Application of Soil Nail Method for Slope Stability Purpose

Faculty of Engineering and Information Technology School of Civil and Environmental Engineering

Application of Soil ailing for Slope Stability Purpose by Victor Yeung

Student Number: 10240810 Project Number S08 – 097 Major: Civil Engineering

Supervisor: Dr. Behzad Fatahi

A 6 Credit Point Project submitted in partial fulfillment of the Requirement for the Degree of Bachelor of Engineering

21 November 2008

Statement of Originality The work contained in this thesis report is the sole work of the author. Fragments of texts that’s that were used from other sources have been properly acknowledged and the theories, results and designs that have been used in this report have been appropriately referenced and all sources of assistance have been acknowledged.

Victor Yeung 21st ovember , 2008

Contents 1.0

Page

Introduction 1.1 1.2 1.3

2.0

Statement of Problem Objective Structure of Dissertation

1 1 1

Literature Review 2.1

3

2.3

Principle Theory of Slope Failure Factors affect the slope stability Slope failure hazard

2.4 2.5 2.6

Landslide hazard identification Consequence-to-life Category Australia Landslide risk zone category

9

2.7

Major landslide in past history (Hong Kong & Australia)

12

2.2

2.7.1 2.7.2 2.8 2.9

3.0

Hong Kong Australia Past method of slope failure prevention Current method of slope prevention in present

4 7

10 11

12 13 14 15

2.9.1 2.9.2 2.9.3

Soil nail Bio-Engineering Soil Re-compaction & No-fine Replacement

15

2.9.4

Other method

17

15 16

Application of Soil nailing for slope stabilization 3.1 3.2 3.3 3.4

Principle theory of soil nailing Soil nail history and Development Function of soil nail Different between soil nail and soil anchor 3.4.1

3.5 3.6 3.7

4.0

Maintenance Soil nail application in different construction purpose Advantage of Soil nail for slope stability Limitation of soil nail

18 19 20 21 21 22 23 24

Construction method of soil nails 4.1 4.2 4.3

The compound of the soil nail Introduce the soil nail construction equipment Soil nail construction procedure summary

25

4.4

The major procedure of soil nails construction

28

26 27

4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.5 4.6 4.6.1 4.6.2 4.6.3 4.7

5.0

Setting out of soil nail position Drilling Soil nail steel bar installation Grouting

28 28 30 30

Procedure of pull-out test nail sample Soil Nail Head Quality Specification Testing on soil nail

30

Soil nail steel bar Cement Grout Soil nails Other type of soil nail installation techniques

34

31 33 34

35 37 43

Design of soil nails 5.1 5.2

Concept of Factor of safety

45

Introduce the Circular slip and Method of slices 5.2.1 Circular slip analysis method

46

5.2.2

47

5.3 5.4 5.5

Method of slices (Ordinary method or Swedish method) Soil nail calculation method Analysis slope stability with soil nail element Slope stability analysis computer program - Slope/W

5.6 5.7

Design parameter Design procedure

51

6.0

46

48 50 50

52

Case Study 6.1 6.1.1 6.1.2 6.1.3 6.1.4

Case study ( Hong Kong) Geotechnical assessment Slope/W Stability Analysis Hand calculation using Swedish Method of Slices Estimated Slip surface

6.1.5 6.1.6

Soil nail design Summary 6.2 Case Study ( Australia) 6.2.1 Geotechnical assessment 6.2.2 6.2.3 6.2.4 6.2.5 6.2.6

Slope/W Stability Analysis Hand calculation using Swedish Method of Slices Estimated Slip surface Soil nail design Summary

53 55 59 60 63 64 69 70 73 76 77 79 80 85

7.0

Conclusion 7.1 Summary and concluding remarks 7.2 Recommendations

8.0

Bibliography

9.0

List of Appendices Appendix A – Previous Boreholes Log Records (Case study 1) Appendix B – Previous Laboratory Test Recods (Case Study 2) Appendix C – Slope/W Analysis Data ( Case study 1) Appendix D – Classification Guide ( Case Study 2) Appendix E – Slope/W Analysis Data ( Case Study 2)

86 87 88

90

List of Figures Figure 1 : Typical circular / rotational shaped slip surface Figure 2 Typical channelisation flow (CEDD ,1990) Figure 3 Typical Slide type landslide (CEDD, 1995) Figure 4 Landslide in main access road of Hong Kong International Airport (Appledaily news ,2008) Figure 5 Figure 6 Figure 7

Landslide in Hong Kong (Appledaily news ,2008) Sau Mau Ping Landslide, (CEDD, 1976) Sau Mau Ping Landslide, (CEDD,1976)

3 7 7 8 8 12 12

Figure 8 Kotewall Road. Landslide, (CEDD,1976) Figure 9 Wong Chuk Hang Landslide, (CEDD,1995) Figure 10 Thredbo 1997 landslide (EMA disaster DB,1997) Figure 11 Thredbo 1997 landslide (EMA disaster DB,1997)

12 12 13 13

Figure 12 Sea Cliff Bridge (http://seacliffbridge.com/) Figure 13 Landslide on Lawrence Hargrave Drive (EMA disaster DB,1988) Figure 14 Shotcrete surface

13 13 14

Figure 15 Figure 16 Figure 17 Figure 18

Masnory surface Chuman surface Typical Soil nailing method (Maunsell.Geotechnical ltd ,2003) Typical Soil nailing method (IECA, 1995)

Figure 19 Root orientation with respect to shallow slope failure (Coppin ,1990) Figure 20 Vetiver Grass System, ( Toyo Greenland Co., Ltd , 2008) Figure 21 No-Fine concrete replacement (Maunsell geotechnical Ltd. ,2005) Figure 22 Completed no-fine replacement slope (After landscaping) ( Maunsell geotechnical Ltd. , 2005) Figure 23 Figure 24 Figure 25

Active and Passive zone (Abramson, 2002) Typical tie-back for deep excavation (deepexcavation.org , 2008) Typical permanent Tie-back wall (Office of Geotechnical, California, 2008)

Figure 26 Soil nail reinforcement bar Figure 27 Typical Centralisers Figure 28 Steel plate and Steel nuts Figure 29 Typical soil nail head reinforcement

14 14 15 15 15 15 16 16 18 22 22 25 25 25 25

Drilling Rig Air compressor Grouting machine

26 26 26

Figure 33 Shotcrete machine Figure 34 Mobile drilling rig Figure 35 Typical drilling rig

26 28 28

Figure 30 Figure 31 Figure 32

Figure 36 Steel bar installation Figure 37 Grouting process Figure 38 Figure 39 Figure 40 Figure 41

Excavated soil nail head Typical buried soil nail head Shotcreting soil nail head Typical detail of soil nail and soil nail head (Hong Kong CEDD standard drawing, 2008 )

30 30 31 31 31 32

Figure 42 Steel bar test sample pieces Figure 43 Bleeding test Figure 44 Flow cone test

34 35 35

Figure 45 Typical section of flow cone test equipment (ASTM C939, 2002) Figure 46 Typical sample record sheet for Bleeding Test and Flow Cone Test ( Maunsell Geotechnical services Ltd , 2008 )

35 36

Figure 47 Square cement grout cube

37

Figure 48 Compressive strength test Figure 49 Pull out test Figure 50 Dial Gauge

37 37 38

Figure 51 Figure 52 Figure 53 Figure 54

Typical sample data sheet for Pull out test (Maunsell Geotechnical services Ltd , 2008)

39

Typical sample plotting sheet for pull out test (Maunsell Geotechnical services Ltd ,2008)

40

Typical sample data sheet for proving test (Maunsell Geotechnical services Ltd, 2008)

41

Typical sample plotting sheet for Proving test (Maunsell Geotechnical services Ltd ,2008)

42

Figure 55 Self drilling (Dipl.-Wirt.Ing, 2008) Figure 56 Jet Grouting (Dipl.-Wirt.Ing, 2008)

43 44

Figure 57 Soil nail launch machine (soil nail launcher Ltd. , 2008 ) Figure 58 Circular slip model (Liu.(2008)

44 46

Figure 59 Swedish Method Model Figure 60 General View of slope Figure 61 p – q plot graph ( Gold Ram Engineering and Development Limited., 2005)

47 54 56

Figure 62 Slope location plan & Bore hole location

57

Figure 63 Critical Cross Section A-A Figure 64 Critical Slip surface Figure 65 Swedish Method of Slices analysis

58 59 60

Figure 66 Figure 67 Figure 68

Estimated Slip Surface Soil nail slope FOS analysis FOS comparison

63 65 66

Figure 69 Soil nail design section detail Figure 70 General view of slope

66 70

Figure 71 Figure 72 Figure 73 Figure 74

Elevation View Side View Silty clay at slope toe Silty clay at slope crest

70 70 71 71

Figure 75 Figure 76 Figure 77 Figure 78

Pocket Penetrometer Pocket Penetrometer Slope location plan Sample collection position

72 72 74 74

Figure 79 Figure 80 Figure 81 Figure 82

Critical Cross Section A-A Critical slip surface Swedish Method of Slices model Estimated slip surface

75 76 77 79

Figure 83 Soil nail slope FOS analysis Figure 84 FOS comparison Figure 85 Soil nail design section detail

81 81 82

List of Tables Typical Examples of Facilities Affected by Landslides in Each Consequence-to-Life Category ( CEDD, 2007)

10

Summary of landslide risk categories and development controls (Wilson ,2004)

11

Table 3 Table 4 Table 5 Table 6

Other drilling method for soil nail (Elias & Juran , 1991) Comparison of Consequence-to-life Category Design parameter Section A-A FOS result

29 53 56 59

Table 7 Table 8 Table 9 Table 10

Two methods FOS result comparison table Swedish Method of Slices Calculation Spreadsheet FOS results table Soil nail parameter

61 61 63 64

Table 1 Table 2

Table 11 FOS result (after soil nail installed) Table 12 Design Assumptions Table 13 Tension Failure of the Steel Bar calculation spreadsheet

65 66 67

Table 14 Table 15 Table 16 Table 17

Bond Failure between Grout and Steel Bar calculation spreadsheet Bond Failure between Grout and soil calculation spreadsheet 1 Bond Failure between Grout and soil calculation spreadsheet 2 Final Soil Nail design schedule table

68 68 69 69

Table 18 Table 19 Table 20 Table 21 Table 22 Table 23

Final result table Hand penetrometer test results Design parameter Section A-A FOS result Two methods FOS result comparison table Swedish Method of Slices Calculation Spreadsheet

69 72 73 76 78 78

Table 24 FOS results table Table 25 Soil nail parameter Table 26 Table 27 Table 28 Table 29 Table 30

FOS results (after soil nail installed) Design Assumptions Tension Failure of the Steel Bar calculation spreadsheet Bond Failure between Grout and Steel Bar calculation spreadsheet Bond Failure between Grout and soil calculation spreadsheet 1

Table 31 Bond Failure between Grout and soil calculation spreadsheet 2 Table 32 Final Soil nail design schedule Table Table 33 Final Result table

79 80 81 82 83 84 84 85 85 85

Abstract Landslides are a common natural disaster which take place around the world. They have claimed many human lives and much damage has occurred from different types of landslides. Through the last couple of decades, different kinds of landslide preventive measures have been developed for reducing these hazards. Each preventive measure involves a unique technique and application benefit. One of the most common slope stabilisation methods is soil nailing. The soil nail application has been developed in the last 30 years. This method is growing rapidly and becoming more popular due to its advantages. Use of the soil nail method for reinforcing unstable slopes is one of the most favourable solutions in geotechnical engineering practice. Thus, there would be a lot of benefit for future use which would be associated with the development of the soil nail application for slope stabilisation. This project will present the application of soil nail for slope stabilisation. The benefits and limitations of soil nail and its construction procedures are described. In addition, design requirements and quality control specifications are explained. Slope stability analysis using “SLOPE/W” code is demonstrated and a design method of soil nail using Slope/W is described in detail. Two selected case studies, located in Hong Kong and Australia, are presented to demonstrate the effectiveness of the soil nail system for slope stabilisation. These case studies present a typical design method used for soil nail walls. A simplified hand calculation method is compared with the limit equilibrium approach used in Slope/W code. It should be noted that soil nailing is one of the methods used for stabilising medium size slopes. Enhancing public education for the landslide hazard is the most desirable way to prevent human loss and property damage in high landslide risk areas. In this study, some recommendations regarding increasing public awareness about landslide hazards are described as well.

Acknowledgements I wish to express my sincere gratitude to my supervisor Dr. Behzad Fatahi for his inspiring discussions, without which I would not successfully been able to complete this thesis. Furthermore, I would like to thank Dr. Behzad Fatahi for his invaluable personal time spent with me through numerous conversations. He has not only taught me how to approach my Capstone project; but more importantly, he has provided invaluable insight which will help me in my journey to be a professional Geotechnical engineer. He has assisted in my development of a large knowledgebase of geotechnical engineering concepts and some interesting ideas such as bio-engineering. I am also thankful to my wife, Maggie Leung. Sharing her Geotechnical experience provided much support and assistance, both of which have contributed in some way to the journey of writing this thesis. Special thanks to my previous employer, Maunsell Geotechnical Services Ltd for the invaluable assistance and some sample data information. Special thanks John Marsh for his advice in proof reading and correcting some grammatical mistake in my thesis. Lastly, I would like to thank all my friends for their continuing and unconditional support and assistance.

Application of Soil Nailing for slope stability purpose

1.0

Introduction

1.1 Statement of Problem This Capstone project topic is Application of soil nailing for slope stability purpose. The project involves the literature review for design, analysis, research related to slope stabilisation methods. This project demonstrates different aspects associated with soil nail application. In Australia and Hong Kong there is high risk of slope failures. Some of them are associated with high risk of hazard for public in densely populated areas. Therefore, slope investigation and classification are important for the community. Thus, both cities developed their own landslip risk reduction programs following a similar independent path, resulting in a large amount of experience gained in dealing with rainfall triggered landslides in densely populated areas.

1.2 Objective This project reviews different methods of slope stabilisation. This project presents the current knowledge and known benefits of soil nail as a slope stabilisation method. In addition, various factors that may trigger slope failure is discussed. Through the use of case studies, the design and construction methods of soil nailing is described in this project.

1.3 Structure of Dissertation In this project, Chapter 2 will discuss how the slope instability can affect society, as well as provide a technical review of the factors that affect the slope instability. This review discusses the different methods used to reduce these hazards will be discussed. Furthermore, a discussion follows, noting the wide range of traditional stabilisation methods available to engineering are presented. This ranges from a simple methods such as to flatten and drain a slope , to more complex methods, such as anchors and soil nail, bio-engineering vegetation and the most common practice of methods involving shotcrete surfaces, masonry facing and so on. A cost-effective solution for stabilisation is the application of soil nailing, which is discussed in chapter 3.0. The first part of this chapter mainly focuses on the literature review of soil nailing and also reviews the principle theory of soil nailing, including it’s history and development of nailing. Furthermore, it will also discuss the soil nail

1

Application of Soil Nailing for slope stability purpose

application for different construction purposes such as deep excavating. The last part of Chapter 3 notes the advantages for selecting soil nailing as an initiative to improve slope instability and this was compared to the other methods are described. Chapter 4 will cover the construction methods and the procedure involved in soil nailing, including the equipment used, and procedures. Quality control is also an essential procedure for soil nail construction. This part will present the quality control criteria in the whole soil nail installation process. Chapter 5 is about design of the soil nail. Here, the design criteria and principle theory are presented. In this part, the use of the computer design program ( SLOPE/W with Morgenstern-price method) is also discussed. Chapter 6 is about case studies for soil nail application. This section concentrates on two separate case studies. The first case study investigates a slope in Hong Kong. The other case study related to application of soil nailing in Australia. Two different design standard have been used for these case studies. The first case study in Hong Kong will use Hong Kong GEOguide for design standard and the second case study Australia Standard AS4678-2002 is used for its design standard. For both case studies, Slope/W computer software is used for stability analysis. The factor of safety is an important outcome for the classification of slopes. Hand calculations using Swedish Method of Slices will also be provided in both case studies. In Chapter 7, conclusion at the study is presented. Furthermore, recommendations for an innovative design method for slope improvement will be briefly described.

2

Application of Soil Nailing for slope stability purpose

2.0 Literature Review 2.1

Principle Theory of Slope Failure

Every year there are approximately a thousand slope failure cases around the globe. globe Onn average, a death toll of many thousands of people, as well as astronomical economic losses related to landslide events are common. Therefore, it is evident that there is a clear need to investigate the cause of devastating slope failures. failure Slope failure is related to various causes, these include include: the rise of ground watertable, soil properties and geological characteristic characteristics of slopes. These causes of slope failures are often interrelated and can influence each other, collectively deteriorating the stability of the slope.. The combination of these failure modes forms the principle elements related to slope failure. Principle Theory Slope failure is driven by slope slip surface which is caused by gravitational and seepage forces that push the slip surface and causes slope instability (Ortigao,2004) According to Abramson (2002), there here are various types of slope failure which are driven by slip surfaces,, namely namely: circular/rotational slip, non-circular circular slip, translational slip and compound slip. The most common type of slope failure mode is circular/rotational rotational slip. slip This is described as a circular shaped slip surface which is mobilised ed across a homogenous & isotropic soil condition, whereas a non non-circular circular slip surface is mobilized mobilize in a non-homogenous homogenous condition (Ortigao, 2004). On the other hand, according to Ortigao, (2004) described that slope failure driven by translational and compound slip surface is developed due to the presence of a rigid layer (for example a bedrock layer), or the presence of discontinuiti discontinuities such as fissures and pre-existing slips.

Figure 1 - Typical circular / rotational shaped slip surface

3

Application of Soil Nailing for slope stability purpose

2.2

Factors Affecting the Slope Stability

There are many factors which affects the slope stability. According to Ortigao, (2004) described that one of the main factors is the geometrical changes. This is described as a change in the gravitational force. The main force responsible for movement is gravity. Gravity is the internal force that acts on body, pulling mass object in a direction toward the center of the earth. If the object is on a flat surface then the gravitational force will act downward. In another words, if the objects is located on the flat surface it will not move under the gravity force. However, in the case of a sloping ground, according to Ortigao (2004) described that the force of gravity can be divided into two vector components, one component is acting normal to the slope and the other component is acting tangent to the slope. The slope gains its stability from the strength properties of the soil. These include the shear strength, frictional resistance and cohesion among the soil particles that make up the soil mass (Ortigao, 2004). As the applied shear stress which occurs under gravitational force becomes greater than the combination of forces holding the soil mass on the slope, the object will move down the slope. In geotechnical engineering, this movement is called slope failure or landslide. Thus, this slope movement is favored by steeper slope angles which increase the shear stresses on the soil. The slope stability is threatened by anything that reduces the shear strength, such as lowering the cohesion among the particles or lowering the frictional resistance. The tenancy of slope failure is expressed in terms of the ratio of shear strength to shear force, which is known as Safety Factor (Cornforth,2005) Safety Factor = Shear Strength/Shear force If the safety factor becomes less than 1.0, slope failure is expected. The other factor that causes slope failure is an increase in water pressure. This is caused by the increase in groundwater level. Consequently, an increase of water pressure adds an increased internal water force inside the slope. Although water is not always directly involved as the transporting medium in mass-wasting processes (Ortigao, 2004), it does play an important role. For exemplary reasons, a sand castle on the beach may be used. If the sand is dry, it is impossible to build a steep face like a castle wall. If the sand is wet, vertical wall can be build. If the sand is too wet, then it flows like a fluid and cannot stay as a wall. For the case of dry sand, the sand can form a slope with a slope angle relative to the flat ground that is equal to its Friction angle. The friction angle is the steepest angle at 4

Application of Soil Nailing for slope stability purpose

which the sand slope can remain stable (Liu ,2008). In this case, the stability of the sand slope is purely dictated by the frictional contact between the soil grains. In general, the friction angle increases with increasing grain size. However, different soil types contain different soil friction angles. This mechanical soil parameter can be usually obtained from experiments, for example, Triaxial test and direct shear test . In the partially saturated soil, water particle and the sand particle are interlocked by an internal suction force between them. This suction force assists in building up apparent cohesion in cohesionless material. It should be noted that, excessive water will break the suction force between the soil particles. The other factor that affects the slope stability is the additional loads (surcharge) applied on the top of the slope. This external loading can increase the disturbing force and cause slope instability. Another reason that affecting slope stability is water pressure. Water pressure is common on a general slope where a watertable might usually exist. When water pressure increases, the effective stresses , shear strength decrease and can lead to slope failure. An increase in the water pressure may be due to many uncertain reasons. Usually, the most common reasons that cause slope failure relate to water pressure increases due to elevated rainfall intensity and increases in the water content in slope, such as water pipe leakage. These are the main factors that can affect the slope stability. These are also the main items which one has to focus on when dealing with reducing the presence of slope instability. There is another factor that can induce instability to a slope, which is an earthquake. However this factor is relatively uncommon when compared to the other factors mentioned above. Slope instability caused by an earthquake only happens during earthquakes in active earthquake zones, such as in China and Japan. This factor causes slope displacement and changes the gravity condition of slope material. During the displacement and change of gravity of slope, the body of slope mass no longer is in a balance condition, and slope will no longer be in a stable condition. In many seismic regions of the world, slope displacements caused by earthquakes have led to disaster situations. Examples of magnitude 7.8 earthquake-induced landslides are the landslide events in the area of Sichuan in China, which were caused by a major earth movement event near the belt of Sichuan region in May 2008.

5

Application of Soil Nailing for slope stability purpose

According to CEDD (2008) & Ortigao, (2004), the causes of slope instability can be summarised as follows: External force that causes slope instability:










Geometrical changes (Undercutting, erosion, changes in slope height, length and steepness) Surcharge (Addition of material, Increase in slope height and increase development at slope crest) Shocks and vibrations (earth quake) Drawdown (lowering of water in lake or reservoir) Change in water regime ( rainfall , increase in weight , pore pressure )

Internal forces that causes slope instability:
Progressive failure (following lateral expansion of fissuring and erosion)
Weathering (reduction of cohesion, desiccation)
Seepage erosion (solution , piping) Moreover, there are some other non-natural factor cause slope instability:
Removal of vegetation;
Interference with, or changes to, natural drainage;




Modification of slopes by construction of roads, railways, buildings, etc; Overloading slopes; Mining and quarrying activities; Vibrations from heavy traffic, blasting, etc; and




Excavation or displacement of rocks.






6

Application of Soil Nailing for slope stability purpose

2.3 Slope Failure Hazard According to J.A.R. Ortigao, 2004 , Landslips can be classified into 3 main types of landslides; these are described below according to their kinematics of the slide. Fall type landslide Usually occurring in rock slopes, rock displacement and rock falls with a very fast movement (EMA ,2008). Usually a topple fall is classified into this category. Slide type landslide Slides are usually caused by mass movements that present a well-defined failure wedge and surface. According to their failure wedge and surface geometry, it can be classified into shallow slides or deep slides (Ortigao, 2004). Flow type landslide A flow landslide is a continuous viscous slide involving soil or rock (Emergency Management Australia - 2008 ). According to Ortigao (2004) explained that if material is clay or fine soil material, this flow is termed a mud flow. Flow slides usually include saturated soil or mud mix with water (also called liquefaction) and are usually initiated from the summit of a hill or mountain due to high rainfall or water leakage and flow downward by channelisation. Sometimes, this slide is also triggered by rapid ground motion and commonly occurs during earthquakes. Unfortunately, the flow will result in major economic loss and major landslide casualties if it happens in a densely populated area.

Figure 2 Typical channelisation flow (CEDD ,1990)

Figure 3 Typical Slide type landslide (CEDD, 1995)

7

Application of Soil Nailing for slope stability purpose

Regardless of the type of landslide failure mode, in some areas of high population density, a landslip can cause a large disaster. If the landslide is a minor one, it might cause damage and displacement of a building’s foundation or break the frame structure of the building. This displacement or settlement can disrupt the building’s structural stability and cause the building to collapse. In the case of a major landslide flow, a whole building can be overwhelmed. Usually this type of major flow will have a high casualty rate if it occurs in a high population density area. For example, Hong Kong has a unique geological environment which mainly consists of volcanic rock with a mountainous region and few flat land areas. This scenarios left many developers with few options, one of which was to build skyscrapers on hillsides. The cost of land is very high as the developers often need to bulldoze mountains to carry out site formation and form more flat lands for the construction of the buildings, which are often over 30 stories. Thus, many of the man-made slopes are very close to buildings, as this helps to save on the land cost, therefore simultaneously stretching the profit margin of a lot of land. At times where land availability is limited, a surplus in population often leads to a city being overdeveloped. This would elevate the risk of landslide failure, as developers are left with no choice but to cut back on the slope to form flat land. By doing so, the new slope would decrease the safety factor, leaving a very steep angle and a lack of surface protection. As this is becoming a widespread global situation, landslides are not unusual in urban areas. This is evident with the even that occurred on 7th June 2008, when a series of landslips occurred in Lantau Island due to heavy rainfall. These serious landslips are mainly located near the main access road of Hong Kong International Airport. This disaster severely affected the operation of the airport.

Figure4 Landslide in main access road of Hong Kong International Airport (Appledaily news ,2008)

Figure 5 Landslide in Hong Kong (Appledaily news ,2008)

8

Application of Soil Nailing for slope stability purpose

2.4 Landslide hazard identification According to Hong Kong housing authority (1999) reported that the identification of landslide hazard involved following procedure:


Desk study- An aerial photograph is an important aspect of landslide hazard identification. The study of aerial photographs assists in cataloguing of historical landslides, describing and evaluating the geomorphology and determining the site history particularly with respect to human activities on natural slopes.




Engineering geological reconnaissance Mapping- The mapping provided additional landslide information data which was not visible on the aerial photos and enables ground truthing of some of the geomorphological interpretations made from aerial photographs.




Ground Investigation – In order to understand the ground model better, ground investigation was carried out to explore the soil properties and the condition of the groundwater regime.




Site investigation – site visits and field measurements were taken of the slope geometry (eg. Slope height, angle, seepage). Therefore, the collected data can be used to provide the most precise information and representative the real slope geometry for further design.




Engineering Geological synthesis – An engineering geological synthesis of the finding from the desk study, engineering geological mapping, ground investigation fieldwork, site investigation fieldwork and laboratory tests was conducted to produce a geological model and representative geological sections

Development of Landslide Risk Assessment in Australia In the recognition of the challenge between development pressures and landslide hazards, in the year 2000, Australian Geomechanics Society Published a series of guidelines called ‘Landslide Risk Management Concepts and Guidelines (AGS2000)’. This is a benchmark technical paper for development of landslide assessment. In 2004, Landslide Likelihood Research had been undertaken to investigate the likelihood of a landslide in residential areas. The aim of this research is to develop the probability estimates for landslide hazards in Australia. Development of Landslide Preventive Measure Program in Hong Kong Prior to 1976, due to the high risk for landslide in Hong Kong, The Geotechnical Engineering Office has been responsible for studies and upgrading works in respect of old (i.e. pre-GEO) substandard slopes under a long term program- Landslip Preventive Measures (LPM) Program. According to CEDD (2008) reported that this long term program will be targeting over 5,000 high-priority substandard Government man-made slopes, and will carry out safety-screening studies for another over 10,000 high-priority private man-made slopes by the year 2010.

9

Application of Soil Nailing for slope stability purpose

2.5 Consequence-to-life Category This guidance is used to identify the level of risk of a human loss in relation to the type of facilities that are affected by landslides. According to the following table, an engineer would be able to improve on the stability of the slope by examining the types of facilities used on the top of the slope. Hence, this would prevent catastrophic damage and the loss of human lives. Table 1 : Typical Examples of Facilities Affected by Landslides in Each Consequence-to-Life Category ( CEDD, 2007) Consequence Group

Facilities

to-life Category

(a) Heavily Used Buildings – residential building, commercial office, store and shop, hotel, factory, school, power station, ambulance depot, market, hospital, polyclinic,clinic, welfare centre 1

(b) Others – cottage, licensed and squatter areas – bus shelter, railway platform and other sheltered public waiting area – dangerous goods storage site (e.g. petrol stations)

1

(High)

– road with very heavy vehicular or pedestrian traffic density (a) Lightly Used Buildings – indoor car park, building within barracks, abattoir, incinerator, indoor games’ sport hall, sewage treatment plant, refuse transfer station, church, temple, monastery, civic centre, manned substation 2

(b) Others – major infrastructure facility (e.g. railway, tramway, flyover, subway, tunnel portal, service reservoir) – construction site (if future use not certain) – road with heavy vehicular or pedestrian traffic density

2

(Middle)

– heavily used open space and public waiting area (e.g. heavily used playground, 3

open car park, heavily used sitting out area, horticulture garden) – road with moderate vehicular or pedestrian traffic density – lightly used open-air recreation area (e.g. district open space, lightly used

4

playground, cemetery, columbarium – non-dangerous goods storage site – road with low vehicular or pedestrian traffic density

5

3

(Low)

– remote area (e.g. country park, undeveloped green belt, abandoned quarry) – road with very low vehicular or pedestrian traffic density

10

Application of Soil Nailing for slope stability purpose

2.6 Australia Landslide Risk Zone Category The Australia Geomechanics subcommittee has developed a classification of consequences of landsliding. AGS(2000) which classified the hazard slope as Exempt (EX), Low(L) , Medium(M0) , Medium(M1), Medium (M2) and High (H) according their different trigger factor, is shown as follows. Table 2 : Summary of landslide risk categories and development controls (Wilson ,2004) Landslide risk zone category

Exempt (Ex)

Low(L)

Medium(M0)

Medium(M1)

Applicable geology & slope

20-50%;

Known

Tertiary>15%;

Landslides and

Rhyodacite

similar terrain.

Tertiary50%

Bedrock50%

Alluvium & Colluvium Alluvium &

Other Bedrock 5-20% ;

Granite 20-40%

Colluvium40%

A simple

Likelihood of

There is a

Landslide Landslide is unlikely without development. The

landslide of

likelihood of a

without likelihood of instability without development is greater

natural slopes is

landslide without

development is in M2 than for the M0 & M1 zones

extremely low

geotechnical information controls

Required site specific

High(H)

Colluvium

development

very unlikely Watch out for

Comply the

You possibly Worry about

springs.

Comply with

guidelines.

You probably

something else

Comply with

the guidelines

They are there

have a problem

have a flooding problem Guidelines

Development

Medium(M2)

for a purpose. Slope stability assessment by an experienced geotechnical practitioner

Confirmation of risk category by the shire using geotechnical information

to confirm or change the regional

submitted to classify the site for soil reactivity.

classification plus additional geotechnical investigation where considered necessary.

site and project specific controls

site and project

If confirmed as

Good Hillside

where applicable including specific

specific controls

High H , it is

practice

attention to drainage & erosion

where

unlikely permit

control

applicable

will be issued.

Good engineering practice

11

Application of Soil Nailing for slope stability purpose

2.7 Major landslide in past history (Hong Kong & Australia) 2.7.1 Hong Kong According to CEDD reported that between 1925 and 2007, more than ten thousand landslides have occurred and every year have about 300 slope failures occur in cut slope areas. In the past half century, at least 300 people died in 24 landslides in Hong Kong. (CEDD,2008) On 18th June 1972, a major landslide took place in Hong Kong at mid-level Kotewall Road. Two high-rise residential buildings collapsed due to a large landslide which was responsible for the death of 67 people (CEDD, 2008). In the same year, another major landslide event caused many fatalities which occurred in Sau Mau Ping village. This devastating landslide event caused major debris flow which overwhelmed a large section of Sau Mau Ping Village (CEDD , 2008).

Figure 6 Sau Mau Ping Landslide, (CEDD, 1976)

Figure 7 Sau Mau Ping Landslide, (CEDD,1976)

In 13th Aug 1995, the large Wong Chuk Hang landslide occurred and the landslide material slipped rapidly down the steep slope and destroyed the seaside shipyards. Two people died in this landslide (CEDD,2008)

Figure 8

Kotewall Road. Landslide, (CEDD,1976)

Figure 9 Wong Chuk Hang Landslide, (CEDD,1995) 12

Application of Soil Nailing for slope stability purpose

2.7.2 Australia According to EMA Disasters Database, 2008, there have been 48 recorded landslide events which have collectively resulted in the death of 39 people , and 19 casualties out of the 7,586 victims in Australian landslide history since 1897. One of Australia’s worst landslides was held in 30th July 1997. A large section of the steep mountainside below the Alpine Way road collapsed and overwhelmed a section of the Thredbo Ski Village in NSW. About 1,000 tonnes of landslide material slipped rapidly down the steep slope and shearing the Carinya lodge off its foundations and slamming it into the Bimbadeen Lodge. It was recorded that 18 people had fallen victims in this disaster which also caused multimillion dollars in damage (EMA ,2008)

Figure 10 Thredbo 1997 landslide (EMA disaster DB,1997)

Figure 11 Thredbo 1997 landslide (EMA disaster DB,1997)

According to Australia National Landslide Database,(2007) reported that on 30 April 1988 in Coledale, a small coal mining town near Wollongong, a landslide resulted from a combination of human interference and two weeks of heavy rainfall. A 20 meter high railway embankment collapsed after earth and rock ballast used to fill an old mine dam became saturated, resulting in severe undermining and subsidence. A sudden rush of mud and rock smashed into a house below, turning it through a 60 degree angle before it was demolished. The occupants a young mother and her baby son, were killed. Some landslide hazards have led to the re-development of infrastructure for some geological reason. For example, the purpose of Sea Cliff Bridge is to replace a section of Lawrence Hargrave Drive that was permanently closed in July 2003 due to a great landslide hazard reason. Therefore, landslides in Australia not only cause human loss, but also cause economic loss which due to leak of landslide hazard assessment .

Figure 12 Sea Cliff Bridge (photo: http://seacliffbridge.com/)

Figure 13 Landslide on Lawrence Hargrave Drive 1988

13

Application of Soil Nailing for slope stability purpose

2.8

Past Method of Slope Failure Prevention

Before 1990, chuman surface and non-reinforcing shotcrete surfaces were a common use of material for slope stability improvement. For some steep slopes, a stone pitching surface was most widely used, or masonry facing for rigid surface cover. Some of them were installed “weepholes” to reduce the pore water pressure inside the slope. However, the main purpose of this was to achieve an impervious interface for prevention of the surface erosion and the rainfall entry into the slope in order to reduce the pore water pressure inside the slope. This method is easy in terms of construction and maintenance and was also cost efficient. However, if the slope had inherent instability due to internal soil, shear failure and sliding would still occur. This method would not provide an enough structural external force against the movement of the slope failure wedge. On the other hand, this method usually uses a concrete or stone base construction material, which is usually grey or white in colour. This triggers an environmental problem, as the finish is very inconsistent with the surrounding natural landscape. The following lists are the conventional slope stabilisation methods. 

Shotcrete surface method:

Shotcrete is a process where concrete is sprayed onto slope surface using a shotcrete feeder gun to form rigid surface. Usually, shotcrete surface slopes have approximate 50-150mm thick and provide wire mash reinforcement to prevent surface crack and shrinkage.



Figure 14 Shotcrete surface

Masnory surface method

Use stone pitching as a rigid surface cover for prevent erosion and surface runoff. This method is easy for maintenance and construction.

Figure 15 Masnory surface



Chuman surface method

Use of cement sand mix material for surface protection. No reinforcement and wire mash required. Poor crack and shrinkage resistance.

Figure 16 Chuman surface

14

Application of Soil Nailing for slope stability purpose

2.9 Current Method of Slope Failure Prevention In the past 20 years, slope improvement technology has advanced significantly. The slope improvements are now focusing on the slope stability design and environmental protection. Many different types of slope retaining methods are used in slope improvement construction and design. The most commonly used methods are Soil-nailing and Bio-Engineering.

2.9.1

Soil ailing is a new technique in which soil slopes, excavations or

retaining walls are reinforced by the insertion steel reinforcing bars. According to Ortigao (2004) noted that the first use of the soil nailing application was in 1972 and now this method is a well-established technique around the world. Sometimes, soil nailing can combine different type of retaining methods such as soil nailing on retaining walls and with greening surfaces. Soil nailing can provide a cost efficient, quick and standard technique for slope improvement solution. Thus, according to CEDD (2008) reported that soil nailing methods dominate about 70% of all soil slope improvement constructions in Hong Kong.

Figure 17 Typical Soil nailing method (Maunsell.Geotechnical ltd ,2003)

Figure 18

Typical Soil nailing method (IECA, 1995)

2.9.2 Bio-Engineering is one of the most innovative technologies for slope improvements in the world. According to Coppin (1990) described that Bio-Engineering includes the use of tree roots or plant roots to retain shallow slope failure. This method has an advantage as it is natural and environmental friendly (Coppin,1990). However, many factors can influence the effectiveness of Bio-engineering for slope stabilisation. This method is in an early stage of development, and needs a period of time for technology proving and development.

Figure 19

Root orientation with

respect to shallow slope failure (Coppin ,1990)

Figure 20 Vetiver Grass System, ( Toyo Greenland Co., Ltd , 2008)

15

Application of Soil Nailing for slope stability purpose

2.9.3 Soil Re-Compaction and o-fine Replacement For some loose material slopes such as fill slope, soil nailing is not a suitable stabilisation method. Some technologies such as soil re-compaction and soil re-placement are more suitable and are usually applied. Soil re-compaction involves the excavation of the loose soil, backfilling and re-compacting to improve the friction angle. However, the soil re-compaction method has some restrictions such as every backfill and re-compaction has to be carried out in a 300mm thick layer (Geoguide 7, 2008), layer by layer, and every single layer needs an individual soil test for compaction ratio checking. Moreover, this method is highly influenced by weather conditions. The soil has to be placed in thinner lifts and requires moisture control for compaction. As a result, this method will increase the construction cost and period. The other method is the soil replacement method. This design approach includes using other materials such as no-fine concrete or gravel to replace the loose soil. Removal of the original loose soil on the slope is carried out, then forming a slope with a design slope angle by backfilling with no-fine concrete or gravel. After that, a thin layer of soil with hydroseeding is applied to the surface as a cover and for landscaping. This method can reduce the construction period, hence alleviating labour costs and operation costs which then compare with the soil re-compaction method. However, these replacement and re-compaction methods are constrained in that the construction sequence has to be scheduled for the dry season when the groundwater levels are lower than they were at the time of active landsliding. Alternatively, temporary groundwater lowering through the use of a raking drain may be needed prior to, and during construction work.

Figure 21 No-Fine concrete replacement (Maunsell geotechnical Ltd. ,2005)

Figure 22 Completed no-fine replacement slope (After landscaping) ( Maunsell geotechnical Ltd. , 2005)

16

Application of Soil Nailing for slope stability purpose

2.9.4 Other Method of Slope Failure Prevention Subsurface Drainage Of all stabilisation methods considered for the prevention of landslides, a reduction of pore water pressure behind the slope is the most important. According to Cornforth(2005) described that the subsurface drainage method can reduce the destabilising hydrostatic and seepage water pressures on the slope as well as the risk of sliding or flow. For large, unstable slopes, a drainage tunnel can be applied to draw down the water table and minimise the risk of slope failure. In Hong Kong, the Lung Fu Shan drainage tunnel and vertical drainage system is under construction. This drainage tunnel can prevent the failure of a 200m high natural slope which could be triggered by water pressure. Other subsurface drainage methods include: Drain blanket, Trenches, Cut-off drains, Horizontal Drains, Relief Drains and Raking Drains. Stone Columns Based on Cornforth, (2005) described that this ground improvement method can increase the average shear resistance of soil along a potential slip surface by replacing or displacing the in situ soil with a series of closely spaced and large diameter columns of compacted stone. However, this method requires the use of a boring machine and material delivery, which would result in an access problem if the slope is inaccessible. This method is not common use in Hong Kong. Usually, vertical soil nailing can provide the same results as stone columns. Shear Piles According to Cornforth, (2005) described that shear piles are reinforced concrete cylindrical piles that pass through the slide plant and anchored at lower end stable soils or bedrock. This shear pile anchorage can provide lateral bearing resistance near the base of ground movement (Cornforth, 2005). This method is effective for a large instability zone and can provide the flexibility of selecting an installation location. However, this method has limitations such as being costly and cannot be installed in moving landslide.

17

Application of Soil Nailing for slope stability purpose

3.0

Application of Soil nailing for Slope Stabilisation

3.1

Principle theory of soil nailing

A slope can be described in terms of geological theories, according to Abramson (2002) described that the soil mass behind the slope surface can be divided into an active and passive zone which are separated by a shear face call slip surface. The slope stability analysis for a soil nailed slope considers the stabilising effect of nails acting on the slip surface, however, this differs with respect to the shape of the slip surface, the forces act on a nail and the method used for calculation of stability.

Figure 23

Active and Passive zone (Abramson, 2002)

Based on Ortigao (2004) described that Soil nailing consists of reinforcing the instable soil mass by the series of elements called nails to resist tension, bending and shear forces. These nail elements are usually made of galvanized steel bar and protected by cement grout. Nails are installed sub-horizontally and closely spaced in a parallel fashion (usually 1.5m to 2.0m in spacing) into soil mass in a pre-drilled hole to improve stability of slope. According to CEDD (2008) described that soil nailing provides pullout resistance force and tension over their entire length. The angle, length and diameter of soil nails are dependent on soil condition and design criteria. Usually, soil nails are installed for permanent slope improvement. Therefore, the corrosion-resistant treatment is similar to soil anchors and requires galvanizing. The soil nail system for mechanical stabilisation against the instability force can be categorised as a limit equilibrium analysis (Abramson,2002). This is a conventional slope stability calculation method with potential slip surfaces modeled, such as circular arc slip surface. Abramson,(2002) stated that this potential slip surface model approximately represents the critical surface of maximum tensile load. Limit equilibrium analysis can examine the slip surface and others to determine the lowest factor of safety after the slope is reinforced.

18

Application of Soil Nailing for slope stability purpose

3.2 Soil nail History and Development Soil nailing methods are widely use in geotechnical construction work. Nowadays, these technologies can be used in Tie-back retailing wall, Temporary support, ground anchor and Tunneling support. Therefore, soil nailing has a great contribution in geotechnical construction. Based on Ortigao (2004) noted that , in the late 60’s, soil nailing developed used in tunneling shotcrete supporting method. This method used a flexible lining that enabled soil deformation around the excavation, which had been reinforced by a number of bolts or nailing. An active zone is formed around the excavation and the lining is subjected to reduced loading (Ortigao, 2004). This technique is the traditional tunneling technique method for preventing soil deformation and reducing the subjected ground pressures of tunnel. As reported by Ortigao (2004), the first time nailing was used in tunneling construction work was in 1970 in Brazil. After that, this nailing method is widely used in France, Canada, Germany, UK and in the USA, among other countries. The soil nailing for slope stability method is similar to the tunneling support method (tieback). The difference is they are installed non-tensioned at a slight downward inclination on slope. Such construction work used soil nailing for slope improvement work in Versailles (France) for first time in 1972 (Cornforth, 2005). According to Ortigao (2004) reported that one of the first national guideline publications for soil nailing was produced in Japan in 1987; the USA has produced national guideline publications through the Federal Highway Administration on this subject in 1996. The Geotechnical Engineering Office (GEO) of Hong Kong extensively uses soil nailing to stabilize thousands of man-made slopes in residual and saprolitic soils and in 1996 presents its prescriptive design method. CEDD, (2008) mentioned, since 1995, over ten thousand of such soil nailing structures have been constructed in Hong Kong through LPM program to stabilize slopes in residual soil. Regarding the development of the soil nail head, in the early 90’s , an exposed soil nail head was commonly used in the soil nailing system. With a large size and exposed head, it was possible to transfer the component of load from the slope face to soil nail. However, buried soil nail heads are now common, since the late 90’s. Hidden into the slopes surface and with a small size, the soil nail head (approximate 0.6m-0.8m) is the main element of the design in soil nailing system. This type of soil nail head can be

19

Application of Soil Nailing for slope stability purpose

covered by hydroseeding surface on top of soil nail head to provide a natural and environmentally friendly slope surface.

3.3

Function of the Soil ail

Soil nail - in general, these are a form of in situ non-tensioned reinforcement, acting similarly to strip reinforcement (Abramson, 2002). Typically, soil nails usually have a diameter of 25-32mm. The length and inclination are both dependent on the design calculation and factor of safety. They are installed in drillholes and bonded into place with low pressure grout. Stress is transferred from the ground to the nail over its full length and there is a shear stress reversals as in reinforced earth. According to Ortigao (2004) described that ,when considering a very steep slope in a granular or cohesive soil, many factors may influence the soil, causing it to not have sufficient internal strength to stand at such an angle. Therefore, for the face to remain stable the force exerted by soil mass sliding must be resisting by a reinforcement structure. In previous chapters, it has been mentioned that the stability method can be achieved through the implementation of structural elements (such as skip wall) , or through the inclusion of reinforcement in the soil (such as soil nail). The aim of the inclusions is to interact with soil mass in a stabilising manner. An active inclusion is like a stressed soil anchor , it exerts a force on the soil mass through the tension in anchor. In the chapter of Principle theory of soil nailing, Abramson (2002) mentioned that the two zones can be identified, an active zone and a passive zone. The stabilising manner relies on the soil frictional force between the soil nail surface and soil which is generated by the surrounding soil mass in passive zone. If the soil mass had to stand at a very steep angle and had insufficient shear strength, the soil mass would deform. Therefore, this deformation may exert a force which would act on any structural element placed in the soil. Based on Cornforth (2005) described that the main aim of the soil nailing method is the structural element which is used to resist this deformation force. Hausmann, (1992) mentioned that Soil nail contains two forces when the soil mass undergoes deformation. The first is friction between the deformation soil mass and the inclusion. This interaction length can be termed the “bond length”. The second is derived from the normal stress which exerted by soil on the inclusion. There are four possible actions of this force: tension, compression , shear and bending. For general slope, bending and shear are commonly used in slope soil nail design.

20

Application of Soil Nailing for slope stability purpose

3.4 Differences between Soil nail and Soil anchor The soil nail and soil anchor are similar in structure. Both of them are able to take tension and resist the soil mass sliding as the earth retailing structure. However, they are two different types of structural. Significant conceptual differences exist, as described in the following section.

Soil Anchor –Anchor structure for slope stability which is only able to resist tension forces. The nail or tendon are usually are Prestressed in a high loads. According to Das, (1990) described that Soil anchor nails contain two parts: Free length and Bonded length. Free length usually are ungrouted length or un-bonded, and bonded length usually are grouted or bonded into the soil (Das, 1990). In this type of structure, tendons are taking the tensile force , which is transmitted from the anchor head to the anchorage zone. As the tendon is located in the free length, it does not have any grout protection. The corrosion protection control of tendon is very important for this reason.

Soil nail- Soil nails involve the rigid reinforcing of a soil mass. These nails can resist tension, shear forces and bending moment which imposed by slope movement. The nail inside the soil is fully grouted and usually Non-prestressed and relatively closely spaced. No force will act on soil nail system until the soil mass failure. Usually soil nails involve a more simplistic installation technique than soil anchors and are easier to construct.

3.4.1 Maintenance Typically, soil anchors need to keep the tensile force in the tendon at a constant level. Many factors can trigger the prestress loss. Therefore, maintenance of re-prestress process may be necessary and thus, result in an increase in the overall maintenance cost. On the other hand, soil nail reinforcement bars inside of the soil are fully protected by cement grout and are usually non-prestressed. If the reinforcement bar corrosion protection control keeps the nail in good condition, the soil nail needn’t be actively maintenaned, hence reducing the maintenance cost.

21

Application of Soil Nailing for slope stability purpose

3.5 Soil nail application in different construction purposes In global construction, soil nails are widely used in construction sites as an anchor system. Not only are soil anchors used as a slope stability retaining structure, but also for other purposes as follows 

Tie- back wall – In this case, the soil nails are used to provide a tension force to the back of the wall to increase the passive pressure of retaining wall system. It’s conceptually very similar to geo-synthetic soil nail (Ortigao, 2004). In order to minimize wall movement and ground settlement, tieback walls are designed to achieve an efficient earth retaining structure within economical considerations.



Ground Anchor – Using soil nails to provide the tensile force in the ground. Typically, they are used to prevent the overturning or floatation of structures such as footing or structures in water.



Deep excavation support - Usually this involves using soil nails as a temporary measure for deep excavation stabilisation. Similar, to the tie-back wall method, in deep excavation, vibration sheet pile will be installed for supporting the vertical cut slope. The deeper the excavation , the higher the active pressure that will be generated and act on the pile wall. Therefore, structural supporting on upper portions of the pile are necessary. Soil nails can provide these external tensile forces to help resist the deformation of pile wall.

Figure 24 Typical tie-back for deep excavation (deepexcavation.org , 2008)

Figure 25 Typical permanent Tie-back wall (Office of Geotechnical, California, 2008)

22

Application of Soil Nailing for slope stability purpose

3.6 Advantages of Soil nailing for Slope Stability Soil nailing presents the following advantages that have be contributed to the widespread use of this technique in many countries in more recent times. 

Economy : Steel bar reinforcement is inexpensive. The concrete or shotcrete for the soil nail head is relatively small and inexpensive. Construction techniques are simple and quick. Skilled labor can be minimized. According to Cornforth (2005) mentioned that, soil nailing can result in a cost saving of 10 to 30 percent when compared to tieback walls.



Rate of construction: Fast rates of construction can be achieved if adequate equipment is employed.



Light construction equipment: Soil nailing can be done using a conventional drilling rig and grouting equipment. Thus, equipment can be delivered to site easily even in areas with difficult access or limited working space constraints.



Adaptability to different soil type : Soil nails can still be used in heterogeneous ground where boulders or hard rocks may be encountered in the soil slope. Soil nailing generally is more feasible than other techniques. This is because it involves only small-diameter drilling for the installation of the inclusions.



Flexibility : Soil nailing retaining structures are more flexible than classical cast-in-place reinforced concrete retaining structures. Soil nails can be incorporated with other earth retaining system such as Tie-back wall, Skill wall etc. Also soil nails can limit the deformation or settlement in the vicinity of existing structures such as a foundation (Cornforth, 2005). This characteristic of soil nailing can help to provide economical retaining structures on unstable slopes.



Reinforcement redundancy: Based on Ortigao (2004) stated that, if any one soil nail becomes overstressed for any reason, it will not cause failure of the slope. It will redistribute the overstress to the adjoining nails system.

23

Application of Soil Nailing for slope stability purpose

3.7 Limitation of Soil ail Although soil nails are widely use for slope stability, there are some limitations regarding the application of soil nail. 

Unsuitable soil: Cohesionless soil slopes are not suitable for soil nails for increasing slope stability. This is because during the drilling of the hole, the un-grouted hole may collapse. Usually, casing drilling may be applied during the drilling process.



Groundwater: Soil nailing has to occur above groundwater level. When soil nail holes are drilled, the drilled hole may collapse because hole surfacing soil is saturated or is filled with water. Therefore, a drilled hole cannot support itself and in result the hole will collapse. Furthermore, when the soil nails are being grouted, groundwater inside the drilled hole may affect the water/cement ratio of the cement grout. This may affect the grout quality and reduce the cement grout strain capabilities.



Utilities: soil nails are drilled inside the slope. Behind of slope may contain utilities such as buried water pipes, underground cables and drainage systems. There are some limitations that state that soil nails must have a safe distance between soil nails and these utilities. Therefore, a soil nail must change its inclination or length or spacing to achieve this distance.



Vibration sensitive structure: During the drilling procedure, vibration may occur and cannot be avoided. Some building structures are vibration sensitive such as Historical Buildings. Therefore, soil nailing is not the suitable method for slope improvement in these cases.



Rock base slope: Some cut slope contain only few meters of top soil. During site investigation the deep layer soil type or a large boulder may be undetected (which would be possible with ground investigation, indicating it’s importance). When drilling the soil nail holes and the rock layer is reached, dust and stone powder may affect the environment and public health.

24

Application of Soil Nailing for slope stability purpose

4.0

Construction method of soil nails

4.1 The Compound of the Soil ails These following component parts are the main elements of the soil nail system:  Soil nail reinforcement – Steel galvanized bar which usually would have a high grade 500N or equivalent strength capability, usually a galvanized steel, deformed bar.  Centralizers – PVC material which are fixed to the soil nail and ensure that soil nail is centered in the drill hole.  Grout tube – Use to transfer the cement grout from grouting machine to the bottom of soil nail.  Steel Plate – A square shape steel plate which use to transfer the bearing from soil nail to the soil nail head.  Steel uts – Used to fix the steel plate on the soil nail steel bar. Usually each soil nail contains 2 steel nuts to fix the position of steel plate.  Soil nail head – A square shape concrete structure which includes the steel plate, steel nuts, and soil nail head reinforcement. This part of structure provides the soil nail bearing strength, and transfers bearing loads from the soil mass to soil nail.

Figure 26 Soil nail reinforcement bar

Figure 28 Steel plate and Steel nuts

Figure 27

Typical Centralisers

Figure 29 Typical soil nail head reinforcement

25

Application of Soil Nailing for slope stability purpose

4.2 Introduce the Soil ail Construction Equipment Some special equipment is using in soil nail construction. The following details describe the essential equipment involved in soil nail construction work. Drilling rig is a machine which creates holes in the slope. Most of them are powered by compressed air. For soil nail purposes, the length of drilling rig is about 2m max length .Therefore, it has a small size, easy delivery and the benefit of high level of mobility.

Figure 30

Drilling Rig

(More drilling methods in Table 3)

Air compressor is a machine that provides compressed air to drilling rig for the power source. It also provides the air wash through the drilling bit which spreads air pressure inside the hole to remove the soil debris.

Figure 31

Air compressor Grouting machine is a machine that provides the grout material and pump into soil nail drill hole. It contains two tanks, one is a mixing tank which used to mix the cement and water to form the liquid grout. The other one is a holding tank which is used to store the grout from mixing tank and high pressure pump to hole

Figure 32

Grouting machine

through grout tube.

Shotcrete machine is used to construct the soil nail head. For some steep slope and high slope, traditional case in situ concrete method is not suitable for concreting the soil nail head. Therefore, Shotcrete is the most suitable in concreting soil nail work because of its flexibility and mobility. Figure 33 Shotcrete machine

26

Application of Soil Nailing for slope stability purpose

4.3 Soil ail construction procedure summary This section will discuss some of the more practical aspects of soil nailing construction and some of the various techniques available for soil nail installation. Soil nail installation involves different parts of a procedure as follows: Trial test process:  Constructed a trial soil nail (Test nail) for a pull-out test to ensure the soil condition is the same as in the design assumption. Preparation work:  Erect and setup the working platform.  Setup all necessary equipment  Fix the centralisers and grout tube on the steel reinforcement. Drilling process:  Use air wash drilling machine or coring machine to drill the hole for the soil nail (Hole diameter approximate 100mm to 150mm) Installation the soil nail:  Use pressure air wash to ensure the drilled hole is clear.  Install the soil nail steel reinforcement into drilled hole. Grouting process:  Use grouting machine to mix the cement and water to a designed water/cement ratio. Bleeding test must be carried out in this stage.  Grouting the soil nail from bottom to top with a suitable grout pressure.  Flow cone test must be carried out during the grouting process. Testing process:  Random proving tests must be carried out after 3 to 7 days of the grouting process to prove the soil nail can withstand the acceptance load. Soil nail head construction:  After the cement grout hardens, install the nuts and steel plate into the soil nail 

steel bar. Install the soil nail reinforcement and concreting the soil nail head structure.

27

Application of Soil Nailing for slope stability purpose

4.4 The Major procedure of Soil nails construction 4.4.1 Setting out of soil nail position The soil nail position should be located identical to that of the working plan, although soil nail locations may be offset within the tolerances of specification agreed by design engineering. Before the drilling stage, cable detection and visual inspection shall be provided to avoid any buried utilities. 4.4.2 Drilling The drilling rig is the equipment for drilling the soil nail drill hole. Before starting the drilling process, the inclination and angle of drill mast should be checked to ensure that they are the same as the design specified angle. Moreover, the drilling bit also needs to meet the diameter requirement according the specification. Usually, most soil nail drilling uses a drill rig machine due to its easy delivery and simple maintenance schedule. Some of them use a small hydraulic and mobile platform. But this equipment is very expensive for operation, especially if the site is not accessible or the space of working area is not available for drilling rig. Coring machines are also suitable for coring the soil nail hole. Traditional drilling rigs are powered by compressed air which is provided by an air compressor machine. Compressed air has two functions in the drilling rig. One is as a power source to drive the rotation of the drill bit and push the drilling mast inward into the slope. The other function is when the drilling rig is operating; the compressed air can force the soil debris out of the hole. The drilled hole diameter usually should be in the range of 100mm to 150mm, depending on the size of the soil nail steel bar. In some case, an iron casing is provided when the soil property is loose dense and the drilled hole collapses easily. Un-grouted drill holes in soil should be kept open only for short periods of time. According to (CEDD GS vol2 section 7, 1992) standard, un-grouted drill holes should not kept open for four days and for only 24 hours according to the Australian Standard. The longer the hole is left open, the greater the risk of collapse of drilled hole. If the hole is collapsed and unable for soil nail installation, a re-drill of the holes is necessary.

Figure 34

Mobile drilling rig

Figure 35

Typical drilling rig

28

Application of Soil Nailing for slope stability purpose

Other type of drilling method

Drill rig Type

Table 3 Other drilling method for soil nail

Drill Open Drilling method

hole

Drill Bit

Cutting

diameter

Type

removal

Cased Hole

Comments (mm)

Lead Flight Kelley Bar Driven Sectional solid-stem Auger

(Elias & Juran , 1991)

Sectional Hollow-stem Continuous Flight Solid-stem Continous Flight Hollow-Stem Single-stem Air

Yes

No Mechanical

Yes

No

Yes

Yes

Yes

No

Yes

Yes

Yes

No

Hydraulic rotary

100-300

Rock, soil ,drag

Mechanical (Air

auger methods for

support)

drilling competent soils or weathered

Mechanical

rock.

Mechanical (Air support) Hydraulic rotary methods for

Duplex Air Rotary

Yes

drilling competent

Yes Button, 100-200

Rotary

Sectional Solid-Stem Augers

Roller,

soils, rock, or Compressed air

Drag Yes

mixed ground conditions

No

(Pneumatic hammers available) Mechanical

Sectional Hollow-Stem Augers

Yes

Yes

100-300

auger methods for

Rock, soil, drag

Hydraulic rotary

Mechanical (Air

drilling competent

support)

soils or weathered rock.

Air track

Pneumatic rotary

Single stem air rotary

methods for

Button, Yes

No

100-300

Roller, Drag

Compressed air

drilling non-caving competent soils or rock

29

Application of Soil Nailing for slope stability purpose

4.4.3 Soil nail steel bar installation According to CEDD Geoguide 7 (2008) requirement, before the steel bar is installed in the hole, the centraliser and grout tube should be provided and fixed tightly to the steel bar to prevent any movement which could be caused

Figure 36 Steel bar installation

by the grouting pressure. Spacing and size of the centralisers needs to meet the requirement of design specifications and ensure that the steel bar is located in the center in the drilled drill

hole. Checking of the galvanizzing, steel bar length and diameter must be carried out before install installing the soil nail steel bar (Geoguide 7, 2008).. After all, the steel bar must be installed in the same direction and angle with the drilled hole and handled carefully to avoid damag damaging the drilled hole and to avoid collapsing the hole hole. 4.4.4 Grouting Grouting is the concreting procedure of the soil nail which provides the bond over the length of the soil nail. The grouting material should be mix of cement and water with an acceptable water/cement ratio. Too high or too low of a water cement ratio may cause a higher risk of failure in the flow cone test and bleeding test. Therefore, according to CEDD Geoguide 7 (2008) requirement, a 0.35-0.45 0.35 range of water/cement ratio is common for use in soil nail grouting. In some cases, Water Reducing educing Admixture Figure 37 Grouting process are added into the grout to provide a higher strength of cement grout in order to reduce the water content. In the grouting process, it must be ensured that the soil nail does not have any void or gap in the grouted column. Hence, the grout should be injected through the grout tube which was previously fixed ed to the steel bar bar. The grout should be delivered with a low pressure pump to the grout tube from the bottom of the drilled hole until the drilled hole fully contains cement grout (Geoguide 7, 2008). This will ensure that the grout evenly and completely fills the hole from bottom to surfac surfacee without any air voids. To make sure the quality of grout met the specification requirement, requirement some tests for cement grout should be carried out in the grouting process. The tests test will be described in the section on Testing on soil nail.

30

Application of Soil Nailing for slope stability purpose

4.4.5 Procedure of pull--out test for nail sample The installation procedure of the pull-out test nail is similar to a general soil nail construction procedure. However, a pull-out test nail needs to provide a free bonded length.. Test nails require partial grouting of the nail to develop a bonded length. Therefore, in the grouting stage stage, the grout should applied to the bonded length only. For the purpose of the test test,, packer should be provided to limit the grout material flow to the free-bonded bonded length. 4.4.6 Soil ail Head This part of the structure provides the soil bearing strength and transfers bearing loads from soil mass to soil nail. Usually the size of soil nail head about 400mm x 400mm and min depth is 250mm. In the soil nail system, there are two type of soil nail head: Exposed soil nail head and buried soil nail head.

Figure 38 Excavated soil nail head

Exposed soil nail heads are located on the slope surface and buried soil nail head are buried inside the slope surface. Therefore, excavation for soil nail heads is necessary with buried option. option The soil nail

head mainly comprises of three components, the bearing plate,, nuts and steel reinforcement. The purpose of the bearing plate is to distribute the force at the nail end to the whole nail head and the steel reinforcement prevents the shrinkage and crack of soil nail. Usually the steel reinforcement for the soil nail head is 16mm diameter mild steel. For the concreting of soil nail head, There are two common methods.. One is shotcrete and the other one is traditional ready mix cconcreting. oncreting. Which method will be used depends on the height of slope, angle of slope, accessibility for ready mix track and material delivery. Both method methods of soil nail head production shall meet the requirements of specification specifications through a compressive pressive strength test. Moreover, steel bearing plates are placed at the half stage of shotcrete process to avoid a gap behind the soil nail head. Therefore, bearing plate plates should be installed when the shotcrete reaches the half depth mark of the soil nail head.

Figure 39 Typical buried soil nail head

Figure 40 Shotcreting soil nail head 31

Figure 41 Typical detail of soil nail and soil nail head

(Hong Kong CEDD standard drawing, 2008 )

Application of Soil Nailing for slope stability purpose

32

Application of Soil Nailing for slope stability purpose

4.5

Quality Specification

Specifications are an explicit set of requirements and standards to be satisfied , set by design criteria on soil nail installation. It must be ensured that the soil nail construction is up to the technical standards and met the quality requirements. According to CEDD General Specification vol 2 (1992) and Australia standard R64 (2007), The main specifications in soil nail installation work are briefly summarised as following:


The tolerances on drilled hole diameters are not in excess of 10mm with minimum thickness of grout cover being 30mm at all locations. (R64, 2007)




The depth of the drilled hole shall not be in excess of 100mm of the designed depth(CEDD GS Vol2, 1992)




Maximum offset to the marked location not excess 100mm vertically and 300mm horizontally. (CEDD GS Vol2, 1992)




The tolerances on outside diameter of centralizers on steel bar shall be within 5mm. (CEDD GS Vol2, 1992)




The spacing of the centralisers shall not be excess 1.5m c/c in Hong Kong (CEDD GS Vol2, 1992) standard and 2m c/c in the Australian standard.




Soil nail installation and grouting shall be carried out within 24 hours after the holes are drilled (Australian standard), or four days after the holes are drilled (Hong Kong standard) (CEDD GS Vol2, 1992)




Water used in grouting shall be clean and free from oil, acids, alkali, organic or vegetable matter and from any ingredients harmful to steel or cement grout.




Water temperature used in grout shall be measured at mixer and shall not be less than 5OC and not more than 27 OC







(CEDD GS Vol2, 1992)

Cement grout shall be passed through a 2.36mm sieve aperture. The Grout shall be used as soon as possible after mixing and within 30 minutes of adding cement. (CEDD GS Vol2, 1992)

33

Application of Soil Nailing for slope stability purpose

4.6 Testing on Soil ails According to CEDD CS2 (1992) noted that different tests are needed to carry out during the soil nail installation stage. These different tests have a main purpose of ensuring that the quality of work is satisfactory in terms of the standard requirements. In the whole soil nail construction process, the following tests must be carried out for quality assurance. 

Soil nail steel bar – tensile test, bending test, re-bend test, galvanizing test.

 

Cement Grout – Bleeding test, flow cone test, compressive strength test. Soil nail – Pull-out test (suitability test ), Proving test (acceptance test)

4.6.1 Soil nail steel bar The soil nail steel bar is the most essential element of soil nail system and withstands tension, bending and shear force in soil mass. Therefore, all

Figure 42 Steel bar test sample pieces

ranges of testing are needed to be carried out before soil nail installation. Usually, take five samples of a 1 meter length from stock of soil nail steel bar for test pieces.

Tensile test Tensile strength, yield stress and elongation are given out in these tests. According to Hong Kong Construction Standard 2 (1995), the tensile stress shall be at least 10% greater than the actual yield stress measured in tensile test. The acceptable elongation shall not be over 12% of 5 time diameter of test piece in high stress steel




bar. (CEDD CS2, 1995) Bending Test This is the test for bending of a steel bar to meet the bending requirements. According to CEDD CS2 (1995) noted that the test piece shall withstand being bent through 180o around a former of a specific diameter. The test specimens shall satisfy the requirement which states that no sign of cracks on visual examination are evident.


Re-bend test This test is for bending in opposite direction after same process of bending test and acceptable require the test specimens shall not break into two pieces.


Galvanizing test This is the measure of the content of galvanized material which has been painted or spread on the steel bar (CEDD CS2, 1995). The galvanized material can prevent the corrosion of the steel bar which may be caused by ground water or saturated soil.




34

Application of Soil Nailing for slope stability purpose

4.6.2 Cement Grout Cement grout is the surrounding material of the soil nail which can protect the steel bar against ground water and transfer the frictional force from soil to steel bar. Therefore, cement grout quality concrete is essential. Bleeding Test - This is the measure of water bleeding from cement grout. According to CEDD CS2 (1992) & AS R64 mentioned that bleeding shall not exceed 1% of volume at 60 minutes after mixing when measured at 20OC temperate in a covered 100mm diameter cylinder. Figure 43 Bleeding test The bleeding rate is dependent on the o humidity and temperature. Therefore, a 20 C constant temperate and covered cylinder are necessary for this test. Moreover, vibration should be avoided during the period of test. Flow Cone Test – According to ASTM C939 – (2002) noted that this test is used to determine the fluidity, or viscosity, of the grout. The fluidity is an indication of how well the grout mix will flow when it is pumped into the grout tube. According to CEDD GS vol2 (1995), the grout mix should pass through this flow cone in at least 15 seconds but should not exceed 30 second. Grout mix that is too thick or too viscous may not be able to be pushed through the length of the tendon, and if the grout mix that is too thin means the grout may contain too much water and affect the water/cement ratio. This test is Figure 44

Flow cone test

typically required to be run twice every two hours or randomly during grouting operations

Figure 45 Typical section of flow cone test equipment (ASTM C939, 2002) 35

36

( Maunsell Geotechnical services Ltd , 2008 )

Figure 46 Typical sample record sheet for Bleeding Test and Flow Cone Test

Application of Soil Nailing for slope stability purpose

Application of Soil Nailing for slope stability purpose

Compressive strength test

quare cement grout cube Figure 47 Square

Through the compressive strength test test,, the specimen sample of cement grout can provide an indication of compressive strength of the material which provides an indication of the mechanical and durability properties, in order to meet the soil nail grout requirement. The specimen sample can be a 100mm sided square cube or 100mm mm dia x 250mm long

Figure 48 Compressive strength test

cylinder cylinder. According to CEDD General Specification vol.2 (1992 1992) and Australia standard R64(2007) (2007) required that six samples are used for square cube samples and three cylinder sample samples.. The compressive strength is calculated from the failure load divided by cross cross-sectional sectional area resisting the load and reported in force per unit area.

4.6.3 Soil nails Soil nail tests are carried out after the completed grouting stage and before the construction of the soil nail head stage stage. According to CEDD CS2 (1992) described that the soil oil nail test involved two type types of tests. One is pull-out out test (also call suitability test ) and the other one is proving test (also call acceptance cceptance test). Both of these tests involve applying ing a force which is trying to pull the soil nail out of the slope and measure the degree of resistance with the soil mass. est on soil nail which is taken to Pull-out test – Test failure to allow the measurement of the ultimate bond strength at the soil mass interface. This test is a destroyable test method because the soil nail steel bar and grout are at failure at the ultimate load. According to CEDD GS vol2 (1995) noted that the t Figure 49

Pull out test

maximum test load should not exceed 90% of the steel bar ultimate tensile capacity, in order to avoid

any accident by sudden failure of steel bar. Therefore, the rate of load application shall be in the range of 33-5kN/minute minute until this 37

Application of Soil Nailing for slope stability purpose

final load is reached. Usually, the period of pull pull-out contains a 3 cycle period. Each cycle’s designed esigned load depend depends on the percentage of maximum working load. For example, for the maximum test load, if the factor of safety for pull pull-out out is 1.5, then the test load must be at 150% of the allowable pull out capacity. Each ach cycle should be hold about 60mins for observed the displacement. Also, the test should be carried out in stages that should not exceed 20% of the maximum load and at each stage one should record the overall displacement at 1-5 min intervals for at least east 30mins to 60mins. When the test reaches the maximum load, it should be unloaded in three intervals. Based on CEDD GS2 (1995) & AS R64(2007),t R64(2007),there are some criteria in pull-out test:  A load test should not be initiated before the grout reaches minimum compression strength of 25MPa in 3 days days, which is established in a compressive strength test series with a minimum of three samples. (CEDD GS vol2, 1995)  A sudden failure of steel bar can cause serious accidents and should be avoided. Therefore, a safe distance for operation from test setup and barriers should be provided.  If the soil nail steel bar are connected together using coupler. It should be ensured that the connect connection of steel bar is secure to avoid the failure of the connection. Displacement Measurement Ortigao, (2004) mentioned that two wo or three gauges are necessary to measure the displacement of soil nail under loading. They are positioned along the axis of the measurements and at the bottom of pull-out pull equipment which reduced the affect of soil compression displacement during pull-out out operation. Figure 50

Dial Gauge

Acceptance criteria 1. Measured displacement stabilises under the maximum test load 2. The test result graph tension load VS displacement are within the range of acceptance range. (CEDD GS vol2, 1995) Pull out test on soil nails are taken up to failure in soil. Therefore, This test also can find out the soil geometry of failure friction. The failure friction (qs) is calculated by


 

 

Eq. (4.1)

where D = soil nail hole diameter, Lb= bonded length , Tf= Failure tension load 38

Application of Soil Nailing for slope stability purpose

Figure 51

Typical sample data sheet for Pull out test (Maunsell Geotechnical services Ltd , 2008)

39

Figure 52 Typical sample plotting sheet for pull out test (Maunsell Geotechnical services Ltd ,2008)

Application of Soil Nailing for slope stability purpose

40

Application of Soil Nailing for slope stability purpose

Proofing Test The method and equipment is similar to the pull-out test but the proofing test is not a destroyable test method. The proofing test is used to ascertain the function of soil nail and prove the soil nail conditions have not changed after construction. This test indicates that the completed soil nail can safely withstand the design loads without any excessive movement or long term creep over its service life. This test is a single cycle test in which the load is applied in increments to a test load. According to CEDD General Specification vol.2, 1992 and Australia standard R64(2007), the design test load should be 150% of the design load capacity and rate of load application shall be in range of 3-5kN/min (same as pull out test). At the maximum test load, the period of observation shall be 60 min for displacement measurement and elongation measurement.

Figure 53

Typical sample data sheet for proving test

(Maunsell Geotechnical services Ltd, 2008) 41

Figure 54 Typical sample plotting sheet for Proving test (Maunsell Geotechnical services Ltd ,2008)

Application of Soil Nailing for slope stability purpose

42

Application of Soil Nailing for slope stability purpose

4.7

Other Type of Soil nail installation techniques

There are different methods of soil nail installation which are used internationally. The common method is the drilled and grouted soil nail method as previously described. The following list will briefly introduce the other methods of soil nail installation. 

Drilled and grouted soil nails method These are approximately 100mm and 150mm diameter nail holes drilled into the slope. The space of the holes typically about 1.5m to 2m, and they are arranged in a staggered pattern on the slope. Steel bars are placed in the center of the holes and use the grouting method to grout the hole (Ortigao, 2004). This method is most commonly used in Hong Kong soil nail construction projects and also for Australian soil nail construction. This method can be used as a temporary and permanent application. Also this method is the most mature technology of soil nail method.



Driven soil nail method According to United States Federal Highway Administration (2006) described that these install method are relatively small in diameter and are mechanically driven into the slope. They are usually spaced approximately 1 to 1.2m apart. This method allows for a faster installation when compared with the drill and grout method. However, This method is not able to provide good corrosion protection. Furthermore, this method cannot be used in narrow construction sites. Therefore, driven nails are only used in the United States for temporary applications. Permanent soil nail cannot be used in this method.



Self-drilling soil nail method Based of Oliver Freudenreich, (2008) described that these soil nails consist of hollow bars that can be drilled and grouted in one operation. In this method, the grout is injected through the hollow bar and drilling takes place at the same time. Therefore, the grout will fill the void from top to bottom of the drill hole. Rotary percussive drilling techniques which are mentioned in table 3 are used with this method. This method allows for a faster installation which compared with drill and grout Figure 55 Self drilling (Dipl.-Wirt.Ing, 2008)

method. Unlike with driven method, some level of corrosion protection is provided. However, this method is similar to the driven method, in that it

43

Application of Soil Nailing for slope stability purpose

cannot be used with permanent soil nails. This method is commonly used for temporary nails. 

Jet grouted soil nail method Ortigao, (2004) introduced that jet grouting is used to erode the ground and allow the hole for the nail and reach the final location. After that, a vibro-percussion drilling method is used to installed the nail bar. However, if the soil is high in plasticity, or Figure 56 Jet grouting (Dipl.-Wirt.Ing, 2008)



clay or bounder inside the slope, this method cannot be used.

Launched soil nail method As introduced by Soil nail launcher Ltd.,(2008), this method involves the launching of the soil nail bar into the slope in a very high speed manner which uses a firing mechanism machine powered by compressed air. Usually, the launch bar diameters are around 19mm to 25mm and up to 8m in length. This method allows for a fast installation with little impact to project site. However, with this method, it may be difficult to control the length of launched nail inside the slope. Also, this method cannot be used in highly plastic clay material. Therefore, this method is only used for temporary nails and widely used for road repair and railroad-related landslides in Figure 57 Soil nail launch machine (soil nail launcher Ltd. , 2008 )

the United Kingdom and Western Europe. (http://soilnaillauncher.com/dnn/ )

44

Application of Soil Nailing for slope stability purpose

5.0

Design of Soil ails

5.1 Concept of Factor of Safety Based on Cornforth, (2005) discussed that in the limit equilibrium design approach, shear strength, pore water pressure, slope geometry and other soil and slope properties are established in soil mechanics calculations. In slope stability calculations, the result needs to obviously ensure that the resisting forces are greater than the force tending to cause slope failure. According to Liu.(2008) defined that the ratio between these relationships is called the factory of safety (FOS) (Liu.2008). In circular slip plate method, FOS is defined as the ratio of total resisting forces to total disturbing forces or total resisting moment. In general, the lower the quality of the site investigation, the higher the value of the FOS, since the degree of risk is influenced by previous experiences. Therefore, the actual magnitude of FOS used in design will vary with requirement of material type and performance.

There are three typical of FOS definitions due to different type of analysis method

τf

FOS =

τ required

FOS =

Eq (5.1)

(Total Stress)

c '+ σ ' tan φ '

τ required

(Effective stress)

Eq (5.2)

FOS =

Summation of resisting force Summation of mobilized force

FOS =

Resisting moment Overturning moment

FOS =

R ∫τ fds Wx

Eq (5.3)

45

Application of Soil Nailing for slope stability purpose

5.2 Introduce the Circular slip and Method of slices 5.2.1 Circular slip analysis method According to Liu.(2008) described, this method is the simplest circular analysis based on assumption that a cylindrical block will fail by rotation about its circular center and the shear strength along the failure plate is defined in an undrained condition. Therefore, under this undrained assumption, the friction angle is assumed to be zero ( Φu=0). The FOS for this method may be analysed by taking the ratio of resisting moment and overturning moment about the center of the circular failure plate. The FOS for this method may be described according this equation : Since Φu=0 , Therefore, Cu=τf

FOS =

τf × L × R W χ + PS − Pw1d − Pw 2 b

Eq (5.4)

If water pressure is below the toe of slip plate, the equation can simplified as follows Figure 58 Circular slip model (Liu.(2008) FOS =

τf × L × R Wχ

Eq (5.5)

where τf = undrained shear strength , R= radius of circular slip surface , W= Weight of sliding mass

χ =Horizontal distance between circle center , and O = center of the sliding mass However , Liu.(2008) mentioned that in some case , when Φu>0, this method is not suitable for analysis in this situation because it is more complicated. Therefore, the method of slices shall be used when Φ is not equal to zero.

46

Application of Soil Nailing for slope stability purpose

5.2.2 Method of slices (“Ordinary” method or “Swedish” method) From the above paragraph, the circular slip method is only applicable for the undrained condition and when the friction angle is to be zero. However, if the strength for cohesive soil and friction angle are to be calculated during site investigation, lab test, etc., the distribution of the effective normal stresses along the failure surface must be known. According to Liu. (2008) stated that this analysis is usually carried out by discretizing the mass of failure slope into smaller slices and treating each individual slice as a unique sliding block. This method is used by most computer programs and many different types of program methods have been developed, such as Janbu’s method and Morgenstern-Price method, etc. However, in its general form, it is a complex method and therefore many procedures have been proposed to simplify. Hence, for hand calculation purposes, the Swedish method of slices will be used in the case study and compared with computer program results in this report and based on Krahn,(2004) defined that the interslice forces are assumed to be zero. In the Swedish method of slices, it is assumed that the FOS value is the ratio of resisting moment to disturbing moment. Any moments are taken around the centre of slip circle plate. The equations of the FOS for this method are as follows:

total stress analysis :

F

Effective stress analysis :

F

∑  !( .  ∝ . ∅ ) ∑  !  .∝ ∑  !( ".  ∝ #$ . ∅" ) ∑  !  .∝

Eq (5.6) Eq( 5.7)

O 7 6 5 4 3 1

1

Figure 59

Swedish Method Model

This method does have several advantages such as different soil layers, water pressure and surcharges can be readily taken into account in the calculations. The distribution of forces around the failure surface is defined and the solution is in equilibrium for the assumed interslice behaviour. However, according to Krahn, (2004) described that this method is only the simplistic method for hand calculations, as the interslice forces are ignored. The slice weight is only resolved into forces which are parallel and perpendicular to the slice base. Therefore, slope analysis may be not accurate and not the most efficient in soil nail design calculation. In

47

Application of Soil Nailing for slope stability purpose

engineering industry, because of the unrealistic factors of safety and consequently should not be used in geotechnical firm.

5.3 Soil nail calculation method Soil nail calculation is an independent part of slope stability improvement design. The result of the calculation is the only suitable soil nail parameter and the FOS of internal stability. Over all stability analysis of soil nail applied slope need to be calculated for final stability checking. Australian Standard AS4678-2002 Appendix C provide the accepted standards for soil nail design procedure. From the design, we can determine the length of soil nail required, bar size, inclination angle and soil nail head size. All soil property data is collected from ground investigations. For the industry based design, a trial and error method will be used for checking the tensile stress of steel bar, the bond failure between grout and steel bar and shear failure of soil nail. The calculation result is based on the FOS in the internal stability analysis. The objective of the internal stability analysis is to ensure that for any failure mechanism the outward thrust of the soil within the failure zone is balanced by the tensile restraint of the soil nail. FOS= Available Force / Required Force In the procedure of soil nail design, slope parameter will be applied according to critical section of the slope. If the slope has different critical criteria, then the design will contain different critical sections of design. Thus, the slope will be separated into different zones in terms of soil nail parameters. In soil nail design, based on Hausmann (1992) and MGSL Ltd (2006) the following equations are noted. Maximum allowable tensile force of steel bar:

Ta = (Φfy) (d - 4)2 × π / 4

Eq ( 5.8)

where Φ = stress reduction factor according to AS 4100:1998 , fy= Yield stress of steel bar. d= diameter of steel bar

48

Application of Soil Nailing for slope stability purpose

Maximum allowable force between steel & grout:

[ β (fcu)1/2 ] × p × (d - 4) × Le / SF

Eq (5.9)

where β = 0.5 for type 2 deformed bars , fcu = cube strength of the cement grout at 7 days = 32MPa SF = factor of safety adopted , Le = effective bond length (grout length) Maximum allowable force between soil & grout:

[(πD C' + 2D Kα σν' tanΦ) Le] / SF where

Eq ( 5.10)

D = diameter of the drill hole, C’= effective cohesion of the soil Kα=coefficient of lateral pressure(α=Inclination angle) = 1-(α/90)(1-Ko)=1-(α/90)(sinΦ) σν'=theoritical vertical stress in soil calculated at mean depth of reinforcement Φ = friction angle

49

Application of Soil Nailing for slope stability purpose

5.4 Analysis of slope stability with soil nail element When the soil nail is applied into the slope, theoretically the slope is stabilised. According to Abramson (2002), the instability force will transmitted across the interslice boundaries through the soil nail element. Therefore, all forces acting on the interslice (including the forces in the soil and water and the reinforcement forces transmitted through the interslice boundary) are represented by a total interslice force (Abramson, 2002). Then the element of the Factor of Safety that is the soil and water force acting on an interslice and a designed soil nail reinforcement force can be determined. Bromhead, (1992) mentioned that the soil and water force acting on an interslice is calculated by subtracting the designed soil nail reinforcement force from the calculated total interslice force. Another way to interpret the relationship of forces in the interslice during the calculation of the factor of safety is to consider the soil and water force acted on the interslice and the reinforcement force acted on the interslice separately (Bromhead, 1992). However, for slope analysis purposes, this analysis is complex and difficult to carry out by hand. Therefore, computer software will be applied for slope reinforced analysis.

5.5 Slope stability analysis computer program - Slope/W Slope/W is the computer software for geotechnical analysis which was developed by GEO-SLOPE International Ltd. According to Krahn, (2004) introduced that there are many methods of analysis which are based on general limit equilibrium methods such as the ordinary method, bishop method, Janbu’s method, spencer method and Morgenstern-price method etc. For industry base, Morgenstern-price method is the most common method for slope analysis. However, based on Krahn (2004) described that the ordinary method of slices is only used for teaching purposes and could not be used in practice due to potential unrealistic FOS values. Morgenstern-price’s method will be used for case study design.

Therefore,

Morgenstern-Price’s method this method was developed and improved by Morgenstern and Price (1965, 1967). According to Ortigao (2004) introduced that the essence of the method is to divide the sliding mass into a relatively small number of linear sections or wedges which are vertical-sided in the conventional way. Within each of these sections, Krahn (2004) explained that interslice forces are considered and the conditions of force equilibrium can be satisfied taking directions normal and parallel to slip surface. Compared with other method, Morgenstern-Price’s method is the closest to the equilibrium approach. Therefore, this method will be used in soil nail design in order to form an economic and efficiently design . 50

Application of Soil Nailing for slope stability purpose

5.6 Design parameters In soil nail calculation and SLOPE/W, soil properties and conditions are the essential components for both of these calculations. Therefore, site investigation and visual inspection are needed for design data collection purposes. Site investigation: Boreholes - Rotary drilling methods are commonly used in slope site investigation. Through the soil data logging, a description of the soil and properties such as soil type, colour, consistency and soil structure are determined. This also can provide an un-disturbed sample for a triaxial test. Ground water levels can also be measured in the boreholes recorded. In-situ test – A standard penetration test or cone penetration test is used to define the relative density of the soil and relative strength. Other in-situ test such as vane shear test and pressuremeter test may be used which vary depending on the soil type and data collection. Laboratory test – Tri-axial tests are more commonly used in ground investigation which determine the soil friction angle value and cohesive value. Moisture content tests are commonly used in Australia which can provide an easy and economical method to estimate the soil type and property through common practice. Visual inspection: Surface Stripping – This is a commonly used method in visual inspection which removes a narrow strip on the slope surface. This is an easy method for determining the skip layer of soil property. Slope geometry - Slope height and slope angle should be measured and this data can be used to model the slope profile in SLOPE/W and soil nail design. Other data

-

Such as surcharge, utilities, slope surface seepage and tension cracks etc. These uncertain data can influence the accuracy of the design and affect the design assumption.

51

Application of Soil Nailing for slope stability purpose

5.7 Design procedure The following procedure will be used for soil nail slope improvement design. 1. Design parameter collection 2. Geotechnical assessment, modeling of the slope profile according to the design parameters 3. Use SLOPE/W program to analyse the critical section Factory of Safety 4. If the analysis is not satisfactory in terms of the required FOS, use trial and error method to determine the failure-resisting force until the slope analysis is satisfactory the required FOS. 5. Use soil nail calculation methods are used to determine the size of steel bar, inclination angle and horizontal and vertical spacing required, the bond length and size of the soil nail head. Also, check the maximum allowable tensile force, max. allowable bond stress and total force mobilised which needs to met the FOS requirements. Use a trial and error method to determine the most efficient design. 6. Input the designed parameter of soil nail into SLOPE/W software to re-analyse the most critical of factory of safety.

52

Application of Soil Nailing for slope stability purpose

6.0

Case Study

Introduction This part of the case study will represent a sample of soil nail application for slope stability improvement. Slope/W software is used as well as the soil nail calculation method to briefly design the soil nail. The first part of the analysis will choose one of Hong Kong Cut slope which is of a high Consequence-to-life Category (Cat 1) . The other part of case study will choose one of Australia Cut slope which also is of a high risk category of Consequence-to-life. Because the type of soil properties in the two geological different areas vary, Case study (Hong Kong) will use Geoguide standard and Case study (Australia) will use Australia Standard AS 4678-2002.

6.1 Case Study Analysis (Hong Kong) There are 3 sample cut slopes which have been selected for this case study. Comparing these 3 cut slopes, the most high risk for slope failure is located next to a sports centre. If slope failure were to occur, soil mass may flow into sport centre and may damage the building structure. The worst case scenario would involve the whole building collapsing. Therefore, I choose this slope for slope stability analysis. Table 4 Compare Consequence-to-life Category Crest facility – Un-development green belt Toe facility – playground , Pavilion

Category group 3 Consequence-to-life Category

2

Middle risk Crest facility - Un-development green belt Toe facility - road with heavy vehicular or pedestrian traffic density

Category group 2b Consequence-to-life Category

2

Middle risk Crest facility – Main Access road Toe facility - indoor games or sport hall

Category group 2a Consequence-to-life Category

1

High risk

53

Application of Soil Nailing for slope stability purpose

Slope Background The selected slope is a soil cut slope which is located at next to sports centre. According to background information from the previous study (CEDD, 1993), the caption slope was formed in 1975 by cutting in association with the opening of access road along crest . Figure 60

General View of slope

Site description This cut slope is located at east of Shek Kip Mei Sports Centre. The slope is about 80m long with a maximum height of 12m. This slope has divided into two portions, an upper portion and lower portion which are separated by a berm. The slope angle in upper portion is approximate 60o and lower portion is approximate 55o. The slope is covered with a vegetation surface which provided a minor surface improvement. The crest facility is low a traffic road which is the main access of the sports centre. The toe facility is an indoor sport hall name Shek Kip Mei Park Sports Centre which is located approximately 5m away from the slope toe.

Visual Inspection The site inspection on the caption slope was carried out in July 2008. During site observation, no seepage or leakage was observed on the slope or surrounding area. The slope has been divided into two batters by a one meter wide berm. The slope is covered with a vegetation surface and no surface erosion has occurred. The slope appears to be in good condition and no adverse signs of distress were observed. Surface channels were found at the berm and toe of slope. The drainage condition appears to be in good condition.

Ground Investigation During the desk study stage, there were three previous ground investigations which were carried out. One in 1984 , one in 1993 and the other in 2005. This previous records are open to the public, as it is able to be accessed at the CEDD Geotechnical Information Unit Library. Combining these investigation records, we got a total of 6 bore holes relative the slope. According to these records, the borehole log had indicated that the slope was composed of completely decomposed granite and highly decomposed granite base on CEDD Geoguide 5 standard. The location of boreholes and borehole log records are shown in appendix A 54

Application of Soil Nailing for slope stability purpose

Laboratory test During the desk study stage, there are previous laboratory tests which have been carried out in 2005 by Gold Ram Engineering and Development Limited. These previous laboratory test reports are open to the public which can collect in CEDD Geotechnical Information Unit Library, in order to obtain soil parameters for further stability assessment and identify the material from ground investigation work. These previous laboratory tests contained information regarding particle size distribution and single stage tri-axial compression tests under undrained conditions. From the result of particle size distribution, the result showed that the completely decomposed granite in the vicinity of the slope was solely composed of sandy materials. From the single-stage triaxial compression tests under undrained conditions. The p’-q plot for completely decomposed granite was generated according to the previous test result which carried out in 2005. The triaxial test results from previous laboratory tests are shown in Appendix B

6.1.1 Geotechnical assessment Critical section According to the site inspection, the minimum distance between slope toe and toe facility had a uniform spacing of around 5 meters and the slope angle has a uniform value of approximately 55-60 degrees. Therefore, the critical section is controlled by the maximum height of 12m. The critical section plan is shown in figure 1. Ground conditions According to the previous borehole log record, From DH1 and DH2, completely decomposed granite (CDG) was found at around 0.04m below the ground. Thus, CDG was found immediately on the slope. The thickness of CDG layer is 14.79m at DH1 and 6.77m at DH2. In the borehole log record, completely decomposed granite is described as extremely weak, brownish yellow, occasionally reddish yellow and brown spotted grey and brown, completely decomposed medium grained granite (silty fine to coarse sand with some angular to subangular fine to medium gravel of granite and quartz) base on Geoguide standard. Groundwater condition According to the borehole record, DH1, DH2, DH3 noted that no groundwater was observed. Therefore, the design groundwater table adopted for this slope stability analysis is to be estimated to be at one-third of the slope height to represent the assumed 1 in 10 year design groundwater table.

55

Application of Soil Nailing for slope stability purpose

Parameter for analysis The soil strength parameters adopted for the stability analysis and soil nail design are based on the laboratory test results of the consolidated undrained single stage triaxial tests on the soil samples. From the results of the p’-q plot in table 6.1, the c’ =0kpa and φ’ =38o. However, according to CEDD Geoguide 1 standard, the recommended minimum value of cohesion for Completely Decomposed Granite (CDG) is 5kPa. Therefore, c’ =5 kpa is recommended in caption slope. The shear strength parameters adopted in the stability analysis for the caption slope are shown as follows: Table 5 Design parameter Soil Type

Unit weight γ’ KN/M3

Cohesion c’ (kPa)

Friction Angle φ’

CDG

20

5

38

500 450 400

q (kPa)

350 300 250 200

DH1

150

DH3

100 50 0 0

100

200

300

400

500

600

700

800

P' (kPa)

Figure 61 p – q plot graph ( Gold Ram Engineering and Development Limited., 2005) Design assumptions  Based on CEDD Geoguide 1 , the ground water table is assumed as a 1 in

 

10 year rainfall intensity case and the groundwater table is assumed to be at 1/3 of the slope height. The surcharge of the crest access road is assumed to be a 20kPa uniform load. Because of a minor highly decomposed granite (HDG) soil layer at the bottom of CDG, c’ and φ’ are assumed to be 5 and 42 respectively

56

Application of Soil Nailing for slope stability purpose

Figure 62 : Slope location plan & Bore hole location

57

Figure 63 Critical Cross Section A-A

Application of Soil Nailing for slope stability purpose

58

Application of Soil Nailing for slope stability purpose

6.1.2 Slope/W Stability Analysis The minimum FOS of slip surface is generated under Morgenstern and Price analysis using SLOPE/W software. The soil layer distribution is made up by previous borehole log records DH1, DH2, and BH6. Other assumptions are according to design assumptions. All design assumptions and soil properties are according to CEDD Geoguide 7 (2008) standards.

Figure 64 Critical Slip surface The critical slip surface is generated under automatic Grid and radius generation, because sliding may occur along any number of possible surfaces. Therefore, computer generated numbers of slip are used to find out the minimum FOS which is recommended. The minimum Factor of safety (FOS) at section A-A obtained are as following table: Table 6 Section A-A FOS result Section Minimum Factor of safety (FOS)

A-A 0.986 In the result, the minimum FOS for soil slope at section A-A does not meet the minimum requirement of 1.4 (according to the Geoguide 7 (2008) standard) for the caption slope having a Consequence-to-Life Category 1. Therefore, further slope stability improvement work is necessary.

59

Application of Soil Nailing for slope stability purpose

6.1.3 Hand calculation using Swedish Method of Slices According to the Morgenstern and Price analysis using SLOPE/W software, critical slip surface plates are generated and the circular arc, centre and radius are computed. In order to compare the FOS with Morgenstern and Price analysis, hand calculation for the method of slices is also carried out to present the basic theory of slope stability analysis for the same slip. This method has assumed that the slip wedge is divided by vertical planes into a series of slices of a certain width. The base of each slice is assumed to be a straight line. For any slice the inclination of base to horizontal is α. In order to make the result more accurate and consistent with mechanics, the slope will be divided into 30 slices and the arc length and inclination angle of each slice is measured. On the other hand, because the ground water table is below the slip surface, no pore water pressure will affect the slope and the boundary water force can be ignored in the method of slices equation.

w

N

Figure 65

T

Swedish Method of Slices analysis 60

Application of Soil Nailing for slope stability purpose

Swedish Method of Slices result The following table lists the results of the hand calculation method and compares with Slope/W calculation results using Morgenstern and Price analysis. Table 7 Two methods FOS result comparison table Morgenstern and Price method Factor of safety (FOS)

Swedish Method of Slices method

0.986

0.802

From the table 6.3, the FOS is not consistent. There is about an 18% difference between each method. It is because in the Method of Slices all interslice forces are ignored. Also, this method is only for c’=0. Therefore, some errors may occur in this analysis. According to Krahn, (2004) described that, usually the error arrange is within the range 5-20% compare with Morgenstern and Price method. The calculation spreadsheet is shown in following table: Table 8 Swedish Method of Slices Calculation Spreadsheet Soil Unit weight of soil=20

Soil friction angle Φ =38

Water table below distance below ground, Slice

Arc

Weight

Angle

No

Length

(W)

α , degree

1

1.04

4.04

2

1.03

3

Tan Φ =0.78128

no pore water pressure

Cos α

Sin α

N=W*cos(α) T=W*sin(α)

N*tan(Φ)

66.00

0.41

0.91

1.64

3.69

1.28

12.11

63.00

0.45

0.89

5.50

10.79

4.29

1.03

20.18

60.00

0.50

0.87

10.09

17.48

7.88

4

0.79

27.71

58.00

0.53

0.85

14.69

23.50

11.47

5

0.69

29.63

56.00

0.56

0.83

16.57

24.57

12.95

6

0.58

26.51

54.00

0.59

0.81

15.58

21.45

12.17

7

0.58

26.30

53.00

0.60

0.80

15.83

21.01

12.37

8

0.52

28.44

51.00

0.63

0.78

17.90

22.10

13.98

9

0.52

27.05

50.00

0.64

0.77

17.38

20.72

13.58

10

0.52

25.65

49.00

0.66

0.75

16.83

19.36

13.15

11

0.52

24.26

47.00

0.68

0.73

16.55

17.74

12.93

12

0.52

22.87

46.00

0.69

0.72

15.89

16.45

12.41

13

0.52

21.48

44.00

0.72

0.69

15.45

14.92

12.07

14

0.52

20.09

43.00

0.73

0.68

14.69

13.70

11.48

15

0.48

18.23

42.00

0.74

0.67

13.55

12.20

10.59

61

Application of Soil Nailing for slope stability purpose 16

0.48

16.49

40.00

0.77

0.64

12.63

10.60

9.87

17

0.46

15.98

39.00

0.78

0.63

12.41

10.05

9.70

18

0.46

18.06

38.00

0.79

0.62

14.23

11.12

11.12

19

0.11

4.70

37.00

0.80

0.60

3.75

2.83

2.93

20

0.51

24.07

36.00

0.81

0.59

19.47

14.15

15.21

21

0.51

26.55

35.00

0.82

0.57

21.75

15.23

16.99

22

0.51

26.16

33.00

0.84

0.54

21.94

14.25

17.14

23

0.51

22.78

32.00

0.85

0.53

19.32

12.07

15.09

24

0.51

19.39

31.00

0.86

0.52

16.62

9.99

12.99

25

0.46

14.40

30.00

0.87

0.50

12.47

7.20

9.75

26

0.46

11.78

28.00

0.88

0.47

10.40

5.53

8.13

27

0.46

9.17

27.00

0.89

0.45

8.17

4.16

6.38

28

0.46

6.55

26.00

0.90

0.44

5.88

2.87

4.60

29

0.46

3.93

25.00

0.91

0.42

3.56

1.66

2.78

30

0.46

1.31

24.00

0.91

0.41

1.20

0.53

0.93

Σ

381.90

306.22

F= Σ(N)*tan(φ)/ΣT (=w*sin(α))= 0.80182

62

Application of Soil Nailing for slope stability purpose

6.1.4 Estimated Slip Surface At the beginning of the soil nail design procedure, we need to estimate the different shapes of the slip surface. This is because if we use the minimum FOS (0.986) to design the soil nail, the reinforced slope may have another shape of slip surface for which the FOS is smaller than 1.4. For example, if the slip no. 1 FOS is smaller than 1 and the slope is not safe, after installing the soil nail with the bond length is just passing through the slip surface of slip no. 1, after that analysis the reinforced slope of slip 1 and the FOS will rise to meet the requirement. The slip 1 seems safe. However, the soil nail may not be contributing a resisting force to slip no. 2,3,4,5. Therefore the overall slope will still not be safe. Therefore, the slope will typically be distributed over 5 different shapes of slip surface and analysed for all of the FOS values (Slope/W FOS analysis data shown in Appendix C )

Figure 66

Estimated Slip Surface Table 9 FOS results table Slip No

1 2 3 4 5

FOS

1.112 0.986 (minimum) 1.157 1.367 1.572

63

Application of Soil Nailing for slope stability purpose

6.1.5 Soil nail design Soil nail length and bond length prediction From the previous analysis, the minimum FOS is for slip no. 2. The FOS for the other slip nos. 1 and 3 are smaller than the required FOS =1.4. For the soil nail concept prediction, if the soil nail bond length passes through the slip 3 failure plate, the soil nail resisting force is then functional to the soil mass. Therefore, for the same reason, the soil nail is also functional for slip no.1, and 2. On the other hand, avoid the reinforced slope minimum FOS fallback to slip no. 3. Thus, the bond length of soil nail will start from Slip no. 3 failure plate. In the trial length estimation, the soil nail resisting force and soil nail length are estimated and a trial and error approach is used to determine the required FOS value. The position of the soil nail is estimated according the slope profile and slope parameters. A 2m horizontal and vertical spacing with a staggered format is recommended. The first row of soil nails is 2m from ground and third row of soil nail is 1m above the berm. Therefore, 5 rows of soil nails are formed uniformly over the 12m slope height.

Estimated design In the preliminary design, soil nail length is estimated as 8m in length for Row A to Row E. However, when checking for shear failure of the adjacent ground (Bond stress between soil and grout), Row A and Row B do not satisfy the safety requirements. Therefore, the bar length is finally changed to 12m to satisfy the shear failure adjacent ground checking. Table 10

Soil nail parameter Nail Length (m)

Bond Length (m)

Inclination

Nail

Design

Angle (degree)

Spacing (m)

resisting force KN

A

12

3.3

15

2

55

B

12

3.8

15

2

50

C

8

4.3

15

2

20

D

8

4.2

15

2

15

E

8

5.7

15

2

8

Row

64

Application of Soil Nailing for slope stability purpose

Soil nail reinforced slope analysis The soil property and soil profile remain as previously modeled. Three rows of 8m soil nails and two rows of 12m soil nail at inclination 15o are added in SLOPE/W slope profile. Bond length and forces are predicted and applied. The minimum FOS of 5 nos slip surface is generated under the Morgenstern and Price analysis using SLOPE/W software.

Figure 67

Soil nail slope FOS analysis

Slip no.1 introduces some errors during Morgenstern and price analysis. The FOS of slip no.1 cannot be generated in this method. The reason for this problem is the row B soil nail design resisting force is too large and causes a force which pushes the soil mass upward towards the berm. However, from the data sheet in Appendix C the FOS for slip no. 1 calculated by ordinary method the FOS is 3.084. Therefore, FOS for slip no. 1 generated by Morgenstern and price analysis is ignored in this case. Table 11 FOS result (after soil nail installed) Slip No FOS 1 error 2 1.534 1.529 3 (Minimum) 4 1.671 5 1.835

65

Application of Soil Nailing for slope stability purpose

Figure 68 FOS comparison Soil ail detail Calculation In order to calculate the soil nail detail parameter parameters, we need to use a trial and error method to find out the nail bar size size, nail length, bond length,, inclination, inclination spacing and required resisting force. It is recommended to use an EXCEL spreadsheet to compare the parameters and check tthe outcome to achieve the most suitable design. assumptions Soil ails design assumption  Because of the caption slope is location at Hong Kong area area, the he design is based ba 

on BS8110 for the reinforcement bar. At the preliminary design stage, the he angle of inclination of soil nail from horizontal is assumedd to be 15° downward.



For internal mode of failure, the following modes of failure were checked and the safety factors were adopted as follows:

Table 12 Design Assumptions

(b)

Modes of Failure

Min. Factor of Safety

(a) Tensile Failure of the steel bar

fmax = 0.5f 0.5 y

Bond Failure between grout and steel bar (c) Shear Failure of adjacent ground

Figure 69

3 2

Soil nail design section detail 66

Application of Soil Nailing for slope stability purpose

Soil nail design calculation for Section A-A (Final Result) This part investigates the tensile strength of the bar, bond failure between grout & steel bar and bond failure between grout & soil. This step is an essential part of the design to avoid the failure of grout and soil bonding and failure of steel bar and grout bonding. Design Parameters Soil Type = CDG , C’=5kPa , γ=20kN/m3 , φ ' = 38ο Drill Hole diameter = 0.1m Soil Nail inclination angle = 15o Unit weight of water = 9.81kN/m3

Part A - Tension Failure of the Steel Bar Yield stress fy = 460Mpa Maximum tensile stress = 0.5 fy = 230Mpa Maximum allowable tensile force of steel bar Ta = (0.5 fy) (d - 4)2 × π / 4

Eq (5.8)

Force per m Width data is Trial and error from Slope/W Bar length is use trial and error from Slope/W analysis Table 13 Tension Failure of the Steel Bar calculation spreadsheet Max. Bar Length

Force per m

Force

Spacing

Width F

Required

Allowable

Bar Size (d)

Level Row No.

Horizontal

(m)

(mm)

L

D

(mPD)

Ta > Tr Tensile

(m)

(kN)

S

F

=F x S Force

Tr (kN)

Ta (kN)

Check

Row E

58.80

8.0

25

2.0

8.00

16.00

79.66

O.K.

Row D

56.80

8.0

25

2.0

15.00

30.00

79.66

O.K.

Row C

54.80

8.0

25

2.0

20.00

40.00

79.66

O.K.

Row B

52.80

12.0

32

2.0

50.00

100.00

141.62

O.K.

Row A

50.80

12.0

32

2.0

55.00

110.00

141.62

O.K.

67

Application of Soil Nailing for slope stability purpose

Part B - Bond Failure between Grout and Steel Bar Cube strength of cement grout at 28 days fcu = 32Mpa For type 2 deformed bar β = 0.5 Factor of safety adopted SF = 3 Allowable bond stress = Ultimate bond stress / SF = [ β (fcu)1/2 ] / SF = 942.81k /m2 Maximum allowable force between steel and grout = [ β (fcu)1/2 ] × π × (d - 4) × Le / SF Where Le : Effective bond length

Eq ( 5.9)

Table 14 Bond Failure between Grout and Steel Bar calculation spreadsheet Max. Bar

Bar Size Horizontal

Free

Bond

Force

Force per

Allowable

Level Row No.

Length

(d)

(m)

(mm)

(mPD)

Spacing length La length Le m Width Required (m)

(m)

(m)

F

Tmax > Tr

Force

(kN) Tr (kN)

Tmax (kN)

Row E

58.8

8.0

25

2.0

4.70

3.30

8.00

16.00

205.26

O.K.

Row D

56.8

8.0

25

2.0

4.20

3.80

15.00

30.00

236.36

O.K.

Row C

54.8

8.0

25

2.0

3.70

4.30

20.00

40.00

267.46

O.K.

Row B

52.8

12.0

32

2.0

3.80

8.20

50.00

100.00

680.06

O.K.

Row A

50.8

12.0

32

2.0

2.30

9.70

55.00

110.00

804.46

O.K.

Part C - Shear Failure of Adjacent Ground (Bond Failure between Grout and soil) Resisting Zone - for soil nail design Mobilisation Force, Inclination Factor,

Tf = (π π D c' + 2 D Kα α σv' tanφ φ) × Le Kα α / 90) (1 - Kοο) = 1 - (α α / 90) (sinφ φ) α = 1 - (α

Completely Decomposed Granite (CDG) Kα α = 0.897 Tf =(π π D c' + 2 D Kα α σv' tanφ φ) × Le = ( 1.571 + 0.140 σ’v ) × Le σ

Eq( 5.10)

Table 15 Bond Failure between Grout and soil calculation spreadsheet 1 Resisting Zone Depth to mid-point of the effective bond length (m) Row No.

Effective bond length in CDG layer (m) Le

CDG Zone CDG

WATER

Row E

3.30

3.40

0.00

Row D

3.80

5.30

0.00

Row C

4.30

7.20

0.00

Row B Row A

8.20 9.70

9.70 9.40

1.40 3.00

68

Application of Soil Nailing for slope stability purpose

Table 16 Bond Failure between Grout and soil calculation spreadsheet 2 Effective Vertical

Force Mobilised

Stress Row No.

σv (kPa)

Tf (kN)= ( 1.571 +

Total Force

Force

Mobilised

Required

F.O.S.

F.O.S. > 2

0.140 σ’v ) x Le

CDG

CDG

Tf (kN)

Tr (kN)

Tf / Tr

Row E

68.00

36.65

36.65

16.00

2.29

O.K.

Row D

106.00

62.45

62.45

30.00

2.08

O.K.

Row C

144.00

93.58

93.58

40.00

2.34

O.K.

Row B

180.27

220.16

220.16

100.00

2.20

O.K.

Row A

158.57

230.92

230.92

110.00

2.10

O.K.

Table 17 Final Soil Nail ail design schedule table Horizontal Level

Bar Length

Bar Size (d)

(mPD)

(m)

(mm)

Row No.

La

Le

(m)

(m)

Spacing (m)

Force per m

Force

Width F

Required

(kN)

Tr (kN) Row E

58.80

8.0

25

2

4.70

3.30

8.00

16.00

Row D

56.80

8.0

25

2

4.20

3.80

15.00

30.00

Row C

54.80

8.0

25

2

3.70

4.30

20.00

40.00

Row B

52.80

12.0

32

2

3.80

8.20

50.00

100.00

Row A

50.80

12.0

32

2

2.30

9.70

55.00

110.00

6.1.6 Summary In this case ase study assessment, the caption slope was in an unstable state in its initial condition before ore adding soil nail nail. The minimum FOS using SLOPE/W software under Morgenstern and Price rice analysis is 0.986 0.986. This is a smaller than the require FOS 1.4 (According to CEDD Geoguide 7 standard ,2008). After applying ing the soil nails, the FOS of the slope is upgrade upgraded to a minimum of 1.529 which has been increased to meet the slope stability requirement requirements. Table 18 Final result table Section A-A

Before Upgrading

Soil nail applied

Minimum FOS

0.986

1.529

The slope is up to require FOS after soil nail installation. 69

Application of Soil Nailing for slope stability purpose

6.2

Case Study Analysis (Australia)

Introduction In Australia, some cut slopes may be discovered near some railway tracks or on a highway road side area. These cut slopes usually formed when highways and railways are constructed. Some of them are sandstone based original cut slope. Due to the soil property of sandstone, there is a lower landslide hazard in sandstone cut slope. However, some of them consist of weak sandstone, silt clay slope, silt sand slope etc. When these soil properties are in a slope which is formed to a steep angle, there may be a high risk of slope failure. In this case study in Australia, a sample of a silt clay slope is presented for demonstration using soil nail for slope stability purpose. This slope is located at Sydney suburban area - Hurstville, which is on the lllawarra line railway side cut slope. The toe facility is a railway and the crest facility is moderate use traffic road. However, if the slope failure, the railway services may be required to stop, or in the worst case this could cause a train derailment. This may cause a loss of human life, as well as substantial economic losses. Crest facility –road with moderate vehicular traffic

Toe facility –railway Category group 1b Consequence-to-life Category 1 High risk Figure 70

General view of slope

Slope Background

Figure 71 Elevation View

Figure 72 Side View

The selected slope is a soil cut slope which is located at the side of a railway track (S33o57’54.91”, E151o05’40.62”). According to background information from a previous information search (SRC, 2008), the caption slope was formed since 1902

70

Application of Soil Nailing for slope stability purpose

by cutting in for railway work construction to Waterfall by Sydney Railway Company (SRC)

Site description This cut slope is located at north of Railway lllawarra line. The slope is about 50m long with a maximum height of 6m which according to GPS height record. This slope toe contains a 1.5m high solid pile wall. The slope has two different slope angles. The slope angle in upper portion is approximate 45o and lower portion is approximate 50 o. The slope is covered with vegetation and the surface is in good condition. The crest facility consists of a moderate vehicular trafficked road about 3m away from the slope crest. The toe facility is a railway line which located adjacent to the slope toe.

Visual Inspection The site inspection on the caption slope was carried out in October 2008. During the site observation, no seepage or leakage was observed on the slope or surrounding area. The slope is covered with a vegetation surface and no surface erosion occurred. The slope appears to be in good condition and no adverse signs of distress were observed. No surface channels were found around the slope. A 1.2m high solid pile wall was observed at toe of the slope.

Site Investigation Because there was no previous study relative the caption slope, no previous ground investigation record was able to be collected for this case study. All soil layers and soil types are according the assumption under Geo standard AS4678-2002. The soil type identification is under the field excavation test. According to the inspection of excavated disturbed soil sample at slope toe and at top of the slope, the slope surface is loose sand material and about 0.01 m depth is the in-situ original soil. The soil sample is classified as silty clay. (Classification guide shown in Appendix D) The sample collection location is shown in figure 78.

Figure 73 Silty clay at slope toe

Figure 74 Silty clay at slope crest

71

Application of Soil Nailing for slope stability purpose

Penetrometer test In order to provide more precise soil property data, a penetrometer in-situ test was taken on site. The equipment used in this field test was Pocket Penetrometer. The pocket penetrometer is a device used by geotechnical engineers to estimate unconfined compressive strengths of in situ soils. The pocket penetrometer is a spring-loaded penetrometer. The spring is calibrated against unconfined compressive strength (typically measured in kg/cm2). The mark at which the indicator is located is taken as the unconfined compressive strength of the soil.

Figure 75 Pocket Penetrometer

Figure 76 Pocket Penetrometer

This Penetrometer test was taken with the data from 10 relative soil layers. An average value of 10 samples was determined and provided an estimated soil properties. The collected data is shown in following table. Table 19 Hand penetrometer test results Reading kg/cm2

1 kg/cm2 = 100kPa

Test no1 -

2.9kg/cm2

290 kPa

Test no2 -

3.2kg/cm2

320 kPa

Test no3 -

2.5kg/cm2

250 kPa

Test no4 -

3.0kg/cm2

300 kPa

Test no5 -

2.8kg/cm

2

280 kPa

2.5kg/cm

2

250 kPa

2.4kg/cm

2

240 kPa

Test no8 -

3.2kg/cm

2

320 kPa

Test no9 -

2.7kg/cm2

270 kPa

Test no6 Test no7 -

Test no10 -

2.3kg/cm

2

230 kPa 2

From this data, the average value is 2.75 kg/cm . From relative analysis, the cohesion value of soil will be high. Due to a lack of more accurate borehole log information and soil laboratory data, And due to conservative reasons, this case study will use the assumption under standard AS4678-2002 table D4 to predict the typical soil property under the soil type identification 72

Application of Soil Nailing for slope stability purpose

6.2.1 Geotechnical assessment Critical section According to the site inspection, the minimum distance between slope toe and toe facility is of a uniform spacing of approximately 0 meters and the slope angle on the surface is uniformly approximate at 45-50 degree. Therefore, the critical section is controlled by the maximum height of 6m with solider pile wall. The critical section plan is shown in figure 6.11. Ground condition According the site investigation, the soil composition of the slope toe and crest under few centimeters is a silty clay material. Therefore, under these parameters, it is assumed that the soil type for the slope from top to bottom is also a silty clay layer. According to the Australian geological property, it is assumed that the bedrock is a sandstone base. Groundwater condition According to site inspection noted that no groundwater and seepage was observed. Therefore, design groundwater table adopted for slope stability analysis is to be estimated at one-third of the slope height to represent the assumed 1 in 50 year design groundwater table in Australia and water pipe leakage at slope crest. Parameter for analysis The soil strength parameters adopted for the stability analysis and soil nail design are based on the typical soil assumptions for the collected soil samples. According to the soil type identification the soil sample is define as a silty clay material. Hence, In the Australian standard AS4678-2002 table D4 the soil group for silty clay is classified as poor grade. According to the typical soil assumption in Table D4 the soil parameter is assume the c’=0-5 kpa and φ’ =17 - 25o. Based on these assumptions, for this caption slope use c’ =5 kpa and φ’ =25o for the soil design parameter. The unit weight of silty clay are base on the assumption according to AS4678-2002 Table D1. The moist bulk weight of silty clay is 18kN/m3 . Therefore, the shear strength parameters adopted in stability analysis for the caption slope are as following list: Table 20 Design parameter (based on assumption according AS4678-2002 ) Soil Type

Unit weight γ’ kN/m3

Cohesion c’ (kPa)

Friction Angle φ’

Silty Clay

18

5

25

73

Application of Soil Nailing for slope stability purpose

Design assumptions  The ground water table is assumed as a 1 in 10 year rainfall intensity and  

the groundwater table is assumed to be at 1/3 of the slope height. The surcharge of the crest traffic road is assuming 20kPa uniform loading. The slope is assumed to consist of one layer of silty clay soil.

Caption slope

Figure 77 : Slope location plan

Section

A-A

Figure 78 : Sample collection position

74

Figure 79 Critical Cross Section A-A

Application of Soil Nailing for slope stability purpose

75

Application of Soil Nailing for slope stability purpose

6.2.2 Slope/W program Stability Analysis The minimum FOS of slip surface is generated under Morgenstern and Price analysis using SLOPE/W software. The soil layer distribution are made up of soil type estimation from practical experience . All design assumptions and soil properties are based on Australia Standard AS 4678-2002 Table D1 and Table D4.

Figure 80 Critical slip surface The critical slip surfaces are generated under automatic grid and radius generation. Because sliding may occur along any number of possible surfaces, computer generation numbers of slip to find out minimum FOS are recommended. The minimum Factor of safety (FOS) at section A-A obtained are as following table: Table 21 Section A-A FOS result Section

A-A

Minimum Factor of safety (FOS)

1.031

In the result, the minimum FOS for soil slope at section A-A does not meet the minimum requirement of 1.5 for the caption slope. Therefore, further slope stability improvement work is necessary.

76

Application of Soil Nailing for slope stability purpose

6.2.3 Hand calculation using Swedish Method of Slices According to the Morgenstern and Price analysis using SLOPE/W software, critical slip surface plates are generated and the circular arc, centre and radius are computed. In order to compare the FOS with Morgenstern and Price analysis, a hand calculation using the method of slices to present the basic theory of slope stability analysis in the same slip is also carried out. This method assumes that the slip wedge is divided by vertical planes into a series of slices of uniform width. The base of each slice is assumed to be a straight line. For any slice, the inclination of base to horizontal is α . Slope will be divided into 6 slices and the arc length and inclination angle of each slice is measured. Because the ground water table is below the slip surface., no pore water pressure effects the slope and the boundary water force can be ignored in the method of slice equations.

Figure 81 Swedish Method of Slices model

77

Application of Soil Nailing for slope stability purpose

Swedish Method of Slices result The following table lists the results of the hand calculations and compares with the Slope/W calculation using Morgenstern and Price analysis. Table 22 Two methods FOS result comparison table Morgenstern and price method Factor of safety (FOS)

Swedish Method of Slices method

1.031

0.639

From the table, the FOS is not consistent. There is about a 38% difference between the two methods. This is because in the Method of slices all of the interslice forces are ignored. Also, this method is only for c’=0. Therefore, some errors may occur in this analysis. Usually the error is within the range 5-20% compare with Morgenstern and Price method. The calculation spreadsheet is shown in following table: Table 23 Swedish Method of Slices Calculation Spreadsheet Unit weight of soil= 18

friction angle Φ = 25

Tan Φ = 0.4663

Water table below distance below ground Slice No

Arc Length

Weight

Angle

Cos

α Sin α

N=W*cos(α) T=W*sin(α) N*tan(Φ)

α 1

1.89

15.60

59.00

0.52

0.86

8.03

13.37

3.75

2

1.54

39.52

50.00

0.64

0.77

25.40

30.27

11.85

3

1.82

67.84

41.00

0.75

0.66

51.20

44.51

23.87

4

1.59

63.90

33.00

0.84

0.54

53.59

34.80

24.99

5

1.47

53.48

25.00

0.91

0.42

48.47

22.60

22.60

6

1.03

24.45

18.00

0.95

0.31

23.25

7.55

10.84

Σ

F= Σ(N)*tan(φ)/ΣT=w*sin(α))=

153.11

0.639406

78

97.90

Application of Soil Nailing for slope stability purpose

6.2.4 Estimated Slip surface At the beginning of soil nail design procedure, we need to estimate the different shape of the slip surface to analyse the FOS. This is because if we use the minimum FOS (1.031) to design the soil nail, the reinforced slope may have another shape of slip surface which the FOS is smaller than 1.5. For example, if the slip no. 1 FOS is smaller than 1 and the slope is not safe, after installing the soil nails with the bond length being just enough to pass through the slip surface of slip no. 1, after that analysis the reinforced slope of slip 1 and the FOS will rise to meet the requirement. The slip 1 seems safe. However, the soil nail may not be contributing a resisting force to slip no. 2,3,4. The slope will still not be safe. Therefore, the slope will be distributed into 4 typical different shapes of slip surface and all will be analysed in terms of their FOS. From the analysis, the FOS for slips 1,2,3 and 4 do not satisfy the FOS requirement of 1.5. Therefore, the soil nail must pass through slip 4 for ensue that slip 1,2,3,4 all of them are in resisting by soil nail. ( Slope/W FOS data sheet shown in Appendix E )

Figure 82

Estimated slip surface

Table 24 FOS results table Slip No

FOS

1

1.241

2

1.107 (Minimum)

3

1.118

4

1.119 79

Application of Soil Nailing for slope stability purpose

6.2.5 Soil nail design Soil nail length and bond length prediction From the previous analysis, the minimum FOS is with slip no. 2. Other slip no. 1,3,4 have a FOS which is smaller than the required FOS of 1.5 .For the soil nail concept prediction, if the soil nail bond length passes through the slips 3 and 4 failure plates, the soil nail resisting force is then functional to the soil mass. Therefore, for the same reason, the soil nail will also be functional to the slip no.1,3 and 4. On the other hand, avoid the reinforced slope minimum FOS fallback to slip no. 3,4. Thus, the bond length of the soil nail will start from the Slip no. 2 failure plate. In the trial length estimation, the soil nail resisting force and soil nail length are estimated trial and error is used to determine the up to standard FOS. The position of the soil nail is estimated according the slope profile and slope parameters. A 1.5m horizontal and vertical spacing with a staggered format is recommended. The first row of soil nails is 1m from the ground and third row of soil nails is 1m high from slope crest. Therefore, 3 rows of soil nails are formed uniformly in 5m slope height.

Estimated design In the preliminary design, soil nail lengths are estimated to be 8m length for Row A to Row C. However, in checking of Shear Failure of Adjacent Ground, all of the rows from A to B are not satisfy the requirement. Therefore, the bar length is adjusted to 12m to satisfy the shear failure adjacent ground checking. Table 25 Row

Soil nail parameter Nail Length (m)

Bond Length (m)

Inclination Angle

Nail Spacing

Design resisting

(degree)

(m)

force KN

A

12

10.03

15

1.5

35

B

12

9.8

15

1.5

25

C

12

10.16

15

1.5

18

80

Application of Soil Nailing for slope stability purpose

Soil nail reinforced slope analysis The soil property and soil profiles remain as previously modeled. Three rows of 12m soil nail at inclination 15o are added in SLOPE/W slope profile. Bond length and forces are predicted. The minimum FOS of 4 nos slip surface is generated under Morgenstern and Price analysis using SLOPE/W software.

Figure 83

Soil nail slope FOS analysis

Slip no.1 introduces some error errors during Morgenstern and Price rice analysis. The FOS of slip no.1 cannot be generated with this method. The reason of this problem is the row C soil nail design resisting force is too large and causes a force which pushes the soil mass upward to slope crest. However, from the data sheet in (Appendix E) the FOS for slip no. 1 calculated by ordinary method the FOS is 9.831. Therefore, Therefor FOS for slip no. 1 generated by Morgenstern and Price rice analysis is ignored in this case. Table 26 FOS results (after soil nail installed)

FOS Change 2.5 2 2.248

1.5

1.712 1.509

Reinforced slope

1 0.5

1.241 1.107 1.118 1.119

Pre-reinforced slope

Slip No

FOS

1

error

2

2.248

3

1.712

4

0 1

Figure

2

3

4

1.509 (Minimum)

84 FOS comparison

81

Application of Soil Nailing for slope stability purpose

Soil ail detail Calculation In order to calculation of soil nail parameter details, we need to use a trial and error method to find out the nail bar size size, nail length, bond length,, inclination, inclination spacing and required resisting force. It is recommended to use an EXCEL spreadsheet to compare the other parameters and to ensure that the most suitable design is achieved. achieved Soil ails design assumption assumptions 

Because of the caption slope is location at Australia area area, the he design is based ba on



AS4100 for reinforcement bar. At the preliminary design stage, the he angle of inclination of soil nail from horizontal is assumedd to be 15° downward.



For internal mode of failure, the following modes of failure were checked and the safety factors were adopted as follows:

Table 27 Design Assumptions

(b)

Modes of Failure

Min. Factor of Safety

(a) Tensile Failure of the steel bar

fmax = 0.5f 0.5 y

Bond Failure between grout and steel bar (c) Shear Failure of adjacent ground

3 2

Figure 85 Soil nail design section detail

82

Application of Soil Nailing for slope stability purpose

Soil nail design calculation for Section A-A (Final Result) This part aims to check the bar and grout bond and tensile strength of steel bar, which is an essential part of checking of soil nail design to avoid the failure of steel bar and failure of the bar and grout bonding. Design Parameters Soil Type = Silty Clay , C’=5kPa , γ=18kN/m3 , φ ' = 25ο Drill Hole diameter = 0.1m Soil Nail inclination angle = 15o Unit weight of water = 9.81 kN/m3

Part A - Tension Failure of the Steel Bar Yield stress fy =500MPa (Assume use OneSteel for steel reinforcement) Maximum tensile stress = 0.5 fy = 250Mpa Maximum allowable tensile force of steel bar Ta = (0.5 fy) (d - 4)2 × π / 4

Eq (5.8)

Force per m Width data is Trial and error from Slope/W Bar length is use trial and error from Slope/W analysis Table 28 Tension Failure of the Steel Bar calculation spreadsheet Max. Bar Length

Force per m

Force

Spacing

Width F

Required

Allowable

Bar Size (d)

Level Row No.

Horizontal

(m)

(mm)

L

D

(mPD)

Ta > Tr Tensile

(m)

(kN)

S

F

=F x S Force

Tr (kN)

Ta (kN)

Check

Row C

74.00

12.0

25

1.5

18.00

27.00

86.59

O.K.

Row B

72.50

12.0

25

1.5

25.00

37.50

86.59

O.K.

Row A

71.00

12.0

25

1.5

35.00

52.50

86.59

O.K.

83

Application of Soil Nailing for slope stability purpose

Part B - Bond Failure between Grout and Steel Bar Cube strength of cement grout at 28 days fcu = 32Mpa For type 2 deformed bar β = 0.5 Factor of safety adopted SF = 3 Allowable bond stress = Ultimate bond stress / SF = [ β (fcu)1/2 ] / SF = 942.81k /m2 Maximum allowable force between steel and grout = [ β (fcu)1/2 ] × π × (d - 4) × Le / SF Where Le : Effective bond length Table 29

Eq ( 5.9)

Bond Failure between Grout and Steel Bar calculation spreadsheet Max. Bar

Bar Size Horizontal

Free

Bond

Force

Force per

Allowable

Level Row No.

Length

(d)

(m)

(mm)

(mPD)

Spacing length La length Le m Width Required (m)

(m)

(m)

F

Tmax > Tr

Force

(kN) Tr (kN)

Tmax (kN)

Row C

74.0

12.0

25

1.5

1.97

10.03

18.00

27.00

623.87

O.K.

Row B

72.5

12.0

25

1.5

2.20

9.80

25.00

37.5

609.56

O.K.

Row A

71.0

12.0

25

1.5

1.84

10.16

35.00

52.5

631.96

O.K.

Part C - Shear Failure of Adjacent Ground

(Bond Failure between Grout and soil)

Resisting Zone - for soil nail design Mobilisation Force, Inclination Factor,

Tf = (π π D c' + 2 D Kα α σv' tanφ φ) × Le Kα α = 1 - (α α / 90) (1 - Kοο) = 1 - (α α / 90) (sinφ φ)

Silty Clay Kα α = 0.93 Tf =(π π D c' + 2 D Kα α σv' tanφ φ) × Le = ( 1.571 + 0.087 σ’v σ ) × Le

Eq(5.10)

Table 30 Bond Failure between Grout and soil calculation spreadsheet 1 Resisting Zone Depth to mid-point of the effective bond length (m) Row No.

Effective bond length in Silty Clay layer (m) Le

Silty Clay Zone Silty Clay

WATER

Row C

10.03

2.62

0.00

Row B Row A

9.80 6.78

4.34 5.35

0.58 1.23

84

Application of Soil Nailing for slope stability purpose

Table 31 Bond Failure between Grout and soil calculation spreadsheet 2

Row No.

Effective

Force

Vertical Stress

Mobilised

Embedded

Tf (kN) (kN)=

Rock Length

Total Bond in Force

( 1.571 +

σv (kPa)

Force Required

F.O.S.

Tr (kN)

Tf / Tr

F.O.S. > 2

rock

0.087 σ’v )

L (m)

Mobilised T ( kN) Tf (kN)

xL Le Silty Clay

Silty Clay

Row C

47.16

56.76

0.00

0

56.76

27

2.10

O.K.

Row B

72.43

76.93

0.00

0

76.93

37.5

2.05

O.K.

Row A

84.23

60.16

3.38

530.93

591.03

52.5

11.26

O.K.

Table 32 Final Soil nail design schedule Table Horizontal Level

Bar Length

Bar Size (d)

(mPD)

(m)

(mm)

Row No.

La

Le

(m)

(m)

Spacing (m)

Force per m

Force

Width F

Required

(kN)

Tr (kN) Row C

74.00

12.0

25

1.5

1.97

10.03

18.00

27.00

Row B

72.50

12.0

25

1.5

2.20

9.80

25.00

37.5

Row A

71

12.0

25

1.5

1.84

10.16

35.00

52.5

6.1.6

Summary

In this second case study assessment, the caption slope in its initial condition before adding the soil nail is un-stab stable. The minimum FOS using SLOPE/W software under the Morgenstern and Price rice analysis is 1.031. This is smaller than the required require FOS of 1.5 for slope stability. After fter the application of the soil nails, the FOS of the slope was upgraded to minimum 1.5009 which meets the requirement. Table 33

Final Result table

Section A-A

Before Upgrading

Soil nail applied

Minimum FOS

1.031

1.509 1.50

The slope is up to require FOS after soil nail installation.

85

Application of Soil Nailing for slope stability purpose

7.0

Conclusions

7.1 Summary and concluding remarks Throughout this project, it has been shown that landslide hazards do not only occur in hill sides. Many cut slopes in urban areas also may face the slope instability problem. There are many solutions for slope improvement and stability work. However, through the literature review we can find that soil nailing is the most efficient, environmentally friendly, and simplest method of slope stability improvement. Soil nail construction has been shown to be a simple technology and does not need complex machines. Therefore, this method can provide a lower construction period and can distribute more resources to stabilise other instable slopes. In conclusion of the two case studies, two slopes are located at two different geological areas. However, it was shown that the soil nail application can be used in these two different cases. Compared which other new technologies such as Bio-engineering, these two case studies have demonstrated that soil nailing is a diverse method that can be applied to any type of soil, in a variety of climates and with any slope angle. Furthermore, using software to analyse the slope is much more accurate than hand calculations and this is the most commonly used method for slope analysis. From the result generated by Slope/W , we can find the different of FOS values before adding soil nails and after adding soil nails. This shows that soil nails can upgrade the FOS for stability purposes and that all parameters of soil nails are designable. That means that the design of soil nails can result in a higher level of efficiency that just using a standardized design. The city is growing and more and more new technologies for slope stability methods are developing. However, the soil nail method can provide some unique aspects over some other methods such as reliability and designability. Therefore, the soil nail method may not be discontinued in the future. The more innovate design of soil nails may combine with other new technologies such as use soil nails for slip prevention measures and be covered with Geotexile or mulching systems for surface erosion and shallow slide preventive measures. From these innovative technologies, There will be more and more slope stability methods combined with soil nail technology and Bio-Engineering technology in the future.

86

Application of Soil Nailing for slope stability purpose

7.2 Recommendations It should be noted that soil nailing is one of the main methods used for stabilising medium sized slopes. Enhancing public education for the landslide hazard is the most desirable way for preventing human loss and property damage in high landslide risk areas. Some recommendations on increasing public awareness about landslide hazard are described as well. The following points are some recommended action items :     

Create a database or slope record system similar to the Hong Kong Slope information System and classification the degree of slope hazard. Consider that the slope hazard information is open to public - let householders know the risks which exist in their surrounding area. Improve the education about the landslide hazards and increase the public alertness. Educate the public in terms of simple inspection of slopes, increasing their slope failure alertness. Educate the private slope householder in terms of maintaining their slope, such as drainage clearance and protecting the slope surface to avoid surface erosion etc.

87

Application of Soil Nailing for slope stability purpose

8.0 Bibliography ASTM C939-(2002) “Standard test method for flow of grout” Civil Engineering and Development Department (CEDD), GEOTECHNICAL ENGINEERING OFFICE. (2000). “ Technical Guidelines on Landscape treatment and Bio-engineering for man-made slope and retaining walls” , Hong Kong. Civil Engineering and Development Department (CEDD), GEOTECHNICAL ENGINEERING OFFICE .(1992). “ General Specification Vol 2”, Hong Kong. Civil Engineering and Development Department (CEDD), GEOTECHNICAL ENGINEERING OFFICE .(1995). “ Construction Specification 2” ,Hong Kong. Civil Engineering and Development Department (CEDD), GEOTECHNICAL ENGINEERING OFFICE ( 2008). “Geoguide 7 Guide to soil nail design and Construction”, Hong Kong. Civil Engineering and Development Department (CEDD), GEOTECHNICAL ENGINEERING OFFICE ( 2008). “Geoguide 5 Guide to Slope maintenance and inspection”, Hong Kong. CHENG LIU. JACK B. EVETT. (2008). “ Soils and Foundations”. 8th edn. Civil Engineering and Development Department (CEDD) “Manuals, Guides and R & D Reports”, viewed 21th September 2008 – 5th October 2008, < http://www.cedd.gov.hk/eng/publications/manuals/index.htm >. DERECK.CORNFORTH (2005) “ Landslides in practice: investigation, analysis, remedial and preventive options in soils.” E.N.BROMHEAD. (1992). “The stability of slopes”. Emergency Management Australia Database, “ Australia Landslide historic events” viewed 1st September 2008, .

88

Application of Soil Nailing for slope stability purpose

GEOSCIENCE AUSTRALIA , “landslide hazard”, (viewed 30th August 2008 ) < http://www.ga.gov.au/hazards/landslide/ > . Guide to the field description identification and classification soil (2007) “Geotechnical site investigation procedure Manual 5/2007”, Australia . Ground investigation report (2005), “Drill hole record”, Gold Ram Engineering and Development Limited, Hong Kong. Ground investigation report (1993), “Drill hole record”, Vibro Limited. , Hong Kong. Hong Kong Slope safety website, “Slope information system”, viewed 11th September 2008 , < http://hkss.cedd.gov.hk/hkss/eng/whatsnew/index.htm >. J.A.R.ORTIGAO. A.S.F.J.SAYAO (2004). “ Handbook of slope stabilisation”. JOHN KRAHN. (2004). “Stability modelling with SLOPE/W” First Edn Revision 1 , GEO-SLOPE/W International, Ltd. , Canada. , pt. 7-57. J.MICHAEL DUNCAN, STEPHEN G. WRIGHT. (2005). “Soil strength and Slope Stability” LEE ABRAMSON,THOMAS S.LEE , SUNIL GLENN M (2002). “ Slope stability and stabilisation methods.” M.R.HAUSMANN. (1992). “ Soil and rock anchorage, rock bolting, soil nailing & dowelling “. N.J. COPPIN , I.G.RICHARDS (1990), “ Use of Vegetation in Civil Engineering” STANDARD AUSTRALIA (2002) “ Earth retaining structures AS 4678-2002” Standard Australia. United states department of agriculture, Soil bioengineering for upland slope protection and erosion reduction (1995), Engineering Field Handbook, chapter 18 pt 4-21

89

Application of Soil Nailing for slope stability purpose

List of Appendices Appendix A -

Previous boreholes Log record ( Case study 1)

Appendix B -

Previous laboratory test record ( Case study 1)

Appendix C –

Slope/w analysis data ( Case study 1)

Appendix D -

Classification guide ( Case study 2)

Appendix E -

Slope/w analysis data ( Case study 2)

90