (2010) provides reference to AS 1397 (2001) for galvanising to Z600 and AS/NZS 1734 (1997) for aluminium. For polymer coatings reference is made to ASTM A742. For aluminised Type 2 coating, the average coating thickness of 48 µm per side of the steel sheet may be assumed (Ault & Ellor 2000). The following should be noted:
Non-aggressive fill materials have pH and resistivity levels within the limit of Table 2.10. Materials with pH and resistivity outside these limits are considered highly aggressive and are not recommended for metal structures. Alternatively, other protection should be considered.
Where abrasion, ponding or drip flows may affect the structure or where the internal surface will be subject to aggressive corrosion, an invert lining should be considered.
The atmospheric corrosion loss on the interior surface of the structure above the level to which water regularly rises is generally considered negligible.
Austroads 2011 — 36 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Additional protective coatings are recommended in coastal installations where extended life is required.
In Table 2.7, the worst case for pH, chlorides or resistivity is used.
In Table 2.8, the worst case for pH and resistivity is used.
Aluminium is vulnerable to soil or water with concentrations of copper greater than 1 ppm and mercury greater than 0.1 ppm. Table 2.10: Expected life and metal loss rates vs. pH and resistivity Resistivity
Acceptable pH range Galvanised steel
Aluminium
≥10 000
5–12
4–9
2 000–10 000
6–10
4–9
500–2 000
5–10
4–9
Galvanising life (years)
Steel loss rates (µm/y)
10–15
1–30
-
-
Aluminium loss rate
Negligible
Source: Draft AS/NZS 2041.1 (2010).
Alternatively, look-up charts can also be used to estimate the year of perforation of a galvanised coated structure. The example in these guidelines is a widely used method from the US, the American Iron and Steel Institute (AISI) chart, as shown in Figure 2.17, and the California Method chart (Ault & Ellor 2000). The AISI is similar to the California chart method except for in-service life values. It assumes the position of 25% perforation metal lost in the invert whereas the California method assumes maintenance-free service life. The AISI chart predicts the service year for a BCMS with 1.6 mm wall thickness. For greater wall thicknesses the multiplication factors in the chart should be used. The same chart can also be used for aluminium coatings by multiplying the service life obtained for galvanised coatings with a factor. The CSP durability guide (NCSPA 2000) suggests a factor of 1.3 in appropriate environmental conditions and the FHWA suggests a factor of 3.5 if only the water side corrosion is considered. There were specific charts developed to estimate the-year-to perforation for aluminised Type 2 coating, such as that developed for the Florida Department of Transportation (Ault & Ellor 2000).
Austroads 2011 — 37 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: NCSPA (2000).
Figure 2.17: AISI chart for estimating average invert life for galvanised BCMS
No studies were found which have investigated the relevance and/or compatibility of these charts for use in Australia. A designer intending to use the above charts should be aware of the expected difference in performance as the charts were developed based on the historical BCMS performance in the US which is different from the Australian environment. Further work on observing the performance of BCMS in Australia and comparing them to the estimate provided by the charts is essential before adopting the chart as a design tool. No data is provided in the draft AS/NZS 2041.1 (2010) for the loss rate of polymer pre-coated structures. The CSP Durability Guide (NCSPA 2000) provides the estimation of service life for polymer pre-coated structures as well as other non-metallic coatings. 2.6.3
Site Investigations/Tests
Site environmental characteristics To adequately assess the site environmental condition, the water and soil pH level and resistivity should be measured. The level of aggressiveness is a measure of how quickly the environmental condition of the site contributes to the development and progression of corrosion and abrasion in the case of a structure designed for flowing water.
Austroads 2011 — 38 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
The detailed investigation of deterioration of buried metal structures in Australia has suggested that three areas of BCMS be considered:
The soil side is the outer surface of the structure which is in direct contact with fill material. The existence of the ground-water table should be investigated and the pH and resistivity of the fill should be determined. Most soils fall in a pH range of 6 to 8 which is favourable for durability. Soils in areas of high rainfall tend to be more corrosive with lower pH values (acid soils). High clay content soils are more corrosive because they tend to retain water longer than granular soils.
The water side is the inner surface of the structure which comes regularly in contact with water, usually the pipe invert. The loss of material thickness at the invert is usually the critical area which controls the service life of BCMS. As a minimum, the following parameters should be determined:
—
water and soil pH and resistivity
—
flow velocity and bed-load.
The atmospheric exposed surfaces – are the inner and outer surfaces of the structure which are not in contact with water or soil but will be exposed only to the atmospheric condition of the site. From historical observation, the corrosion rate at this location of the pipe is minimal and can be neglected. The exception should be made when the structure is expected to be continuously inundated where the atmospheric conditions can be treated as being of moderate corrosivity. The exception should also be made when the structure is close to the sea.
The level of corrosion potential of the soil and water can be determined based on the result of the pH and resistivity test within the range set in Table 2.11. The most severe corrosion due to pH or resistivity should be considered for durability design. Table 2.11: Corrosive level determination pH
Resistivity (ohm – cm)
Normal condition
5.8–8
> 2000
Mildly corrosive
5.0–5.8
1500–2000
4.6 m/s
Austroads 2011 — 39 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
2.7
Detailing
The following details should be taken into consideration when designing the BCMS:
footing details
longitudinal stiffeners
end treatment, including end stiffening ring beams and batter protection and headwalls
invert lining.
2.7.1
Footings
Design of footings shall include the following:
geotechnical investigation of the foundation material
full coordination of the design of the metal structure and the footing
design of the footing using an accepted method.
Forces acting on footings for arch structures For arch structures the design of footings should use the vertical and horizontal components of compression force acting on the bottom part the wall (Fb) as illustrated in Figure 2.18. The vertical and horizontal forces are therefore defined using Equation 4 as follows:
Fv = Fb Cosθ Fh = Fb Sinθ where Fb
=
arch footing force, in kN/m run of structure
FV
=
vertical arch footing force, in kN/m run of structure
Fh
=
horizontal arch footing force, in kN/m run of structure
θ
=
arch re-entrant angle, in degrees.
The following factors should also be considered when designing BCMS footings:
variation in footing design along the length of the BCMS as a result of load variation
potential scouring of footing when continuous water flow is to be expected
differential settlements
—
provision of a construction joint to separate concrete lining from the footings
—
between footings
vertical and horizontal forces during construction.
Austroads 2011 — 40 —
4
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
It is undesirable to make the corrugated metal arch stiffer or unyielding compared to the adjacent side fill. The use of massive footings or piles to prevent any settlement of the arch is generally not recommended. Providing for some arch settlement helps to induce positive soil arching and avoids possible drag down due to consolidation of the adjacent side fill.
b
b
b
b
b
b
Source: Draft AS/NZS 2041.1 (2010).
Figure 2.18: Arch footing forces
Austroads 2011 — 41 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
2.7.2
Longitudinal Stiffeners
Longitudinal stiffeners are used with bolted plate structures. The stiffeners transfer transverse compression in the wall to the fill. The fill resists forces by passive pressure against the vertical face of the longitudinal stiffener and a portion of the side wall of the structure. Details of a typical stiffener are illustrated in Figure 2.19.
Source: Draft AS/NZS 2041.1 (2010).
Figure 2.19: Typical longitudinal stiffener detail
For the design of the longitudinal stiffener, the draft AS/NZS 2041.1 (2010) provides a suitable method. 2.7.3
End Treatments
End treatments are critical to the longevity of BCMS. One of the fundamental requirements for adequate performance is to protect the culvert from piping/erosion of the backfill. Aside from perforated inverts due to corrosion, erosion around the ends of the culvert is one of the major problems with protecting the integrity of the backfill material. End treatments are a key element to ensuring erosion does not occur. While it is acknowledged that using concrete headwall structures with cut-off walls is very effective in controlling these piping/erosion effects, sometimes these ‘standard details’ do not work. The designer needs to investigate and understand the site so as to control these effects. Key factors that must be considered include the likelihood of overtopping, debris load and the potential for culvert blockage. There are a number of end treatments designers can consider for BCMS: (a)
(b)
Headwalls – are entrance structures that protect the embankment from erosion and improve the hydraulic efficiency of the culvert. They provide embankment stability and protection from buoyancy. Properly designed, they shorten the required structure support length and reduce maintenance damage. They also provide structural protection to inlets and outlets. Headwalls should be considered for multiple hydraulic installations (BCMS which are expected to accommodate flowing water) with dh ≥ 900 mm. For installation where dh < 900 mm, batter protection is sufficient. Cut-off walls and aprons – are used at the inlets of the culvert to prevent scouring and undermining from high headwater depths or from approach velocity in the channel to eliminate clogging by vegetation growth. They are used to improve hydraulic efficiency at the inlets. Most aprons include a cut-off wall to protect them from undermining. Cut-off walls
Austroads 2011 — 42 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
should be considered for the upstream end of all hydraulic structures, and where necessary on the downstream end. They are typically reinforced concrete with minimum depth of 600 mm. A typical cut-off wall and apron is illustrated in Figure 2.20.
Source: Draft AS/NZS 2041.1 (2010).
Figure 2.20: Typical cut-off wall and apron details
(c)
Batter protection – provides stability for the embankment adjacent to and around the ends of BCMS and to protect the ends from adverse hydraulic effects. Batter protection includes use of gabions, armoured rocks, mass concrete and reinforced concrete (Figure 2.21).
Source: CSPI (2007).
Figure 2.21: End treatment using gabions
(d)
End-stiffening ring beams are not required for structures with dh < 900 mm. For structures with dh ≥ 900 mm, Table 2.13 should be consulted for the end-stiffening ring beam requirement. A typical end-stiffening ring beam is illustrated in Figure 2.22.
Austroads 2011 — 43 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Table 2.13: Requirements for end stiffening ring beam End treatment Skew number*
Vertical square cut end
< 55
Stepped or bevelled to embankment slope 1V:1H
1V:1.5H
1V:2H
Flatter than 1V:2H
This skew number is not recommended
Skew in degree
35
55–74
Yes
Yes
Yes
Yes
Yes
16 to 35
75–84
No
No
Yes
Yes
Yes
6 to 15
85–95
No
No
No
No
Yes
5
96–105
No
No
Yes
Yes
Yes
6 to 15
106–125
Yes
Yes
Yes
Yes
Yes
16 to 35
> 125
This skew number is not recommended
* For detail on determining the skew number see Figure 2.23. Source: Draft AS/NZS 2041.1 (2010).
Source: Draft AS/NZS 2041.1 (2010).
Figure 2.22: Typical end stiffening ring beam/headwall
Austroads 2011 — 44 —
> 35
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: Draft AS/NZS 2041.1 (2010).
Figure 2.23: Diagram for indicating skew number
(e) 2.7.4
Manufactured end finishes – are made to suit the site conditions. Standard end finishes are square ends, step bevels, skews, full bevels and skew bevels. Invert Lining
The most common invert protection method is by lining the invert with reinforced concrete. The lining serves as a protection of the surface from abrasion and corrosion. The installation of invert lining is recommended to take place after initial flexure and settlement ceases, typically six months after BCMS installation. The connections between the lining and the wall are usually achieved by welding of reinforced bars. There is a concern, however, about heat damage on the BCMS surface around these connections. As an alternative, the use of reinforced concrete shear keys can be considered (Luczak et al. 2009). The shear keys are installed by cutting a rectangular hole into the wall of the BCMS to form a small hole in the backfill which is filled with reinforced concrete that is continuous with the invert lining. 2.7.5
Spacing
Multiple barrels are often used to obtain adequate hydraulic capacity under low embankments or for wide waterways. Sometimes multiple barrels may be prone to clogging as the area between barrels tends to catch debris and sediment. When a channel is artificially widened, multiple barrels placed beyond the dominant channel may be subject to excessive sedimentation (Connecticut DOT 2000). For multiple installations, a minimum space between structures is required to ensure that adequate backfill support is provided to the structure and the fill above. This is also to ensure an adequate room for compaction. Table 2.14 represents the required minimum spacing for multiple installations of different span ranges.
Austroads 2011 — 45 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Table 2.14: Minimum spacing for multiple structures Span dh (mm)
Minimum spacing between structure (mm) Select and modified fill
Flowable fill
dh ≤ 900
300
150
900 < dh ≤ 3000
dh 3
150
3000 < dh ≤ 5000
1000 (2000 for bolted plate structure)
200 (400 for bolted plate structure)
5000 < dh ≤ 8500
1000 (2000 for bolted plate structure)
300 (600 for bolted plate structure)
To be determined by limit state or finite element analysis
To be determined
dh > 8500 Source: Draft AS/NZS 2041.1 (2010).
2.7.6
Cover
A minimum cover is required to ensure that bending moments due to live loads are restricted to levels which may be safely neglected in the design. For bolted plate structures with shallow corrugations, the minimum cover is the greater of:
600 mm for highway application and 700 mm for railway applications
dh 6
d 400 h . dv
dh dv 2
where dh and dv are the effective horizontal and vertical geometrical dimension of the BCMS, in millimetres, respectively. For bolted plate structures with deep corrugations, the minimum cover is the smaller of 1500 mm and the minimum design cover for shallow corrugations for the same structure size. For the other imposed loads and during the construction stage, the minimum cover is determined based on following considerations:
The minimum cover should be calculated for the design loads.
The minimum cover for these loads is significantly higher than the relevant highway or railway design loads and should be determined using specialist engineering advice.
Prior to construction commencing a design check shall be undertaken for the actual construction equipment to determine if additional cover is required.
2.7.7
Location and Alignment Considerations
Aspects of location and alignment are critical to the overall success of BCMS. The Canadian handbook of steel drainage design (CSPI 2007) provides some useful information on this topic of which the key aspects are summarised below. Proper location is important because it influences the adequacy of the opening, maintenance of the culvert, protection from flooding of adjoining improvements, and possible washout of the roadway. It is necessary to consider the adjoining property both upstream where ponding may be an issue
Austroads 2011 — 46 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
and ensuring the downstream side has safe exiting velocities in order to avoid undue scour or silting downstream. Since a culvert is a fixed line in a stream, engineering judgement is necessary to locate the structure particularly due to the variable nature of streams over time. The key principles of alignment can be summarised as follows:
Provide the stream with a direct entrance and direct exit to avoid retarding the flow. To achieve a direct entrance and exit the channel may need to be modified or a skew culvert alignment adopted.
Reasonable precautions are needed to prevent the stream changing course near the ends of the culvert. This may include suitable end treatments like steel or concrete end sections, riprap, grass or paving to assist in avoiding erosion.
Culverts for drainage of cut and fill sections on long, descending grades should be placed on a skew of about 45 degrees across the roadway to ensure the flow of water will not be retarded.
A broken alignment under a roadway may be appropriate in long culverts where differing alignments at the entry and exit would be beneficial.
Figure 2.24 shows two examples of improved alignments through channel changes. Figure 2.25 presents various methods of obtaining correct culvert alignment.
Source: CSPI (2007).
Figure 2.24: Improved alignments through channel changes
Austroads 2011 — 47 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: CSPI (2007).
Figure 2.25: Various methods of obtaining correct culvert alignment
The ideal grade for a culvert will ensure neither silting nor excessive velocities and scour. The reader is referred to CSPI (2007) for additional information on suitable hydraulic performance. A slope of 1 to 2% is advisable. In general a minimum slope of 0.5% will avoid sedimentation. A culvert should be long enough so that the ends do not clog with sediment or become covered with settling, spreading embankment material. Pre-cambering needs to be considered for culverts under high fill situations to ensure that excessive settlement does not result in a low spot in the centre of the culvert which may result in water ponding or excessive sediment build-up, both of which may result in accelerated deterioration of the culvert. Multiple barrels are often used to obtain adequate hydraulic capacity under low embankments or for wide waterways. Sometimes multiple barrels may be prone to clogging as the area between barrels tends to catch debris and sediment. When a channel is artificially widened, multiple barrels placed beyond the dominant channel may be subject to excessive sedimentation (Connecticut DOT 2000).
Austroads 2011 — 48 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
3
CONSTRUCTION GUIDELINES
Construction is a critical aspect of the successful long-term performance of BCMS. In order for the design theories in Section 2 to be valid, proper construction procedures for BCMS will need to be followed as outlined in this section. The construction procedures should consider:
material handling
site preparation
backfilling
invert protection
final structure shape.
In addition to these guidelines the designer should also consider appropriate standards and manufacturer specifications, such as AS1762-1984, AS 3703.1-1989, AS/NZS 2041 (1998) and when it is released AS/NZS 2041.1, which will supersede the previous document.
3.1
Material Handling
3.1.1
Material Delivery
The flexible nature of BCMS should be considered in all aspects of handling, including the process of unloading onto the site. The pipes can generally be moved around the site using one of the following methods described by the Australian Rail Track Corporation Ltd (ARTC 2006) and shown in Figure 3.1:
Slings, which can be installed in one of two ways —
for small diameter pipes, in short lengths, a sling can be attached to either end of the pipe using end hooks. Care should be taken in lifting and lowering of the pipe to avoid damage at pipe ends
—
for larger diameter pipes, a sling can be installed around the circumference of the pipe at two different locations. The pipe is then lifted at each of these points. The two slings should not be separated by any more than 15 pipe diameters.
If a pipe is being removed off a truck, a support rope should be slung around the pipe to ensure that the rolling speed can be controlled. Using the restraining rope, the pipe can then be rolled down a timber ramp onto a flat surface. Pipes may also be rolled across a flat foundation; however, the path should be free from all abrasive obstacles to prevent damage occurring to the coating.
Bolted structures typically arrive on site nested in bundles and can be unloaded using a crane.
Austroads 2011 — 49 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: ARTC (2006).
Figure 3.1: Pipe unloading arrangement
3.1.2
Handling Damage
Before the BCMS is installed it should be checked for any damage to the structure or protective coating. Although BCMS may still be able to function effectively with minor damage, damage around either end will result in problems in creating a watertight connection using the coupling bands (ARTC 2006). Damage to the protective coating may also cause problems for the corrosion resistance of the structure. Therefore, any damage in the coating should be repaired before installation. To repair the coating the damaged area should first be cleaned to remove any existing dirt, loose or cracked coating and any corrosive residue, which will impede the reapplication of the coating. This removal can be done using sand blasting, power disk sanding, or wire brushing. A solvent can also be used if oil or a grease material is present on the structure. After the surface has been cleaned and left to dry, the coating may be reapplied. The American Natural Research and Conservation Service, (NRCS 2001), suggests that if more than 0.2% (or approximately 77 cm2) of the metallic coating is damaged then it should be rejected. For non-metallic coatings, the following is suggested (NRCS 2001):
For bituminous coatings: the re-application should be a minimum of 1.30 mm thick after hardening. The structure should be rejected when the total area of breaks exceeds 0.5% of the total surface area (or approximately 230 cm2) for a given pipe.
Austroads 2011 — 50 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
For polymer coatings: the re-application should be a minimum of 0.25 mm thick after drying. The structure should be rejected when the total area of breaks exceeds 0.5% of the total surface area (or approximately 230 cm2) for a given pipe.
3.2
Site Preparation
3.2.1
Installation Type
There are two common methods of installation of BCMS, a trench or an embankment installation. A trench installation, as seen in Figure 3.2, occurs when the culvert is placed within a trench of a controlled width, along the natural ground surface or compacted fill. AS/NZS 2041 (1998) specifies the minimum trench width on either side of the structure as 300 mm and 150 mm for select fill and flowable fill respectively. When released the draft AS/NZS 2041.2 standard should be consulted.
Source: AS/NZS 2041-1998.
Figure 3.2: Trench installation
In comparison, an embankment installation, as seen in Figure 3.3, occurs when a culvert is placed along the natural ground surface, or compacted fill, and has an embankment constructed above it. AS/NZS 2041 (1998) states that for installing a structure in an embankment, the select fill should extend a minimum distance equal to the span of the structure on each side, subject to geotechnical investigation. For very large diameter structures with minimum cover, the dh min at mid-height of the culvert is not considered acceptable for defining the selected backfill zone. A lateral distance of dh/2 from the wall of the culvert at a vertical location of 10% of dn below the crown should be used to ensure there is adequate select fill in the upper quadrant areas.
Austroads 2011 — 51 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: AS/NZS 2041 (1998).
Figure 3.3: Embankment installation
AS/NZS 2041-1998 assumes that all installations are considered to be of the embankment type, unless the soil that surrounds the select fill, used in the backfilling operations, has a strength and stiffness higher than or equal to that of the select fill. Installation of multi-structures should consider the minimum spacing between structures as specified in the draft AS/NZS 2041.1. 3.2.2
Grade
The grade of the pipe should be developed in the design process, taking into account the site conditions. A minimum fall rate of 0.5% is suggested to prevent sediment forming within the culvert, whilst at the same time preventing scouring at the outlet. The height at which the culvert is installed in relation to the river bed is also important, as incorrect installation will result in scouring. 3.2.3
Camber
Pipes which are installed on top of foundations that are expected to settle, which is often the case for those constructed using high levels of embankment fill (over 4.0 m), will need to incorporate a camber into their initial installation process (ARTC 2006). To establish a camber the pipe is installed using two different grades, usually a flat grade on the upstream section and a steeper grade on the downstream section of pipe (Connecticut DOT 2000). This causes a high point to form in the centre, as seen in Figure 3.4. Over time, as the weight of embankment causes settlement, the camber will flatten out, resulting in a consistent grade.
Austroads 2011 — 52 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: Connecticut DOT (2006).
Figure 3.4: Camber under a high fill
It is important to consider the installation and soil conditions to determine if a camber is required and if so, the appropriate grade at which to install it. Lack of a camber in high-settling foundations may result in a disruption to the watercourse over time, as the pipe slumps in the centre. 3.2.4
Foundation Requirements
To ensure the long-term structural integrity of BCMS it is important to install them onto a firm, stable and uniform foundation. Therefore, before commencing installation it is important to check the quality of the natural foundation and take remedial action if required. Although the foundation material should provide adequate support for the entire length of the structure it should not be stiffer than the undisturbed ground that supports the backfill on either side of the structure. In addition to this it is also important that the structure is not as stiff as the associated side fill. Installing side fill that is less compressible than the structure, allows positive soil arching to occur. It is generally not recommended that massive footings or piles be used to compensate for the settlement of the foundation. It is, however, acceptable to reduce some of the structure’s settlement, using other means such as a camber within the installation. This helps to induce positive soil arching and may reduce the possibility of drag down, caused by consolidation of the side fill. Before commencing installation the natural foundation should be assessed by the appropriate geotechnical personnel, in regard to its ability to provide uniform support over the entire structure. It should consist of a bed of dense, finely and evenly graded material, which has been well consolidated. If the foundation is deemed to be suitable, it should be prepared to a level 75 mm below the invert level of the structure. This is known as the bedding layer (Section 3.2.5) as shown in Figure 3.5. Rock foundations can be a source of major problems as they can cause the structure to flatten out along its invert. As a result the rock should be excavated to a depth of 250 mm or dh/4, whichever is less, below the structure’s invert, as seen in Figure 3.5 (b). The minimum width of the excavation should be equal to the structure’s internal span or diameter, ensuring that no part of the structure is able to bear directly on rock, which would result in a non-uniform loading. The excavated foundation should be filled with a compacted select material to a level 75 mm below the invert.
Austroads 2011 — 53 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Soft or unstable soil foundations (such as highly plastic clays and silts) may result in excessive areas of slumping along the structure, which can cause:
deformation of the structure
high tensile forces that may induce separation of the seam along the invert
ponding
scouring of the underlying bedding, due to ponding and seam separation
sinking of the bedding and backfill into the underlying layers.
To prevent these impacts from occurring, it is suggested that the soft or unstable soil should be removed and replaced with an appropriate compacted material which may provide adequate support. Depending on the strength of the soil, geotechnical advice may need to be sought to determine the extent of soil which should be replaced. As a general recommendation a minimum depth of 250 mm of the inadequate soil should be removed and replaced with a suitable compact material, such as quarry waste, leaving a 75 mm depth for the bedding. The width of the excavation should be two times the internal span, or diameter, of the structure width, as seen in Figure 3.5 (a). When released the draft AS/NZS 2041.2 standard should be consulted for any variations of these parameters. To prevent the material from sinking into the underlying soil, a layer of geotextile fabric may need to be installed to separate the new and existing layers of material.
Source: AS/NZS 2041 (1998).
Figure 3.5: Bedding on soft, rock and firm foundations
Austroads 2011 — 54 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
All foundations, regardless of material, need to be free of protruding stones, large hard lumps, roots and other foreign matter (Connecticut DOT 2000). All soil lumps larger than 75 mm should be removed if they are in the top 75 mm of the foundation to ensure correct bedding requirements are met (ARTC 2006). All foundations should be excavated so that they conform to the curvature along the structure’s invert. This may allow also for more efficient compaction of the backfill under the structure’s haunch zones. 3.2.5
Bedding
An appropriate bedding material is required under all installed structures. The purpose of the bedding is to ensure that the load is distributed evenly, and to even out irregularities in the foundation. It is recommended that a uniformly deep 75 mm layer of uncompacted coarse granular bedding is placed over the top of the foundation. It should be levelled and contain suitable material which is able to fill in along the structure’s corrugations. In particular structure types, such as a pipe arch, the bedding should not be placed in the haunch zones. A suitable bedding material should be a well graded coarse sand or gravel, with a maximum particle size of 12 mm. The minimum width of the bedding under the structure should be one-third of the structure’s effective horizontal dimension (dh). If the foundation material is acceptable it may be racked to loosen the foundation material to create a suitable bedding layer. The bedding layer should also be shaped to ensure it conforms to the curvature of the structure to ensure efficient compaction. Appropriate precautions and techniques should be incorporated into the structure design to ensure that the bedding is safe from erosion and scouring. This can be achieved using cut-off walls at the end of the structure to prevent water ingress or using other suitable devices (see Section 2.7.3 for suitable end treatment).
3.3
Pipe Assembly
3.3.1
Assembly Instructions
To ensure correct assembly procedures the pipe should be installed according to the manufacturer’s specifications. For helical lock seam corrugated steel pipes the first step in the process is to lay the first pipe at the downstream end. The coupling band is then fitted around the laid pipe before the next pipe is butted up against it (usually allowing a gap of around 5 to 10 mm depending on the location of the corrugations). The coupling band should overlap both pipe sections equally. This process should be continued for the laying of each pipe. When it is certain that the pipes have been correctly aligned, the bolts on the coupling band should be tightened (ARTC 2006). For multi-plate structures there are four methods of assembly (CSPI 2007): 1
Pre-assembly of rings – where circumferential rings of round structures have been pre-assembled off-site. The rings are then transported to the site for connection along their circumferential seams.
Austroads 2011 — 55 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
2
Complete pre-assembly. The pre-assembly can be done at the factory, usually for relatively short pipe lengths and for small span projects (size is limited by shipping/transportation). The pre-assembly can also be done on the project site for structures to be lifted or skidded intact into place.
3
Plate-by-plate assembly. This is the most common method of assembly where structures are assembled directly on the prepared bedding in a single plate-by-plate erection sequence, starting from the invert, then the sides and finally the top. As few bolts as possible should be used to align the plates. Only after part of the structure has been assembled into shape by partial bolting, can the remaining bolts be inserted and hand tightened. Once all the parts have been aligned, the bolts should be torqued with a power wrench.
4
Component sub-assembly. The components are usually divided to the bottom plates, the side plates and the crown plates. These are assembled away from the bedding allowing the assembly process and the construction of the foundation and bedding to be carried out at the same time. The process is illustrated in Figure 3.6 which is also useful in demonstrating the backfilling sequence.
Austroads 2011 — 56 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: CSPI (2007).
Figure 3.6: Component sub-assembly method for multi-plate structure
Austroads 2011 — 57 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
The bolts used should have an acceptable bolt torque as specified by AS/NZS 2041 (1998) and listed in Table 3.1. Reference to AS 4100 (1998) should be made for information on the inspection of bolt tightness. To ensure that these specifications are met throughout the structure, 1% of randomly selected bolts along the longitudinal seams should be tested. If any of these bolts fail, 5% of the bolts in the circumferential and longitudinal seams shall be tested to ensure that they fall within the allowable range. Table 3.1: Bolt torque Torque range (Nm)
Structure class
Plate thickness (mm)
Steel
Aluminium
1 (bolted)
1.2–3.5
20 ± 5
10 ± 2
2
2.5–5
310 ± 40
170 ± 15
2
6–8
395 ± 25
170 ± 15
Source: AS/NZS 2041 (1998).
AS/NZS 2041 (1998) also notes that in using this table:
Bolt torque values at the lower end of the allowances are preferable to those values at the higher end, as they allow for the corrugations of lapping plates to be closely nested and aligned without being damaged by excessive bolt tightening.
Bolts and nuts used in Class 1 flanged-type structures and to connect arch structures to base channels should be hand-tightened only.
3.3.2
Shape Tolerances
Before backfilling commences the structure should be checked to ensure that it is within the required tolerances and manufacturer specifications. The structure length should not differ from the specified structure length by any more than 1%. In regard to the cross-sectional shape, the effective horizontal (dh) and vertical (dv) dimensions of the structure should not differ by more than ± 2% to the values stated in AS/NZS 2041 (1998) or specified by the designer if is a nonreferenced shape. When released the Draft AS/NZS 2041.2 standard should be consulted for additional/revised information. If the structure is considered to be a long span structure, the shape should conform to the following tolerances throughout the construction process:
Horizontal ellipses should not have diameters that vary from those specified by more than ± 2%.
High profile, low profile and pear arches should not have vertical or horizontal centreline dimensions that differ from those specified by more than ± 1%.
Inverted pear shapes should not have vertical or horizontal centreline dimensions that differ from those specified by more than ± 2%.
Austroads 2011 — 58 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
3.4
Backfilling Specifications
Due to the nature of BCMS and how loads are carried through the structure, the implementation of an effective compaction process is essential to ensure that the design performance is achieved. Incorrect compacting and soil type may result in insufficient stiffness within the backfill. This stiffness is essential in preventing the pipe from bulging when loads travel over the top of the structure. 3.4.1
Material Selection
Material selection should be in line with the manufacturer’s specifications, appropriate standards and the local availability of material for procurement. As a general guideline the ideal backfill is a well graded granular material. Care, however, should be taken if a site encounters a very fine natural material in conjunction with a high groundwater table. This situation is conducive to scouring and piping as a result of the moving water. The infiltration of the surrounding material into the side fill may also result in a loss of structural stability. These occurrences may be reduced by separating the areas with the use of a geotextile fabric. Other materials which should be avoided in backfilling operations are:
large rocks or lumps that will not pass through a 75 mm sieve
vegetation
shale, slate, clay and peat black soils as they tend to be unstable
ferrous sulphate, salts or any other substances which may be corrosive
soils with an excessive moisture content, which may hinder compaction to appropriate dry density.
Crushed sandstone with a pH between 5 and 10 is the preferable backfill material. AS/NZS 2041 (1998) suggests the select fill requirements as seen in Table 3.2, with a maximum linear shrinkage of 8%. The silt and clay component should have a maximum liquid limit of 30%. When released the draft AS/NZS 2041.2 standard should be consulted for revised/additional information. Table 3.2: Select fill requirements Sieve aperture (mm)
Mass of sample passing (%)
53
100
9.5
50 to 100
2.36
30 to 100
0.075
0 to 25
Source: AS/NZS 2041 (1998).
Other materials may be approved if they have a consistency and moisture content which still allows compaction to the specified density. The other exception is in trench installations, or where free draining fill is required, as both of these situations may allow for the use of a single sized granular material.
Austroads 2011 — 59 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Flowable or cement modified fill may also be utilised in backfill operations as a replacement for select fill. The fill should meet appropriate strength requirements of 0.6 to 3 MPa (28 days) and a modulus of 25 to 100 MPa. The definition of flowable fill can be found in Appendix H of AS/NZS 2041 (1998). The draft AS/NZS 2041.1 (2010) specifies the following soil types as select fill materials in accordance with AS 1726 (1993): 1
Importance Level 1 – soil classifications GW, GP, SW, SP, GC, SC, SM (Groups I and II)
2
Importance Level 2, where the span does not exceed 3000 mm – soil classifications GW, GP, SW, SP, GC, SC, SM (Groups I and II)
3
All other structures – soil classifications GW, GP, SW, Sp only (Group I only).
3.4.2
Compaction Process and Equipment
Compaction process Although the processes are similar for different types of BCMS, specifications within the standards do vary. The manufacturer’s specifications should also be consulted for the recommended compaction process. The process should start with compaction of fill material (select fill or natural soil) up to the haunch at the five and seven o’clock locations. It is essential that this area is compacted correctly despite the awkwardness of the location. Pneumatic compactors and other hand equipment, which are light and easy to manoeuvre, are preferred tools for the compaction around these areas. The use of granular fill material is ideal as it provides ease of compaction under the limitations compared to a clayey fill. To prevent overloading on one side, the height difference of the compacted backfill on both sides should be kept to a minimum. Each layer of compacted fill should be placed evenly on each side, with a maximum height difference at any one time of 300 mm recommended. When released the draft AS/NZS 2041.2 standard should be consulted for revised/additional recommendations. The fill material should be placed in horizontal, uniform layers, with the maximum height of each layer of fill allowable varying slightly depending on the standard. The recommendations are as follows:
helical lock-seam pipes (AS 1762-1984), each layer should be no more than 150 mm deep when compacted
long span corrugated steel structures (AS 3703.2-1989), each layer should be no more than 200 mm deep before compaction
all other buried corrugated metal structures (AS/ NZS 2041-1998); each layer should be no more than 300 mm deep before compaction.
When the draft AS/NZS 2041 standard is released reference needs to be made to this code for layer depths. Care should be taken when compacting along the side of the BCMS. It is important to ensure that there is no major change to the horizontal diameter of the structure. Horizontal props may need to be incorporated to help reduce the possibility of distortion.
Austroads 2011 — 60 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
The top third of the structure requires a great deal of care as the final layers of material are compacted. Lighter and more controllable equipment may need to be used as it is in this stage where the vertical diameter will decrease, leading to an increase in the horizontal diameter as the structure goes into ring compression. The structure should be monitored for excessive deformation. Vertical supports may need to be incorporated during this stage to limit deflection. The plumb-bob method is an effective way of monitoring the deformation. This is achieved by suspending the plumb-bob from the shoulder position (2, 10, and 12 o’clock) prior to backfilling, so that the points of the bobs are a specific distance from a marked point on the invert as shown in Figure 3.7.
Source: CSPI (2007).
Figure 3.7: Backfilling with plum-bob monitoring
Other important factors which should be considered within the compaction process are:
Compaction of fill material by pudding or jetting is not recommended.
Correct and careful tamping of the backfill is important to achieve the required quality of the compaction.
Moving construction equipment that is delivering material should travel parallel to the pipe rather than at right angles.
Compaction along the long side of the BCMS is dependent on whether or not the structure has restraining headwalls. If the structure does have headwalls the compaction process should begin at either side of the headwalls and move inwards. If, however, the structure does not have headwalls the compaction process should start at the centre of the structure and move outward.
Compaction equipment The following compaction equipment is recommended as required in AS 1762 (1984):
Hand equipment is essential for tampering under the haunches of the structure, and other small areas. It should preferably not weigh less than 10 kg and have a tamping face not larger than 150 mm x 150 mm.
Austroads 2011 — 61 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Mechanical tampers are usually satisfactory in most areas, except for the confined spaces. They should, however, be used carefully and completely over the entire area at each layer to obtain the desired compaction. Striking the structure with tamping tools should be avoided.
Tamping rollers, such as sheeps-foot and rubber-tyred rollers may be used to compact the backfill if space permits. If these rollers are used, the backfill adjacent to the structure should be stamped with hand or hand-held power equipment.
Vibrating compactors may be used to compact granular backfills.
When released the draft AS/NZS 2041.2 standard should be consulted.
3.5
Construction Loads
During the construction process it is important to consider the impact of construction equipment on the BCMS. As the backfill has only just been installed, it generally has not had time to fully consolidate and therefore reach maximum strength. In addition to this, the heavy construction equipment may impart loads well in excess of the structure’s designed service loads. If heavy equipment needs to travel across the BCMS, the structural capability under this new load should be checked. If the current structure is unsuitable for this increased loading, temporary additional cover should be installed. Only light compaction equipment should be used above the structure until the cover is equal to 0.6 m. Heavy machinery should not travel on the structure or proceed past the face of the continuous longitudinal stiffeners (long-span) until at least 0.6 m has been placed and compacted.
Austroads 2011 — 62 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
4
STRUCTURAL MANAGEMENT AND INSPECTION OF BCMS
4.1
Structure Management Planning
BCMS are high risk structures due to the high corrosion potential. This usually occurs in the invert which is critical for the structural stability of the structure. Due to the potential seriousness of a complete failure or washout, inspections of buried corrugated metal pipes are critical to the safe and effective management of these higher risk structures. All structures that are 1200 mm or greater should be part of a typical bridge/culvert inspection program. BCMS smaller than 1200 mm should also be considered due to the potential of washout; however, inspections of culverts smaller than 1200 mm become problematic due to the inability to get inside the culvert for inspection purposes. When culverts are located in medium/high risk situations (environmental hazards are significant or the consequences of failure are significant), then a structural management plan should be developed as part of the initial design and planning of a project. This should include a full program of inspection and monitoring (non-destructive evaluation to measure loss of steel plate section) to effectively manage the sacrificial plate thickness of the culvert and any other aspects that may be critical to the useful life of the culvert. With an appropriate management plan in place it is possible to optimise the materials and processes. For example, the invert can be effectively managed to maximise the life of this critical element. Through the initial period the structure evaluation team responsible for the implementation of the structural management strategy will undertake the prescribed inspections and monitoring to determine the rate of decay of the sacrificial material in the invert. This can then be compared to initial design assumptions. Using this information the rate of deterioration can be used to estimate the likely time that the sacrificial thickness will be consumed. The structural management plan can then be adjusted accordingly. For instance, the structural management plan may call for the invert to be concrete lined at a certain date to maximise the use of material. The monitoring and inspection plan will provide the information to update the plan by providing more realistic dates for such action based on the actual performance of the culvert. Structural management plans should have the following considerations and information:
design assumptions used in the initial design
requirements for initial commissioning/handover inspection. This should include a geometrical survey to assess and deformations etc. that have occurred during the construction phase. This also serves as a baseline for future measurements and management
required Level 1 and Level 2 inspection frequencies and methodology
review action dates
proposed maintenance/repair dates, e.g. invert lining
feedback loop to evaluate the maintenance/repair dates based on the inspection information
feedback loop to the design of new culverts to provide experience in design assumptions.
If suitable structural management plans are implemented, the Level 3 inspections and risk management strategies described later in this section should not be required.
Austroads 2011 — 63 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
4.2
Workplace Health and Safety
Once a structure is less than 1800 mm in height, it needs to be considered as a confined space. If the structure is longer than 20 m, confined space protocols should also be considered. Typically, for entry into a confined space an inspector needs:
appropriate training and a ticket indicating competence from a certified trainer
two or more people present on the site with only one in the BCMS at a time
consideration of the use of a safety line to prevent the rescuer from being affected
a gas detection meter.
Meters should be calibrated according to manufacturer’s recommendations and need to be ‘challenged’ before each typical entry. Challenge kits consisting of the typical dangerous gases are available from the manufacturer. The challenge kit allows the meter to be checked ensuring that it effectively detects critical gases. If on initial entry the structure is found to be in poor condition with considerable corrosion that has the potential to affect the stability of the structure, the inspection should cease until the structure is inspected and assessed by a certified structural engineer to be safe to enter. If it is deemed unsafe to enter, all work to develop a remedial strategy must be done outside the structure until suitable safety measures are put in place, such as propping.
4.3
Level 2 Structural Inspections: Defect Identification
Typical defects that affect BCMS include joint defects, invert deterioration, corrosion, shape distortion and soil migration. The cause of the defect can be a result of the construction process, in-service loading or environmental conditions. These defects should be identified as part of a Level 2 structural inspection. The following points explain the most common defects: (a)
Helical structure The most common problems associated with helical steel joints are misalignment, water exfiltration, backfill infiltration and joint separation. Misalignment of the joints may be a sign of settlement in the supporting soil structure. This settlement may have occurred during construction and stabilisation. The more serious problem is if progressive settlement is continuing to occur while in-service. Misalignment can also lead to undermining of the BCMS, water exfiltration or infiltration of backfill material. Exfiltration occurs when leaking joints allow water flowing through the culvert to leak into the supporting material. Exfiltration can result in piping where supporting soil material is easily eroded. Infiltration is the opposite problem to exfiltration and occurs where water from the backfill material is seeping through the culvert joints. Infiltration can cause settlement and misalignment problems if the water carries fine-grained soil particles from the backfill material. Joint separation may also occur due to external loads and changing soil conditions and this allows backfill infiltration and water exfiltration.
Austroads 2011 — 64 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
(b)
Bolt defects Joint defects for multi-plate structures occur at the bolt lines typically from construction damage. The bolt lines are weaker than the plate itself, and some construction specifications call for the joint to be offset in each ring to avoid a line of weakness. If this was not done, the backfilling operation could put high bending moments on the bolted joint lines, causing local cracking in the plates where excessive tension occurs (Figure 4.1). This is a defect that should be avoided in construction; however, once the BCMS is completed and backfilled, the cracked joint should go into compression and not be a long-term failure initiator. It is essential, though, that the cause of such defects be determined. If they are due to continuing vertical loading they may indicate the start of structural failure.
Figure 4.1: Cracks in metal plate probably caused by excessive side pressures during backfill
(c)
Invert deterioration Invert deterioration is usually due to a combination of corrosion and abrasion. Once the galvanising layer is abraded from material carried by the flow of the water, corrosion then attacks the bare steel and is accelerated by further abrasion that constantly removes the protective oxide layer formed by corrosion. The continuation of this action will ultimately lead to the loss of the invert and the creation of scour holes under the BCMS. Continued deterioration could result in the complete washout of the structure. It should be noted that the progression of scour holes to full washout can occur in a matter of minutes. For BCMS to withstand significant fill loads and heavy repetitive live loads, an effective soil structure interaction is necessary. This composite behaviour uses the compressive strength of the structure wall, with the compressive or bearing strength of the well-compacted soil surrounding the structure. As loads are applied to the structure, the flexible structure attempts to deflect, with the vertical diameter decreasing and the horizontal diameter increasing. The change in horizontal diameter is resisted by the lateral soil pressure and results in the relatively uniform radial pressure around the structure that creates a compressive thrust in the structure walls, hence ring compression theory can be assumed (Conn DOT 2000). As described in Section 2.1 the metal structure behaviour is reliant on uniform radial pressure around the pipe, and loss of the invert may result in severe distortion and collapse of the BCMS.
Austroads 2011 — 65 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
(d)
Corrosion Corrosion of the BCMS can ultimately cause failure by reducing the material thickness. This can occur at the invert level due to the removal of the galvanising through abrasion and standing water, or on the external face of the metal structure from chloride or sulphate attack due to the content of the backfill material. As the external face of the metal structure is not visible, the determination of metal thickness is important to establish the aggressiveness of the environment and the necessary repairs. This can be undertaken using destructive methods (hole drilling) or non-destructive methods such as ultrasonics. If holes are drilled as part of testing program, each hole must be fitted with a galvanised screw.
(e)
Shape distortion The construction process for BCMS requires care and attention to the backfilling procedure. During construction, metal structures are flexible and will distort if excessive earth pressure or construction loads are applied. In extreme cases, the metal structures can collapse or be severely distorted during construction (Figure 4.2). In assessing a BCMS in-service, it is essential to consider the cause of any distortion (out-of-roundness). Construction damage should be assessed separately to overloading, distortions or soil movements under current service conditions. Significant distortions occurring during construction should be recorded in construction files or earlier inspection reports. Unfortunately, construction records may be difficult to access after several decades. Determining when damage occurred may require structural engineering advice. As part of a structural management plan, a geometry survey plan should be undertaken as part of a commissioning inspection undertaken by the team responsible for future inspection and management of the culvert.
Source: TMR (2010).
Figure 4.2: New culvert damaged at joint during backfill, probably due to construction overload
(f)
Soil migration Soil migration occurs when there is a loss of backfill support due to water eroding fine material from the trench side walls. For migration to occur the backfill material must be erodible and there must be a flow path for the water. Due to the fact that granular material is typically used as BCMS bedding, any perforation of the structure by corrosion can lead to floodwater washing the bedding sand out. Any significant flow of water behind the metal structure can lead to severe loss of backfill followed by embankment collapse, so repairs to structure perforations should be undertaken as quickly as possible before the next wet season.
Austroads 2011 — 66 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
(g)
Crimping of the wall (CSPI 2007) Crimping is local bucking of the shell into a large number of waves, each of relatively small length. It predominately occurs in the compression zone of the wall where the culvert undergoes large bending deformations. This type of crimping usually takes place in culvert wall segments of relatively small radius of curvature. It indicates that the soil behind the segment is not dense enough to prevent excessive bending deformations. Crimping can also occur throughout an entire culvert wall section subject to excessive thrust while being supported by a very well compacted backfill. This situation is rare but can occur in culverts built on relatively yielding foundations. In this instance the long-term foundation settlement is thought to induce negative arching thus subjecting the wall to greater thrust loads than assumed in design, resulting in buckling of the wall.
(h)
Distortion of bevelled ends (CSPI 2007) Bevelled ends are particularly vulnerable to damage by horizontal pressures due to the inability to develop ring compressions. They are also vulnerable to heavy pieces of equipment falling on them or impact from debris.
(i)
Excessive silt build-up Excessive silt can result in reduced flow volumes through the culvert and provides a medium to ensure that moisture remains in contact with the metal structure for longer periods of time. This results in accelerated corrosion of the culvert invert. Such build-ups need to be removed as part of ongoing maintenance and assessments need to be made as to why it is occurring. It might be the sign of a more pressing problem. Mitigation strategies should also be suggested.
(j)
Surrounding soil condition Erosion and undermining of the culvert need to be identified including the condition of the various forms of end treatments.
4.4
Level 2 Structural Inspections: Condition States
The following is an extract from the Bridge Inspection Manual (VicRoads 2004). It provides suitable descriptions for the various condition states. The following condition states apply to all steel pipes, painted or galvanised, circular, elongated or elliptical. Condition state 1. There is no evidence of rust or corrosion and the paintwork or galvanising is in good condition. The line and invert of the pipe is straight with no water being retained in the pipe. Condition state 2. Surface or spot rusting may be evident and the paint system is no longer effective. There is no corrosion of the metal occurring. The line of the pipe is straight, but minor settlement may be allowing some water to be retained in the pipe. Condition state 3. The paint system has failed and pitting corrosion is prominent especially at normal water level. Loss of section has occurred but there is still adequate section left to not affect serviceability of the pipe. There may be some deviation of the line of the pipes due to local buckling, or moderate settlement of the pipe may be allowing a significant amount of water to be retained in the pipe.
Austroads 2011 — 67 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Condition state 4. Heavy corrosion is occurring and the invert of the pipe may have corroded out in areas. There may be large deviation of line of the pipe due to buckling of plates or plates may have crinkled at the bolt line in large diameter pipes. An excessive amount of water may be retained in the pipe. Bolts may have torn through the plates or split the plate edges allowing differential movement and buckling of plates. Condition state 5. Immediate closure failure is imminent.
4.5
Level 3 Structural Inspections: Information Collection
Level 3 structural inspections are typically undertaken when a Level 2 inspection has identified components that are in a condition state 3 or 4 and a structural inspection is needed to assess:
safety of the structure
causes of stability issues
repair methods
replacement.
When inspecting BCMS on site, the following information should be collected to aid in selecting a suitable repair method and the design process:
BCMS type (multi-plate/helical)
size and shape
corrugation (pitch x depth)
height of fill material (m)
material thickness (galvanising and base metal)
extent of corrosion (invert/full height)
maximum outside diameter (if relining)
voids present in fill
estimated maximum sag in pipe due to settlement
waterway description (is BCMS in standing water?)
environmental conditions (e.g. marine environment, local factors e.g. cow sheds upstream)
water/soil samples (to assist with durability design)
other defects and cause (construction/in-service)
sketches.
4.5.1
Type of BCMS
There are two main types of metal structures –multi-plate or helical corrugated. Multi-plate structures These structures are constructed by hand bolting a number of circular plate segments together to form a ring (Figure 4.3). Metal thickness is 4–5 mm or more.
Austroads 2011 — 68 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: TMR nd a.
Figure 4.3: Multi-plate culvert
Helical corrugated structures These structures are helically wound by machinery which decreases installation time (Figure 4.4). Metal thickness is typically thinner than for plate structures, and is typically 3 mm or less. They are therefore more susceptible to corrosion and to distortion during backfilling operations.
Source: TMR nd a.
Figure 4.4: Helically wound culvert
4.5.2
Size and Shape
To determine the shape, mark 1 m square grid with spots of paint on a side wall starting at the upstream end. Measure and record the horizontal and vertical diameter at 10 or more points to determine its shape. If there are particular defects such as open joints, extra diameters should be measured. Typically the size and shape are of particular importance when assessing the maximum diameter of the pipe that can be inserted for relining purposes.
Austroads 2011 — 69 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
4.5.3
Corrugations – Pitch and Depth
The pitch (P) and depth (D) of corrugations should be measured at a few places to determine which shape was used in the BCMS. Figure 4.5 details the standard corrugation dimensions in mm.
Source: AS 1762 (1984).
Figure 4.5: Corrugation profile for steel pipes
Additionally, non-sinusoidal corrugations may be present and the shape of these corrugations should be recorded or obtained from the manufacturer. 4.5.4
Height of Fill Material
Measure the height of fill material above the BCMS, as this information may be required to determine the suitable repairs and in the design process for determining the earth/vehicle loads. 4.5.5
Material Thickness
Where an edge piece can be found at the ends or joints, measure the thickness using a micrometre or other thickness gauge to an accuracy of 0.1 mm. If this can be measured at a relatively clean corrosion-free point, it will indicate the original thickness. A non-destructive thickness gauge is useful in measuring residual metal when extensive corrosion is obvious. It is important to determine whether sufficient metal thickness is left in the bulk of the BCMS to justify a repair, even if the invert is seriously corroded. If metal thickness measurements indicate severe corrosion on the outside of the structure, a concrete invert repair is not a suitable treatment. 4.5.6
Maximum Outside Diameter
If relining is a possible repair method, then measure the straightness of the structure with string lines, or with appropriate levelling equipment (Section 5.2.7). The straightness of the structure will determine the largest possible diameter that will fit through the entire BCMS length. 4.5.7
Voids Present in Fill
If the BCMS has been perforated by corrosion, typically by standing water in the invert, then some of the granular backfill around the structure may have been flushed out in flood flows. Determine this by first tapping around the structure, starting from near the invert where a void is most likely. If voids are detected, then paint a line between the hollow and solid sounding sections and map them using the structure dimension grid. The depth of cavities can be checked by drilling small
Austroads 2011 — 70 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
holes and measuring the depth of the void. Holes should be sealed with an appropriate galvanised screw. 4.5.8
Estimated Maximum Sag in Pipe due to Settlement
This can be achieved using the methods similar to that described in Section 5.2.7. 4.5.9
Waterway Description
A general description of the upstream and downstream waterways needs to be made particularly noting the presence of standing water, presence of scour, relative gradients before and after, any pollution or other factors that may affect water aggressiveness and typical sediment size and properties. 4.5.10
Environmental Conditions
The environmental conditions need to be assessed to determine the potential for corrosion e.g. marine environment. Additional local factors should also be considered such as proximity to farm areas e.g. cow sheds upstream. 4.5.11
Water/Soil Samples
The chemical make-up of the water needs to be assessed to assist with the durability assessment of proposed retrofit or re-design. The required tests are presented in detail in Section 2.6.3. 4.5.12
Other Defects and Cause (Construction/In-service)
As identified in Section 4.3. 4.5.13
Sketches
A sketch containing summary information of the investigation can also be useful to record defect locations.
4.6
Risk Assessment Method and Treatment Action
Level 2 structure inspections will identify structures in poor condition (condition states 3 and 4). A risk assessment should be conducted on these structures and a treatment plan with appropriate action timeframes developed. Note that if an appropriate structural management plan has been in place for a structure through its life then this risk assessment approach should not be necessary. Figure 4.6 presents a flowchart of the overall process to determine the risk level for a given situation and appropriate treatment timeframes. The following section on risk is based upon the information found in TMR nd a.
Austroads 2011 — 71 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Identify Metal Culvert Gather Basic Information - Culvert Type - Size and Shape - Corrugations - Height of Fill - Material Thickness - Extent of Corrosion - Max possible OD (if relining) - Voids Present in Fill - Other Defects
Determine Typical Situation
Situation 1 - Standing water in culvert
Treatment Unknown Risk
- Allow culvert to drain - Assess culvert condition when dry
Very Low Risk
- Plan to install concrete invert or paint system - Repair within 2 years
Low Risk
- Inspect culvert after rain for erosion of back fill through invert perforations - Install concrete invert within 1 year
Medium Risk
-Prop and repair ASAP -Check road surface levels fortnightly, after short or during long rainfall events -Speed restriction and hazard signs
High Risk
- Prop and repair ASAP - Check road surface levels weekly, after short or during long rainfall events - Speed restriction and hazard signs
Very High Risk
- Prop immediately -Repair urgently - Speed restriction and hazard signs
Situation 2 - Corrosion appearing in the invert - Substantial metal thickness remains
Situation 3 - Considerable level of corrosion in the invert - Small holes appearing
Treatment
Treatment
Situation 4 - Culvert Invert rusted through in large sections. - Culvert remaining circular -No significant flows - Constant HV traffic volume
Treatment
Situation 5 - Culvert Invert rusted through in large sections. - Culvert remaining circular -Local flooding occurs -HV traffic volume increases
Treatment
Situation 6 - Culvert invert rusted through in large sections. - Culvert ring movement evident -No road surface settlement
Treatment
Situation 7 - Culvert invert rusted through in large sections. - Culvert ring movement evident -Road surface settlement
Extreme Risk
Treatment - Close road immediately - Repair urgently
Source: TMR nd a.
Figure 4.6: Flowchart for risk assessment and treatment
Austroads 2011 — 72 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Sections 4.6.1 to Section 4.6.7 illustrate these situations with detailed descriptions of typical conditions that will be encountered when inspecting metal structures and an indication of the Level 2 inspection conditions state, along with a risk ranking (depending on a number of factors) and the action required to ensure the protection of the asset and the public. Figure 4.6 applies primarily to buried corrugated metal pipes. Buried corrugated metal arches should be considered in a similar way to the management of the rest of the authority’s bridge stock. Similarly, if the structure is a pipe but carries no water flow it too could be considered in a similar way to the typical bridge stocks. 4.6.1
Situation 1
Unknown risk condition
standing water in the BCMS, typically caused by a blockage on the downstream end (Figure 4.7)
structure will rapidly corrode and it is difficult to inspect its condition. A decision must be made quickly on how to make the BCMS durable, easy to inspect and safe.
Immediate treatment Survey downstream levels and consult the land owner. If practical, cut a low flow channel that will allow the BCMS to drain. If a blockage is caused by cattle or vehicles pushing the bank into the stream bed, negotiate a preventative strategy such as:
constructing an alternate crossing further downstream
installing a low flow pipe under a low-level concrete ford
putting up a fence around the stream for sufficient length to maintain exit drainage.
Hydraulics specialists can be consulted if there are concerns regarding this. Later treatment
assess BCMS when clean and dry to determine whether additional repairs are needed
check BCMS after each wet season to ensure they remain self draining.
Note: If a BCMS has deep standing water and downstream surveys show it is not practical to drain the BCMS, the owner must be informed and a management plan developed.
Austroads 2011 — 73 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: TMR nd a.
Figure 4.7: Standing water in BCMS
4.6.2
Situation 2
Very low risk condition
significant corrosion appearing in the invert of the BCMS, but substantial thickness of metal remains (Figure 4.8)
BCMS is retaining circular shape or may have distorted during construction process but is remaining stable
the BCMS is assessed as being condition state 3.
Treatment
monitor BCMS annually
plan to install a concrete invert or use a paint system, while the BCMS still has adequate metal in the invert and before the structure perforates due to corrosion
complete repairs within 2 years.
Austroads 2011 — 74 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: TMR nd a.
Figure 4.8: BCMS has significant invert corrosion and will need a reinforced concrete invert in the next 2 years
4.6.3
Situation 3
Low risk condition
considerable level of corrosion in the invert and small holes appearing in the invert of the BCMS (Figure 4.9 and Figure 4.10)
no evidence of material loss from soil behind the BCMS and no soil cavities evident
BCMS is retaining circular shape or may have distorted during construction but is remaining stable
BCMS is assessed as being condition state 4.
Treatment
monitor and plan for installation of a concrete lining of the invert within the next 12 months. If corrosion extends above the level where it is practical to install a concrete invert consider relining
after a significant rainfall event, check the BCMS for erosion of backfill material through the invert perforations
repairs should be carried out within 1 year.
Austroads 2011 — 75 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: TMR nd a.
Figure 4.9: Heavy corrosion in invert with small perforations to metal structure
Source: TMR nd a.
Figure 4.10: Heavy corrosion – considerable loss of metal thickness
4.6.4
Situation 4
Medium risk condition
BCMS invert rusted completely through over a large portion of the length
loss of backfill material below invert
BCMS is retaining circular shape (loads are being carried by soil arch) or may have distorted during construction but is remaining stable
BCMS has no significant flows
heavy vehicle traffic loading remains constant.
Austroads 2011 — 76 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Treatment
prop immediately and repair as soon as possible within two months or before the next wet season starts
check BCMS and pavement levels fortnightly, after short rainfall events and during extended rainfall periods for any signs of ring compression failure or settlement of the soil arch over the BCMS
put speed restrictions and hazard identification signage in place to ensure adequate stopping sight distance, should a hazard develop.
Safety
if a dip in the pavement occurs the road is to be closed immediately.
4.6.5
Situation 5
High risk condition
BCMS invert rusted completely through over a large portion of the length
loss of backfill material below the invert (Figure 4.11)
BCMS is retaining circular shape (loads are being carried by soil arch) or may have distorted during construction process but is remaining stable (see notes in Section 4.3)
local flooding event occurs creating the risk that the BCMS and embankment could be washed out
heavy vehicle traffic loading increases.
Treatment
BCMS must be repaired as soon as practical (within one month)
props should be installed immediately
BCMS and pavement surface should be checked weekly, after short rainfall events and during extended rainfall periods for any signs of ring compression failure or dips in the pavement caused by soil arch settlement
if replacement is determined to be the most appropriate repair method and the replacement BCMS cannot be obtained immediately from the manufacturer alternative options that may be considered are:
—
temporarily replacing the BCMS with a readily available low flow reinforced concrete pipe. The low flow pipe can then be replaced with the appropriately sized pipe when it is available from the manufacturer
—
completing the works under full road closure if sidetracks or diversions are an issue
—
backfilling with concrete can be undertaken if necessary to decrease the time required for installation if the road is not available for extended closure
put speed restrictions and hazard identification signage in place to ensure adequate stopping sight distance, should a hazard develop.
Safety
if a dip in the pavement occurs the road is to be closed immediately.
Austroads 2011 — 77 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Note: The helical lock seams have not distorted and BCMS remains circular with no evidence of ring compression failure. Source: TMR nd a.
Figure 4.11: BCMS invert corroded away (loss of granular bedding material in invert)
4.6.6
Situation 6
Very high risk condition
BCMS invert rusted through over large portion of the length
compression ring movement is obvious (metal has buckled or is overlapping), but soil arch is mostly intact (Figure 4.12)
voids possibly present behind BCMS lining but no dip in road
BCMS structure has failed, embankment soil arch at high risk of imminent failure (condition state 5 – close structure)
no visible settlement of road pavement over BCMS.
Treatment
inform the owner immediately
seek structural engineering advice – send photos of BCMS
prop the BCMS immediately if safe to do so
put speed restrictions and hazard identification signage in place to ensure adequate stopping sight distance, should a hazard develop
undertake urgent repairs.
Safety
BCMS is structurally unsafe and can fail under heavy traffic loading or in a flood due to embankment erosion
if a dip in the pavement occurs the road is to be closed immediately.
Austroads 2011 — 78 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
4.6.7
Situation 7
Extreme risk condition
BCMS invert rusted through over large portion of the length
compression ring movement is obvious (metal has buckled or is overlapping) but soil arch is mostly intact (Figure 4.12)
voids possibly present behind BCMS lining but no dip in road
condition state 5 – close structure, BCMS structure has failed, embankment soil arch at high risk of imminent failure
road pavement above BCMS shows obvious signs of settlement (Figure 4.13).
Treatment
as given for Situation 6 – very high risk.
Safety
BCMS has structurally failed
road is to be closed immediately.
Source: TMR (2010).
Figure 4.12: BCMS ring movement
Austroads 2011 — 79 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: TMR (2010).
Figure 4.13: BCMS soil arch failure
Austroads 2011 — 80 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
5
MAINTENANCE AND REPAIR PROCEDURES
5.1
Emergency Propping
Install emergency propping for immediate temporary stabilisation of the structure when safety issues become evident. Propping of the BCMS can only slow the complete collapse of the structure and there is still a future risk to the travelling public if a failure is allowed to occur. Since most conditions that require propping are caused by excessive corrosion of the invert, vertical propping is often not an option. However, propping at 45 degrees (Figure 5.1) can be effective. For safety, 6 mm self-tapping screws can be inserted in the BCMS to hold timber spreader beams with heavy wire ties until the props are installed. Each sleeper should have a minimum of two props. Timber sleepers should be used to distribute the load from the props across numerous BCMS corrugations. The timber sleepers will be held in place against the wall by friction, but a suitable connection of the props to the sleepers should be considered to prevent the props from being washed out. Seek advice from a qualified structural engineer for the required safe working load and arrangement of the props.
Note: This culvert had not failed and props were installed pending a full structural assessment.
Figure 5.1: Example of emergency propping
An appropriate installation method, taking into consideration all applicable safety risks, and which ensures that no disturbance of the culvert walls occurs, would need to be developed prior to installing props.
5.2
Repair Methods
The following are a range of repair treatments that should be considered, taking into account:
remaining life of the non-corroded portion of the culvert
durability and expected service life of the repair
size of embankment over culvert
Austroads 2011 — 81 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
traffic volumes
water flow
ability to detour traffic
cost.
The products and methods detailed in the repair methods should be sourced from reputable suppliers. 5.2.1
Repair and Maintenance Methods
The condition of the BCMS determines the appropriate rehabilitation or maintenance method required. Preventative maintenance measures are recommended either at construction or during its service life. Two common methods of preventative maintenance are concrete lining of the invert (Section 5.2.2) and/or a paint membrane that will prevent internal rusting (Section 5.2.3). Both of these methods are dependent on the inverts being of sound condition with no rust evident. 5.2.2
Concrete Lining of Invert
This is the most common repair for larger culverts with sufficient working space and high embankments where it is difficult to remove the culvert. The concrete lining of culverts can be either reactive maintenance or planned maintenance. Ideally, concrete lining is used as part of a structural management plan to optimise the life of a structure through careful management of the sacrificial metal thickness in the invert. Once the sacrificial thickness is used up the invert is lined to protect the remaining required structure thickness in the invert. In a reactive situation the invert may be seriously deteriorated and in these instances invert lining can be used as a reactive approach to extend the life of a culvert. When using this treatment it is essential that the concrete invert liner forms a structural bond with the uncorroded metal above the invert. Thin layers of concrete that are not structurally fixed to the metal culvert are not effective repairs, as shown in Figure 5.2. Voids that may be present beneath the invert or in the backfill material will need to be grouted. The rest of the metal culvert (excluding the corroded invert) must be in a reasonable condition. The inside face should have adequate remaining galvanising metal. The thickness should be checked with a non-destructive thickness gauge or by drilling a small number of holes and checking with a thickness gauge reading to 0.2 mm or better, and holes must be plugged with a galvanised screw afterwards. Where there are significant concerns regarding collapse due to the invert being corroded out, it is often not safe to enter the culvert to prop it. In this situation the invert lining can be conducted in a progressive manner by the following procedure:
Starting at one end, use curved plate or reinforcing bars welded to the shell at sufficient centres to reinstate the hoop capacity of the section. Only a small length is repaired at a time.
Once the appropriate reinforcement has been installed, the lining section under consideration is poured. It is important that the reinforcement has appropriate cover, therefore the hoop strengthening should not be considered as reinforcement.
This process is then continued in section lengths deemed appropriate to ensure safety.
Austroads 2011 — 82 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
If there is significant thickness loss due to corrosion of the external face of the culvert, a concrete invert may not be effective as a long-term repair and relining should be considered.
Figure 5.2: A thin concrete invert lining which has separated from the culvert and washed away in flood
VicRoads specifies the following for concrete linings (VicRoads 2009):
The minimum thickness of concrete lining shall be 130 mm above the crest of corrugations.
The minimum height of lining shall be normal water level plus 300 mm or one-third height of the structure, whichever is greater.
Top edges of concrete lining shall slope towards the centreline of the structure to prevent ponding of water against the wall of the structure.
At both ends of the structure the concrete invert lining shall terminate with a 900 mm deep reinforced concrete cut-off wall. The cut-off wall depth shall be measured below the finished invert level, and the wall shall be detailed to connect to the reinforced concrete headwall if this is present.
Concrete for the lining shall be special class performance concrete having a grade not less than VR 330/32 as specified in VicRoads (1997).
The concrete lining shall be reinforced with a steel fabric having a minimum steel area of 500 mm2/m in both directions and mesh dimensions not greater than 200 mm and bar size not less than 8 mm.
Cover to the mesh at the edges of the concrete lining shall be not less than 50 mm and not more than 100 mm. Minimum cover shall be 50 mm to all other faces, including to the crest of the BCMS corrugations.
For steel BCMS, reinforcement in the concrete lining shall be lapped and welded for electrical conductivity and supported by steel bars welded or bolted to the structure at 1.0 m maximum spacing in both directions.
The following steps are recommended for preparation of an existing BCMS for concrete lining. This information has been extracted from the Bridge Technical Note 2005/009, (VicRoads 2009):
temporarily divert water flow
remove sediment in culvert
pressure wash to remove sediment and debris
Austroads 2011 — 83 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
abrasive sweep blast area to be lined to equivalent to class 1 finish to AS 1627.9 (2002)
for steel BCMS, paint penetrating primer 50 microns DFT over area to be lined using Xymax MonoLock PP, Wasser MC-Prebond, Zinga or other approved equivalent
for aluminium BCMS, paint with an approved bitumastic coating.
Advantages
concrete can be applied in situ
cement mortar linings have been found to dramatically reduce internal corrosion
abrasion forces on the BCMS are reduced
the afflux can be controlled by using more/larger rock baffles
depressions can be made in the concrete layer to allow rest areas for fish and marine animals
increased service life of the BCMS
safe progressive methods can be used
it is a low-cost repair method.
Disadvantages
BCMS under 900 mm in diameter have restricted access for personnel to lay the concrete
approach and departure aprons will need to be raised or constructed to maintain inflows and outflows
the diameter of the BCMS will be reduced.
5.2.3
Painting the Invert
This repair method can be used where the culvert is not extensively corroded and significant metal thickness remains. The type of paint system used will depend on the abrasive conditions the culvert is subject to. If a significant volume of debris flows through the culvert regularly, high abrasion resistant systems can be used. Alternatively, paint systems with lower abrasion resistance can be used in culverts which do not flow regularly or if the flow does not contain debris. Painting of the invert of a BCMS was traditionally done using a coal tar epoxy coating. This method is no longer used due to carcinogenic and environmental concerns. The preferred method would be to obtain coated BCMS before installation. If repairing or prolonging the life of an existing structure, a number of paint options are available including:
bituminous paint
polymer coatings
metal oxide based paints.
All metal surfaces need to be in sound condition and free of dust, dirt and moisture. Application of the selected product can be done with a sprayer unit, brush or roller. This method could be used with a concrete liner as mentioned above, to add better protection to the invert of the BCMS. Advantages
extends the life of the BCMS by increasing resistance to corrosion and abrasion.
Austroads 2011 — 84 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Disadvantages
impossible to get complete coverage on moist inverts
diversion of stream waters required to allow BCMS to dry
breathing apparatus may be required due to confined space and lack of ventilation.
5.2.4
Joint Repairs
Separated joints can be repaired using a chemical grout such as polyurethane foams. This will stop joints from leaking and prevent the erosion of backfill material behind the culvert. Where backfill material has been lost, the voids can be grouted using a low pressure cement based grout. 5.2.5
Replacement of the Culvert
Removing and replacing a culvert should be considered when culverts:
have large distortion from original shape (greater than 5%)
experience significant reduction in waterway area due to the culvert shape not being circular (they can be oval etc. in shape) or not having a consistent diameter along the length
have significant voids in the embankment material and grouting is not practical
are considered to have structurally failed.
Removing and replacing a culvert is the most practical option when:
culverts are smaller than 1.5 m diameter making it difficult to work inside
embankment height is low and traffic can be easily diverted
larger culverts have failed in ring compression, are badly corroded or have distorted significantly, so they cannot be easily relined.
All of the issues described in Section 2 on appropriate selection of BCMS need to be considered when selecting the replacement material and structural system. Advantages
culvert can be redesigned to perform adequately over the appropriate design
significant site-specific experience can be gained from the performance of the old culvert which can be used in the design of the new culvert.
Disadvantages
road will need to be closed for the replacement, possibly requiring staged construction
costs may be higher.
5.2.6
Shotcrete Lining
Shotcrete lining is performed by pneumatically applying cement plaster or concrete to the area where relining is required. A gun operated by compressed air is used to apply the cement mixture. Water is controlled and added to the dry material as it passes the nozzle of the gun. Shotcrete lining is considered to be stronger that the hand-placed mortar of the same aggregate-cement proportion because it permits placement with a lower water-to-cement ratio.
Austroads 2011 — 85 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
It is recommended that the shotcrete thickness should be between 50 mm to 100 mm. A minimum area of steel of 0.4% of the area of lining in both directions is recommended. Advantages
culvert can be retrofitted in place with minimal interruption to the traffic above.
Disadvantages
skilled labour is required to successfully implement
cost may be high; this is a trade-off with traffic delays of other options.
5.2.7
Slip Lining
Where significant metal thickness has been lost from the entire circumference of the culvert, a repair option is to reline the culvert with a new culvert of smaller diameter and grout the void between the new and old culvert. Slip lining involves inserting a sleeve inside the existing pipe, in situ. Before the designer makes a final choice of material type it is recommended that tests are conducted on both the ground water and flowing water in order to determine their chemical content and, hence, the potential for corrosion. Where the ground water contains concentrations of pH levels lower than 5 (acidic), steel may not be suitable and other materials such as concrete. High density polyethylene (HDPE) or aluminium may be more appropriate subject to consideration of cost and time. It is also recommended that the chemical composition of the backfill material is specified in order to reduce the risk of corrosion (VicRoads 2008). In short, all the issues relevant to selecting a new BCMS are relevant to the selection of a suitable liner. Materials Slip lining can be undertaken using the following materials:
HDPE
aluminium
steel
stainless steel
concrete.
Techniques A HDPE lining is a push or pull technique where a new lining is inserted into the existing BCMS (Figure 5.3). Once the new lining is in place an annulus grout is poured between the existing BCMS and the new lining. Systems which rely on the host pipe for some measure of structural support are sometimes known as ‘interactive lining’ techniques.
Austroads 2011 — 86 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Figure 5.3: HDPE lining being installed
The culvert must be closest to its theoretical shape for efficient relining. If there is significant distortion, then a much smaller pipe liner would be required for relining which can cause several problems such as:
The void between the new liner and the old culvert or in the backfill material can be very large and expensive to grout.
The smaller diameter culvert may not carry the design flood flow, resulting in the road embankment overtopping and washing away.
Relining needs careful consideration and relevant experts should be consulted. Figure 5.4 illustrates the relining process.
Figure 5.4: Relining process
In order to line the culvert, the largest possible liner size (outside diameter) needs to be determined. This can be achieved by the following method:
At either end of the culvert, establish the horizontal and vertical centre point with a level and straight edge of suitable length to fit inside the culvert.
Put self-tapping screws through the culvert and establish horizontal and vertical stringlines at each end with a laser at one end and aim it at the centre of the other end.
At 2.0 m centres, measure the horizontal and vertical diameters and the laser centreline intercept for both.
Austroads 2011 — 87 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
The dimensions can be plotted, and the largest straight internal tube can be measured.
Plot all horizontal and vertical diameters and laser intercepts.
Measure the largest included circle (Figure 5.5).
Choose a liner based on suitable construction clearances and grout thickness requirements.
Figure 5.5: Estimating largest liner diameter
Advantages
slip lining allows for the repair of the BCMS to be conducted with minimal disruption to traffic flows, deep excavations together with the associated safety issues (VicRoads 2008)
makes use of the existing alignment, which may be the only viable alignment where the site is restricted (VicRoads 2008)
suitable for a wide range of pipe types and diameters
avoids the need to build new end-walls where these already exist (VicRoads 2008)
relatively cheap, simple process.
Disadvantages
loss of cross-sectional area may be significant and it may be necessary to check the flow-capacity of the BCMS in the lined condition (VicRoads 2008)
severe bends cannot usually be negotiated, especially at larger diameters
launch and reception pits must be dug
lateral connections must be excavated and re-built.
Austroads 2011 — 88 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
5.2.8
Pipe Jacking Around the Existing Culvert
Pipe jacking can be used to replace almost any culvert size (Figure 5.6). A jacking unit is needed at the portal to push the pipe into place. This jacking unit is of significant size and requires a suitable working area and a stable backstop to react against the jacking unit.
Source: Tenbusch and Tenbusch. (2008).
Figure 5.6: Typical pipe jacking set-up
Pipe jacking around the existing culvert is an expensive option, but may be necessary when there are no other alternatives. Examples include failed culverts that are unsafe to work in and culverts under high fills or major roads where traffic diversion and excavation are impractical. Pipe jacking is a specialist contracting skill and advice should be sought from experienced practitioners. Advantages
can be used for consumption or parallel construction
allows direct installation of concrete or other pipe material that does not need a secondary lining
original or slightly larger replacement pipe can be used
can eliminate sag in an existing culvert.
Disadvantages
requires large diameter for personnel entry
specialist skills and equipment needed.
5.2.9
Filling the Culvert
Filling the culvert should be considered if the pipe is too badly deteriorated and distorted to repair and/or there is also a high safety risk to personnel due to collapse if the pipe were disturbed during repair. In these situations flowable fill is used to fill the culvert preventing collapse or disturbance to the overlying roads. A satisfactory procedure for this operation would need to be developed prior to undertaking these works. The definition of flowable fill can be found in Appendix H of AS/NZS 2041 (1998).
Austroads 2011 — 89 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
6
CONCLUSIONS
These guidelines have collated extensive information on each of the key areas of BCMS including design, construction, management/inspection and rehabilitation. They provide a valuable overview of information suitable to assist engineers with all aspects of BCMS. The information presented in the guidelines should not be used in isolation from other sources of information on BCMS. Australian Standards should always be used to form the basis for any work on BCMS in Australia. These standards can change over time and need to be referenced regularly. The design of the guidelines has incorporated the information in the draft standard AS/NZS 2041.1 (2010) which is to replace the previous version of AS/NZS 2041 (1998). The new standard will cover design methods, installation, helically formed sinusoidal pipes and bolted plate structures. Once released, all BCMS will need to be designed in accordance with these new standards and all information in these guidelines should be interpreted with consideration of the new standard requirements. Design of BCMS should be carried out by experienced practitioners or by engineers under the direct supervision of an experienced practitioner with significant knowledge in BCMS and soil structure interaction. The rehabilitation of corroded culverts should be carried out in accordance with proven techniques following comprehensive design analysis of the deteriorated structure. Each road authority should ensure that rehabilitation works on corroded culverts are undertaken according to their own guidelines. If none are available, guidelines of other road authorities and/or the guidelines contained in this document should be consulted. Culvert inspections should be carried out in accordance with the individual road authority proformas. If not available, suitable methods can be developed using the information provided in these guidelines.
6.1
Future Directions
The design section of these guidelines has been developed using information from the current draft standard which is to replace AS/NZS 2041 (1998). The guidelines should be reviewed for consistency with this standard once it has been published. For future improvement, the durability design can be further simplified by development of a service life look-up chart similar to the AISI chart in Figure 2.17 or a study into compatibility of the use of US based charts for use in Australia. Further study should also be conducted to include durability information on other types of coatings such as polymer pre-coated and bituminous-coated materials. Currently, only sufficient information for the durability design of galvanised coatings is available. Information such as the corrosion rate for aluminium Type 2 coating is limited and no data is available in the draft AS/NZS 2041.1 (2010) for other types of coatings. Although the draft AS/NZS 2041.1 (2010) introduces the limit state design, the finite element analysis (FEA) design method is still briefly explained. Further works will need to include a more detailed explanation on how to use FEA for BCMS design by possibly adopting the freely available FEA software and providing design examples.
Austroads 2011 — 90 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
The management and inspection section of these guidelines has discussed a number of defects that should be identified in a Level 2 structural inspection. Additional work is required to identify condition states for the various defect severities needs to be developed in accordance with typical bridge component condition states as covered in most of the road authority bridge inspection manuals.
Austroads 2011 — 91 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
REFERENCES Australian Rail Track Corporation 2006, Buried corrugated metal structures: installation, RC 4012, issue A, revision 1, ARTC, Adelaide, SA, viewed 15 February 2010, . Ault, JP & Ellor, JA 2000, Durability analysis of aluminized type 2 corrugation metal pipe, FHWA-RD-97-140, Federal Highway Administration, Office of Infrastructure Research and Development, McLean, VA, USA. California DOT 2007, Method for determining field and laboratory resistivity and pH measurements for soil and water, method 643, California Department of Transportation, Sacramento, CA, USA, viewed 26 July 2011, . Connecticut DOT 2000, Drainage manual, Connecticut Department of Transportation, Newington, CT, USA, viewed 15 February 2010, . Corrugated Steel Pipe Institute 2007, Handbook of steel drainage and highway construction products, 2nd edn reprinted, CSPI, Cambridge, Ont, Canada. Curtice, DK & Funnell, JE 1971, ‘Comparative study of coatings on corrugated metal culvert pipe’, research project 07-2733, Southwest Research Institute, San Antonio, TX, USA. Luczak, H, Walker, A & Zhang, J 2009, ‘Buried corrugated metal structures: the Victorian perspective’, th Austroads bridge conference, 7 , Auckland, New Zealand, Convention Management New Zealand, Auckland, NZ, 9 pp., viewed 26 July 2011, . Meacham, DG, Hurd, JO & Shisler, WW 1982, Culvert durability study, ODOT/L&D/82-1, Ohio Department of Transportation, Columbus, OH, USA. Mlynarski, M, Katona, MG & McGrath, TJ 2008, Modernise and upgrade CANDE for analysis and LRFD design of buried structures, NCHRP report 619, National Cooperative Highway Research Program, Transportation Research Board, Washington DC, USA. National Corrugated Steel Pipe Association 2000, CSP durability guide, Jensen Bridge & Supply, NCSPA, Sandusky, MI, viewed 15 February 2010, . Natural Resources Conservation Service 2001, Corrugated metal pipe, construction specification 751, NRCS, Washington, DC, viewed 15 February 2010, . Petersen, DL, Nelson, CR, Li, G, McGrath, TJ & Kitane, Y 2010, Recommended design specifications for live load distribution to buried structures, NCHRP report 647, National Cooperative Highway Research Program, Transportation Research Board, Washington, DC, USA. Pritchard, RW 2008, ‘Behaviour of helical steel culverts’, PhD thesis, University of Queensland, Brisbane, QLD. Pritchard, R & Muller, W 2003, 'Level 3 inspection: helical spun steel culvert under Gateway Arterial (100m north of Deagon Interchange)', Road Systems and Engineering Group, Queensland Department of Main Roads, Brisbane, Qld.
Austroads 2011 — 92 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Sandford, TC 2000, Soil-structure interaction of buried structures, Transportation Research Board, Washington, DC, USA, viewed 5 May 2011, . Tenbusch, AA & Tenbusch, AF 2008, Failing culverts: solution options: with special attention to the solution of replacement by tunneling, Tenbusch, Louisville, TX, USA, viewed 26 July 2011, . Transport and Main Roads 2010, Road drainage manual: appendix 14a: inspection and treatment of metal culverts, Department of Transport and Main Roads, Brisbane, Qld, viewed 26 July 2011, . Transport and Main Roads nd a, ‘Corrugated steel culverts: managing the risk’, BAM advice note no. 88, Department of Transport and Main Roads, Brisbane, Qld. Transport and Main Roads nd b, 'Design criteria for rehabilitating metal culverts using steel reinforced polyethylene liners to MRTS', draft technical note, Department of Transport and Main Roads, Brisbane, Queensland. VicRoads 1997, Structural concrete, standard specification section 610, VicRoads, Kew, Vic. VicRoads 2004, Bridge inspection manual, VicRoads, Kew, Vic. VicRoads 2008, ‘Draft refurbishment of buried corrugated metal structures’, bridge technical note 2008/001, version 1.0, VicRoads, Kew, Vic. VicRoads 2009, Buried corrugated metal structures, bridge technical note 2005/009, version 2.0, VicRoads, Kew, Vic, viewed 26 July 2011, . VicRoads 2010, Buried corrugated metal (steel) structures, standard specification section 632, VicRoads, Kew, Vic.
Standards Australia AS 1170.0-2002, Structural design actions: general principles. AS 1170.4-2007, Structural design actions: earthquake actions in Australia. AS 1289.4.3.1-1997, Methods of testing soils for engineering purposes: soil chemical tests: determination of the pH value of a soil: electrometric method. AS 1289.4.4.1-1997, Methods of testing soils for engineering purposes: soil chemical tests: determination of the electrical resistivity of a soil: method for sands and granular materials. AS 1289.5.3.1-2004, Methods of testing soils for engineering purposes: soil compaction and density tests: determination of the field density of a soil: sand replacement method using a sand-cone pouring apparatus. AS 1289.5.3.5-1997, Methods of testing soils for engineering purposes: soil compaction and density tests: determination of the field dry density of a soil: water replacement method.
Austroads 2011 — 93 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
AS 1289.5.1.1-2003, Methods of testing soils for engineering purposes: soil compaction and density tests: determination of the minimum and maximum dry density of a cohesionless material: standard method. AS 1289.5.5.1-1998, Methods of testing soils for engineering purposes: soil compaction and density tests: determination of the minimum and maximum dry density of a cohesionless material: standard method. AS 1289.5.6.1-1998, Methods of testing soils for engineering purposes: soil compaction and density tests: compaction control test: density index method for a cohesionless material. AS 1289.5.8.1-2007, Methods of testing soils for engineering purposes: soil compaction and density tests: determination of field density and field moisture content of a soil using a nuclear surface moisture density gauge: direct transmission mode. AS 1397-2001, Steel sheet and strip – Hot-dip zinc-coated or aluminium/zinc-coated. AS 1627.9-2002, Metal finishing – preparation and pre-treatment of surfaces: part 9: pictoral surface preparation standards for painting steel surfaces. AS 1726-1993, Geotechnical site investigations. AS/NZS 1734-1997, Aluminium and aluminium alloys – Flat sheet, coiled sheet and plate. AS 1761-1985, Helical lock-seam corrugated steel pipes. AS 1762-1984, Helical lock-seam corrugated steel pipes: design and installation. AS/NZS 2041-1998, Buried corrugated metal structures. AS/NZS 2041.1 forthcoming, 'Buried corrugated metal structures: part 1: design methods', draft no. DR 10015 CP (2010). AS 3703.1-1989, Long-span corrugated steel structures: part 1: materials and manufacture. AS 3703.2-1989, Long-span corrugated steel structures: part 2: design and installation. AS 4100-1998, Steel structures. AS/NZS 4680-2006, Hot-dip galvanized (zinc) coatings on fabricated ferrous articles. AS 5100.2-2004, Bridge design: design loads.
Austroads 2011 — 94 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
APPENDIX A
BCMS MANUFACTURERS AND COMPANIES PROVIDING REHABILITATION SERVICES
This section is provided for information only and does not make any recommendations regarding any particular company.
A.1
Humes
Humes makes steel reinforced concrete pipes, culverts and a wide range of engineered precast concrete and environmental products. Products SRCP (steel reinforced concrete pipe). AKS – is a high-density polyethylene lining. All Humes’ precast products can be by supplied incorporating AKS, and the liner is suited for in situ applications. AKS (combined with ALS Secugrout) also enables a superior solution for post-construction installation and renovation. CMP (corrugated metal pipe) is a helically wound, lock-seamed corrugated metal pipe and is available in galvanised steel, aluminium or stainless steel. Humes CMP manufacturing facility is also fully mobile, allowing for on-site manufacture of pipes and culverts up to 5.1 metres in diameter. Locations Office locations in all capital cities. Phone: 1300 361 601 Web: www.humes.com.au
A.2
Atlantic Civil Products
Atlantic Civil Products Pty Ltd is an Australian company that develops, designs, manufactures and supplies products for the civil, mining and forestry industries. Its products include corrugated metal pipe, corrugated structural plate, stabilised earth walls and steel girder bridges. Products Hel-Cor corrugated metal pipe – available in aluminium, aluminised steel, galvanised steel and polymer-coated galvanised steel (Trenchcoat). On-site milling is available. Hiflo corrugated metal pipe – smooth internal wall for low hydraulic friction. Locations Head office in Garbutt, Townsville other offices in WA, NSW. Phone: 1800 99 77 54 Web: www.atlanticcivil.com.au Email:
[email protected]
Austroads 2011 — 95 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
A.3
Bluescope Steel (Bluescope Lysaght)
Bluescope Steel supplies many steel products including tanks and pipes including culverts up to 900 mm in diameter. Products HYDRORIB® pipe – is a lightweight, galvanised steel pipe, laminated inside and out with a polymer protective coating film. The pipe features a smooth bore for excellent flow characteristics and a ribbed outer wall for stiffness and strength. HYDRORIB® galvanised pipe is available in 300 mm to 900 mm diameters provides a high-quality and economic alternative to traditional culvert products. Locations NSW, QLD, Vic. Phone: 1800 654 774 Web: www.bluescopewater.com.au
A.4
Roundel
Roundel produces the STILCOR™ range of corrugated steel products. Its product range is used in road, rail, stormwater, drainage, irrigation, forestry and mining applications. Products STILCOR™ – utilising the strength of sinusoidal corrugations and using a double offset ‘lock seam’, galvanised coil is roll formed into a continuous helical barrel. Sizes range from 300 mm to 3600 mm in diameter. ALUCOR™ – aluminium coil is roll formed into a continuous helical barrel to produce an extremely robust yet economical piping solution. Sizes range from 300 mm to 2400 mm in diameter. Locations Neerabup, WA. Phone: 618 9404 5391 Web: www.roundel.com.au
A.5
Interflow
Interflow Pty Ltd provides repair, restoration and renewal services for deteriorated underground non-pressure pipelines. Pipeline diameters can range from 100 mm to over 2400 mm, and the full range of ancillary services are offered. Processes Expanda consists of a single, continuous strip of PVC, which is spirally wound into the existing pipeline via a winding machine positioned in the base of an existing manhole or access chamber. The edges of the strip interlock as it is spirally wound to form a continuous watertight liner inside the host pipe. Once a section of Expanda liner is installed, a mechanical process is used to
Austroads 2011 — 96 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
radically expand it until it fits tightly against the wall of the host pipe. This minimises the loss in cross-sectional area. The ends of the liner at each manhole are sealed and rendered to the host pipe whilst lateral connections are reconnected by robotic cutting. Rotaloc is a full-bore spirally wound PVC liner that restores the structural integrity, reliability and efficiency of aging sewers, gravity pipelines and culverts with diameters from 800 mm to 1500 mm. Like Expanda, the Rotaloc PVC profile strip is supplied to site on spools so the size of the site footprint is minimal. The profile is available in a range of sizes and thicknesses to enable selection based on design requirements for the project. Cementitious grouting of the void between the liner profile and the host pipe can be offered in order to meet specification requirements. Ribline is a fixed diameter full-bore structural liner that restores the integrity, reliability and efficiency of aging pipes and culverts. It is suitable for pipe diameters from 400 mm to 3000 mm. Ribline is made of a composite steel reinforced high density polyethylene profile. The combination of the strength of steel and the durability of plastic result in a liner with a high strength-to-weight ratio. Locations Head Office – Girraween NSW. Offices in Brisbane, Melbourne, Perth and NZ. Phone: 1800 251 240 Web: www.interflow.com.au Email:
[email protected]
A.6
Veolia Environmental Services
Veolia provides pipe laying and pipe rehabilitation services. Processes Veolia provides services in pipe lining, pipebursting and coatings. Locations National Office – Pyrmont NSW. Offices in all capital cities Phone: (02) 8571 0000 Web: www.veoliaes.com.au Email:
[email protected]
Austroads 2011 — 97 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
A.7
UEA Group
UEA's Trenchless Division offers a full range of trenchless and conventional construction technologies. Processes Thrust boring – installation of steel casing from 300 mm to 1000 mm to accommodate the appropriate carrier pipes. Pipe ramming – Used for large pipe installation in difficult ground conditions – cobble, running sand, soft ground. Steel pipe up to 2 m diameter and up to 60 m in length. Pipe bursting – pneumatic and hydraulic bursting installations. Installation sizes from 100 mm to 250 mm. Locations Offices in Sydney and Canberra Phone: (02) 9851 3000 and (02) 6228 1199 Web: www.uea.com.au Email:
[email protected]
A.8
INSITUFORM® PACIFIC Pty Ltd
Insituform Technologies®, Inc., is a provider of cured-in-place pipe (CIPP) and other technologies and services for the rehabilitation of pipeline systems. Processes CIPP (cured-in-place pipe) – Insituform CIPP restores structural integrity to damaged pipes. Repair sizes range from 100 mm up to 2500 mm in diameter. Locations St Mary’s, NSW Phone: (02) 9484 5944 Web: www.insituform.com Email:
[email protected]
A.9
ITS Trenchless (formerly CLM)
ITS Trenchless (previously known as CLM Trenchless) provides a broad range of processes for the installation and renovation of pipelines and structures. Processes Pipebursting – Pipebursting involves replacement of an existing pipe by pneumatic or hydraulic means with minimal disruption to the environment.
Austroads 2011 — 98 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Swagelining – is the process of building sections of polyethylene (PE) pipe that are butt fused together to form a continuous string; the pipe is pulled through a reducing dye to temporarily reduce diameter. This allows the pipe to be easily pulled through the host pipe. After the pipe is inserted, the pulling force is removed, allowing the pipe to return naturally toward its original diameter until it presses closely against the wall of the host pipe. In situ spray lining – has been developed as a rapid-setting high-build polymeric lining system that will confer structural properties to drinking water pipelines. Point-linings – point-liners in the industry are also referred to as pipe patches, patch lining and short form pipe liners. Sliplining – is similar to pipebursting, except a pipeline smaller than the existing main is installed with no displacement of the host pipe. This is ideal for pipeline renewals where the flow capacity can be reduced. Installation is either by towing in a product pipe, or pipe-jacking, depending on the project specifics. Locations NSW and QLD Phone: (02) 8603 2000 and (07) 3865 6100 Web: www.itstrenchless.com.au Email:
[email protected]
Austroads 2011 — 99 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
APPENDIX B B.1
REVIEW OF STATE ROAD AUTHORITY EXPERIENCE
Roads and Traffic Authority, NSW (RTA)
Extent of use and configurations Corrugated steel structures have been used on RTA’s road network for a number of decades, with round pipes of all sizes used for road drainage purposes with the larger corrugated steel arch structures used for stock or pedestrian underpasses. There are approximately 70 bridge size corrugated steel structures i.e., those with a combined span of 6m or larger, and approximately 62 000 small size culverts on RTA’s road network. RTA has the results of inspections of 41 430 culverts in its culvert database, and from the inventory currently collected, the reported number of corrugated steel culverts is:
spiral wound – 387 culverts (0.9% of the total)
multi-plate – 372 culverts (0,9% of the total).
From the inventory currently collected, the reported number of corrugated aluminium culverts is:
spiral wound – 10 culverts (0.02% of the total)
multi-plate – 4 culverts (0.01% of the total).
Many of the spiral wound steel pipes were probably installed in the 1970s and 1980s giving a serviceable life of 30 to 40 years, well short of the life of 100 years now specified for permanent drainage structures, and which would be expected from concrete pipes. Following the fatal corrugated steel pipe culvert failure at Somersby in June 2007 and initial findings from subsequent RTA culvert condition inspections, RTA Road Design Branch issued Road Design Technical Directions RTD 2009/001, RTD 2009/002 and RTD 2009/002 ANNEX on the selection and the rehabilitation of corrugated steel structures with can be found at: . RTD 2009/01 imposes restrictions on the use of steel culverts for new works and specifies a product assessment process for proprietary steel culverts, in lieu of a site-specific assessment and approval process requiring sign-off by the Principal Road Design Engineer on the use of any steel culvert products on projects. Failures Five people were killed when the council-owned three-cell corrugated steel pipe culvert carrying Piles Creek under the Old Pacific Highway near Somersby on the NSW Central Coast collapsed during a severe storm event on Friday 8th June 2007. The culvert failure was caused by washout of the surrounding embankment fill arising from severe corrosion and subsequent failure of the pipe walls. This event prompted a number of actions by the road authorities in relation to their stormwater and other culvert structures. The majority of the RTA’s corrugated steel pipe structures inspected well before and after the Somersby collapse have been found to have corroded inverts with damage ranging from mild to severe, with various forms of remediation works subsequently undertaken by the responsible Regional office. Austroads 2011 — 100 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
From the inventory records for various aluminium pipes, little evidence of corrosion was reported. No RTA corrugated metal structures are thought to have failed catastrophically. Generally, a progressive decline and signs of deterioration reflected in the road pavement in advance of total failure would be expected. Design specifications The design of new corrugated steel structures and the rehabilitation of existing ones have generally been carried out in accordance with the relevant Australian Standards. RTA does not have its own design specifications for such structures. Inspections Following the corrugated steel pipe culvert failure at Somersby, the RTA issued the following comprehensive requirements for the condition inspections of all types of culverts on RTA’s road network: . Repair and maintenance methods RTD 2009/02 and RTD 2009/02 ANNEX provide details of procedures deemed suitable by RTA for the rehabilitation of corrugated steel pipe culverts. Remediation treatments in spirally wound steel pipes in recent years have generally been by relining the pipe with a smaller pipe, or installing a concrete base to the bottom third of the pipe. Techniques known to have been used to date for the rehabilitation of failed or corroded structures include:
Full structural shotcrete lining – Bangalow Bypass on Pacific Highway – RTA Northern Region – repair of failed new 2.4 m diameter three-cell pipe culvert during construction in 1994
Full structural shotcrete lining – Tumblong Creek on Hume Highway – RTA South-Western Region – rehabilitation of corroded large diameter pipe culvert
Concrete invert lining and approach treatment – Pyes Creek – RTA Sydney Region – design analysis and rehabilitation of corroded 3.7 m diameter two-cell pipe culvert built in 1960
Interflow lining with grouting of annulus and provision of new headwalls and wingwalls – George Creek on Newline Road – RTA Sydney Region – design analysis and rehabilitation of corroded 3.3 m diameter two-cell pipe culvert built in 1960
Precast concrete pipe insertion and grouting with new wingwalls and head walls – Campbelltown culvert – RTA Sydney Region – analysis and rehabilitation of corroded 2.7 m diameter two-cell pipe culvert.
Austroads 2011 — 101 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Construction specifications RTA QA Specifications R22, Corrugated Metal Structures, currently withdrawn, specifies the requirements for installation of corrugated metal structures, and refers to RTA QA specification R11, Stormwater drainage, and to the relevant Australian Standards. RTA R22 will be reinstated following publication of the new AS/NZS 2041 series of Standards.
B.2
Roads Corporation, Victoria (VicRoads)
General experiences The majority of BCMS on state-controlled roads in Victoria are tubular of circular, elliptical or compound cross-section and either assembled from rectangular steel plates or is spirally-wound on site. VicRoads also has a number of metal arch structures. There are 2265 culverts on the Victorian state-controlled network which makes up to one-third of the total number of structures on the network. 142 of these culverts are BCMS, including 33 steel arches. A further 25 BCMS are also present in the network but have not been assessed, bringing the total number of BCMS to 167. Of the 142 BCMS, 13 are reported to be either programmed for lining in the current year or to be under consideration for lining. Table B 1 summarises the condition status of BCMS in Victoria. Table B 1:
BCMS condition summary in Victoria Condition and lining status
Condition state
Lining status
Number
C1
Unlined
79
C1
Lined
20
C1
Replaced
0
C2
Unlined
26
C2
Lined
2
C3
Unlined
13
C4
Unlined
2
Sub-Total
142
Other BCMS
25
Total
167
Experience has found that the arch type structure is less prone to severe corrosion in circumstances where the concrete channel carries the majority of the flow and the arch springing is above the level of the water flow. Typically arch structures are in condition state 1. VicRoads also has a small number of aluminium BCMS. This culvert type has been found to have very few corrosion issues, and no real abrasion issues have been experienced despite the common belief that abrasion is more critical in aluminium alloy pipes. Inspections over recent years have revealed that the majority of steel BCMS in Victoria show signs of corrosion to varying degrees. Many culverts have suffered a significant loss of wall thickness and some are perforated. In the most severe cases the invert has corroded away and water is flowing below the original invert level. It has been found that it is not uncommon for a BCMS that
Austroads 2011 — 102 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
has been in service for 25 years to be severely damaged and culverts of 10 to 15 years of age to be showing evidence of serious corrosion with significant loss of thickness already occurring. Significant corrosion in a culvert installed in 2002 and inspected in 2007 was found despite the absence of continuously flowing water in the culvert. In summary, VicRoads has found that the age at which corrosion commences is much earlier than might reasonably be expected and that the rate of loss of thickness also exceeds expectations. For these reasons the expected design life of this type of structure is typically not being achieved. Design specification Standard specification section 632 – Buried Corrugated Metal (Steel) Structures (VicRoads 2010) defines design and construction parameter limitations for all VicRoads BCMS. VicRoads allows bolted steel plates or sheets in accordance with AS/NZS 2041 (1998) and long span corrugated steel structures in accordance with AS 3703.1 (1989) and AS 3703.2 (1989). Pipes made from helically-wound galvanised steel strip with lock seams or other proprietary helically-wound profiles are not permitted typically due to the very thin sections required to allow the rolling process to occur. The thin section is considered inadequate for durability/corrosion resistance and would result in a design life that is shorter than required. BCMS are not permitted if the culvert is permanently inundated to any depth. Typically BCMS are to have a design life of 100 years with the residual thickness at 100 years to be sufficient to safely support the current design dead and live load relevant at the time of design. BCMS of less than 1200 mm are not permitted. If the design life cannot be achieved using steel, consideration must be given to using materials such as aluminium alloy or pre-cast concrete pipes or box culverts.
B.3
Department of Transport and Main Roads, Queensland (TMR)
Failures Pritchard and Muller (2003), provide a detailed report on the failure of a helical spun steel culvert located under the Gateway Arterial (100 m north of Deagon Interchange) (Figure B 1 to Figure B 3). Significant structural failure was noted in all three pipes due to standing water causing corrosion and subsequent loss of steel section in the culvert walls. Deformation was also attributed to mechanical damage during installation. As shown in Figure B 2, the pipe wall cannot support the ring compression in the most corroded locations causing the pipe to fold in on itself, grossly distorting the pipe cross-section and reducing the pipe’s capacity. This defect appears to be directly below the Gateway Motorway and has most likely been exacerbated by the additional dead load of the overlying embankment material.
Austroads 2011 — 103 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Corrosion at these locations
Standing water Source: Pritchard and Muller (2003).
Figure B 1: Failure mechanism observed during the inspection and proposed propping measures
Source: Pritchard and Muller (2003).
Figure B 2: Gross structural failure and distortion of the pipe at a joint between pipe segments
Austroads 2011 — 104 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Source: Pritchard and Muller (2003).
Figure B 3: Deformation of the pipe wall possibly occurred during culvert installation
Recommendations include that as a long-term solution, the existing pipes be abandoned and filled with flowable fill to prevent adverse effects to the overlying roadways. New pipes would be installed on a new alignment, chosen on the basis of practicality and cost of installation. To prevent collapse in the central pipe in the short-term, temporary propping was recommended. Figure B 4 shows a failure that occurred on the Bruce Highway which resulted in closure for several months.
Source: TMR nd a.
Figure B 4: Culvert soil arch failure
Austroads 2011 — 105 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
Inspections TMR has developed bridge asset management (BAM) advice note no. 88 which is a draft inspection and treatment document for metal culverts. This includes required culvert information to be collected and various risk conditions assessed and suitable actions taken. A regular inspection and maintenance program will keep culverts free-flowing and provide early notice when attention is needed. When inspection reveals signs of distress, such as excessive invert wear, the most practical response is early rehabilitation. Preventive care is the best way to achieve long life. Virtually all culvert failures result from insufficient maintenance or unidentified harmful changes in service conditions. If the need arises, the hydraulic performance of buried culverts typically can be restored with liners. The characteristics of corrugated steel culverts make relining relatively fast and economical, saving the cost of excavation and replacement without sacrificing drainage capacity. TMR in the Road Drainage Manual (TMR 2010) provides guidelines on inspecting corrugated metal pipe culverts and these include:
inspecting the pipe for signs of rust or corrosion
ensuring the painted or galvanised surface is in good condition
checking that the line and invert of the pipe is straight with no water being retained in the pipe
making sure that connection bolts have not torn through the plates or split the plates.
B.4
Department of Lands and Planning, Northern Territory (DLP)
Typically, BCMS have not been used much by the DLP because of corrosion concerns. It has had one example of a badly deformed pipe due to a high embankment resulting in a compression type failure. The use of BCMS in the Northern Territory dates back to the early 1960s or earlier. They are generally only galvanised. The DLP has experienced a few examples of external corrosion (soil side) of the BCMS, but typically it is most common that the inverts are lost within 10 years or so due to erosion and corrosion. There have also been a few examples of culverts being scoured out and ending up downstream.
B.5
ACT Department of Territory and Municipal Services (ACT TAMS)
ACT has provided information on four corrugated steel culverts. They are all over 30 years old. Three are pedestrian underpasses with no water and the fourth has a small creek flowing through it. All are in good condition and no particular issues have been identified.
Austroads 2011 — 106 —
Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures
B.6
Summary of Areas of Concern/Interest to State Roads Authorities
Table B.2 lists the topics related to BCMS that road authorities are seeking advice. Table B.2:
Topics related to BCMS that road authorities are seeking advice Issue
Raised by
Durability (corrosion rates)
VicRoads
Non-destructive testing
VicRoads
Repair methods (less invasive and safe)
VicRoads
Standard end treatments
TMR
Austroads 2011 — 107 —
INFORMATION RETRIEVAL Austroads, 2011, Guidelines for Design, Construction, Monitoring and Rehabilitation of Buried Corrugated Metal Structures, Sydney, A4, pp. 113. AP-T196-11 Keywords: Buried, corrugated, soil structure interaction, culvert, asset management, structural design, culvert rehabilitation, construction Abstract: Buried corrugated metal structures (BCMS) have been used in Australia as an attractive solution to under road drainage requirements due to the low cost and fast construction times achievable. Several incidents of significant failures of BCMS, however, have been reported in current practice. These guidelines provide essential information regarding BCMS from the design process, installation, in-service monitoring, through to maintenance and repair procedures. The guidelines include: (i) methods for designing of BCMS which include structural and durability considerations, (ii) methods of installation and construction required to satisfy the design performance, (iii) guidelines for structural management and inspection of BCMS which include the defect identification condition rating system and structural management plans, and (iv) repair methods for damaged BCMS.
