dissertation proper work (1)

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dissertation submitted to

UNIVERSITY OF EAST LONDON

SCHOOL OF ARCHITECTURE, COMPUTING AND

ENGINEERING

by

ALEXIO MATHIAS MUSIMBE

(REG NO) 1101393

Course year and course name

Degree: BENG CIVIL ENGINEERING

Module: FINAL YEAR PROJECT CE6216T

Dissertation Title: INVESTIGATION AND ANALYSIS OF SOIL SLOPE

FAILURE AND SOIL SLOPE STABILITY AT THE

GIANT’S CAUSEWAY VISITORS CENTRE

Course Leader: DR JOHN WALSH

Student Year Number: YEAR THREE

University of East London

Date of Submission: 01 December 2015

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Acknowledgements

At first I want to express gratitude and praise to God that my project was completed in

time. I would like to thank the Geological Survey of Northern Ireland for offering ground

investigation and geotechnical report of the Giant’s Causeway Visitor’s Centre, without

their help it would had been impossible to start this research project.

I would like to thank Dr John Walsh for introducing me to modern slope stability analysis

software (Oasys Slope 19.0) and I am grateful for his enduring advice, interest and help

towards such an interesting project. I also want to express my profound gratitude to Mr

Richard Freeman, the supervisor for this research project for his valuable assistance,

advice, guidance, interest and supervision for all stages of this research project. I

appreciate his guidance and help to this research project.

I would also like to thank my friends and family members for their continuous support

throughout this research project.

Alexio Musimbe, December 2015

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Abstract This research project focused on the investigation into slope failure and soil slope

This research project focused on the investigation into slope failure and soil slope stability

methods at the Giant’s Causeway Visitor’s Centre in Northern Ireland. In this research

project Oasys Slope 19.0 was used to analyse the possibility of slope failures. Bisho

Method with variable inclined interstice method was used for analysis. The acceptable

minimum factor of safety, according to BS6031:2009 was set as 1.3.

Basic soil parameter is needed to be used for slope stability analysis by Oasys Slope 19.0

and the results indicated the importance of shear strength in stability of slopes. A further

distinction should also be made of drained and undrained conditions whereby drained

condition refers to a situation whereby drainage is allowed whilst undrained condition

mean drainage is restricted.

The shear strength of the soils at the Giant’s Causeway Visitor’s Centre have a huge role

in stabilisation of the slopes as most of the soils there are coarse-grained soils. The critical

failure surface and the factor of safety will be part of the output which will be produced by

Oasys Slope software. These will be analysed and if necessary, slope remedial measures

will be suggested as to stabilise the slopes.

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Table of Contents

Acknowledgments………………………………………………………………………………ii

Abstract………………………………………………………………………………………….iii

Chapter 1: Introduction………………………………………………………………………..01

1.1 Overview of the Research Project……………………………………………….02

1.2 Statement Problem of research project…………………………………………03

1.3 Objectives of the research study…………………………………………………03

1.4 Scope of the research project…………………………………………………….04

1.5 Significance of the research project…..………………………………………….05

Chapter 2: Literature review…………………………………………………………………..06

2.1 Background of the Giant’s Causeway Visitor’s Centre.…………………….07

2.2 Ground Investigations of the area…………………………………………….07

2.3 Soil’s geotechnical parameters………………………………………………..08

2.3.1 SPT Tests………………………………………………………………...09

2.3.2 SPTCorr v.2.2.…………………………………………………………...11

2.3.3 Unit weight of soil ……………………………………………………….12

2.3.4 Cohesion of soil………………………………………………………….12

2.3.5 Soil friction angle…………………………………………………………12

2.3.6 Slope geometry………………………………………………………….13

2.4 Soil types…………………………………………………………………………13

2.5 Slope stability analysis basic requirements…………………………………..14

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2.6 Drained and Undrained Strength…………………………………………………15

2.6.1 Drained and undrained soil conditions………………………………..…...15

2.6.2 Analysis of drained soil conditions……………………………………….…16

2.6.3 Undrained soil conditions analysis…………………………………………..17

2.7 Short term analysis…………………………………………………………………17

2.8 Analysis of long term conditions…………………………………………………..18

2.9 Pore water pressure analysis……………………………………………..............18

2.10 Circular Surface’s Slip…….………………………………………………………19

2.11 Factor of Safety of the slopes………………………………………………...….19

2.12 Load on the slopes…………………………………………………………………21

2.13 Oasys Slope 19.0 analysis………………………………………………………..21

2.14 Conclusion of Literature review…………………………………………………..21

Chapter 3: Data Collection for the research project………………………………………..22

3.1 Introduction…………………………………………………………………………..23

3.2 Geometry of slope…………………………………………………………………..23

3.3 Ground water levels and name of soil layer………………………………………25

3.4 Unit weight of soil……………………………………………………………………25

3.5 Cohesion and Shear Strength……………………………………………………..32

3.6 Angle of Internal friction…………………………………………………………….33

3.7 Summary of Data……………………………………………………………………35

Chapter 4: Methodology……………………………………………………………………....39

4.1 Oasys Slope 19.0……………………..…………………………………………….40

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4.2 Problem definition…………………………………………………………………..40

4.3 Modelling of analysis problem……………………………………………………..43

4.4 Type of analysis……………………………………………………………………..43

4.4.1 Methods of analysis…………………………………………………………….43

4.4.2 Ordinary Method of Slices……………………………………………………..45

4.4.3 Simplified Bishop Method……………………………………………………..45

4.4.4 Janbu Method…………………………………………………………………..45

4.5 Oasys Slope: Method of Iteration………………………………………………….46

4.5.1 Relation of Oasys Slope 19.0 to Factors of Safety…………………………46

4.5.2 Analysis of Horizontal Intersliced Forces…………………………………....46

4.5.3 Analysis of Constant Inclined Intersliced Forces……………………………47

4.5.4 Analysis of Variably Inclined Intersliced Forces…………………………….47

4.6 Positioning of Slices………………………………………………………………….46

4.7 Research Project Adopted method…………………………………………………48

4.7.1 Bishop Methods………………………………………………………………......48

4.7.2 Horizontal Interslice Forces Simplified Method………………………………..48

4.7.3 Parallel Inclined Intersliced Forces……………………………………………...48

4.7.4 Bishop’s Method: Variably Inclined Interslice Forces………………………….49

4.8 Units of Data…….. …………………………………………………………………..49

4.9 Verification of data and Computation of Factor of Safety…………………………52

5.0 Result and Discussion……………………………………………………………………53

5.1 Slope A……………………………………………………………………………..….54

5.2 Slope B……………………………………………………………………………..….57

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5.3 Slope C……………………………………………………………………………..…60

5.4 Conclusion of results and discussion………………………………………………65

6.0 Future recommendations and conclusions.……………………………………………67

6.1.1 Changing slope geometry…………………………………………………….....68

6.1.2 Retaining structures………………………………………………………………69

6.1.3 Geotextiles………………………………………………………………………...70

6.1.4 Grassing the slope………………………………………………………………..71

6.1.5 Drainage…………………………………………………………………………...71

6.2 Conclusions………………………………………………………………………….72

References…………………………………………………………………………………..…73

Appendix A: Oasys Slope 19.0 Graphical Outputs…………………………………………77

Appendix B: Giant Causeway Visitor’s Centre Map Area………………………………….90

Appendix C: Soil laboratory tests, Borehole data and Geotech Data Tables……………95

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List of Figures

Figure 1.1 Mudflow slope failure at the Giant’s Causeway Visitors Centre………………04

Figure 2.2 Borehole data at the Giant’s Causeway Centre………………….…………….10

Figure 2.2 SPTCorr v.2.2.1.11 ……………………………………………………………….11

Figure 2.3 Output from Oasys Slope 19.0 Software……………………………………….20

Figure 3.1 Slope geometry of slope A………………………………………………………..24

Figure 3.2 Slope geometry of slope B………………………………………………………. 24

Figure 3.3 Slope geometry of slope C………………………………………………………..24

Figure 3.4 Gravelly Silt Soil Slope failure at Giant Causeway.........................................25

Figure 3.5 Showing soil profile of Slope A……………. ……………………………………..27

Figure 3.6 Measurement of V: H ratios at Giant’s Causeway………………………………27

Figure 3.7 Slope failures at Giant’s Causeway…………………………………………….28

Figure 3.8 Soil Profile of slope B..……………………………………..…………………....29

Figure 3.9 Unit weights of slope C……………………………………………………………31

Figure 4.1 Theory of slices……………………………………………………………………44

Figure 4.2 Showing Inputs of Slip Surfaces of Slope B…………...………………………50

Figure 4.3 Showing Inputs of Partial Factors of Slope B. . .…………………………….…51

Figure 4.4 Output of Oasys Slope 19.0……………………………………………………...52

Figure 5.1 Slope A: Undrained analysis in Oasys Slope 19.0……………………………..55

Figure 5.2 Slope A: Drained analysis in Oasys Slope 19.0…………………………………55

Figure 5.3 Slope B: Undrained analysis in Oasys Slope 19.0………..……………………58

Figure 5.4 Slope B: Drained analysis in Oasys Slope 19.0…………………………………58

Figure 5.5 Slope C: Undrained analysis in Oasys Slope 19.0……………………………..61

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Figure 5.6 Slope C: Drained analysis in Oasys Slope 19.0…………………………………61

Fig 5.7 Showing graphical output of Slope C ……………………………………………….63

Fig 5.8 Showing tabular output of Slope C ………………………………………………….64

Figure 6.1 Showing modification of slope geometry………………………………………..68

Figure 6.2 Showing modification of slope geometry and stability of slope……………….69

Figure 6.3 Slope stability method of walls…………………………………………………...69

Figure 6.4 Slope stability methods of walls………………………………………………….70

Figure 6.5 Showing use of geogrids in slope stability………………………………………71

Figure 6.6 Demonstrating use of drainage systems in slope stability……………………..72

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List of Tables

Table 3.1 Unit weights of Slope A…………………………………………………………26

Table 3.2 Unit weights of Slope B…………………………………………………………28

Table 3.3 Unit weights of Slope C…………………………………………………………30

Table 3.4 Cohesion values of Slope A……………………………………………………32

Table 3.5 Cohesion values of Slope B……………………………………………………32

Table 3.6 Cohesion values of Slope C……………………………………………………32

Table 3.7 Angles of internal friction of Slope A………………………………………….33

Table 3.8 Angles of internal friction of Slope B…………………………………………..34

Table 3.9 Angles of internal friction of Slope C…………………………………………..34

Table 3.10 Data summary of slope A……………………………………………………..35

Table 3.11 Data summary of slope B……………………………………………………..36

Table 3.12 Data summary of slope C……………………………………………………..37

Table 5.1 Geotechnical Parameters for slope A………………………………………….62

Table 5.2 Geotechnical Parameters for slope B…………………………………………..64

Table 5.3 Geotechnical Parameters for slope C…………………………………………..66

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List of Symbols and Abbreviations

Symbols

Greek Symbols

φ angle of internal friction

φ' effective angle of friction

γ bulk unit weight of soil (kN/m3)

γd dry unit weight of soil (kN/m3)

γw unit weight of water (kN/m3)

ρ mass density of soil (g/cm3)

ρs grain density of solids (g/cm3)

ρw density of water (mg/m3)

σ’ effective normal stress (kPa)

τ shear stress or mobilized shear stress (kPa)

τf shear strength of soil (kPa)

Roman symbols

b: the width of a slice

c: total apparent cohesion value (kPa)

c’: the effective cohesion value (kPa)

e :Void ratio

g : acceleration due to gravity

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ru:the pore pressure ratio

Abbreviations

ASTM: American Standard for Testing Materials

BS: British Standards

FOS: Factor of Safety

Gs: Grain Specification Gravity

GwT: Ground water table

SLIDE: The limit equilibrium software which is used for groundwater and slope stability

analysis

SPT: Standard Penetration Test

Sr: Degree of saturation

V: Height of a slope

H: Horizontal length of a slope

V: H Ratio: Slope geometry ratio of a slope / Gradient of slope

Ws : Weight of solids

Ww: Weight of water

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CHAPTER 1.0: INTRODUCTION

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1.1 Overview of the research project ggggggggggggggggggggggggggggggggThe

The research project is going to be focused on the Giant’s Causeway Visitor’s Centre

regarding soil slope stability analysis at the area. The Giant’s Causeway is one of the

only three natural world heritage sites in the British Isles according to the research

conducted by Queens University Belfast (Queens University Belfast, 2015).The Giant’s

Causeway coastal environment is a combination of slopes, high rainfall and active mass

movements. In geotechnical engineering it is arguably possible to identify or as well

locate an increased risk of slope failure, however it is not possible to predict the overall

stability of a slope without any form of evaluation or analysis. Therefore Oasys Slope

19.0 software will be used in evaluation of slope stability of selected slopes at the

Giant’s Causeway Visitor’s Centre, in this research project.

According to Abramson et al. (2002) slope instability of soil is a very important and

challenging aspect in the history of civil engineering. The instability of the slopes is part

of geo-dynamic process that shapes the geo-morphology of the earth. In his analysis

Venkataramaiah (2006,p.318) he stated that the instability of the slopes might have a

negative effect on the safety of people as well as their property, that’s why it is important

to have a full understanding of the complex soil’s behaviour when it comes to slope

stability. He stated an example of slope failures during the construction of Panama Canal,

which led to better research of failed earth slopes in Sweden.

Over past years in geotechnical engineering ,the failure of slopes have led to better

understanding of soil properties and better ways of stabilising the slopes. The

emergence of new instruments for observation of slopes’ behavior as well as increased

knowledge of soil mechanics principles have led to better slope stability analytical

methods according to Chen and Liu (1990).

This research project provides general information required when it comes to slope

stability analysis. They are many slope stability evaluation methods which are available

for slope stability analysis but this research study has focused on the use of limit

equilibrium- computer software (Oasys Slope 19.0) and involves analysis of examples of

slope stability problems at the Giant’s Causeway Visitor’s Centre in Northern Ireland.

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1.2 Statement of research project

Regarding slope stability, in his analysis Abrasom et al. (2002) he stated that it is

important to fully understand slope behavior and failure mechanism of slopes. This is

essential when it comes to designing and application of appropriate measures which are

required to stabilise the slope.The application of proper method for stabilising the slope

will depend on the mode of failure of the slope. The financial aspect of designing the

slope is important as well, this is useful as to avoid over designing the slope or

burdening the client.

1.3 Objectives of the research study

The primary objective of any slope stability analytical problem is to contribute towards

safety of people or property. Preliminary analysis in a slope stability project is helpful in

identification of the following according to Murthy (2002, p.379).

1. Critical geological information of the slopes

2. Material making up the slopes

3. Environmental parameters

4. Economic parameters

Evaluation of slope stability analysis is a combined effort of contribution of

1. Engineering geology

2. Mechanics of soil

In this research project the stability of soil slopes which will be evaluated will be located

at the Giant’s Causeway Visitor’s Centre.

For this research study, the core research topics will be

1. To assess the stability of the slopes at the Centre under short term condition and

long term condition.

2. To analyse landslides on the Centre as well as failure mechanism and the effect

of environmental factors towards slope stability.

3. To be able to suggest appropriate slope stability remedial measures in the event

of slope failure.

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Summary of the Objectives

1. Using Oasys slope 19.0 software to determine the minimum factor of safety

values of the slopes.

2. Determining the critical failure of slope’s surface and application of engineering

judgment in determining whether the slopes will be stable or not stable as

recommended by BS.

3. Suggestion of appropriate slope stability remedial measures that can be applied

in the event of slope failure.

An example of slope failure at the Giant’s Causeway is shown below on fig 1.1

Fig 1.1 Mudflow slope failure at Giant’s Causeway, Queens University Belfast (2015)

1.4 Scope of the research project

The research project is going to be carried out at the Giant’s Causeway Visitor Centre

This will be done by

1. Analytical studying of possible failures of selected slopes by use of geotechnical

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software (Oasys Slope)

2. Suggestion of appropriate /suitable remedial slope works in the event of slope

failure

1.5 Significance of the research project

The significance of the research study is the presentation of case study of slopes by use

of software analysis at the Giant’s Causeway Visitor’s Centre .The landslides are natural

hazards that can threaten the properties or people who come and visit the Centre. With

technology development, the negative impacts/effects of landslides can be minimised by

application of effective slope stabilisation techniques according to McDonnell and Smith

(2000).

It should however be noted that a slope with a different mode of failure will require

different slope stability method. Slope failures can be treated by different stabilisation

methods. The appropriate slope stability method which is suitable for a slope failure will

always be a questionable problem in geotechnical engineering, therefore full knowledge

of information regarding the causes of failure of slopes and relevant appropriate

treatment is essential in ensuring slope stability and slope maintenance according to

Lancellota (2009,p.414)

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CHAPTER 2.0: LITERATURE REVIEW

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2.1 Background of the Giant’s Causeway Visitor’s Centre The gggggg The The

The coastal environment occupying the Giant’s Causeway is generally made of steep

slopes, rocks which are fractured, marine erosion as well high rainfall. Therefore it is not

with a huge surprise that this region is characterised by landslides or active mass

movements of soil.

In a research study which was carried out by McDonnell and Smith (2000) they confirmed

a history of slope failures at the Giant’s Causeway. The research study indicated that the

main causes of failure of slopes at the site are

1. Moisture

2. Steepness of slopes

3. General geological structure of slopes

The following points will be considered for this research project regarding the area

1. Slope stability calculations with Oasys Slope 19.0, this included computing the

factor of safety of the slopes and the acceptable factor of safety was set as 1.3 as

recommended by BS6031:2009.

2. Determining the critical failure of the slope’s surface

3. Suggestion of remedial measures that can be applied as to stabilise the slopes if

they happen to fail.

2.2 Ground Investigations of the area xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxThe

The data for this research project was based on the report which was done by Glover Site

Investigations Limited on behalf of the Geological Survey of Northern Ireland. The report

was done on the Giant’s Causeway Visitor’s Centre. The report is specified in the design

of manual for roads and bridges, Volume 4 Geotechnics and Drainage Section 1

Earthworks, Part 2 and HD 22/08: Managing Geotechnical Risk (Glover Site

Investigations Limited, 2009, p.2)

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The report offers the following information (Glover Site Investigations Limited, 2009, p.4)

1. Listing of relevant collated existing information at the Giant’s Causeway Site

2. Proposed remedial solutions to the slopes

3. Desk study of area

4. Description of field operations and laboratory tests carried out

5. Description of ground water types and conditions in the area

6. Information about implications of data and of geotechnical design of structures

2.3 Geotechnical Parameters xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxBefore

Before analysis of a slope or generally the ground where the slope exists, essential

borehole data is required according to Knappet and Craig (2012). The borehole data will

provide valuable soil parameters which are essential for slope stability analysis such as

1. Soil strata layers

2. Soil moisture content levels

3. Ground water levels

4. The presence of any particular plastic layer in the soil in which there is high chance

of shear occurring, will be easily noted.

For slope stability analysis at the Giant’s Causeway Visitor’s Centre the following ground

investigations could be used

1. Soil laboratory tests

2. Site aerial photographs

3. Studying of site’s geological maps or memoirs which could be used to indicate as

well as predict soil conditions

4. Observing as well as visiting the slope.

For this research project the ground investigations have been done by Glover Site

Investigations Ltd on behalf of the Geological Survey of Northern Ireland. Standard

Penetration Tests (SPT) were used by the company for evaluation of soil parameters as

well as laboratory tests. The tests were done to obtain soil’s particle size distribution,

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index property of soil, specific gravity tests, bulk density, and water content of soils as

well as shear strength of soils (Glover Site Investigations Limited, 2009, p.13).

2.3.1 SPT tests aaaaaaaaaaaStandard Penetration Test (SPT) is one of the main tests

Standard Penetration Test (SPT) is one of the main tests which were conducted by Glover

Site Investigations Limited to obtain geotechnical soil parameters. The technical

standards which govern the use of the tests are ASTM D1586 (United States of America)

and EN ISO 22476, Part 3 (United Kingdom and Europe).

The Standard Penetration Test is an in situ test, it is used in determination of geotechnical

engineering soil properties. It is a useful test in geotechnical engineering as it can be used

to estimate relative density of a soil as well approximate values of shear strength

parameters according to Smith (2013,p.441). These are useful parameters which are

required by Oasys Slope 19.0 to determine the factor of safety of slope or slope’s stability

according to Oasys Ltd (2012).

The SPT works by driving a sample tube into the ground. This is a standard practice and

the tube will be thick walled. Blows from a hammer are used to drive the tube into the

ground. To standardise the test, the slide hammer which is used has a standard weight

and a known falling distance. The tube will then be driven from a depth of 0.150m into the

ground to the depth of 0.450m.The number of blows which are required for the thick

walled tube to penetrate each depth of 0.150m up to a depth 0.450m of soil are then

recorded (Geotechnical Information, 2015).

The sum of blow numbers which are required for second and third 0.150m of penetration

is reported as SPT blow count value (Geotechnical Information, 2015). This is commonly

known in geotechnical engineering as the “N-value” or the standard penetration

resistance. The N value number will provide or indicate relative density of soil’s

subsurface according to Knappet and Craig (2012). This is then used empirically to

correlate as well estimate shear strength properties of soil.

Soil Properties which can be correlated by SPT-N value according to Bodo and Jones

(2013) are

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1. Soil packing either loose, compact ,dense and very dense

2. Soil’s relative density in percentage

3. Soil’s friction angle

4. Strength of the soil.

Borehole log and test results data

Fig 2.1 Showing a Borehole data of one of the slopes at the Giant’s Causeway (Glover

Site Investigations Limited, 2009, p.40)

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2.3.2 SPTCorr v.2.2 this is a simple software which can be used for estimation of this this

This is a simple software which can be used for estimation of geotechnical soil parameters

using the SPT-N values or the SPT blow count. SPTCorr v.2.2 can be used for soils such

as weak clays, hard clays or loose to hard sand. The software offer the following

(Geologismiki, 2015)

1. Calculations of corrected SPT blow count N60 and N1,60

2. Soil’s relative density, Dr

3. The internal angle of friction of soil ,phi

4. The elasticity modulus of soil ,Es

5. The undrained strength values of soil,Su

6. Summary report showing all correlations.

Fig 2.2 showing an output of SPTCorr v.2.2.1.11 software

The software will be used to verify correlations of geotechnical parameters such as angles

of friction of soil which are derived from correlation using “N” value. The report by Glover

Site Investigations Limited provides “N” values of each soil layer.

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2.3.3 Unit weight of soil the unit weight of soil is defined as the ratio of the total weight of

The unit weight of soil is defined as the ratio of the total weight of the particular soil to the

total volume of that soil according to Bodo and Jones (2013). Generally unit weight of soil,

(γ) is determined in the laboratory. This is done by measurement of the volume and weight

of soil sample which is relatively undisturbed. Measurement of soil unit weight in the field

is normally done by these procedures such as

1. The sand cone test

2. Rubber balloon method

3. The nuclear densiometer.

For this research project soil unit weights are going to be calculated from dry density

(mg/m3) and bulk density (mg/m3) values which were obtained by Glover Site

Investigations Limited. These will be calculated according to Bodo and Jones

(2013).The results will produce both the unit weight and dry unit weight of soil, which are

essential parameters to be used for slope stability analysis according to Oasys Ltd

(2012).

2.3.4 Cohesion of Soil Soil cohesion values, c, are normally obtained by the Direct Soil

Soil cohesion values, c, are normally obtained by the Direct Soil Shear Test in the

laboratory. Compressive strength which is not confined can be obtained in the

laboratory. This is done by either the unconfined compressive strength test or the

common triaxial test.In geotechnical engineering they are also correlations for

unconfined shear strength soil, as generally estimated from field using the Vane Shear

Tests (Geotechnical Information, 2015). Glover Site Investigations Ltd have already

obtained soil cohesions values for this research project.

2.3.5 Soil friction angle The soil internal angle values can be determined by either the

The soil internal friction angles can be determined by either the triaxial test or the direct

shear test in the laboratory according to Das (2008, p. 374-392).Soil friction angle

according to Bodo and Jones (2013) is defined as the soil’s shear strength parameter,

this definition is derived from the use of the Mohr-Coulomb failure criteria. This is used

to describe the soil’s friction shear resistance together with the normal stress according

13

to Knappet and Craig (2012) .For this research project, values of soil friction angles

which are going to be correlated from SPT values determined by Glover Site

Investigations Ltd will be used. The correlation will be done using the angle of friction

correlation table (Geotechnical Information, 2015) and the SPTCorr v.2.2.1.11 software.

2.3.6 Slope Geometry

The slope geometry is very important in slope stability analysis according to Abrasom et

al. (2002) as it can alter the overall factor of safety of the slope. Critical height of any

slope will depend on the density, the bearing capacity and the shear strength of the

slope foundation. As the height of the slope increases, the shear stress within the

slope’s toe will also increase. This is due to increased added weight. It should also be

noted that the shear stress will also be related to the material making up the slope as

well as the slope angle.

It can be therefore be concluded that as the slope angle increases, this will result in

increase of tangential stress .This will also result in increment of shear stress as well as

decrease in stability of a slope (Indian Institute of Technology,2015). The slope

geometry will involve the determination of the V: H ratios of each slope to be studied

and the researcher had to be measure the slopes as part of site visiting. The V: H ratios

will be used when importing data into Oasys Slope 19.0 for slope stability analysis.

2.4 Soil types

Soil classification is a core part of this research project and generally soil is classified in

geotechnical engineering based on its properties as either a building material or its use

in foundation supports. Using simple laboratory tests or tests on the field engineering

properties as well as soil behavior can be obtained.

The common soil types at the Giant Causeway Visitor’s Centre (Glover Site

Investigations Limited, 2009, p.45)

1. Gravelly silt soil.

2. Sandy gravelly silt soil.

3. Sandy soil.

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2.5 Basic Information Requirement for Slope Stability analysis In geotechnical

In geotechnical engineering regarding slope stability analysis there is a distinction

between drained conditions and undrained conditions according to Lancellota

(2009,p.414-416). The most important requirement or principle is that equilibrium will

need to be achieved when it comes to total stresses (Community, 2014).

During the analysis of a slope, body weights and external loads should be included. This

also includes loads which are caused by water pressures according to Murthy (2002,

p.367 -368). All these loads should be included in the analysis. The analysis will provide

important results which are as follows according to Smith (2013, p.279)

1. Total normal stress which will be acting on the shear surface

2. Total shear stress that will be required for the equilibrium to be achieved.

In geotechnical engineering, the factor of safety for a shear surface is defined as the ratio

of soil’s shear strength divided by shear stress of the soil required to achieve equilibrium

according to Smith (2013,p.281). In order to successfully evaluate the shear strength of

a soil, it should be noted that the values of normal stresses which will be acting on the

slip surface are needed. This however does not apply to soils with a friction angle of zero

as their strength depend on the normal stress that will be on potential plane of failure

according to Das (2008, p.374-382).

In analysis of effective stress according to Das (2008, p.379-382), the shear strength of

soil is needed to be fully evaluated. This is done by subtracting “pore pressures” which

will be acting on shear surface from the total stresses of the soil. This will result in

determination of effective stresses which are then used in evaluation of effective shear

strengths. Hence forth when it comes to analysis of effective stress, it is essential

requirement to know or if not to estimate pore pressures which will be at every point along

the shear surface.

When it comes to drained soil conditions, the pore pressures of soil can be analysed as

well as evaluated generally with a good degree of accuracy. This is because the pore

pressure values are obtained by either hydrostatic or by steady seepage boundary

15

conditions way according to Bodo and Jones (2013). In geotechnical engineering pore

pressures are rarely evaluated accurately for undrained soil conditions. This is due to

the fact their values can be determined by the response of soil to external load(s)

according to Bodo and Jones (2013).

When it comes to analysis of total stress of a soil, pore pressures will not be deducted

from total stresses. This is due to the fact that shear strengths of soils are related to the

total stresses. This means it will no longer be a necessity either to evaluate or to

subtract pore pressures as a way of analysing total stress of a soil. Henceforth, it should

be noted that total stress analysis is only applicable to soil conditions which are

undrained according to Knappet and Craig (2012).

The basic principle behind total stress analysis according to Knappet and Craig (2012)

is that pore pressures which are caused by undrained loads are normally determined by

soil’s behavior. For instance for any total stress value given on a potential plane of

failure of soil,usually there is a pore pressure value which is unique.Therefore the

effective stress value of soil will be unique as well

Shear strength in geotechnical engineering is generally approved that it is controlled by

effective stress. According to Bodo and Jones (2013) there is analysis that it is possible

under undrained conditions to have to relate shear strength to the normal stress. This is

possible because in soil mechanics according to Knappet and Craig (2012), total stress

and effective stress are both uniquely related under undrained conditions. Therefore it is

also important to stress that this principle cannot be applied to drained conditions as

pore pressures will be controlled by the hydraulic conditions and not from the response

which comes from soil’s external loads.

2.6 Undrained and drained condition

2.6.1 Analysis of drained and undrained soil conditions

The strength of both drained and undrained strength of cohesive soil is an important

16

factor which require analysis in slope stability. Cohesive soils or clay soils to be more

specific generally possess less or have less permeability as compared to coarse

grained soils such as sand soil .Therefore it means they will be restriction for water

movement whenever there is a change in volume (Community, 2014).

For soils such as clay soils, they require a number of years before equilibrium is

achieved. According to Duncan et al. (2005) he stated that soils such as clay, dissipation

of the excess pore pressure will take years before equilibrium will be achieved in soil. In

general drained condition can be defined as a condition were drainage is allowed and

also undrained condition is defined as a condition where there is restriction in drainage.

One important factor to note is that in both drained soil and undrained soil conditions of

cohesive soils such as clay soils, they is a reduction in cohesive soil’s strength from

their peak strength to their residual strength .This is mainly due to restructuring of soil

according to Duncan et al (2005)

2.6.2 Soil’s drained conditions

This in geotechnical engineering is referred to a condition were load changes are slower

enough or where they have being in place for a long time so that equilibrium in a soil

can be reached. This is also not applicable when excess pressure are caused by loads.

Under drained soil conditions ,pore pressures are known to be controlled by boundary

hydraulic conditions according to Duncan et al (2005).In his analysis Bodo and Jones

(2013), there was analysis that water within the soil may either be static or it may be

steadily seeping, this can be without a change in seepage over a period of time. There

is no decrease or increase in the percentage of water in the soil.

For instance if these conditions prevail on a site or if possible approximation of condition

is possible, drained analysis is applied. The drained analysis is done by using

1. Total unit weights of soil

2. Soil’s shear effective strength parameters.

3. Pore pressures which are determined from use of hydrostatic levels of water or

either using seepage (steady) analysis.

17

2.6.3 Undrained soil conditions analysis

In undrained soil condition, the changes in loads of soil will occur in much rapid rate

than the rate in which water can either flow into or out of the soil according to

Knappet and Craig (2012).The behavior of soil will control the pore pressures of soil,

this is in relation to the changes in external loads. If undrained conditions prevail at a

site, in geotechnical engineering, undrained analysis is appropriate way of analysis.

Total unit weights and total shear strengths parameters will be used in undrained soil

analysis according to Bodo and Jones (2013).

2.7 Short term analysis

This in geotechnical engineering, in their analysis Bodo and Jones (2013) referred it to a

soil condition such as after a construction has occurred (generally the time which is

immediate after load changing).To illustrate an example of short term soil condition is an

example of an embankment construction, for an embankment which is made of sand

soil and has a foundation on clay soil. In soil mechanics according to Knappet and Craig

(2012) the short term condition will be referred to the time required for the construction

of the embankment or the time when the construction ends. For instance if the

construction takes 3 months, the duration for the short term condition for that

embankment will be 3 months.

It would be reasonable within this particular period to assume that on drainage would be

occurring in the clay foundation as compared to the embankment which is made of sand

where full draining would have occurred. For instance in clay soils, it will take a longer

time before there in an elapse or significant total dissipation of pore water pressure.

Therefore the soil will be still undrained. Hence when it comes to analysis of total stress,

the undrained shear strength symbol CU is used.

Therefore in undrained tests the parameters will always be expressed in terms of total

stresses whilst in drained soil conditions the parameters will be represented by C’u and

. In sand soils, drainage will occur instantly and certainly before construction ends,

therefore effective stress parameters C’u and are used (Community, 2014).

18

In his analysis, Walsh (2014, p.203) he stated that when a soil is purely cohesive, the

shear resistance values at all points on the arc will equate to Cu.

2.8 Analysis of long term conditions

For instance after a certain period of time, the foundation made of clay will reach the

drained condition state. The analysis of this particular state will be done or performed as

mentioned previously on chapter 2.6.2 “Soil’s drained conditions” as both long term and

drained conditions offer same meaning. They refer to a soil condition where drainage

equilibrium has being achieved and they are no available excess pore pressure which

are due to external loading according to Bodo and Jones (2013).For instance after a

long time, the soil will have reached a fully drainage state. The effective stress

parameters, C’ and should be used. In this report, CU values which were obtained by

Glover Site Investigations Limited will be used for this research project.

2.9 Pore water pressure

To analyse effective stress well on the basis of description of water pressure. The

following could be done to describe it fully according to Atkinson (2014, p.324-327).

1. If pore water pressures are measured based on ground water levels of borehole

or either piezometers, the data that has being measured should be fully

described as well as summarised using suitable and appropriate tables or

figures.

2. It should be noted that if seepage analysis is to be done so that pore water

pressures will be computed, they should be full description of the method as well

as the full description of the computer software method which will be used for

analysis

3. They should be appropriate summary of flow nets or either pore water pressure

contours, total head or pressure head. The results will be expected to be

presented well for analysis.

19

2.10 Circular surface’s slip

In geotechnical engineering specific to limit equilibrium slope stability analysis, it is

useful requirement to analyse as may trial slip surfaces of the slope and hence finding

the slip surface that will hence offer or give the lowest factor of safety. Oasys Slope 19.0

has a wide variety of options when it comes for specification of trial slip surfaces


The critical slip surface’s position is affected by the soil’s strength at the slope. The

position of critical slip surface for a pure frictional soil i.e. (c =0) is different than for a soil

with a soil which assigned strength (φ = 0) according to Oasys Ltd (2012). This will result

in complication of the situation when it comes to analysis because in order to analyse or

locate the critical slip surface’s position, it is important as well to ensure that the

properties of the soil are well defined in terms of its effective strength parameters

accurately according to Lee et al. (2002).

2.11 The factor of safety

After the geometry of the slope and all subsoil conditions of a slope have been

measured or determined, evaluation of slope stability will be done by Oasys Slope 19.0

Software according to Oasys Ltd (2012). The primary objective of any slope stability

analysis in geotechnical engineering according to Abrasom et al. (2002) is

1. Evaluation of slope’s safety and to compute/calculate the factor of safety of

safety before the slope fails

2. To find the mechanism which is behind the slope failure, this will be important

when designing the slope to bring it to required factor of safety

Generally in slope stability analysis in the area of geotechnical engineering, there is

some valid argument or to say a doubt in the actual correct shear strength value of soils.

The loading of the soil is more known accurately due to the fact that it merely consists of

slope’s selfweight.The factor of safety is chosen as the ratio of the current available

shear strength of soil to that of the required shear stress which is required to stabilise

20

the slope . In this research project the stability of slopes will be done using methods of

limit equilibrium. The output will be a graphical output of critical slip surface of the slope.

𝐹𝑎𝑐𝑡𝑜𝑟 𝑜𝑓 𝑠𝑎𝑓𝑒𝑡𝑦 = 𝑆ℎ𝑒𝑎𝑟 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ

𝑆ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 𝑤ℎ𝑖𝑐ℎ 𝑖𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑏𝑟𝑖𝑛𝑔 𝑒𝑞𝑢𝑖𝑙𝑖𝑏𝑟𝑖𝑢𝑚 (2.2)

This can be shown as follows on Oasys Slope 19.0

Fig 2.2 Showing Factors of Safety from Individual failed slices according to Walsh

(2014, p.219)

In this report the acceptable FOS value for a stable slope, for both the drained and

undrained conditions will be 1.3.According to Walsh (2014,p.220) it is not easy in

geotechnical engineering to specifically assign a specific acceptable factor of safety for

all slopes. In his analysis he stated that the acceptable value of safety factor for each

slope will depend on slope failure causes versus the costs of achieving the required

/given factor of safety for that slope.

In general terms when it comes to slope stability analysis in geotechnical engineering,

the following guidelines show a range of acceptable safety factors according to Walsh

(2014, p.220)

1. Standard Slope – FOS will range from 1.20 – 1.40

2. Critical Slope -FOS will be 1.50

3. Marine Slope – FOS will be 2.00

BS6031:1981 Earthworks recommends a factor of safety between 1.30 and 1.40 for

a slope failure which will not consequence in fatal problems and were acceptable level

of ground investigation has been carried out. The acceptable factor of safety for this

research project will be 1.30 as recommended by BS6031:2009

21

2.12 Load on the slopes

This is referred to as the load on the slope and since that they will be no action of traffic/

carriageway structuring, load on top of the slopes will be ignored in this report according

to Oasys Ltd (2012)

2.13 Oasys Slope 19.0 Analysis

Slope stability analysis for this research project will be done with Oasys Slope software

by inputting parameters. The inputs will be as follows according to Oasys Ltd (2012)

1. Heterogeneous types of soils

2. Surface geometry of soils

3. Pore water pressure conditions.

2.14 Conclusion of Literature review

According to BS6031:2009 the acceptable minimum factor of safety of the slopes is 1.3

in this research project. Therefore it means if a slope produces a factor of safety below

1.3 it will be unstable and hence remedial slope stability methods should be applied and

if the minimum factor of safety is more than 1.3 the slope will be stable.

According to Lancellota (2009, p.423) in an undrained soil, reduction of mean stress will

occur. This will result in development of negative pore pressure in soil and after a

certain period of time the pore pressure will be dissipated and migration of pore water

will occur in surrounding areas. This will result in swelling and softening of soil which will

ultimately reduce the strength of the soil and hence the minimum factor of safety will be

expected to be achieved in long term.

22

CHAPTER 3.0: DATA COLLECTION FOR THE

RESEARCH PROJECT

23

3.1 IntroductionxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxFor

For slope stability analysis to occur, essential data is required to be imported into the

Oasys Slope 19.0 software for analysis to occur. According to Oasys Ltd (2012), the

following parameters are required

1. The geometry of the slope or the V:H ratio of the slope

2. The name of each soil layer

3. The groundwater level if it exists

4. The values of unit weights of soil

5. The condition of soil whether it is drained or undrained.

6. The shear strength parameters of the soil.

7. Angle of internal friction values.

The research study is going to be based on three selected slopes at the Giant

Causeway Visitor’s Centre and Glover Site Investigations Limited have provided a

report which provides ground investigation information as well as geotechnical report of

the site.

The ground investigations included (Glover Site Investigations Limited, 2009, p.8)

1. 10 boreholes which are from cable percussion boring to 6.9m to 14m

2. Rotary coring up to a depth of 14.1m at one borehole located as BH6,as shown

in Appendix B

3. 3 Trial pits were done.

4. Laboratory tests were done on soil‘s moisture content, density, particle size

distribution and contamination maxi suite.

3.2 Geometry of the slope

The geometry of the selected slopes, was measured during site visit at the Giant

Causeway by the researcher with the help of his fellow civil engineering students’

colleagues. This was the only way of obtaining the slope geometries' ratio data as they

were not provided in the report by Glover Site Investigations Limited.

24

Slope A geometry ratio: 1:1.78 (V: H)

Fig 3.1 showing the layout of the V: H ratio of slope A

Slope B geometry ratio: 1:2 (V: H)

Fig 3.2 Showing the layout of the V: H ratio of slope B

Slope C geometry ratio: 1:9 (V: H)

Fig 3.3 showing the layout of the V: H ratio of slope C

Existing Ground Level

V=2.50m

H=4.45m

2.50m

1:1.78 V: H ratio


V=2.30m

H=4.60m0

2.30m

1:2 V: H ratio


V=2.9m

H=5.5m0

2.90m

1:9 V: H ratio

25

3.3 Ground water levels and Name of soil layers

The names of the soil layers were provided by Glover Site Investigations Limited, the

main types of soils at the 3 slopes (Glover Site Investigations Limited, 2009, p.30 –

p.45)

1. Sandy soil

2. Gravelly silt soil

3. Sandy gravelly silt soil

Fig 3.4 Gravelly Silt Soil at a Slope failure at the Giant Causeway Visitor’s Centre

(Queens University Belfast, 2015)

The ground water levels are provided on 2 of the 3 slopes which are going to be studied

in this research project. Groundwater levels are as follows

1. Slope A : Groundwater level located at 3.50m below ground surface

2. Slope B: Groundwater level located at 4.90m below ground surface

3. Slope C: No ground water level available

3.4 Unit weight of soil

Glover Site Investigations Limited have provided bulk densities (mg/m3) and dry

densities (mg/m3).The unit weight of soil and the dry unit weight of soil will be calculated

from bulk and dry densities which were provided. According to Bodo and Jones (2013)

density of a soil is defined as the measurement of quantity of a mass in a unit volume of

the material.

26

Dry density is a measurement of the amount of solid particles per unit of volume whilst

bulk density is the measurement of amount of solid added to water per unit of volume

according to Smith (2013,p.585)

Dry density,pd =𝑀𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑠

𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 =

𝑀𝑠

𝑉 =

𝐺𝑠𝑃𝑤

1+𝑒 (3.1)

Bulk density,p = 𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠


𝑀𝑠+𝑀𝑤

𝑉 =

𝐺𝑠𝑃𝑤+𝑆𝑟𝑒𝑝𝑤

1+𝑒 (3.2)

The units which will be used in this research project are mg/m3.

Dry unit weight, yd. =𝐷𝑟𝑦 𝑤𝑒𝑖𝑔ℎ𝑡


𝑊𝑠

𝑉 =

𝐺𝑠𝑌𝑤

1+𝑒 =9.81pd (3.3)

Unit weight, y =𝑇𝑜𝑡𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡


𝑊𝑠 +𝑊𝑤

𝑉 =

𝐺𝑠𝑌𝑤+𝑆𝑟𝑒𝑦𝑤

1+𝑒 =9.81p (3.4)

Example using slope A data

On slope A, the second layer of soil is firm brown sandy gravelly silt with numerous

cobbles. The dry density is 1.93mg/m3 and bulk density is 2.23mg/m3.

Dry unit weight = 9.81 x 1.93mg/m3 = 18.93kN/m3

Unit weight of soil = 9.81 x 2.23mg/m3 =21.87kN/m3

Table 3.1: Slope A Unit weights

SOIL

Dry density

Bulk density

Dry Unit weight

Unit weight

Gravelly silt 1.84mg/m3 1.89 mg/m3 18.0kN/m3 18.50 kN/m3

Firm sandy

gravelly silt

1.93 mg/m3

2.23 mg/m3

18.93 kN/m3

21.88 kN/m3

Stiff gravelly silt

1.96 mg/m3

2.27 mg/m3

19.23 kN/m3

22.27 kN/m3

Very stiff

gravelly silt

2.01 mg/m3

2.37 mg/m3

19.72 kN/m3

23.25 kN/m3

27

SLOPE A: Soil Profile

.

Fig 3.5: Showing soil profile of slope A

Fig 3.6: Showing measurement of V: H ratios at Giant Causeway Visitors Centre


1.3 m

V=2.5m

H=4.45m

2.5m

Gravelly Silt Soil sat =18.5Kn/m3 bulk=18kN/m3

Gravelly Silt sat =18.5Kn/m3 bulk=18kN/m3

1.7 m Sandy Gravelly Silt Soil sat =21.9Kn/m3 bulk=18.9kN/m3

0.50m Stiff Sandy Gravelly Silt Soil sat =22.3Kn/m3 bulk=19.27kN/m3

3.50m Very Stiff Sandy Gravelly Silt Soil sat =23.3Kn/m3 bulk=19.7kN/m3

28

Table 3.2 Slope B Unit weights

SOIL

Dry density

(mg/m3)

Bulk density

(mg/m3)

Dry Unit weight

(kN/m3)

Unit weight

(kN/m3)

Loose brown

sand and stone

fill

1.98 2.22 19.42 21.78

Stiff brown

sandy gravelly

silt

2.10

2.35

20.60

23.05

Very stiff brown

gravelly silt

2.18

2.48

20.60

24.33

Fig 3.7 Slope failure at the Giant’s Causeway Visitor’s Centre (Queens University

Belfast, 2015)

29

SLOPE B: Soil Profile

.

Figure 3.8 Soil profile of Slope B


0.40mm

V=2.3m

H=4.60m

2.3m

Sand Soil sat =21.78Kn/m3 bulk=19.42kN/m3

Sand sat =21.78Kn/m3 bulk=19.42kN/m3

4.3 m Stiff Brown Sandy Gravelly Silt Soil sat =23.065Kn/m3

bulk=20.6kN/m3

0.20m Very Stiff Brown Sandy Gravelly Silt Soil sat =24.33Kn/m3

bulk=20.6kN/m3

2.30m Very Stiff Brown Sandy Gravelly Silt Soil sat =24.33Kn/m3

bulk=20.6kN/m3

30

Table 3.3: Unit weights of Slope C

SOIL

Dry

density(mg/m3)

Bulk

density(mg/m3)

Dry Unit weight

(kN/m3)

Unit

weight(kN/m3)

Loose sand 1.98 2.22 19.42 21.78

Firm brown

sandy gravelly

silt

1.72

2.10

16.87

20.60

Very stiff grey/

brown sandy

gravelly silt

1.91

2.18

18.74

21.39

Very stiff brown

sandy gravelly

silt

2.26

2.50

22.17

24.53

31

Slope C: Soil Profile

.

Figure 3.9 Soil Profile of Slope C

Condition of Soil

The slope soil conditions are either drained or undrained conditions of soil. The data

that has been provided by Glover Site Investigations Limited is of undrained soil

conditions and since under drained conditions ,which is long term and full drainage will

have occurred the values of shear strength will be expected to have a value of 0 for all

soil layers of the slopes.


0.4 m

V=2.9m

H=5.5m

2.9m

Loose Sand Soil sat =21.78Kn/m3 bulk=19.42kN/m3

Loose sand sat =21.78Kn/m3 bulk=19.42kN/m3

1.6 m Firm brown Sandy Gravelly Silt Soil sat =20.60Kn/m3

bulk=16.87kN/m3

1.40m Very Stiff Sandy Gravelly Silt Soil sat =21.39Kn/m3

bulk=18.74kN/m3

4.10m Very Stiff Sandy Gravelly Silt Soil sat =24.53Kn/m3

bulk=22.17kN/m3

32

3.5 Cohesion and Shear Strength

Table 3.4: Cu and C’ values of Slope A

SOIL

Cu values (undrained)

(kN/m2)

C’ values (drained)

(kN/m2)

Gravelly silt 0 0

Firm sandy gravelly silt

79

0

Stiff gravelly silt

125

0

Very stiff gravelly silt

308

0

Table 3.5: Cu and C’ values of Slope B

SOIL

Cu (undrained)

(kN/m2)

C’ (drained)

(kN/m2)

Loose brown sand and

stone fill

117 0

Stiff brown sandy

gravelly silt

184

0

Very stiff brown

gravelly silt

40

0

33

Table 3.6: Cu and C’ values of Slope C

SOIL

Cu (undrained) (kN/m2)

C’ (drained) (kN/m2)

Loose sand 0 0

Firm brown sandy

gravelly silt

5

0

Very stiff grey/ brown

sandy gravelly silt

87

0

Very stiff dark grey

slightly sand gravelly silt

223

0

3.6 Angle of Internal Friction

Glover Site Investigations Limited have provided “N” values of the soils which will be

correlated to their respective friction angles using the software. The correlation will be

done using the angle of friction correlation table (Geotechnical Information, 2015) and

the SPTCorr v.2.2.1.11 software.

Table 3.7: Angle of Internal friction values of Slope A

SOIL

SPT N number

Angle of Internal

friction

Gravelly silt 11 32

Firm sandy gravelly

silt

14

38

Stiff gravelly silt

35

42

Very stiff gravelly silt

50

44

34

Table 3.8: Angle of Internal friction values of Slope B

SOIL

SPT N value

Angle of Internal

friction

Loose brown sand and

stone fill

38

35

Stiff brown sandy

gravelly silt

48

38

Very stiff brown sandy

gravelly silt

50

45

Table 3.8: Angle of Internal friction values of Slope C

SOIL

SPT N number

Angle of Internal friction

Loose sand 34 32

Firm brown sandy

gravelly silt

34

41

Very stiff grey/ brown

sandy gravelly silt

49

44


gravelly silt

61

47

35

Table 3.10: Summary of geotechnical soil properties of slope A

V: H ratio of slope =2.50:4.45 simplified to 1:1.78

Cu u C' ' bulk sat

(kN/m2) (°) (kN/m2) (°) (kN/m3) (kN/m3)

Gravelly Silt 0 32 0 32 18.0 18.5

Firm sandy gravelly silt 79 38 0 38 18.93 21.88

Stiff gravelly silt 125 42 0 42 19.23 22.27

Very stiff gravelly silt 308 44 0 44 19.72 23.25

3.7 Summary of Data

36

Table 3.11: Summary of geotechnical soil properties of slope B

V: H ratio of slope =2.30:4.60 simplified to 1:2

Cu u C' ' bulk sat

(kN/m2) (°) (kN/m2) (°) (kN/m3) (kN/m3)

Loose brown sand and stone

fill

117 35 0 35 19.42 21.78

Stiff brown sandy gravelly silt 184 38 0 38 20.60 23.05


gravelly silt

40 45 0 45 20.60 24.33

37

Table 3.12: Summary of geotechnical soil properties of slope C

V: H ratio of slope =2.9:5.5 simplified to 1:1.9

Cu u C' ' bulk sat

(kN/m2) (°) (kN/m2) (°) (kN/m3) (kN/m3)

Loose sand 0 32 0 32 19.42 21.78

Firm brown sandy gravelly

silt

5 41 0 41 16.87 20.60

Very stiff grey/brown sandy

silt

87 44 0 44 18.74 21.39

Very stiff brown slightly

sand gravelly silt

223 47 0 47 22.17 24.53

39

CHAPTER 4.0: METHODOLOGY

40

4.1 Oasys Slope 19.0

Over the years in geotechnical engineering they are many different techniques

regarding slope stability analysis such as hand calculation or software analysis using

software such as Oasys Software or Slide. In this project Oasys Slope 19.0 Software

will be used for slope stability analysis.

According to Oasys Ltd (2012), Oasys Slope 19.0 software is an example of a modern

limit equilibrium software and is able to handle ever increasing analytical slope stability

problems. The software is capable of dealing with

1. Complex strati graphical data

2. High irregular pore water pressure soils

3. Any kind of slip shape surface

4. Either non-linear or linear shear strength models of slopes.

5. Either distributed or concentrated loads

6. Structural reinforcements

The Oasys Slope 19.0 software operates on the principle that the method of slices will

be applied more and more as a way of slope stability analysis (equilibrium formulations).

4.2 Problem Definition

For slope stability analysis the limit equilibrium was carried out by Oasys Slope 19.0 for

slope stability analysis at the Giant’s Causeway Visitor’s Centre. The data of the slope’s

geometry and soil parameters will be imported into the software, the analysis will then

be selected and the Oasys Slope 19.0 also give results of factor of safety for Swedish

(Fellenius), Janbu and Bishop Type of analysis, the factor of safety result will depend on

the analysis type which was selected.

Oasys Slope 19.0 has being primarily designed for slope stability analysis as well as

offering the option to include reinforcement for soil according to Oasys Ltd (2012). The

software can also be used in analysis of earth pressures as well as problems regarding

bearing capacities. Oasys Slope 19.0 also checks for both non-circular and circular

failures according to Oasys Ltd (2012). Therefore it means it allows calculations for rock

and soil slopes but this research project is mainly focused on stability analysis of soil

41

slopes.

Oasys Slope 19.0 offers the following three methods of analysis

1. Bishop’s method

2. Swedish Circle or Fellenius Method

3. The Janbu’s method

These three methods of analysis listed above will mean Oasys Slope 19.0 is capable of

computing both circular as well as non-circular surfaces. The location or position of the

circular surfaces is then defined through use of rectangular grids of centers as well as

different numbers of radii and common point whereby all the entire circles will pass or

either which tangential surface will touch. However non – circular slips are individually

defined in series of x co-ordinates and y co-ordinates according to Oasys Ltd (2012).

1. The actual ground section is built or imported into the software by specification of

each material’s layer. This is done from the surface downwards, in terms of

x-coordinates and y co-ordinates series.

2. To specify the strength of each material, this is done by specification of soil’s

cohesion value and the angle of shear resistance of soil. Cohesion’s linear

variations with depth will also be entered.

3. The ground water profile as well as distribution of water pressure can also be

individually set for each stratum of the soil. This is done either by

Applying a phreatic surface with the use of hydrostatic distribution of

pore pressure

Using a phreatic surface with a piezometric (user-defined) distribution

of pore pressure

Using a coefficient value of Ru which is an overall value

For each stratum the maximum suction of the soil can also be

specified.

4. Use of combinations such as reinforcements ,which consists of either geotextiles

(horizontal),rock bolts ,soils nails which are inclined as well as ground anchors

can be specified. According to BS8006:1995 the moment of restoration is

42

contributed through the use of reinforcement.

5. The soil slopes which are either partially submerged or full submerged can also

be analysed

6. Through the application of external forces onto the ground surface, these can be

used as a way of representing loads such as building loads

7. Through the use of slip mass’s horizontal acceleration, earthquake loading can

be included.

8. Finally, the factor of safety which will be calculated will then be applied to the

strength of the soil or the magnitude relating to the loads applied ,either by

Causing failure of slope (as a way of bearing capacity representation)

Preventation of slope failure

The three methods which are available for slope stability analysis are

1. Fellenius or Swedish method

2. Bishop method

The variably inclined interslice force method

The horizontal interslice force method

The parallel inclined interslice force method (also known as spencer’s

method)

3. Janbu method

The horizontal interslice force method

The variably inclined intersliced force method

The parallel inclined intersliced force method (also known as spencer’s

method)

However it should be noted that the use of method of slices is used , as to determine

the factor of safety of the slopes regarding stability.

43

4.3 Modelling of analysis problem

Shear strength is one of the most important geotechnical parameter when it comes to

slope stability analysis and one of the ways of used to describe shear strength, is by use

of the Coulomb equation according to Abrasom et al. (2002).

τ = c + σn´tanφ … … … … … … … ( 4.1)

Whereby τ will be the shear strength

c is the cohesion value,

σ´n is the normal stress on shear plane

Φ is the angle of internal friction value

The failure envelope is usually determined by the use of triaxial test and the results are

usually presented in terms of half-Mohr circles according to Knappet and Craig (2012).

The strength parameters of the soil occupying the slope such as c and φ can be used as

total strength parameters or effective strength parameters. Oasys slope software cannot

do this type of input but the user has to input the parameters, however it should be

noted that Oasys Slope 19.0 is not able to distinguish these 2 sets of data parameters.

In slope stability analysis in geotechnical engineering, when it comes to slope stability

analysis, using effective strength parameters will offer a more realistic solution. This is

particularly important when considering the position of critical slip surface according to

Oasys Ltd (2012)

4.4 Oasys Slope’s Methods of Analysis

4.4.1 Type of analysis

The hypothesis in slope stability analysis begins with the fact that stability of any slope

depends on the result of downward also called motivating forces such as gravitational

forces and upward or resisting forces. For the slope to be stable enough the resisting

forces must be greater than the forces caused by motivating forces according to Murthy

(2002,p.368).

44

The stability of slopes in geotechnical engineering is determined by analysing the factor

of safety

Factor of safety = ∑ 𝑅

∑ 𝑀 (4.2)

Equation 4.2 states that the factor of safety is defined as the ratio between forces or

resisting moments (R) and the forces or moments which are motivating (M).

Theory of slices according to Oasys Ltd (2012)

Fig 4.1 Showing basic annotation and sign convention of methods of slices (Oasys Ltd,

2012)

F: Represents the factor of safety value

Ph: Represents horizontal component of external loads

Pv: Represents vertical component of external loads

E: The horizontal interslice force

X: The vertical interslice force

W: Soil’s total weight

N: Total normal forces which will be acting along a slice base

R: Distance between the slice base to the moment centre

S: Shear force that will be acting along slice base

h: Slice’s mean height

b: Slice’s width

L: The length of slice’s base (b/cosx)

45

u: The pore pressure that will be at slice base

- Represents the slice base angle which is to horizontal

x – Represents the horizontal distance between the slice and moment centre y - Represents the vertical distance between the slice and moment centre

-Represents the unit weight value of soil

c – Represents the cohesion value at base - Represents the angle of friction value at base

4.4.2 Ordinary Method of Slices

The ordinary method of slices will neglect all forces which are inter-slice forces. It also

fails to satisfy equilibrium forces and this includes individual slices as well as slide

masses. According to Fellenius (1936) the ordinary method of slices is one of the

simplest procedures when it comes to slope stability analysis. The ordinary method of

slices is also known in geotechnical engineering as Swedish method of slices

4.4.3 Simplified Bishop Method

The Simplified Bishop Method is a slope stability analytical method which works by

assuming that vertical interslice shear forces will not exist. According to Bishop (1955)

the interslice forces which are resultant forces will be horizontal. According to Oasys Ltd

(2012) this will result in satisfaction of the equilibrium of the moment but will not result in

the equilibrium of forces.

4.4.4 Janbu method

For analysis the Janbu method uses horizontal forces of equilibrium equation according

to Oasys Ltd (2012).This will be done as to obtain FOS value. However it should be

noted that the Janbu method will not include interslice forces as part of analysis, but it

will account this for its correction factor. The correction factor will be related to

1. Cohesion value

2. Friction angle value.

3. The shape of failure surfaces.

46

4.5 Oasys Slope: Method of Iteration

In slope stability analysis, the Oasys Slope will use iteration as a way of convergence

for each of either of Janbu method and Bishop methods.

4.5.1 Relation of Oasys Slope 19.0 to Factors of Safety

For slope stability analysis according to Oasys Ltd (2012), each iteration which will refer,

Oasys Slope 19.0 will compute/calculate the factor of safety. This is done by use of the

ratio between the restoring moments to the disturbing moments (known as a function of

Fi -1). The calculation will be complete when the difference between the 2 factors of

safety are within tolerated specified value. The FOS is defined as the ratio between

restoring moment to disturbing moment. An iterative solution is always necessary, as

this ratio is always a function of F, therefore it should be noted that this is not applicable

to Swedish Circle Method.

4.5.2 Analysis of Horizontal Intersliced forces

According to Oasys Ltd (2012) the horizontal intersliced forces will be analysed as

follows

1. The slope will start at slice numbered as 1.The slices are numbered from left

direction to right direction. Through maintenance of vertical equilibrium, the

resultant horizontal force is then calculated

2. Oasys Slope 19.0 will then use the force as interslice force from slice number

2.This process is then continued until the very last slice, this will end up with a

calculated resultant force.

This particular method of analysis is based on the basis that each slice and the slope as

a whole will be in vertical equilibrium, with 0 vertical interslice forces. Therefore,

horizontal equilibrium will not be achieved within each slice or as a whole slope.

Therefore it can be concluded that the only force check within each slice in the slope will

be for vertical equilibrium according to Oasys Ltd (2012)

4.5.3 Analysis of Constant Inclined Intersliced forces

In this particular method according to Oasys Ltd (2012), the slope being analysed will

vary with ratio, which is always constant between vertical interslice forces and horizontal

47

interslice forces. In this particular method the slice will not be in equilibrium, only the

entire slope will be in equilibrium. Therefore when calculating, equilibrium will be

effectively maintained for each slice which will be in the direction normal to the

intersliced forces according to analysis done by Oasys Ltd (2012).

4.5.4 Analysis of Variably inclined intersliced forces

According to Oasys Ltd (2012) this particular method is superior as compared to other

methods, as it will maintain every slice in vertical equilibrium and horizontal equilibrium

at all times. Therefore it should be noted that, it may result in exceeding the strength of

the soil which will be along the slice interface. This is because the method doesn’t check

vertical interslice forces against the materials’ shear strength. Therefore it means results

will have to be validated or checked for this particular method.

The interslice forces are adjusted both for vertical direction and horizontal direction

separately. This is done by addition of fraction of residual values which are from the

previous iteration. To determine the fraction, this is done through the use of horizontal

length of the slip surface. This is the slip surface that the slice will be representing.

Therefore the interslice direction will vary by this particular method, but however it

should be noted that each slice will be in equilibrium at all times as in the whole slope


4.6 Positioning of the slices

According to Oasys Ltd (2012) Oasys Slope 19.0 will divide each slip mass into slices.

The slice boundaries which are a result of this dividing process are located on the

following points on the slope

1. When there is a gradient change of stratum

2. At each stratum’s intersection or slip surface

3. At each’s phreatic intersection

4. At slice’s midpoint whose width will be greater than known average slice width.

This is given by the following formula

𝑋𝑟𝑖𝑔ℎ𝑡−𝑋𝑙𝑒𝑓𝑡

𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑠𝑙𝑖𝑐𝑒𝑠 (4.4)

48

4.7 Research project adopted method

4.7.1 Bishop methods

According to Bishop (1955), the bishop’s methods mentioned above can be used on

circular surfaces. It should be noted that if reinforcement is specified, one of Bishop’s

methods should be applied.

3 methods of solution are available under Bishop. The methods are

1. Parallel Interslice forces

2. Variably Inclined Intersliced forces

3. Horizontal Intersliced Forces

4.7.2 Horizontal Interslice forces: Simplified Method

This method can be used to all circular slip forces according to Oasys Ltd (2012). They

are several assumptions which are made for this method, the assumptions are as

follows

1. The shear forces on the interslice are all assumed that the sum is 0.This will

satisfy vertical equilibrium but it will not satisfy horizontal equilibrium

Whereby Xn – Xn+1 =0 (4.5)

This will lead to errors in the final values of factor of safety. The errors are usually

small and as well on the safe side

2. The overall moment of equilibrium is satisfied by each method

4.7.3 Parallel Inclined Intersliced Forces

This analytical method is also known in geotechnical engineering as Spencer’s Method,

this method can be applied to circular slip surfaces. It is a better method than Bishop’s

Simplified Method and it satisfies the horizontal equilibrium, moment equilibrium and

vertical equilibrium as a whole according to Oasys Ltd (2012).

The assumptions which are made of this method are

1. The Oasys Slope 19.0 Software will assume that all interslice forces will be

parallel but however not horizontal.

2. The method also satisfy the overall horizontal equilibrium and vertical

49

equilibrium

3. This analytical method will also satisfy overall moment of equilibrium

The difference between the 2 methods mentioned above depends with the increase or

decrease of slope angle. For steeper slopes Spencer’s method is recommended and it

is more accurate than the other method. However they maybe problems of interlock and

if this is suspected, the variably inclined interslice force method will be used.

4.7.4 BISHOP’S METHOD: VARIABLY INCLINED INTERSLICE FORCES

This analytical method can be applied on circular slip surfaces. It is a better method

than the previous Bishop methods above, it has being designed to overcome interlock

problems. The assumptions made for this method according to Oasys Ltd (2012) are

that

1. The Oasys Software will compute the interslice forces as to maintain horizontal

equilibrium and vertical equilibrium of slices

2. The inclinations of the interslice forces will also be varied until overall horizontal

equilibrium, vertical equilibrium and moment equilibrium are achieved in each

iteration.

The Bishop’s variably inclined interslice forces method will be used in this research

project as it offer analysis on circular slips as well the advantage of overcoming

interlocking problems.

4.8 Data entry and FOS calculation

The data will be imported into the Oasys Slope 19.0 as follows

1. Units of data will be imported

2. Parameters will be imported

3. Analysis option chosen

4. Method of Partial Factors is chosen

5. Titles of work tasks assigned

6. Materials imported into slope

7. Groundwater levels assigned

8. Strata levels assigned

9. Slip surfaces assigned

50

10. Reinforcement/Loads on slopes.

Units of Data

Direction of slip: Chosen direction: Downhill

Minimum slip weight: 100kN/m

Analysis type: Static

Analysis

Chosen factor of safety on: Shear strength

Number of slices (minimum):25

Method chosen: Bishop-variably inclined interslice forces

Maximum iterations chosen (number):100

Reinforcement (Yes/No): Not Active

Fig 4.2: Showing Inputs of Slip surfaces of Slope B

Partial Factors

Selected: SLS

Factor number on dead load: 1.0

Factor number on live load: 1.0

Factor number on the unit weight of soil: 1.0

Factor number on the cohesion value of drained soil: 1.0

51

Factor number on the cohesion value of undrained soil: 1.0

Factor number on the friction angle of soil: 1.0

Correct moment factor: 1.0

Reinforcement pull out factor: 1.0

The economic ramification of failure: 1.0

Sliding reinforcement factor: 1.0

Fig 4.3: Showing inputs of Partial factors of Slope B

Materials

Column 1: Soil description

Column 2: Unit weight of soil either above or ground water level

Column 3: Soil condition (either drained –linear or undrained)

Column 4: C or Co

Specification of Slip surface

Circle Centre: Grid

Left grid (bottom): x =45

Y=45

Grid centers: 40 in x –direction at 2.50m spacing

20 in y –direction at 2.50m spacing

Extension of grid: Grid fully extended as to find the minimum factor of safety

Initial radius: 2.5m chosen

Incrementation: Increased by 1.50m until entire consideration of all circles

Reinforcement used: No reinforcement used

52

4.9 Verification of data and Computation of Factor of Safety

When the slip surface has been specified the Oasys Slope 19.0 will run several checks

as to verify the data. When there is satisfaction of verification and if they are no errors

the Oasys Slope software will then compute the factor of safety. This is done according

to method of slice selected according to Oasys Ltd (2012). In this research project,

Bishop Method with variably inclined forces is used. The minimum factor of safety will

then be displayed as part of output of the software as well as displaying critical slip

surface

Fig 4.4 Showing output from Oasys slope 19.0 according to Oasys Ltd (2012)

53

CHAPTER 5.0: RESULTS AND DISCUSSION

54

5.1 Results and Discussion

The stability of the three selected slopes at the Giant Causeway Visitors Centre were

analysed for both drained and undrained conditions, limit equilibrium method (Oasys

Slope 19.0) was used for analysis of stability. The graphical outputs, tabular outputs and

the factor of safety results for slope stability analysis of the three slopes are presented

in Appendix A.

The Appendix shows the safety factors of the slope which have being computed by

Oasys Slope 19.0 both for undrained and drained soil conditions. Modified Bishop

Method with variably inclined intersliced method was used. In this research study there

was analysis of both drained and undrained soil conditions, because drained conditions

refer to condition in soil where drainage is allowed and undrained condition refer to a

condition where drainage is restricted, this will result in different soil strength between

drained and undrained soil.

Slope A

Soil properties were evaluated from a combination of SPT test, SPTCorr v.2.2.1.11

software, correlation soil’s properties table (Geotechdata, 2015) and ground

investigation data provided by Glover Site Investigations Limited. The geotechnical soil

properties are shown on fig 5.1 and fig 5.2.The ground water table which was found in

this area was approximately 3.50m below ground level and the V: H ratio or the gradient

of the slope was 1:1.9 .The measured height of slope was 2.50m and the horizontal

distance was 4.75m,hence

𝐺𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑙𝑜𝑝𝑒 𝑜𝑟 𝑉: 𝐻 =2.50

4.00 =1:1.78(5.1)

Soils at the slope are as follows (Glover Site Investigations, 2009, p.5)

1. Gravelly Silt

2. Firm Sandy Gravelly Silt

3. Very Stiff Gravelly Silt

55

Fig 5.2 Slope A: Drained analysis in Oasys Slope 19.0

Fig 5.1 Slope A: Undrained analysis in Oasys Slope 19.0

56

Factor of Safety

The factor of safety for undrained conditions is 1.112 and for the drained conditions is

1.112 this shows that the slope is not stable both in short term (undrained) and long

term (drained) conditions. The graphical output are shown in Appendix A. The minimum

factor of safety for both conditions is less than 1.3. This is less than the suggested

acceptable factor of safety for a stable slope in this research project according to

BS6031:2009. Therefore remedial measures have to applied for the slope to be stable.

In accordance with BS6031:1981 Earthworks both conditions for drained and undrained

conditions do not satisfy the range of 1.20 -1.40 and also less than the suggested

adopted factor of safety (1.3) for this research project for a stable slope as

recommended by BS6031:2009.The instability of the slopes according to Duncan et al.

(2005) can be reached by a decrease in the shear strength of the soil at the slopes or by

an increment in the shear stress that is required for equilibrium.

According to McDonnell et al. (2000) she stated that one of the causes of slopes failures

at the site was heavy rainfall. High rainfall will result in increased pore pressure which

will ultimately result in decrease of effective stress in soil. This will affect all soil types

according to Duncan et al. (2005).The difference in permeability of the soils, will result in

different time length required for pore pressures to change. Therefore due to decrease

in effective stress the slopes will become less stable.

High rainfall will also result in increment of the weight of the soil. The infiltration as well

as the seepage of the water into the soils at the slope, will ultimately increase the soil’s

water content and thereby the weight of the soil will be increased. The ultimate effect

will be the increase of the shear stress according to Duncan et al. (2005). This will result

in less stability of the slope. As the slope fails, there is usually a precedence of crack

development. The cracks will usually develop near the slope’s crest. The ultimate result

in cracking of the soil is losing the strength on the crack’s plane. This will result in

reduction in the stability of the soil

57

5.2 Slope B


software, correlation soil’s properties table (Geotechdata, 2015) and ground

investigation data provided by Glover Site Investigations Limited. The geotechnical soil

properties are shown on fig 5.3 and fig 5.4.The ground water table which was found in

this area was approximately 4.90m below ground level and the V: H ratio of the slope

was 1:2.The measured height of the slope was 4.60 and the

𝐺𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑙𝑜𝑝𝑒 𝑜𝑟 𝑉: 𝐻 =2.30

4.60 =1:2 (5.2)

Soils at the slope are as follows (Glover Site Investigations Limited, 2009, p.6)

1. Loose brown sand

2. Sandy gravelly silt

58

Fig 5.3 Slope B: Undrained analysis of slope by Oasys Slope 19.0

Fig 5.4 Slope B: Drained analysis of slope by Oasys Slope 19.0

59

Factor of Safety


1.918 this shows a decrease in safety factor as well as stressing that the slope is stable

both in short term (undrained) and long term (drained) conditions. The factor of safety

for both conditions is more than 1.3 which is more than the acceptable factor of safety

for a stable slope (1.3). Therefore no remedial measures have to apply for the slope to

be stable.

In accordance with BS6031:1981 Earthworks both conditions for drained and undrained

conditions do satisfy the acceptable range of factor of safety 1.20 -1.40 and also

satisfies the factor of safety of 1.3 for this research project as recommended by

BS6031:2009.It can be concluded that Slope B is stable and no remedial measures are

required to stabilise the slopes.

According to Duncan et al (2005) the decrease of factor of safety of a slope is caused

by two major points which are decrease in shear strength of soil or the increase of shear

stress required to achieve equilibrium. The factor of safety values for slope B values

have decreased from 6.816 (undrained) to 1.918 (drained).The shear strength values of

the soil occupying the slopes have decreased from undrained to drained state and

ultimately according to Duncan et al (2014) this will decrease the factor of safety in long

term. The decrease in shear strength of soil will affect the overall factor of safety value

of the slope but in this instance according to BS6031:2009 the slope is stable in both

long and short term.

60

5.3 Slope C


software and ground investigation data provided by Glover Site Investigations Limited.

The geotechnical soil properties are shown on fig 5.5 and fig 5.6.There was no ground

water table which was found in this area and the V: H ratio of the slope was 1:9 .The

measured height of the slope was 2.9m and the measured horizontal distance was

5.5m, hence

𝐺𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑙𝑜𝑝𝑒 𝐶 𝑜𝑟 𝑉: 𝐻 =2.9

5.5= 1: 9 (equation 5.3)

Soils at the slope are as follows

1. Loose brown sand

2. Sandy gravelly silt

61

Fig 5.5 Slope C: Undrained analysis of slope (short term)

Fig 5.6 Slope C: Drained analysis of slope (long term)

62

Factor of Safety


1.413 this shows an increase in safety factor as well as stressing that the slope is not

stable in short term (undrained) and stable in long term (drained) conditions. The factor

of safety for short term (undrained) is less than 1.3 which is less than the acceptable

factor of safety for a stable slope (1.3) hence the slope is not stable in short term hence

remedial measures have to be applied for the slope to be stable according to

BS6031:2009.

Generally when it comes to geotechnical engineering specifically to BS6031:1981

Earthworks a required factor of safety value for a standard slope is between 1.20 – 1.40

and based on that information the safety factor produced under drained conditions

(0.720) is not stable and stable to support a slope. The factor of safety of the slope

under drained conditions is more than 1.3 which means the slope will be stable in long

term.

According to Lancellota (2009, p.423) in a saturated soil for instance, reduction of mean

total stress will occur. A negative pore pressure in soil will develop and as time goes on

the pore pressure will be dissipated. Migration of pore water will occur in surrounding

areas in the soil. This will result in swelling and softening of soil which will reduce

strength hence the minimum factor of safety will be achieved in long term conditions

according to Abrasom et al (2002).

63

Fig 5.7 Showing graphical output of Slope C (undrained /short term condition)

The Oasys Slope 19.0 will display the FOS of the slope as shown on fig 5.7 as

0.720.The software also shows the critical slip surface as well respective soil layers

occupying the slope.

Graphical Output of Slope C (Undrained /Short term condition)

64

Fig 5.8 Showing tabular output of Slope C (undrained /short term condition)

The Oasys Slope 19.0 software also show results in tabular form as shown on fig

5.8.The tabular form will show inputs such as soil strata layers ,analysis options as well

as displaying the minimum factor of safety of the slope.

Tabular Output of Slope C (Undrained /Short term condition)

65

5.4 Conclusion of Results and Discussion

After the computation of factor of safety of all the slopes, according to the suggested or

recommended factor of safety of 1.30 by BS6031:2009, we can conclude that

1. Slope A – the slope is not stable in both short term and long term

2. Slope B – the slope is stable in both short term and long term

3. Slope C – the slope is not stable in short term but stable in long term

After the output of safety factors by Oasys Slope 19.0 it can be concluded that remedial

works will be required for Slope A and Slope C as to bring the slopes to the required

safety factor (1.3) which will mean the slopes will be stable.

The remedial works required for each slope will depend on the required factor of safety

for each slope versus the cost of achieving that particular factor of safety, as to stabilise

the slope according to Walsh (2014,p.220).Generally in geotechnical engineering they

are two main methods which can be applied as to stabilise the slope

1. Primary method :These remedial works will immediately take action in stopping

the slide from further occurring

2. Secondary method: These remedial works are useful in ensuring longevity in

stabilisation of the slope and can also be useful in preservation of primary

treatments.

The primary methods available for stabilising the slope are as follows, they are arranged

in order of preference according to Community (2014)

1. Regrading the slope : This has effective effect and also there is high probability

that it will become uneffective with time

2. Drainage: This particular method can be put into use if regrading of the slope is

considered impractical to be applied. This is mostly effective immediately in soils

which are highly permeable. This method will take more effect on soils which are

fine grained.

3. Corporating structural components into the slope: These structures can be

active and effective such as stressed nails or anchors. They can also be passive

such as walls or sheet piling. Generally passive schemes will only take effect on

66

further movement of slopes which is not desired in slope remedial application.

Secondary methods maybe applied as well in slope stability if required. They are useful

in maintaining the stability of the slopes for long term. They can also be useful in

preserving primary treatments. The following secondary methods maybe used

1. Shallow and deep methods

2. Geotextiles

From the results produced by Oasys Slope 19.0 as shown in Appendix A, it can be

concluded that there is a distinction between drained and undrained strength of soils

especially cohesive soils. This is due to the fact that there is restricted movement of

water in cohesive soils as compared to coarse grained soils such as clay soils. For

cohesive soils such as clay soils it may take a long time before there is a complete

dissipation of excess pore water pressure, this is before the achievement of effective

equilibrium.

In drained and undrained analysis of soils according to Lancellota (2009, p.423), he

stated that in a saturated soil they will be a reduction of mean total stress. This is

followed by development of negative pore pressure in the soil and as time goes on the

pore pressure will be dissipated and migration of pore water will occur in the soil. This

will effectively results in swelling and softening of the soil which will reduce the strength

of the soil hence the minimum factor of safety is expected to be achieved in long term

conditions according to Abrasom et al (2002).

67

CHAPTER 6.0: FUTURE RECOMMENDATIONS

AND CONCLUSION

68

6.1 FUTURE RECOMMENDATIONS AND CONCLUSIONS

Slope A and Slope C will require application of remedial works as both their factor of

safety values are under the suggested FOS of 1.3, as recommended by

BS6031:2009.According to Walsh (2014,p.220) the acceptable remedial works for each

slope will depend on the remedial work versus the costs accumulated to bring that

required factor of safety.

Suggested Remedial works for the slopes

6.1.1 Changing Slope geometry

Generally slope stability decreases with increase in height of the slope, as the slope

height increases, the shear stress which is within the toe of slope will increase due to

extra added weight. Shear stress is also affected by slope angle. If the slope angle is

decreased or the gradient of the slope is decreased, the shear stress will decrease and

according to Duncan et al. (2005) the factor of safety will increase. This will increase the

stability of the slope.

Changing geometry of the slopes A and C will increase the stability of the slopes. This is

done by either by

1. Excavation as to unload the slopes

2. Filling as part of the slopes

3. Reduction of the overall height of slope.

However it should be noted that when excavation and or filling are used as part of slope

remedial measures, it is important to ensure correct positioning and obliging to the

neutral point concept (Environment, 2015)

Fig 6.1 Showing modification of slope geometry (Environment, 2015)

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Fig 6.2 Showing modification of slope geometry as to stabilise the slope (Environment,

2015)

6.1.2 Retaining Structures

Retaining structures such as use of piles, walls or anchors maybe used as a way of

stabilising slopes A and C. It should be however be noted that as well as appreciated

that the forces as well as the moments that these forces are subjected to maybe very

large. Henceforth, engineers will need to be careful when it comes to designing them.

Fig 6.3 Slope stability method being used in form of walls (Gabion1, 2015)

70

In slope stability analysis regarding retaining structures, it should be noted that retaining

structures are not really considered the most effective remedial measure. This is due to

the fact they are very difficult to implement on an already moving slide according to

Menzies and Murphy (2001). It should however be noted that they are commonly used

in ensuring complete stability of already existing landslide, which may be reactivated in

future.

The interslice forces from stability analysis which has being mentioned in chapter 4.0,

will be used to estimate the forces that will be acting on the retaining wall. The retaining

wall will provide required resistance which is only actively mobilized by the further slope

deformation according to Duncan et al. (2005). The force will then act along the line of

action as shown on Fig 6.4 into either the soil or rock slope, but specifically to soil

slopes at the Giant Causeway Visitors Centre.

Fig 6.4 Demonstrating use of retaining wall in Slope Stability (Community, 2014)

6.1.3 Geotextiles

These are manmade, usually they are plastic based soil reinforcement materials. In

slope stabilisation, geogrids are usually used. One use of them is to apply an

embankment fill, this will effectively reduce the amount of landslide movement as well

keeping the slope in good place according to Abrasom et al .(2002). Geogrids are

occasionally used as anchors, this will provide a reaction against the disturbing

71

moments. They are occasionally used in repairing small engineering earthworks and

they are usually effective if applied well.

Fig 6.5 Showing use of geogrids in slope stability (Community, 2014)

6.1.4 Grassing the slope

This method can be applied to all the slopes ,this including Slope B.The grassing

method is a slope remedial method whereby the slope is covered by grass or sand ,this

will effectively as well as immediately result in reduction of the amount of water that can

infiltrate into the slopes according to Knappet and Craig (2008). This is an inexpensive

method which if applied at slopes will be simple as well effective in long term whilst

effectively stabilising the slopes.

6.1.5 Drainage

Drainage is the least effective method that can be used as a remedial measure at the

Giant Causeway’s slopes due to the fact that although drainage is effective in stabilising

the slopes in short term ,in long term these drains will require lots of maintenance as

well as repair according to Duncan et al. (2005) . This is often expensive as well as

difficult to perform, making it less desirable remedial measure. The drainage method is

effective in soils were regrading of the slope is considered impractically impossible to be

done according Abrasom et al. (2002). Drainage also has effective use in high

permeable soils and also will take more time to be fully effective in fine grained soils and

the most common drainage remedial work in slopes is surface drainage.

72

Fig 6.6 Demonstrating use of drainage systems in slope stability (Community, 2014)

6.2 CONCLUSIONS

Natural slope instability is a major problem and concern at the Giant Causeway Visitor’s

Centre. The failure of the slopes might result in a dangerous destruction of this natural

monument. The failures of the slopes can be concluded as being triggered by internal or

external factors which will result in internal changes of soil such as the rise in pore water

pressure or forces which are imbalanced according to Duncan et al (2014). These are

forces which may be caused by external loads.

It can also be concluded that the there is a distinction between drained and undrained

conditions of soil. Shortly, undrained condition is referred to as a condition where

drainage is restricted and drainage condition, to a condition where drainage is

permitted. The factor of safety for undrained conditions was different to that of those of

drained conditions for all the slopes, but the factor of safety for all the slopes was below

1.3 recommended by BS6031:2009 for the 2 out of 3 evaluated slopes.

73

Therefore it can be concluded that 2 out of 3 slopes at the Giant Causeway Visitors

Centre which have been studied in this research project are not stable. Therefore it

means remedial measures have to be applied to stabilise the slopes.

The primary duty of an engineer is to design a structure economically without affecting

its strength. In this research project specific to slopes, it can be concluded that steep

slopes will require less earth work and less cost but this will also mean the overall factor

of safety of the slope will be reduced according to Abrasom et al .(2002). For instance

when the V: H ratios of the slopes are altered, the factor of safety of the slopes will

change.

At the Giant Causeway Visitors Centre another option will be to have a provision of

reinforcement to the slopes or to apply retaining walls to the slopes. This will effectively

decrease amount of volume of earth work required, however the cost of applying these

structures can be expensive according to Chandler (1991, p.77-101.). It should be noted

also, the construction of any structure will depend on the cost of land. The Giant

Causeway is located outside urban area, therefore cost of land might be cheaper but

applying adequate reinforcement or retaining walls might also decrease the cost

financially (Belfast City Council, 2015)

74

References

Abramson, L., Lee, T., Sharma, S., Boyce, G. (2002) Slope Stability and

Stabilization methods. 2nd edn.USA:John Wiley and Sons

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edn.London:CRC Press

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and Split –Barrel Sampling of soils, West Conshohocken, PA: , American Society

for Testing and Materials

Belfast City Council (2015) Council rates. Available at:

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(Accessed:6 September 2015)

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Slope.Geotechnique,Vol.5 (No. 1),pp.7-17

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United Kingdom: British Standards Institution

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Earthworks.London:BSI Geotechnical Design

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London: Thomas Telford

Chen,W. , Liu,X. (1990) Limit Analysis in Soil Mechanics.USA:Elsevier Science

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June 2015)

Das, B.M. (2008) Advanced Soil Mechanics. USA :Taylor and Francis

Duncan,J. Wright,S. Brandon,L. (2005) Soil strengths and slope

stability.Canada:John Wiley and Sons

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Visitor’s Centre Ground Investigation and Geotechnical Report. Northern Ireland:

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Slope/Pdf/02%20Causes%20of%20slope%20failure.pdf (Accessed : 1 July 2015)

Knappet, J. and Craig, R. (2012) Craig’s soil mechanics .8th edn. London:Spon

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engineering.London:Thomas Telford

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Mechanics and Foundation Engineering. New York: Marcel Dekker

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Oasys Ltd (2012) Slope Version 19.0.London:Oasys Ltd

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Appendix A: Oasys Slope 19.0 Graphical and Tabular Outputs

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Appendix A

Drained Conditions for Slope A (Graphical Output)

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Tabular Output of Slope A – Drained Condition

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Slope A: Undrained Conditions (Graphical Output)

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Tabular Output of Slope A – Undrained Condition

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Slope B: Drained soil conditions

83

Tabular Output of Slope B– Drained Condition

84

Slope B: Undrained Soil Conditions

85

Tabular Output of Slope B – Undrained Condition

86

Slope C: Drained Soil Conditions

87

Tabular Output of Slope C – Drained Condition

88

Slope C: Undrained Soil Conditions

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Tabular Output of Slope C – Undrained Condition

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Appendix B: Giant Causeway Visitor’s Centre Map Area

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APPENDIX C:Soil laboratory tests, borehole

data and Geotech data table

102

co

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The soil friction angle values for this research were compiled using the SPT-N values obtained by Glover Site

Investigations Limited. SPTCorr v.2.2.2 software was used with (SPT-N) – soil’s friction angle correlation table which is

according to Geotechdata (2015).The table is shown on next page (page 103 and page 104)

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