P06-09 Unsealed Pavements

GUIDE TO PAVEMENT TECHNOLOGY

Part 6: Unsealed Pavements

Guide to Pavement Technology Part 6: Unsealed Pavements

Guide to Pavement Technology Part 6: Unsealed Pavements Summary Part 6 of the Guide to Pavement Technology addresses unsealed pavements including operational demands of unsealed road surfaces, pavement configurations, floodways, cuts, fills and mine haul roads, the identification of suitable pavement materials including commercially produced products and natural gravel sources, improvement of unsealed road pavement materials using modified stabilised materials, pavement design, including determination of required pavement thickness over the subgrade, drainage and erosion protection, and environmental considerations and performance expectation, including surface condition assessment. It is based on material contained in the ARRB Unsealed Roads Manual together with technical information contained in other relevant reports and documents. Keywords unsealed roads, unsealed road surfacings, pavement design, pavement materials, pavement performance, stabilisation, surface condition, pavement maintenance, pavement rehabilitation, life cycle costing, evaluation/assessment First Published September 2009 © Austroads Inc. 2009 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. ISBN 978-1-921551-52-9

Austroads Project No. TP1565 Austroads Publication No. AGPT06/09

Project Manager Chris Mathias Prepared by Bob Andrews Published by Austroads Incorporated 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 This Guide is produced by Austroads as a general guide. Its application is discretionary. Road authorities may vary their practice according to local circumstances and policies. 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.

Guide to Pavement Technology Part 6: Unsealed Pavements

Sydney 2009

Austroads profile Austroads purpose is to contribute to improved Australian and New Zealand transport outcomes by: 

providing expert advice to SCOT and ATC on road and road transport issues



facilitating collaboration between road agencies



promoting harmonisation, consistency and uniformity in road and related operations



undertaking strategic research on behalf of road agencies and communicating outcomes



promoting improved and consistent practice by road agencies.

Austroads membership Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure, Transport, Regional Development and Local Government in Australia, the Australian Local Government Association, and New Zealand Transport Agency. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its 11 member organisations:           

Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Transport and Main Roads Queensland Main Roads Western Australia Department for Transport, Energy and Infrastructure South Australia Department of Infrastructure, Energy and Resources Tasmania Department of Planning and Infrastructure Northern Territory Department of Territory and Municipal Services Australian Capital Territory Department of Infrastructure, Transport, Regional Development and Local Government 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 sector.

ACKNOWLEDGEMENT The author acknowledges the significant compilation and editing of Chapter 5 Pavement Material Sources by William G Harvey (Department of Primary Resources Sth Aust).

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

CONTENTS 1

INTRODUCTION ............................................................................................................ 1

1 1.2 1.3

Scope of Guide to Pavement Technology Part 6............................................................ 1 Guide to Pavement Technology ..................................................................................... 2 Unsealed Road Network Operation ................................................................................ 3

2

TYPES OF UNSEALED ROADS ................................................................................... 5

2.1 2.2

Pavement Configurations and Classifications ................................................................ 5 Selection of Pavement Type........................................................................................... 8

3

PAVEMENT MATERIALS............................................................................................ 10

3.1

Basic Pavement Material Principles for Unsealed Roads............................................. 10 3.1.1 Stability ........................................................................................................... 10 3.1.2 Resistance to Wear ........................................................................................ 12 3.1.3 Impermeability ................................................................................................ 12 3.1.4 Workability and Compaction ........................................................................... 13 Unbound Granular Specifications ................................................................................. 14 3.2.1 Maximum Size and Particle Size Distribution ................................................. 14 3.2.2 Base and Subbase ......................................................................................... 18

3.2

4

PAVEMENT THICKNESS ............................................................................................ 19

4.1

Thickness Design Methodology.................................................................................... 19 4.1.1 Design Traffic.................................................................................................. 20 4.1.2 Thickness Design ........................................................................................... 20

5

PAVEMENT MATERIAL SOURCES ........................................................................... 22

5.1

Borrow Pit Geological Sources ..................................................................................... 22 5.1.1 Residual Deposits........................................................................................... 22 5.1.2 Colluvial Deposits ........................................................................................... 22 5.1.3 Alluvial Deposits ............................................................................................. 23 5.1.4 Concretionary Deposits .................................................................................. 23 5.1.5 Volcanic Deposits ........................................................................................... 23 Winning of Local Materials from Borrow Pits ................................................................ 23 5.2.1 Overview of Regulations................................................................................. 23 5.2.2 Pit Operation and Rehabilitation ..................................................................... 25 Processing Material from Borrow Pits........................................................................... 28 5.3.1 Processing on the Road Bed .......................................................................... 29 5.3.2 Mobile Plant Crushing..................................................................................... 30

5.2

5.3

6

STABILISATION OF UNSEALED ROADS ................................................................. 35

6.1 6.2 6.3

Types of Stabilised Materials........................................................................................ 35 Application of Stabilisation............................................................................................ 36 Granular Stabilisation by Blending Materials ................................................................ 36 6.3.1 Granular Mix Design ....................................................................................... 36 Stabilisation Using Chemical Binders ........................................................................... 38 6.4.1 Types of Chemical Stabilisation Binders ........................................................ 38 6.4.2 Applications .................................................................................................... 41 6.4.3 Product Selection and Mix Design.................................................................. 42 Stabilisation with Lime .................................................................................................. 43 Stabilisation with Cementitious Binders........................................................................ 43 Stabilisation with Powder Binders................................................................................. 43

6.4

6.5 6.6 6.7

Austroads 2009 — i—

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

6.8

Methods of Applying Stabilisation Binders in the Field ................................................. 44 6.8.1 Powder Binders .............................................................................................. 44 6.8.2 Liquid Binders ................................................................................................. 44 6.8.3 Methods of Mixing........................................................................................... 45 6.9 Logistical Selection of Stabilisation Binders for Construction ....................................... 46 6.10 Technical Evaluation of Stabilisation Binder Performance ........................................... 46 6.10.1 Laboratory Evaluations ................................................................................... 46 6.10.2 Field Trials ...................................................................................................... 47 7

UNSEALED SURFACE WEARING CHARACTERISTICS .......................................... 48

7.1 7.2

Introduction ................................................................................................................... 48 Types of Surface Wear ................................................................................................. 48 7.2.1 Loss of Fine Material (Dust)............................................................................ 48 7.2.2 Loose Gravel .................................................................................................. 52 7.2.3 Corrugations ................................................................................................... 53 7.2.4 Potholes.......................................................................................................... 55 7.2.5 Dry Rutting in Wheelpaths .............................................................................. 56 7.2.6 Surface Gouging............................................................................................. 56 7.2.7 Surface Scour ................................................................................................. 57 7.2.8 Ice Formation on Surface ............................................................................... 58

8

UNSEALED ROAD SURFACE MANAGEMENT......................................................... 60

8.1 8.2

8.6

Introduction ................................................................................................................... 60 Surface Maintenance.................................................................................................... 60 8.2.1 Patrol Grading................................................................................................. 60 8.2.2 Reshaping and Shallow Stabilisation.............................................................. 62 Resheeting (Wearing Course Replacement) ................................................................ 63 8.3.1 Measuring and Estimating Gravel Loss .......................................................... 63 8.3.2 Predicting Gravel Loss.................................................................................... 64 Unsealed Road Condition Monitoring ........................................................................... 66 Visual Pavement Condition Rating Systems ................................................................ 67 8.5.1 South Africa .................................................................................................... 67 8.5.2 USA ................................................................................................................ 68 Quantitative Pavement Condition Rating...................................................................... 73

9

COST–BENEFIT CONSIDERATIONS......................................................................... 75

9.1 9.2

Concept ........................................................................................................................ 75 Life Cycle Analyses for Selection of Wearing Course and Associated Maintenance Management Strategies .......................................................................... 75 9.2.1 Introduction ..................................................................................................... 75 9.2.2 Life Cycle Cost Analyses ................................................................................ 76 9.2.3 Grading Intervention Frequency and Sheeting Life ........................................ 78

8.3

8.4 8.5

REFERENCES ...................................................................................................................... 79

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

TABLES Table 1.1: Table 1.2: Table 2.1: Table 2.2: Table 3.1: Table 3.2: Table 3.3: Table 3.4: Table 3.5: Table 3.6: Table 6.1: Table 6.2: Table 8.1: Table 8.2: Table 9.1:

References to unsealed pavement technology in the Austroads Guide to Pavement Technology and ARRB Unsealed Roads Manual............................ 2 Key websites pertinent to unsealed road technology ....................................... 2 Unsealed road classification ............................................................................. 6 Indicative pavement type selection................................................................... 9 Suggested CBR values for pavement materials for unsealed roads .............. 11 Typical Clegg Impact Value (CIV)................................................................... 11 Base strength versus Clegg Impact Value...................................................... 12 Indicative permeability values (100% standard compaction) .......................... 12 Typical properties for unsealed road wearing course ..................................... 15 Typical specifications (South Africa)............................................................... 16 Types of stabilisation ...................................................................................... 35 Example calculation – blending two materials ................................................ 38 Comparative rates of annual gravel loss ........................................................ 66 US Army URCI scale and condition rating...................................................... 68 Example life cycle analyses............................................................................ 77

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

FIGURES Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 2.5: Figure 2.6: Figure 3.1: 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 5.1: Figure 5.2: Figure 5.3: Figure 5.4: Figure 5.5: Figure 5.6: Figure 5.7: Figure 5.8: Figure 6.1: Figure 6.2: Figure 6.3: Figure 6.4: Figure 6.5: Figure 6.6: Figure 6.7: Figure 6.8: Figure 7.1: Figure 7.2: Figure 7.3: Figure 7.4: Figure 7.5: Figure 7.6: Figure 7.7:

Layers associated with an unsealed road pavement ........................................ 5 Class U1 road ................................................................................................... 6 Class U2 road ................................................................................................... 7 Class U3 road ................................................................................................... 7 Class U4 road ................................................................................................... 7 Class U5 road ................................................................................................... 8 Laboratory CBR test and 4.5 kg Clegg impact field test ................................. 11 Loose material and coarse texture due to surface wear ................................. 12 Moisture range variation for compaction......................................................... 13 High quality unsealed road surface ................................................................ 15 Suggested PSD range for unsealed wearing course...................................... 16 Relationship between shrinkage product, grading coefficient and performance of wearing course gravels.......................................................... 17 Workability attributes of granular materials..................................................... 18 Pneumatic traffic counting .............................................................................. 20 Traffic counts obtained from vibration sensors ............................................... 20 Design for granular pavements (80% confidence).......................................... 21 Pit material raised and transported to road bed.............................................. 25 Operation of ‘Rockbuster’ plant ...................................................................... 29 Static grid roller............................................................................................... 30 Jaw crusher .................................................................................................... 31 Gyratory crusher ............................................................................................. 32 Cone crusher .................................................................................................. 32 Impact crusher ................................................................................................ 33 Vertical shaft impact crusher .......................................................................... 34 Example combination particle size analysis ................................................... 37 Schematic of insoluble polymer encapsulating soil particles .......................... 39 Electron micrograph of acrylimide copolymer coating soil particles ............... 40 Vertical saturation test .................................................................................... 43 Purpose-built stabilisation binder spreader..................................................... 44 Application of liquid stabilisation binder with water truck ................................ 45 Adding granulated polymer using patented eductor ....................................... 45 Stabilisation binder mixing with purpose-built recycler ................................... 46 Advisory sign for dust hazard ......................................................................... 49 Mobile and static dust monitoring apparatus .................................................. 49 Schematic diagram of Colorado State University Dustometer........................ 50 Loss of fines increasing surface texture ......................................................... 51 Slurrying unsealed surface ............................................................................. 51 Loss of fine material leaving coarse gravelly surface ..................................... 52 Loose material between wheelpaths (note centre overlap from trafficking in both directions) ........................................................................................... 52 Austroads 2009 — iv —

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Figure 7.8: Figure 7.9: Figure 7.10: Figure 7.11: Figure 7.12: Figure 7.13: Figure 7.14: Figure 7.15: Figure 7.16: Figure 7.17: Figure 7.18: Figure 8.1: Figure 8.2: Figure 8.3: Figure 8.4: Figure 8.5: Figure 8.6: Figure 8.7: Figure 8.8: Figure 8.9: Figure 8.10: Figure 8.11: Figure 8.12: Figure 8.13: Figure 9.1: Figure 9.2:

Measurement of loose material on pavement surface.................................... 53 Corrugation formation in dry climates ............................................................. 54 Corrugations in gravel surface (left) and sandy surface (right) ....................... 54 Corrugation formation in wet climates ............................................................ 55 Potholes on flat crossfall................................................................................. 55 Dry rutting in wheelpath .................................................................................. 56 Surface gouging.............................................................................................. 56 Longitudinal scour on steep gradient.............................................................. 57 Transverse scouring on horizontal curve ........................................................ 58 Longitudinal scouring between wheelpaths .................................................... 58 Snow and ice formation .................................................................................. 59 Patrol grading ................................................................................................. 60 Tow-behind steel drum roller and multi-tyred roller ........................................ 61 Surface slurrying during compaction .............................................................. 62 Wet compaction and slurrying (left) and dry compaction (right) ..................... 62 Surface after scarifier grading......................................................................... 63 Ground penetration radar (GPR) with horn antenna....................................... 64 Condition deduct values (drainage, cross-section, corrugations, dust) .......... 69 Condition deduct values (potholes, ruts, loose aggregate)............................. 70 US Army URCI calculation.............................................................................. 71 Pavement assessment example ..................................................................... 71 US Army unsealed road condition assessment form ...................................... 72 Roughometer .................................................................................................. 73 Laser Profilometer .......................................................................................... 73 Example life cycle analysis ............................................................................. 77 Life cycle analysis of sheeting life and grading intervention ........................... 78

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

1

INTRODUCTION

1.1

Scope of Guide to Pavement Technology Part 6

Part 6 of the Guide to Pavement Technology addresses unsealed pavements technology and complements the ARRB Unsealed Roads Manual (Giummarra, Ed, in press). It also includes relevant technical information contained in other reports and documents. Note that the ARRB Unsealed Roads Manual is a compilation in the one document of all aspects pertaining to the design, construction and management of unsealed roads for use by all authorities associated with unsealed roads e.g. Austroads members, local government, national parks, forestry commissions, etc. In contrast, Part 6 of the Austroads Guide to Pavement Technology covers unsealed roads technology. Part 6 outlines those aspects of unsealed roads pertinent to unsealed road pavement technology, including: 

operational demands of unsealed road surfaces



pavement configurations, floodways, cuts, fills and mine haul roads



identification of suitable pavement materials including commercially produced products and natural gravel sources



improvement of unsealed road pavement materials using modified stabilised materials



pavement design, including determination of required pavement thickness over the subgrade, drainage and erosion protection, and environmental considerations



performance expectation, including surface condition assessment



maintenance and rehabilitation and life cycle operating cost evaluations.

Topics related to unsealed roads and addressed in other parts of the Guide to Pavement Technology are listed in Table 1.1.

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Table 1.1:

References to unsealed pavement technology in the Austroads Guide to Pavement Technology and ARRB Unsealed Roads Manual Part 4: Pavement Materials Part 4E: Recycled Materials Part 4I: Earthworks Materials Part 10: Sub-Surface Drainage

Pavement material selection

Part 4: Pavement Materials Part 4A: Granular Base and Sub Base Materials Part 4J: Aggregate and Source Rock

Crushed unbound granular materials

Part 4: Pavement Materials Part 4D: Stabilised Materials Part 4L: Stabilising Binders

Stabilisation

Part 8: Pavement Construction and Construction Assurance Part 9: Pavement Work Practices ARRB Unsealed Roads Manual

Construction practice/specifications

Part 7: Pavement Maintenance Part 9: Pavement Work Practices ARRB Unsealed Roads Manual

Maintenance practice Geometric design

ARRB Unsealed Roads Manual Part 5: Pavement Evaluation and Treatment Design ARRB Unsealed Roads Manual

Asset management

Part 3: Pavement Surfacings Part 4: Pavement Materials Part 4K: Seals

Bituminous sealing of unsealed road pavements

Table 1.2 lists relevant websites from which pertinent publications on technologies associated with unsealed roads such as technical notes, guidelines, work tips and safety data can be obtained. Also included in the guide is a bibliography of relevant publications. Table 1.2:

Key websites pertinent to unsealed road technology

Austroads

1.2

www.austroads.com.au

ARRB

www.arrb.com.au

NZ Transport Agency

www.nzta.govt.nz

Australian Asphalt Pavement Association

www.aapa.asn.au

Cement Concrete & Aggregates Australia

www.concrete.net.au

Australian Stabilisation Industry Association

www.auststab.com.au

PIARC

www.piarc.org

Materials Safety

www.msds.com.au

South African National Roads Agency

www.nra.co.za

Guide to Pavement Technology

The Guide to Pavement Technology consists of the following 10 parts: 

Part 1: Introduction to Pavement Technology



Part 2: Pavement Structural Design



Part 3: Pavement Surfacings



Part 4: Pavement Materials —

Part 4A:

Granular Base and Subbase Materials

Austroads 2009 — 2—

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

— — — — — — — — — — —

Part 4B: Part 4C: Part 4D: Part 4E: Part 4F: Part 4G: Part 4H: Part 4I: Part 4J: Part 4K: Part 4L:

Asphalt Materials for Concrete Road Pavements Stabilised Materials Recycled Materials Bituminous Binders Geotextiles and Geogrids Test Methods Earthworks Materials Aggregate and Source Rock Seals Stabilising Binders



Part 5: Pavement Evaluation and Treatment Design



Part 6: Unsealed Pavements



Part 7: Pavement Maintenance



Part 8: Pavement Construction



Part 9: Pavement Work Practices



Part 10: Subsurface Drainage

1.3

Unsealed Road Network Operation

Australia has about 800,000 km of roads, of which about two-thirds are unsealed (Austroads 2000) whilst New Zealand has about 92,700 km of roads of which about 40% are unsealed (Transit New Zealand, Road Controlling Authorities and Roading New Zealand 2005). The unsealed road network serves the community by providing: 

access to rural and local communities, often in isolated locations



freight routes servicing primary and secondary industries



haul roads servicing the mining and timber industries



recreational, social and tourist pursuits



links for military use



emergency services access (e.g. fire fighting) in national parks, etc.

Compared to sealed roads, the performance of unsealed roads is typified by: 

higher operating costs associated with surface maintenance and replenishment



restricted or no access during and after periods of heavy rainfall



higher accident risks per vehicle-kilometres travelled associated with corrugated, potholed, dusty, slippery (when wet) and loose (dry) surfaces



higher environmental and heritage (i.e. historical and indigenous) impacts associated with a high consumption of natural materials extracted from natural gravel pits



high demand for water associated with frequent maintenance operations.

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

The purpose of this guide is to assist road authorities in the efficient management of unsealed road pavements. As well as addressing fundamental design issues, advice is provided on operational and strategic management issues such as: 

braking and skidding associated with loose gravel on the road surface



visibility issues associated with the generation of dust



damage to vehicles (e.g. windscreens) associated with flying stones



optimising routine patrol grading to maintain an adequate riding surface



conservation of natural materials associated with maximising periods between re-surfacing



reduced environment and heritage impacts associated with less material extraction



reduced impacts on the roadside habitat associated with loose material.

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

2

TYPES OF UNSEALED ROADS

2.1

Pavement Configurations and Classifications

The quality levels of unsealed roads, and the associated levels of maintenance, vary widely across the network. They are principally based on the volume of daily traffic, the composition of the traffic (e.g. road trains), and accessibility and remoteness issues: pavement configurations vary from twolane, multiple granular layers and shoulders constructed over the subgrade, to a single lane shaped subgrade. In all cases in this guide, unsealed pavements refer to full depth granular pavements. Other pavement layers or wearing surfaces such as stabilised bound layers or light duty bituminous surfaces are not addressed. There are four types of pavement layers associated with unsealed roads (Figure 2.1): 

wearing course: sometimes referred as the ‘sheeting layer’; it is maintained with patrol grading and replenished after some years as its thickness is reduced and/or when a large amount of fine material has been lost as dust



base: provides structural support to the wearing course and protects against subgrade deformation



subbase: adds to the structural capacity of the pavement and makes up the desired thickness indicated from empirical thickness design charts



subgrade: the in situ soil or fill upon which the pavement is founded; it may also be used as a wearing course on access tracks.

wearing course (sometimes referred to as sheeting layer) base subbase subgrade Figure 2.1: Layers associated with an unsealed road pavement

At this time there is no agreed hierarchy of unsealed pavement types. However, a suggested Austroads classification is shown in Table 2.1 (Austroads 2006a). Photographs of typical roads associated with the five classes shown in Figure 2.2 to Figure 2.6.

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Table 2.1: Class

U1

U2

U3

Daily traffic (typical veh/ day)

>200

Unsealed road classification

Description

Material quality and typical configuration

 All-weather formed pavement with adequate drainage provided. At least two pavement layers over subgrade.  Granular or modified materials may be adopted in the base and wearing course. Dust suppressants may be incorporated in maintenance strategies.

    

100 – 200

 Mostly all-weather formed pavement with some drainage. Two pavement layers over subgrade.  Granular or modified materials may be adopted in the wearing course. Dust suppressants may be incorporated in maintenance strategies.

20 – 100

 Formed pavement with surface drainage. Max. of two pavement layers over subgrade.  Granular or modified materials may be adopted in the wearing course. Dust suppressants may be incorporated in maintenance strategies.

 

 



Crushed quarry materials or in situ processed natural gravels. 20 mm max. size** wearing course, min. 100 mm thick. 40 mm max. size base, min. 150 mm thick. 55 mm max. size subbase, min. 150 mm thick. Crushed quarry materials, crushed pit material, ‘on road’ processed natural gravels. 40 mm max. size wearing course, min. 100 mm thick. 55 mm max. size base, min. 150 mm thick. Natural gravels, pit materials or quarry wastes. 40 mm max. size wearing course, min. 100 mm thick. 55 mm max. size subbase, min. 150 mm thick.

U4

< 20

 Unformed pavement with single pavement layer over subgrade.

 

Natural gravels, pit materials or quarry waste. 50 mm max. size wearing course, min. 150 mm thick.

U5

< 10

 Unformed pavement comprising subgrade only.



Vegetation cleared subgrade1.

In some circumstances, a wearing course may be incorporated as a thin binding course over subgrade (armouring to improve trafficability). ** refers to maximum stone size

Source: ARRB Group

Figure 2.2: Class U1 road

Austroads 2009 — 6—

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Source: ARRB Group

Figure 2.3: Class U2 road

Source: ARRB Group

Figure 2.4: Class U3 road

Source: ARRB Group

Figure 2.5: Class U4 road

Austroads 2009 — 7—

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Source: ARRB Group

Figure 2.6: Class U5 road

2.2

Selection of Pavement Type

With the exception of mine and forestry haul roads and some access tracks, in most cases the construction of new unsealed roads is unlikely as the network is essentially established. Therefore the selection of a particular type of unsealed road pavement will generally be associated with upgrading an existing road surface to meet the intended use. The main issues to consider in the selection of an unsealed road pavement are: 

the volume and type of traffic (e.g. road trains) as this will govern the pavement thickness and quality of wearing course required



desired speed of traffic in relation to safety and dust emissions



the importance of the pavement in terms of all-weather access which may have social or economic impacts on communities and industries



the availability of local materials for the wearing course because the provision of inadequate materials can result in high maintenance costs.

As a guide, Table 2.2 suggests typical situations associated with the selected pavement types.

Austroads 2009 — 8—

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Table 2.2: Pavement type

Traffic spectrum

Indicative pavement type selection Attributes

Typical applications 

Main unsealed roads carrying significant freight or livestock. Links to major resource developments, e.g. mines, gas fields, etc.

U1

>200 veh/day and/or >20% heavy vehicles2

Up to 100 km/h1 two lanes plus shoulder

U2

100-200 veh/day and/or >10% heavy vehicles

Up to 100 km/h two lanes plus shoulder



Main links between communities, national parks, recreational areas, haul roads.

20-100 veh/day and/or 5% (subgrades and formations)

 

Addition of lime. Addition of chemical binder.



Granular

40% < CBR1 < +100% (subbase and basecourse)

Blending other granular materials which are classified as binders in the context of this guide.

Common binders adopted

 Modified

0.7 MPa < UCS2 < 1.5 MPa (basecourse)

 

Addition of small quantities of cementitious binder. Addition of lime. Addition of chemical binder.

Anticipated performance attributes   

Improved subgrade strength. Improved shear strength. Reduced heave and shrinkage.

  

Improved pavement stiffness. Improved shear strength. Improved resistance to aggregate breakdown.

  

Improved pavement stiffness. Improved shear strength. Reduced moisture sensitivity, i.e. loss of strength due to increasing moisture content. At low binder contents can be subject to erosion where cracking is present.



  Bound

UCS2 > 1.5 MPa (basecourse)



Addition of greater quantities of cementitious binder. Addition of a combination of cementitious and bituminous binders.

 

Increased pavement stiffness to provide tensile resistance. Some binders introduce transverse shrinkage cracking. At low binder contents can be subject to erosion where cracking is present.

1.

Four day soaked CBR.

2.

Values determined from test specimens stabilised with GP cement and prepared using standard compactive effort, normal curing for a minimum 28 days and 4 hour soak conditioning.

Source: Austroads 2006b

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

6.2

Application of Stabilisation

It is unlikely that traditional subgrade stabilisation would be linked to unsealed road construction unless it formed part of a staged construction approach in which the road was intended to be sealed in the short term. Stabilisation of unsealed road wearing surfaces is generally limited to granular stabilisation. However, modified stabilisation (particularly lime and chemical binders) may be used to enhance surface wear characteristics, slow down the rate of deterioration – mainly manifested as the generation of dust and loose, gravelly surfaces which may lead to potholes and corrugations – and reduce subsequent asset management costs, e.g. decreasing the number of patrol grading interventions and wearing course replacement (re-sheeting). The possible use of stabilisation for improving wearing course attributes cannot be assessed quantitatively in a laboratory. However, there are some simple tests for binder suitability and these are presented in this section of the guide. Ideally, laboratory indicators should be supported by effective field trials, bearing in mind that the realisation of any cost benefit demonstrated by the laboratory result can be reduced as traffic volumes increase. A process for these evaluations is provided in Section 6.9. Bound stabilisation techniques have generally been associated with specific unsealed sites such as floodways; however, recent use of lime stabilisation for unsealed roads has led to the successful production of bound pavement materials.

6.3

Granular Stabilisation by Blending Materials

Granular stabilisation by blending materials involves: 

mixing of materials from various parts of a deposit at the source of supply



mixing of selected, imported material with in situ materials



mixing two or more selected imported natural gravels, soils and/or quarry products on site or in a mixing plant.

Some typical applications of granular stabilisation are: 

correction of grading generally associated with gap graded or high fines-content gravels



correction of grading and increasing plasticity of dune or river-deposited sands which are often single sized



correction of grading and/or plasticity in crushed products, quarry wastes and environmentally acceptable industrial by-products



decrease in particle breakdown of soft aggregate by the addition of harder aggregate.

Generally, granular stabilisation is adopted to align with, as far as is possible, the classification properties of an unbound granular material defined in standard road authority specifications. 6.3.1

Granular Mix Design

Granular stabilisation is adopted when it is necessary to change the intrinsic characteristics of the existing material to suit its intended purpose or to meet a particular specification. Some examples of the application of granular stabilisation are: 

fine sand added to crushed rock in a pugmill, when the PI of the crushed rock material exceeds specification

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

coarse aggregate added in situ to a fine pavement material for the purposes of in situ stabilisation



harder rock may be quarried and added to soft rock to meet hardness-abrasion specifications.

The mix design, in terms of grading, can be determined using simple proportion calculations of the constituent materials passing respective sieves: ((A% x Apass)/100) + ((B% x Bpass)/100) where

A%

=

percentage of material A being added

Apass

=

percentage of material A passing allocated sieve

B%

=

percentage of material B being added

Bpass

=

percentage of material B passing allocated sieve.

1

A worked example is shown in Figure 6.1, in which 70% of material ‘A’ (coarse product) is combined with 30% of material ‘B’ (fine product) to achieve a combination grading to meet a typical basecourse specification. Simple spreadsheets can be developed to perform these analyses as shown in Table 6.2. 100.000 90.000 80.000

Percent Passing

70.000 60.000 50.000

MATERIAL B

40.000 COMBINATION 70% A + 30% B 30.000 MATERIAL A 20.000 10.000 0.000 0.010

0.100

1.000

10.000

Seive Size mm

Figure 6.1: Example combination particle size analysis

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Table 6.2:

Example calculation – blending two materials Sieve size (mm) and per cent finer by mass

Material type

Mix proportions (%)

0.075

0.300

1.18

2.36

4.75

9.5

19.0

26.5

Grading of material A

70

8.0

14.0

27.0

35.0

47.0

74.0

99.0

100.0

Grading of material B

30

12.0

27.0

58.0

86.0

100.0

100.0

100.0

100.0

9.2

17.9

36.3

50.3

62.9

81.8

99.3

100.0

Combination 70/30

6.4

Stabilisation Using Chemical Binders

Many products have been tried and evaluated as chemical binders. Some have been proved ineffective while others such as petroleum products, if used excessively, may have adverse environmental effects. Chemical stabilisation binders act as surface stabilisers providing stability to otherwise unstable surface materials. The benefits of chemical stabilisation of unsealed wearing courses are: 

prevention of particles becoming airborne



resistance to traffic wear



retention in pavement, i.e. not lost through evaporation or leaching



resistance to ageing



environmental compatibility



easily applied with common road maintenance equipment



workable and responsive to maintenance



cost competitive.

The primary function of chemical stabilisation is to bind the fine fractions such that they hold the aggregate fractions in place for a longer period of time. This is enhanced by the binder providing bonding and waterproofing of the fines to maintain the dry strength of the fine material. 6.4.1

Types of Chemical Stabilisation Binders

The categories of the mainstream chemical binders used in unsealed roads, and their reaction with subgrade soils and pavement materials, may be categorised as follows: 

synthetic polymers



natural polymers



ionic compounds



salts.

Synthetic polymers may be grouped into water soluble and water insoluble. Most synthetic polymers in Australia and New Zealand are sold in a dry powdered format (commonly termed DPP, i.e. dry powder polymers).

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Insoluble Dry Powdered Synthetic Polymers A water insoluble dry powdered synthetic polymer is a manufactured material that is thermally bound to a very fine carrier such as fly ash. Typically it is classified as a stabilising binder rather than a dust suppressant. The fine powdered product, when mixed with hydrated lime, has the effect of flocculating and coating clay particles within the pavement material. The fly ash, which is encapsulated by the polymer, is effectively inert and does not react chemically in the stabilisation process. Its only function is to facilitate the distribution of the polymer throughout the pavement material. This polymer is used only in the powdered format and remains in a powder form during the pavement material mixing process. Figure 6.2 illustrates the action of an insoluble dry powdered synthetic polymer (IDPSP) coated with a fly ash carrier surrounding soil particles to induce lower permeability and hence retard the loss of strength with wetting. Three IDPSP blends are commercially available and spread at a rate typically 1% to 2% by dry mass of pavement material: 

a synthetic polymer thermally bonded to a fine powder carrier (i.e. fly ash)



a blend of 2:1 synthetic polymer-coated fly ash/ hydrated lime for medium plasticity materials (PI < 12)



a blend of 1:1 synthetic polymer-coated fly ash/ hydrated lime for higher plasticity materials (12 < PI < 20).

Source: Polymix Industries

Figure 6.2: Schematic of insoluble polymer encapsulating soil particles

Synthetic Soluble Polymers These products are manufactured in granulated or liquid form and added to the compaction water to form the polymer chain which is an acrylimide or urethane copolymer. They encapsulate soil particles with a thin film of polymer and, upon drying, bonding and water insolubility is achieved. Figure 6.3 illustrates an acrylimide copolymer coating soil particles to induce bonding and low permeability and hence retard the loss of strength when the moisture content is greater than OMC.

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Source: Biocentral Laboratories

Figure 6.3: Electron micrograph of acrylimide copolymer coating soil particles

Natural Polymers These products include tall oil pitch, sulphonated lignin and di-limonene which bind fine particles to interlock with larger aggregates. In addition, they often have surfactant properties enhancing compaction by dilation of fine material when compacted with a vibrating roller. Their success is dependent upon both plasticity and particle size distribution. Cement or lime can be added as a secondary binder for increased stiffness. These products are mostly obtained as resin by-products from the pulping industry. They are often highly acidic in addition to remaining soluble and subject to leaching over time. Their use should be strictly supervised in terms of worksite safety. Due consideration should also be taken with respect to any environmental impact associated with leaching. Ionic Compounds These products are generally produced by the petroleum industry. They produce an ionising action in water which induces cation (+ ions, e.g. Ca++, Na+, K+, Mg++, H+) exchange at the surface of negatively-charged clay particles. By the process of ionic exchange, water that would normally be electrostatically bound to the clay particles is replaced by ions, allowing much of this water to be expelled as free water. Other processes occur including coagulation and flocculation of clay particles after compaction and some cementing action through formation of insoluble salts. Salts The most commonly used salt is water-attracting (hydroscopic) magnesium chloride; other salts include sodium chloride and calcium chloride. They require moisture (humidity) to be effective. They also require frequent re-application following rainfall. The consideration of salt leaching effects on the roadside environment must again be considered. Of the chlorides used as a chemical binder, calcium, sodium and magnesium are used, with calcium and magnesium being deliquescent substances and sodium hygroscopic. The deliquescent substances absorb moisture from the atmosphere and liquefy. Hygroscopic substances, on the other hand, depend on exposed surfaces to absorb moisture. Salts such as those mentioned above control dust by keeping road surfaces damp, but have little or no cementing action. Sodium chloride is of little value in arid regions as it absorbs moisture at high humidity (e.g. 70%). Likewise, calcium and magnesium chlorides cease to absorb moisture at humidity levels below 30-40%. Austroads 2009 — 40 —

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Roads treated with calcium chloride should only be graded after rainfall, and then only lightly from the edges to the centre, then reversing the operation feathering the material to the road edge. Limiting the sections to be treated will enable compaction before the surface dries, allowing bonding of the surface. Maintenance of the crown is essential with all treatments to ensure adequate drainage. 6.4.2

Applications

The main applications of chemicals in stabilisation are either as compaction aids and stabilisation binders which are mixed into a pavement layer or surface treatments for dust suppression, i.e. 

chemical binders used in stabilisation: synthetic polymers — natural polymers — ionic compounds chemical binders used to improve compaction: —



wetting agents, soaps synthetic polymer — natural polymers chemical binders used for dust suppression: — —



— — — — — —

wetting agents, soaps hygroscopic salts (e.g. calcium, magnesium or sodium chloride) natural polymers (e.g. ligno-sulphonate, molasses, tannin extracts) synthetic polymer emulsions (e.g. polyvinyl acetate (PVA), polyvinyl chlorate (PVC), polyacrylamide copolymers (PAM) modified waxes petroleum resins.

A study on dust control techniques, including a performance evaluation of numerous chemical dust suppressants (Foley, Cropley and Giummarra 1996), concluded that dust control methods available fell into three main categories: 

good construction and maintenance practice



use of mechanical stabilisation to form a good wearing course that forms a hard surface crust



use of chemical binders as an adjunct (not replacement) to the above methods.

The sequence of remedies should follow the order given above, with possibly all methods being used to reduce dust emissions to a satisfactory level. It is considered of little value to use a chemical dust suppressant if some of the basic roads’ building requirements are not first addressed. Short of sealing a road, there are no known ways to eliminate dust emissions effectively on a long term basis by using a single process or just one application of a chemical binder (Foley et al. 1996). However, on a life cycle basis, they can lead to lower maintenance costs through less frequent patrol grading and longer sheeting life. Benefits from chemical stabilisation include extended periods between resurfacing, lower levels of surface roughness and hence vehicle operating costs, a reduction in accidents, higher quality primary produce and an improved amenity for nearby residents.

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Prior to using a chemical binder, unstable areas and poorly-graded material should be removed and replaced with selected material at optimum moisture content. Adequate drainage is the single most important characteristic for long-term results because it ensures the subgrade does not become saturated and therefore weakened. Surface gravel should be added and a proper crown formed to facilitate surface water runoff. 6.4.3

Product Selection and Mix Design

The selection of the type of chemical binder should be made bearing in mind the quantity of fines in the surface material or the subgrade (if there is no surfacing structure), climatic conditions and traffic volumes and construction logistics (e.g. transportation of stabilisation binder). The consideration of proprietary chemical binders is generally based on the determination of their suitability to the parent material rather than the determination of the required application rates. Basic information is generally available from product literature together with field examples. In some cases quantitative measurement of performance or attribute improvement is available. Chemical binders are generally separated into either dust palliatives or stabilisers and the following performance properties need to be considered: 

resistance to abrasion (effect of traffic and wind on treated surfaces)



resistance to erosion



resistance to leaching



increased shear strength (all weather trafficability)



long-term durability.

The large variety of proprietary products available and classified as chemical or polymer binders, coupled with varying degrees of quality performance data, make them less definitive in their selection compared to cement, cementitious, lime or bituminous binders. It is suggested in Part 4D of the Guide to Pavement Technology – Stabilised Materials (Austroads 2006b) that a simple capillary rise or vertical saturation test is the most appropriate (and economical) way to evaluate the suitability of a material in the laboratory. These two tests are conducted on material screened on a 2.36 mm sieve since the chemical binder is associated with the fine fractions. Figure 6.4 shows the vertical saturation test in which a compacted specimen is prepared with and without binder, allowed to cure and dry and then subjected to saturation from dripping. The annular mass is used to induce collapse. As can be seen, the use of a binder in this particular case has shown that it could be of some value for field trialling or adoption.

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Source: ARRB Group

Figure 6.4: Vertical saturation test

6.5

Stabilisation with Lime

Lime stabilisation of unsealed roads located in dry to moderate climatic conditions of Australia has been successful in increasing the strength of the pavement material in both dry and wet conditions, reduce dust generation and lower the frequency of resheeting to about five to 10 years. The application rate of lime is at least that quantity required to reach the lime demand criterion such that long term stabilisation can take place. Additional lime has been used at intersections to improve the durability of the surface from turning traffic conditions. In sections of weak subgrade the existing formation material has been in situ stabilised with lime to provide longer performance to the pavement material of unsealed roads.

6.6

Stabilisation with Cementitious Binders

In addition to lime stabilisation of unsealed roads, cementitious binders have been successfully used on unsealed low volume roads and as localised treatments, such as floodways, bends, intersections, etc. Circumstances where the use of cementitious binders may be considered include: 

improving the subgrade strength to significantly reduce pavement depth or where saturated subgrades are encountered



modifying poor materials to make them suitable as a pavement layer



enhancing wear resistance and/or reduce dust emissions from the wearing course.

More comprehensive detail on stabilisation using these types of binders may be found in Part 4D of the Guide to Pavement Technology (Austroads 2006b).

6.7

Stabilisation with Powder Binders

Stabilisation of unsealed roads with powder binders is generally limited to the use of lime or cement as localised treatments such as floodways, bends, intersections, etc. However, in some circumstances, the use of cement or lime may be considered where: 

improving the subgrade strength may significantly reduce pavement depth or where saturated subgrades are encountered



poor materials are modified to become a suitable pavement layer

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

wear resistance can be enhanced or dust emissions from the wearing course reduced.

More comprehensive detail on stabilisation using these types of binders may be found in Part 4D of the Guide to Pavement Technology (Austroads 2006b).

6.8

Methods of Applying Stabilisation Binders in the Field

6.8.1

Powder Binders

Powder binders require spreading on the road surface with a purpose-built spreader where the application rate can be closely monitored and controlled (Figure 6.5).

Source: AustStab

Figure 6.5: Purpose-built stabilisation binder spreader

Uncontrolled applications may be undertaken on short lengths (e.g. floodways) with 20 kg bags laid in a grid pattern and spread by rake. In this situation workers must be equipped with personal protective equipment. In all cases, thorough mixing of powder binders can only be achieved using a purpose-built road recycling machine. The use of grader mixing will result in poor distribution of the stabilisation binder in addition to requiring mixing times which may exceed the hydration time for cementitious binders. 6.8.2

Liquid Binders

Liquid binders are best added by pumping the mixture into the spray bar contained in the mixing chamber. A low quality and uncontrolled approach is to spray the mixture to the road bed via a water truck. They can be pumped directly into the water truck or a venturi system adopted which is attached to the water filling system however there is no method to ensure that the binder is well displaced within the water truck (Figure 6.6).

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Source: ARRB Group

Figure 6.6: Application of liquid stabilisation binder with water truck

In the case of powder or granulated binders (e.g. polyacrylamides) that are added to the water truck, a recirculating system or patented eductor mixing system is required (Figure 6.7).

Source: ARRB Group

Figure 6.7: Adding granulated polymer using patented eductor

6.8.3

Methods of Mixing

Purpose-Built Recyclers In all cases, thorough mixing of binders into the road material can only be achieved using a purpose-built road recycling machine (Figure 6.8). The use of grader mixing will result in poor distribution of the stabilisation binder in addition to requiring mixing times that may exceed the hydration time for cementitious binders. In addition, the application of water through the mixing chamber can greatly improve the compaction of the stabilised material.

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Source: ARRB Group

Figure 6.8: Stabilisation binder mixing with purpose-built recycler

6.9

Logistical Selection of Stabilisation Binders for Construction

In addition to laboratory mix design considerations, the selection of a particular binder needs to be considered in terms of the following: 

The quantity of stabilisation binder required for logistical transportation to the site and to meet the construction demands.



The cost of the binder, including transportation to the site and on-site handling, e.g. bulker bags or tanker transfer to the spreader. Typical (ex-bin) costs of liquid chemical binders range between $8,000 and $12,000 per kilometre and $4,000 (cement and lime) to $50,000 (some polymers) with 2% by mass addition of powder binders.



Any additional plant required for powder binders, e.g. a mechanical spreader and mixer.



Any potential reduction in compaction moisture content that can realise a saving in water carting costs; waiting for water to be delivered to a site can be critical during construction



OH&S issues associated with some binders affecting the health of workers (e.g. dust) and skin damage associated with quicklime and sulphonated lignin products.



Chemical residues left in water tankers including those (mostly acidic) which preclude the tanker from being used for domestic water supply in construction camps and those which accelerate rusting of construction plant.

6.10

Technical Evaluation of Stabilisation Binder Performance

6.10.1

Laboratory Evaluations

The process for establishing the suitability and quantity of stabilisation binder in the laboratory is as follows: 1.

Identify local materials proposed for construction and consider blending materials to match, as closely as possible, the suggested PSD provided in Table 3.5.

2.

Review binder product literature, placing particular emphasis on the results of field trials (particularly independent trials) and quantitative stabilised pavement data.

3.

Prepare test samples at the binder content recommended in the manufacturer’s specification.

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4.

If the product is designed to provide water-proofing properties, binder suitability and content confirmation by capillary rise, vertical saturation or permeability testing is recommended.

5.

If the product is designed to provide increased strength characteristics, binder suitability and content confirmation using CBR or UCS testing is recommended.

6.

If the product is designed to provide improved compaction characteristics, binder suitability and content confirmation by laboratory compaction testing is recommended.

7.

Depending on the outcomes of the laboratory testing, adjust the binder content as appropriate if the binder appears suitable.

8.

If insufficient field performance data is available, undertake a field trial, ensuring that an untreated (or proven treatment alternative) section is included as a base measure or ‘control’. Alternatively, if the laboratory testing suggests that the product may be suitable, adopt it for the project and monitor performance for future application.

9.

Monitor in-service performance over the length of the treatment life and document changes to all sections in the trial.

6.10.2

Field Trials

Many case histories exist but it is common to find that many trials lack a design method which would isolate and define the effect of the product. A more definitive assessment of the effectiveness of a product or process should take into account materials safety data sheets associated with the proposed binders, material type, construction achievement, climate and nature of traffic and comparisons with a base product/process (generally traditional practice). Control sections A full scale road trial should incorporate at least one control section, which is constructed at the same time as the experimental sections. The control sections must be identical in all respects to the experimental sections, except that no additive is used. The difference in performance between the experimental and control sections is then used to determine the effectiveness of the additive. A control section also helps eliminate any advantages resulting from extra supervision provided during the construction of the experimental sections. Care needs to be taken to ensure that construction follows recommended practice as normal practice may differ from that required for a specific binder. In addition, care needs to be taken to ensure that any improvement in the performance of a trial section is solely due to the binder and not different construction or increased compaction applied due to more intense surveillance. The performance of the control section, using the same materials (but with no additive) and laid under the same conditions, will indicate whether the observed performance of the experimental sections is due to the additive, or other causes.

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7

UNSEALED SURFACE WEARING CHARACTERISTICS

7.1

Introduction

This section describes the various wearing and environmental factors influencing the quality of the riding surface of an unsealed road. For aspects associated with maintenance and construction of unsealed roads, the reader is referred to the ARRB Unsealed Roads Manual – guidelines to good practice, 3rd Edition (Giummarra, Ed, in press). Unsealed surface wear is a characteristic which is dependent upon the different types of surfacing materials, traffic volumes and axle configurations, climatic conditions, quality of construction and frequency of maintenance applied. Surface defects are commonly considered to represent deterioration in unsealed road pavements. They often control maintenance intervention strategies (e.g. patrol grading) in association with considerations on the importance of the road. Deterioration models have been developed based upon either gravel loss (sheeting life) or surface roughness (rideability) which predicts both sheeting life and maintenance intervention for application in life cycle analyses.

7.2

Types of Surface Wear

Wearing of the surface can be: 

traffic induced: dusty surface when trafficked, resulting in loss of fines and the development of coarse texture — loose aggregate pulled out of the surface due to loss/lack of fine material binder — loss of crossfall (crown elevation) through loss of fines and aggregate — rough corrugated surfaces where very sandy surfaces are encountered rain induced: —



— — — —

potholes formed from permeable surfaces and poor crossfall, allowing water to pond lateral erosion on crossfalls total loss of trafficability during floods, particularly fine grained surfaces (silts and clays) surface gouging on soft surfaces during/after rainstorms.

The ARRB Unsealed Roads Manual (Giummarra, Ed, in press) provides greater detail with respect to unsealed surface wear, defects and suggested treatments for repair and avoidance. 7.2.1

Loss of Fine Material (Dust)

The loss of fine material is the first sign of the wearing of an unsealed road surface. It is manifest as dust generation and aggregate exposure, resulting in coarse surface texture, roughness and noise. Dust is caused both by the loss of fine particles (finer than 0.425 mm) from the road surface arising from the loosening of the pavement materials, and disturbance to the wearing course caused by the action of traffic and climatic conditions.

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A loss of fines leads to an increase in the permeability of the surface, resulting in early pavement deterioration and accelerating the need for re-surfacing. Loss of fines also exposes a coarser textured surface, leading to higher levels of irregularities and hence increases in vehicle operating costs. It also contributes to road safety issues. A typical advisory sign for dust hazard is shown in Figure 7.1.

Source: ARRB Group

Figure 7.1: Advisory sign for dust hazard

It is important to note that any dust suppression treatment on an unsealed road surface is not permanent but forms part of an overall road management strategy which may imply regular applications of the palliative (i.e. water or water plus an additive). The US Department of Agriculture (1999) provides guidance on the use of dust palliatives. Quantitative measurement of dust generation can be made using specific detection apparatus developed by the US Army Research and Development Centre (Rushing 2006) (Figure 7.2).

Source: Rushing (2006)

Figure 7.2: Mobile and static dust monitoring apparatus

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The Colorado State University developed the ‘Dustometer’ which can be described as a moving dust sampler that provides a real-time quantitative dust emission measurement for a section of a road. Its dust measurements are precise, reproducible, and easily obtained (Sanders and Addo 2000). It provides a uniform procedure for gathering and comparing data from many test sections; many data points can be generated within a short period of time. The device consists primarily of a fabricated metal box designed to hold a 10 x 8 in (approx. 250 x 200 mm) glass fibre paper which is mounted to the bumper of a pick-up truck behind the driver's side rear tyre, an electric power generator, a high-volume vacuum pump, and a flexible plastic tube connecting the suction pump to the filter box. The fabricated filter box has a 12 x 12 in (approx. 300 x 300 mm) opening that is covered with a 450 m mesh sieve screen that faces the tyre. The screen prevents any non-dust particles from being drawn onto the filter paper during dust measurement. The filter paper is supported near the bottom of the fabricated box by a sieve mesh. A schematic diagram of the ‘Dustometer’ is shown in Figure 7.3.

Source: Sanders and Addo (2000)

Figure 7.3: Schematic diagram of Colorado State University Dustometer

Surface texture Surface texture is developed by the gradual loss of fines on the surface exposing the aggregate which, depending on the strength of the fines (binder), can result in the aggregate producing a loose surface (Figure 7.4).

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Source: ARRB Group

Figure 7.4: Loss of fines increasing surface texture

Texture can be measured using the sand patch method (ASTM E965, 2006) or the automated laser profilometer. During construction, it is common to reduce the rate at which fine material is lost by slurrying the surface with water (or water plus an additive) (Figure 7.5) such that sufficient fines surround the aggregate to hold it in place.

Source: ARRB Group

Figure 7.5: Slurrying unsealed surface

The surface can be restored periodically using routine patrol grading to remove loose material to the side windrows of the pavement. However, at some stage the loss of fine material is such that the surface becomes very coarse (Figure 7.6) and ‘boney’.

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Source: ARRB Group

Figure 7.6: Loss of fine material leaving coarse gravelly surface

7.2.2

Loose Gravel

The generation of loose gravel under traffic, termed ravelling, may be a significant safety and economic problem. Loose gravel may be distributed over the full width of the road but is commonly concentrated in windrows between the wheelpaths (Figure 7.7). The problems caused relate to safety hazards, damage to vehicles and windscreens, increased fuel consumption and lack of adequate lateral drainage.

Source: ARRB Group

Figure 7.7: Loose material between wheelpaths (note centre overlap from trafficking in both directions)

The measurement of the rate of deterioration of loose surface material has been undertaken in a number of unsealed road studies (e.g. Andrews 2000). This rudimentary test involves the removal of the loose material within a square metre of pavement and weighing it (Figure 7.8). The loss of material over time allows surfaces to be rated as shown in Figure 7.8.

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6

R o ugh

Sur f ace g r ad ed f o r f ir st t ime af t er 16 mo nt hs ser vice

4

S e rv ic e a ble 2

Sur f ace g r ad ed af t er r ai n

Smo o th 0 0

200

400

600

800

10 0 0

12 0 0

D a ys

T o tal

D us t

Gra v e l

Source: ARRB Group

Figure 7.8: Measurement of loose material on pavement surface

7.2.3

Corrugations

Corrugations are mostly formed through loose surface material being displaced as a result of tyre action coupled with the mass and speed of the vehicle. Loose surface material arranges itself into parallel ridges which lie at right-angles to the direction of traffic. Spacing (wavelength) can vary from 500 mm to 1 m and depths can range up to 150 mm (Figure 7.9). Any irregularity in the surface can initiate the process which then develops at a rate dependent upon the traffic, speed and tyre pressure.

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Source: OECD (1987)

Figure 7.9: Corrugation formation in dry climates

Granular materials with particle sizes greater than 5 mm, low plasticity and limited fines, or materials which have lost fines due to traffic action, are susceptible to corrugations (Figure 7.10). In dry climates only the material that forms the ridges is affected, with the underlying material remaining in place.

Source: Giummarra (in press)

Source: ARRB Group

Figure 7.10:

Corrugations in gravel surface (left) and sandy surface (right)

In wet climates corrugations generally develop during the dry season. However, if the pavement and basecourse become soft enough due to saturation, deformation through the full pavement structure can occur as shown in Figure 7.11. The absence of a tight surface, combined with coarse sandy material, if present in high proportions, can lead to the rapid formation of corrugations.

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Source: OECD (1987)

Figure 7.11:

7.2.4

Corrugation formation in wet climates

Potholes

Potholes play a significant role in the development of ride quality, or roughness, of unsealed roads. They can cause substantial damage to vehicles if allowed to develop and increase in size. The effect of potholes on vehicles depends on both the depth and diameter of the pothole. Potholes which affect vehicles the most are between 250-1500 mm in diameter with a depth of more than 50-75 mm. Roads particularly susceptible to potholing are those with flatter grades and crossfalls, particularly at bridge approaches, alignment changes from ‘left to right’, superelevation at ‘S’ bends, and intersections where water can lie on the surface, particularly in wheelpaths. Pothole occurrence is rare on gravel roads with correct crossfall and superelevation. The development of potholes is triggered by stripping of the surface material and the infiltration of water. Solids in suspension are carried away by wheel action on the surface and, as water penetrates the pavement, the action continues, forming a hole in the pavement (Figure 7.12).

Source: Giummarra (in press)

Figure 7.12:

Potholes on flat crossfall

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7.2.5

Dry Rutting in Wheelpaths

Rutting on unsealed roads in dry environments is mostly due to the loss of material in wheelpaths caused by trafficking (Figure 7.13). However, if insufficient pavement thickness exists over a soft subgrade rutting (refer Figure 4.3) can be due to vertical deformation under traffic or, in the case of a weak wearing course or basecourse, shear failure (shoving) of the layer.

Source: ARRB Group

Figure 7.13:

Dry rutting in wheelpath

Dry season rutting occurs in sand and gravels that have low plasticity such that loose material is displaced sideways and traffic continues to travel in the same wheelpath. 7.2.6

Surface Gouging

Surface gouging (Figure 7.14) occurs in those materials where the strength is sensitive to water ingress such as wearing course materials with high clay and/or silt contents. Surfaces become instantly slippery and, with continued saturation (or inundation), this is the greatest cause of road closures and inaccessibility issues associated with the strategic function of the road.

Source: ARRB Group

Figure 7.14:

Surface gouging

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7.2.7

Surface Scour

Scour is the loss of surface material caused by the flow of water along and/or over the road. It is related to a lack of compaction, excessive longitudinal grade and the build-up of debris on shoulders preventing surface water from flowing off the pavement (Figure 7.15). Both transverse and longitudinal scouring can occur. Transverse scouring commences at the edge of the shoulder or on areas where the level of compaction is lower and works towards the road pavement (Figure 7.16). Alternatively, a lack of adequate shoulder slope may lead to water standing on the road and eventually finding an escape route. Plant growth on shoulders, and the consequent entrapment of debris and earth, prevents water draining from the pavement. An area where the prevailing longitudinal grade encourages water to flow along the pavement in preference to the direction of the crossfall gives rise to longitudinal scouring (Figure 7.17). Longitudinal scouring is more likely to occur on areas having steep vertical grades.

Source: ARRB Group

Figure 7.15:

Longitudinal scour on steep gradient

Scouring of the surface not only creates adverse driving conditions but also leads to further deterioration of the pavement through exposure to the environment. Scouring can be pronounced when combined with material susceptible to rutting. The ability of the surface material to resist scour depends on the shear strength of the material subject to the water flow. Pavements with high fines contents and small aggregates are more inclined to scour than those consisting of a well-graded mix containing crushed stone 19 mm in size or larger.

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Source: Giummarra (in press)

Figure 7.16:

Transverse scouring on horizontal curve

Source: Giummarra (in press)

Figure 7.17:

7.2.8

Longitudinal scouring between wheelpaths

Ice Formation on Surface

The presence of ice on the surface results in a reduction in the coefficient of friction between the tyre and the surface to almost zero, leading to unpredictable vehicle movements and very hazardous driving conditions (Figure 7.18). Surfaces subjected to ice or frost are either closed or treated with grit or chemicals such as calcium or sodium chloride.

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Source: Giummarra (in press)

Figure 7.18:

Snow and ice formation

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8

UNSEALED ROAD SURFACE MANAGEMENT

8.1

Introduction

The management of unsealed roads surfacings is predominantly undertaken based on local knowledge of the performance of specific pavements within a network. Surface condition is generally maintained by routine patrol grading initiated (based on local knowledge) by known performance or in response to complaints by road users. Resheeting operations involve replacing the wearing course when it has worn away in a similar manner to a granular overlay on a pavement. Given that these resheeting operations are normally based on local knowledge, or complaints by road users, it is difficult to develop formal methodologies for the management of unsealed roads surfacings that could be applied at the network level. However, given the large costs associated with resheeting unsealed pavements, greater emphasis is now being directed towards the development of network-level maintenance intervention strategies, including surface deterioration models which more accurately predict surface life.

8.2

Surface Maintenance

8.2.1

Patrol Grading

Patrol grading normally consists of grading the surface to side windrows to improve the smoothness of the surface Figure 8.1. However, heavier grading (i.e. more material moved) is adopted where the road requires reshaping, generally after periods of heavy traffic (e.g. destocking stations or mine hauls) or severe surface damage caused during wet weather trafficking. In these circumstances the pavement surface is typified by high quantities of loose material, rutting and loss of crossfall and, after rainfall, gouging and potholes.

Source: ARRB Group

Figure 8.1: Patrol grading

In these operations, the material is generally shallow-graded to side windrows and, when the surface becomes ‘boney’, i.e. coarse texture, fine material from the windrow is brought back over the surface. However, when undertaking this operation, care should be taken not to produce a thin skin of material over the old surface as this will delaminate rapidly, leading to the formation of potholes.

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In addition to grading, the performance of the surface can be significantly improved by compacting the surface after grading. Tow-behind rollers are commonly used to compact surfaces which have been disturbed by grading operations. They can be used behind the grader or as a separate operation using a small tractor. Types of tow-behind rollers include static and vibratory rollers, steel drum and rubber-tyred rollers, and combination rollers (Figure 8.2). The level of compaction achieved using tow-behind rollers will not be as high as the level provided by a separate roller.

Source: Earthco Projects Pty Ltd

Figure 8.2: Tow-behind steel drum roller and multi-tyred roller

In addition to normal compaction, it is also quite common to slurry the surface (Figure 8.3) which, under the pore pressure developed beneath the static roller, brings fine material to the surface. The fine material assists in retaining the aggregate fractions in place through cohesion, particularly if it has medium plasticity. This operation can also be associated with the incorporation of liquid stabilisation binders. However, care needs to be taken when slurrying not to produce a highly slippery surface (often achieved when some liquid stabilisation binders with high surfactant properties are used). Appropriate warning signs must be placed on new work until the surface has thoroughly dried. Figure 8.4 compares a typical surface subject to wet compaction and slurrying (left) and dry compaction (right).

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Source: ARRB Group

Figure 8.3: Surface slurrying during compaction

Source: ARRB Group

Figure 8.4: Wet compaction and slurrying (left) and dry compaction (right)

8.2.2

Reshaping and Shallow Stabilisation

Reshaping, involving the scarification of the road surface and remixing of the aggregate base, can yield a proper blending of fines and aggregates and the restoration of an appropriate crowned road surface. Scarifying operations can also be adopted in thin stabilisation applications where liquid binders are used. A typical surface after scarifier grading is shown in Figure 8.5.

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Source: Giummarra (in press)

Source: Earthco Projects Pty

Figure 8.5: Surface after scarifier grading

8.3

Resheeting (Wearing Course Replacement)

Gravel loss and subsequent replacement by re-sheeting is the most significant factor affecting the life cycle operating costs of an unsealed road pavement. Typically a 150 mm thick unsealed wearing course will be lost within 8 to 12 years, after which a new wearing course will be required. The loss of wearing course material on unsealed roads results from: 

traffic abrasion and loss of fine binding material



degradation of stone due to weathering and polishing



climatic conditions, i.e. wind and rain introducing scouring and erosion



patrol grading loose material to windrows and over-cutting the surface



pavement material selection.

8.3.1

Measuring and Estimating Gravel Loss

The ability to correctly estimate gravel loss is very useful to a manager scheduling resheeting operations because it helps identify where resheeting is required and the amount of material required. However, there is currently little information available regarding actual gravel loss, how much gravel is on the road and therefore what gravel is to be added. It would seem that many practitioners wait for the subgrade to show through before resheeting, which is far too late. Gravel loss can be estimated by: 

monitoring core levels of gravel depth over time



taking spot levels on various representative sections of roads and measuring annual wear loss



measuring the rate of loose material generated between wheelpaths (Andrews 2001)



applying a formula, calibrated to local conditions, to estimate loss



using technology based on ground penetrating radar to measure existing gravel depth



differentiating between the materials used in the base and wearing course.

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Ground penetrating radar (GPR) An innovative approach to estimating gravel loss is the use of GPR technology. The technology uses a pulse of energy fired into the road surface and the time delay of its reflection to calculate the distance to the object and thereby the thickness of different layers in a pavement. GPR equipment mounted on a vehicle can travel at moderate speed. GPR antennas can come in various forms, including above, and close to, the surface. A horn-based antenna, which is an above-surface type, is shown in Figure 8.6.

Source: Giummarra (in press)

Figure 8.6: Ground penetration radar (GPR) with horn antenna

Trials on the suitability of different GPR systems, on both sealed and unsealed roads, have provided favourable results (Giummarra 1998). However, care should be taken when using GPR equipment to ensure that the gravel and subgrade dielectric properties are suitable. An initial test section should be used to ascertain the suitability of this equipment to local pavement conditions. 8.3.2

Predicting Gravel Loss

International studies of the performance of unsealed roads have led to the development of a number of models, in particular relating to World Bank projects in developing countries. These models consider the inter-relationships between construction, maintenance and vehicle operating costs. Factors considered include: 

the impact of gravel loss on resheeting intervention



the impact of surface looseness on vehicle operating costs (VOC)



the impact of surface roughness on maintenance intervention



the impact of rut depth on maintenance and re-sheeting intervention strategies



journey time as an indicator of road condition



traffic volumes (both ways) as an indicator of pavement wear



the impact of climate on surface dust and erosion characteristics



geometry (slope and camber) as an indicator of erosion.

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In the absence of historical data to provide the most accurate determination of future gravel loss from the sheeting material along an unsealed road, various predictive formula have been developed, three of which are now discussed. Where possible, these models have been adjusted to minimise the number of parameters, example calculations are provided in Table 8.1. Predictive Model 1: TRRL The Transport and Road Research Laboratory (TRRL) (now TRL) model was originally based on the results of a study in Kenya and developed by Jones (1984). The model is described by the formula: 1

where

GLA

=

f(0.133(ADT)2/((0.133ADT2 + 50)) x (4.2 + 0.0336ADT + 504MMP2 + 1.88VC)

GLA

=

annual gravel loss (mm/year)

ADT

=

average daily traffic in both directions (veh/day)

MMP

=

mean monthly precipitation (metres/month)

VC

=

gradient (%) for uniform road length

f

=

constant for various gravels (laterite: 1.3,quartzite:1.5, volcanic:0.96, coral:1.5, sandstone:1.4, calcretes: 2.0-4.5)

Predictive Model 2: HDM-4 The HDM-4 model, as described by Paterson (1987) and can be described as follows: 2 GLA = 12.63 + 0.898(MMP x G) + 3.65(KT x ADT)) where

GLA (MLA in original formula) = predicted annual material loss (mm/year) MMP

=

mean monthly precipitation (mm/month)

G

=

average longitudinal gradient of the road (%)

ADT

=

average daily traffic in both directions(veh/day)

KT

=

traffic-induced material whip-off coefficient.

and 3 KT = MAX(0, 0.022 + 0.969(KCV/57300) + 0.00342(MMP x P075) – 0.0092(MMP x PI) – 0.101(MMP)) where

PI

=

Plasticity Index

KCV

=

average horizontal curvature of the road (deg/km)

P075

=

amount of material finer than the 0.075 mm sieve.

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Predictive Model 3: Paige-Green (1990) This model is described by the formula: GLA = 3.65(ADT(0.059 + 0.0027N – 0.0006P26) – 0.367N – 0.0014PF + 0.0474P26) where

GLA (GL in original formula) = annual gravel loss (mm) Fawcett et al. (2001) ADT

=

average daily traffic in both directions

N

=

Weinert N-value (a climate index value)

PF

=

plastic factor (plastic limit x per cent passing the 0.075 mm sieve)

P26

=

per cent passing the 26.5 mm sieve.

It was recommended that the particle size distribution be recalculated assuming that 100% was passing the 37.5 mm sieve and that the Weinert N-value (12 x evaporation in the hottest month(mm)/annual precipitation(mm)) be limited to a maximum value of 11 (Jones, Sadzik and Wolmarans 2001). Table 8.1:

Comparative rates of annual gravel loss

Location: Unsealed Road through Flinders Ranges, South Australia Predictive Model 1: TRRL

Predictive Model 2: HDM-III

Predictive Model 3: Paige-Green

Material f = 1.4

PI = 20, P075 = 22

P26 = 95, P075 = 23, PL = 15, PF = 345

Traffic ADT = 92 vehicles per day

ADT = 92 vehicles per day

ADT = 92 vehicles per day

Rainfall MMP = 0.026 metres/month

MMP = 0.026 metres/month

N = 11

Alignment VC = 0

G = 0, KCV = 0, KT = 0.017

Input Parameters:

Annual Gravel Loss GLA = 10 mm

GLA = 18 mm

GLA = 11 mm

It was noted that the gravel loss along the unsealed road was believed to be between 7 mm and 14 mm per year.

8.4

Unsealed Road Condition Monitoring

The benefits of efficient unsealed road network management systems are as follows: 

They provide an indication of the current level of service provided by the network and the associated costs of maintaining current service levels.



They allow estimates to be made of the costs associated with increased levels of service to meet socio-economic and environmental demands on the network.



Patrol grading maintenance interventions can be planned so that they are deployed where and when required to meet the functionality of a particular road.



They provide an indication of the life of the wearing course (sheeting) in order to forward plan re-sheeting schedules which, because of high initial capital outlay, represent the highest unit costs associated with road operating costs.

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

They provide data identifying high operating-cost roads that may be candidates for upgrading, i.e. improved unsealed wearing course material, stabilisation applications or bituminous sealing.



They provide a rational platform upon which planning for funding submissions can be made.

8.5

Visual Pavement Condition Rating Systems

There is no commonly adopted pavement condition rating system in Australia and New Zealand for unsealed roads although there are a number of commercial products available from various sources. The easiest and most common system is based upon visual assessment by trained inspectors identifying the severity and extent of the particular condition attribute from which a numerical rating system is applied to establish a condition index. Two systems (South Africa and the US) are now described. 8.5.1

South Africa

The pavement condition assessment part of the system (Jones and Paige-Green 2000) can be applied routinely to maintenance operations as a basis for: 

predicting gravel loss and patrol grading frequency



prioritising maintenance actions (e.g. defects with a severity of four or five should be given immediate attention, whilst defects with a severity of three should be considered as a warning that will require attention in the near future)



monitoring improvement or deterioration in the overall road network as a result of funding fluctuations



direct comparisons of the performance of various roads



the identification of specific problems



project level investigations.

The assessment is undertaken on segments of the road network visually and numerically rated on a scale of one (very good) to five (very poor). To assist in making the rating system more uniform, Jones and Paige-Green (2000) provide example pictures of each attribute in terms of its rated category. The inspection is made from a vehicle travelling at 40 km/h and gathers data relating to: 

general performance of the road to meet road function and road user satisfaction



moisture condition at the time of assessment



wearing course thickness and quality



road profile (loss of crossfall)



drainage adequacy



ride quality (roughness (IRI))



dust (visibility and safety)



trafficability or accessibility, classified as acceptable or not



potholes – estimates of depth and extent



rutting: estimate of depth

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

stoniness (texture) : a measure of texture where stone is embedded and the presence of loose stone in windrows between the wheelpaths



slipperiness or skid resistance: classified as acceptable or not



cracks: as observed in the dry clay surfaces.

8.5.2

USA

This system established an ‘unsealed road condition index’ (URCI) as a numerical indicator based on a scale of 0 to 100 as shown in Table 8.2 (Department of the Army 1995). Table 8.2:

US Army URCI scale and condition rating

0 - 10

10 - 25

25 - 40

40 - 55

55 - 70

70 - 85

85 - 100

failed

very poor

poor

fair

good

very good

excellent

The inspection is undertaken from a vehicle traveling at 40 km/h in sections (sample units) to determine, firstly, the density of the specific defect (per cent of section area or length) and, secondly, its severity (deduct values) based on the density and whether there is a low, medium or high impact on road function. The defect attributes are: improper cross-section (loss of crown or flat crossfall), inadequate roadside drainage, corrugations, dust, potholes, rutting and loose aggregate. Deduct value curves have been developed for each distress mode as shown in Figure 8.7 and Figure 8.8.

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Source: United States Department of the Army (1995, Figures C-2, C-3 & C-4)

Figure 8.7: Condition deduct values (drainage, cross-section, corrugations, dust)

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Source: United States Department of the Army (1995, Figures C-5, C-6 & C-7)

Figure 8.8: Condition deduct values (potholes, ruts, loose aggregate)

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From summing all the deduct values, the URCI is determined using Figure 8.9.

Source: United States Department of the Army (1995, Figures C-8)

Figure 8.9: US Army URCI calculation

The URCI is calculated as shown in the following example: 1.

Determine the density of the defect.

2.

Calculate the density as a percentage of the section (sample unit).

3.

Determine the deduct values from Figure 8.7 and Figure 8.8.

4.

Sum the deduct values and determine the ‘q’ value.

5.

Determine the URCI from Figure 8.9 and the overall condition rating is obtained from Table 8.2.

As an example calculation of the pavement shown in Figure 8.10 follows.

Source: ARRB Group

Figure 8.10:

Pavement assessment example

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

A pavement inspection of a 150 m sample unit gave the following rating: — — — —



improper cross-section – 30 m: density = 30/150 x 100 = 20; severity: low (i.e. low crossfall) corrugations – 80 m: density = 80/150 x 100 = 53; severity: medium dust – medium: deduct value = 4 loose aggregate – 80 m: density = 80/150 x 100 = 53; severity: low

Determine the deduct values from Figure 8.7 and Figure 8.8, viz. — — — —

improper cross-section deduct value = 14 corrugations deduct value = 45 dust deduct value = 4 loose aggregate deduct value = 18



Total Deduct Value = 14 + 45 + 4 +18 = 81 and q = 3



URCI (determined from Figure 8.9) = 46 and, from Table 8.2, the pavement is rated as FAIR.

A typical assessment and condition calculation sheet is shown in Figure 8.11.

Source: United States Department of the Army (1995, Figure 3-2)

Figure 8.11:

US Army unsealed road condition assessment form

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8.6

Quantitative Pavement Condition Rating

Quantitative measurements of the condition of unsealed road surfaces have been developed to rate a range of attributes of the road surface by direct measurement. More recently, the collection of IRI roughness, rutting and texture data in an automated manner at highway speeds has become common in Australia and New Zealand. Pavement roughness can be measured using a Roughometer which can be fitted to a vehicle as shown in Figure 8.12.

Source: ARRB Group

Figure 8.12:

Roughometer

In addition, rutting and roughness can be determined using a laser profilometer mounted to the front of a vehicle as shown Figure 8.13.

Source: ARRB Group

Figure 8.13:

Laser Profilometer

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An alternative system known as ‘Optigrade’ was developed by the Forest Engineering Research Institute of Canada originally to manage the grading maintenance of unsealed forest hauls roads. The system comprises an accelerometer and GPS hardware mounted on a haul truck routinely traveling the road, and software designed to assist managers making decisions on grading frequency.

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9

COST–BENEFIT CONSIDERATIONS

9.1

Concept

Effective asset management demands that a systematic approach be taken to the whole-of-life (life-cycle) management of any infrastructure component in order that an efficient and an effective management system are provided to the users. Road asset management strategies involve the establishment of a program of systematic monitoring of pavement condition, physical treatments (construction, maintenance and rehabilitation) applied to roads, and controls on how the roads are operated. These actions directly influence the level of service provided to the road users and, ultimately, community benefits. Economic evaluation is an objective asset management tool used to demonstrate accountability and the effective management of road assets. Economic evaluation is used, once all the consequences have been quantified, to assist in the selection of new road investments and the physical treatments and controls to be applied to existing roads. The adoption of cost-benefit analyses, in the form of life cycle cost models, can provide definitive information regarding the likely benefits associated with materials selection and blending or stabilisation as well as different construction options. In the past, the selection of options was based solely on the initial prime costs of construction for new works or resheeting or, in the case of maintenance, the prime cost of patrol grading. Both of these are generally poorly defined in terms of actual costs. Criteria other than construction and maintenance costs are also often used in asset management. For example, the majority of unsealed roads exist primarily to provide access for the local community and freight movement, functions which are not incorporated in current economic evaluations. In addition, road safety is a critical consideration in the management of the road network, including unsealed roads. In the absence of suitable data on other road operating factors applicable to the sealed road network, the life cycle methodology presented in this Guide is limited to construction and maintenance considerations.

9.2

Life Cycle Analyses for Selection of Wearing Course and Associated Maintenance Management Strategies

9.2.1

Introduction

In the context of unsealed roads, life cycle analyses can be used to evaluate any benefits gained from processes which increase sheeting life or reduce patrol grading intervention (i.e. termed ‘performance’ in this Guide). Examples could include: 

the benefits of incurring additional prime costs associated with improving materials by blending materials from different locations to improve performance



the benefits of incurring additional costs through the incorporation of a chemical binder in the wearing course to improve performance



the evaluation of the location of water supplies and water reducing agents on the prime construction costs.

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9.2.2

Life Cycle Cost Analyses

Life cycle analysis is based on Net Present Worth (NPW) and Equivalent Annual Cash Flow (EACF), which are defined by the following formulae: NPW =

where



 1  $C n  n  (1 + r ) 

 r(1 + r )N  EACF = NPW   N  (1 + r ) − 1

and

$Cn

=

treatment cost in year ‘n’

r

=

discount rate of future expenditure (taken as 6%, including the net effects of inflation)

n

=

number of years projected into the future

N

=

life of the strategy.

Typical uses for life cycle analyses include: 1.

Determining the operating costs of an existing pavement to assist in forward planning funding.

2.

Blending materials to improve wearing course performance, increase sheeting life and reduce patrol grading. In this analysis the additional costs of mixing or bringing a second material to site is considered.

3.

Adding a stabilisation binder to improve wearing course performance, increase sheeting life and reduce patrol grading. In this analysis the cost of the stabilisation binder and any ancillary equipment (recyclers and binder spreaders) are considered.

4.

Determining if it is viable to seal a pavement in the event that maintenance costs are high. In this analysis the cost of maintaining and replacing the wearing course at intervals determined from gravel loss estimates against the cost of sealing (which may include the cost of providing an improved basecourse material) is considered.

A typical example analysis is shown in Table 9.1 and Figure 9.1 where the cost of maintaining a pavement with an estimated 10 year wearing course life and annual patrol grading intervention are assessed: life cycle analysis period

20 years

initial construction cost of wearing course

$25,000/km

patrol grading annually

$150/km th

replace wearing course every 10 year

$25,000/km

discount rate

6%

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Table 9.1: Year

Activity

Sheeting cost

0

Resheet

$25,000

1

Patrol grade

2

Patrol grade

3 4 5 6 7 8 9 10 11 12 13 14

Patrol grade Patrol grade Patrol grade Patrol grade Patrol grade Patrol grade Patrol grade Patrol grade Resheet Patrol grade Patrol grade Patrol grade

15 16 17 18 19 20

Patrol grade Patrol grade Patrol grade Patrol grade Patrol grade Patrol grade

Example life cycle analyses

Stabilisation Grading Patrol Discount Analysis binder cost intervention grading cost rate period 6 months Sheeting life 10 years

$25,000

20

Annual NPW $

$150

0

$150

0.06

133

$150

0.06

126

$150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150

0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06

119 112 106 100 94 89 84 79 12499 70 66 63

$150 $150 $150 $150 $150 $150

0.06 0.06 0.06 0.06 0.06 0.06

59 56 53 50 47 150

$30,000

25150

Net present Equivalent annual worth $ cash flow $

39303

Discount rate: Life cycle period: Net Present worth: Equivalent annual cash flow:

$25,000

$25,000 New surface

3427

6% 20 years $39303 $3427

$25,000 Resheet

Annual Cost

$20,000 Construction activity

$15,000

$10,000

$5,000

Maintenance activities $150 per year $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150 $150

$0 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Year

Figure 9.1: Example life cycle analysis

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16

17

18

19

20

21

22

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This example indicates that the annual cost of maintaining this pavement is $3,427 per kilometre. This cost should be compared to other strategies and the most appropriate strategy adopted. Other examples may include methods of increasing the life of the wearing course by, for example: 

extending sheeting life to greater than 10 years



increasing the time between routine patrol grading.

9.2.3

Grading Intervention Frequency and Sheeting Life

$4,000

$3,500

$3,000 10 Years $2,500

12 Years

$2,000

15 Years

`

20 Years $1,500 0

3

6

9 12 15 18 21 24 27 Months between patrol grading intervention

30

33

36

Figure 9.2: Life cycle analysis of sheeting life and grading intervention

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Period between re-sheeting

Annual Capitalised Cost Per Kilometre

Using the life cycle analysis methodology, the relationship between grading intervention and sheeting life on the operating costs of a pavement can be estimated as shown in Figure 9.2. It can be seen that increasing grading intervention frequencies beyond 12 months had little impact on life cycle costs. On the other hand, the benefit of increasing sheeting life was very significant. Therefore any process, e.g. blending materials or stabilisation that increases sheeting life, can have a significant cost benefit even though the initial prime costs are greater.

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

REFERENCES American Society for Testing and Materials 2006, Standard test method for measuring pavement macrotexture depth using a volumetric technique, ASTM E 965-96, ASTM International, West Conshohocken, PA, USA. Andrews, RC 2000, Surface longevity treatments for unsealed roads, MTRD report 97/PA/056, Transport South Australia, Walkeley Heights, SA. Andrews, RC 2001, ‘Opportunities for improved unsealed road asset management with chemical stabilisation’, ARRB Transport Research conference, 20th, 2001, Melbourne, Victoria, ARRB Transport Research, Vermont South, Vic., 21pp. ARRB Transport Research 1998, Guide to the design of new pavements for light traffic: a supplement to Austroads pavement design, APRG report no. 21, ARRB Transport Research, Vermont South, Vic. Australian Government 2004, NOHSC declares amendments to the exposure standards for crystalline silica: media release December 31, 2004, Australian Government, National Occupational Health and Safety Commission, Canberra, viewed 9 January 2009, Austroads 2003, Control of Moisture in Pavements During Construction, APRG technical note 13, Austroads, Sydney, NSW. Austroads 2006a, Asset Management of Unsealed Roads: Literature Review, LGA survey and workshop (2000-2002), by L Dowling, AP-T46/06, Austroads, Sydney, NSW Austroads 2006b, Guide to Pavement Technology: Part 4D – Stabilised Materials, by R Andrews & G Vorobieff, AGPT04D/06, Austroads, Sydney, NSW Austroads 2007, Guide to Pavement Technology: Part 4 – Pavement Materials, by G Youdale & K Sharp, AGPT04/07, Austroads, Sydney, NSW. Austroads 2008a, Guide to Pavement Technology: Part 4A – Granular Bases and Subbase Materials, by B Vuong, G Jameson, K Sharp & B Fielding, AGPT04A/08, Austroads, Sydney, NSW. Austroads 2008b, Guide to Pavement Technology: Part 2 – Pavement Structural Design, by G Jameson, AGPT02/08, Austroads, Sydney, NSW. Foley, G, Cropley, S & Giummarra, G 1996, Road dust control techniques: evaluation of chemical dust suppressants' performance, Special report no. 54, ARRB Transport Research, Vermont South, Vic. Giummarra G (ed.) (in press), Unsealed roads manual: guidelines to good practice, 3rd edn, ARRB Group, Vermont South, Vic. Giummarra, G 1998, ‘Better management of unsealed roads: estimating gravel loss: an innovative approach’, Road and Transport Research, June, vol.7, no.2, pp.84-85 Jones, TE 1984, The Kenya maintenance study on unpaved roads: research on deterioration, TRRL laboratory report 1111, Transport and Road Research Laboratory (TRRL), Crowthorne, UK. Jones, D & Paige-Green, P 1996, ‘The development of performance related material specifications and the role of dust palliatives in the upgrading of unpaved roads’, Roads 96: Combined ARRB Transport Research conference, 18th, and Transit NZ land transport symposium, 1996, Christchurch, New Zealand, ARRB Transport Research, Vermont South, Vic., vol.3, pp.199-212 Austroads 2009 — 79 —

GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

Jones, D & Paige Green P 2000, TMH12: Pavement management systems: standard visual assessment manual for unsealed roads: version 1, Contract report CR-2000/66, CSIR Transportek, Pretoria, South Africa. Jones, D, Sadzik, E & Wolmarans, I 2001, ‘The incorporation of dust palliatives as a maintenance option in unsealed road management systems’, ARRB Transport Research conference, 20th, Melbourne, Victoria, ARRB Transport Research, Vermont South, Vic., 16pp. National Association of Australian State Road Authorities 1980, Pavement materials: part 2: natural gravel, sand clay and soft and fissile rock, NAASRA: Sydney, NSW. (Note : superseded in 2008 by Austroads Guide to pavement technology: part 4A: granular bases and subbase materials). Organisation for Economic Cooperation and Development 1987, Maintenance of unpaved roads in developing countries: final report, OECD, Paris, France. Paige-Green, P 1990, The economic optimisation of unpaved roads by improved material selection and construction techniques: final report, research report DPVT 106, CSIR Division of Roads and Transport Technology, Pretoria, South Africa. Paterson, WDO 1987, Road deterioration and maintenance effects: models for planning and management, John Hopkins University Press for World Bank, Baltimore, MD, USA Rushing, JF 2006, ‘Influence of application method on dust palliative performance’, ARRB conference, 22nd, 2006, Canberra, ACT, ARRB Group, Vermont South, Vic., 12pp. Sanders, TG & Addo, JQ 2000, ‘Experimental road dust measurement device’, Journal of Transportation Engineering, vol.126, no.6, March, pp.530-5 Sossic, P 1987, ‘The Clegg hammer’, South Australia Highways Department, Road Construction Course 1987. South Africa Department of Transport 2009, ‘Unsealed roads: design, construction and maintenance’, Report draft TRH 20, South African Department of Transport, Committee of State Road Authorities, Pretoria, South Africa Standards Australia 2001, Method of testing soils for engineering purposes: soil strength and consolidation tests: determination of permeability of a soil: falling head method for a remoulded specimen, AS1289 6.7.2, SA, North Sydney, NSW United States Department of Agriculture 1999, Transportation systems, November 1999, San Dimas Technology and Development Center, San Dimas, CA. USA United States Department of the Army 1995, Unsurfaced road maintenance management, Technical manual TM 5-626, Department of the Army, Washington DC. USA Wooltorton, FLD 1947, ‘Relation between the plastic index and the percentage of fines in granular soil stabilization’, Proceedings Highway Research Board, vol. 27, pp. 479-490.

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GUIDE TO PAVEMENT TECHNOLOGY PART 6: UNSEALED PAVEMENTS

FURTHER READING Barwick, PJ 1992, Energy efficient maintenance of roadside vegetation, Greening Australia, Hobart, Tas. Cock, D 1993, ‘Managing unsealed roads in South Australia’, Legal Liability of Road Authorities, Seminar, 1993, Adelaide, South Australia, Australian Institute of Traffic Planning and Management, Thornleigh, NSW, 13pp. Department for Industry, Tourism and Resources 2006, Mine rehabilitation: leading practice sustainable development program for the mining industry, Australian Government. Department for Industry, Tourism and Resources, Canberra, ACT. Ferry, AG 1986, Unsealed roads: a manual of repair and maintenance for pavements, Technical recommendation TR-8, New Zealand National Roads Board, Road Research Unit, Wellington, New Zealand. Ferry, AG & Major, NG 1997, ‘Strategies for grader maintenance of gravel roads’, National Local Government engineering conference, 9th, 1997, Melbourne, Victoria, Institute of Municipal Engineering, Sydney, NSW, pp.75-80 Fossberg, PE, Harral, C & Fiaz, A 1988, ‘Technical options and economic consequences for road construction maintenance’, IRF Middle East regional meeting, 3rd, Riyadh, Saudi Arabia, International Road Federation, Washington DC, pp. 3.57-3.69. Moll, J 1993, ‘Paving of corrugated metal pipe inverts for repair and fish passage’, Engineering Tech Tips, July, 1993. PIARC, TRL & Intech Associates 2002, Rural road surfacing: gravel/laterite (surface option no. 3), viewed 12 January 2009, Poyhonen, A 1995, ‘Methods for repairing frost damaged gravel roads’, International conference on lowvolume roads, 6th, 1995, Minneapolis, Minnesota, Transportation Research Board, Washington DC, pp.149-154 Provencher, Y 1995, ‘Optimising road maintenance intervals’, International conference on low-volume roads, 6th, 1995, Minneapolis, Minnesota, Transportation Research Board, Washington DC, pp.199-207. Standards Australia (various years), Manual of uniform traffic control devices, AS1742, SA, North Sydney, NSW

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