Rock Slope Stability of Cliff End

Rock Slope

Stability of

Cliff End

University of East London

Nima Golzar Soufiani

U0737756

Acknowledgments

I would like to first of all thank my Mother and Father and sister for endlessly

supporting and believing in me even when I didn’t believe in myself. Without

their support, encouragement and belief, I would never be where I am today nor

would I be the man that I am today.

I would also like to thank Mr Richard Freeman for giving me the chance to take

part in this project and for giving me the chance to be supervised by him. Mr

Freeman’s advice as well as he’s encouragement and enthusiasm to help us in

any way possible was truly a source of inspiration for me to complete this

project to the best of my ability.

It is no exaggeration to say that without the help and advice from Mr Trevor

Rhoden, this project may not have been completed. He’s help, advice, and

patience with us in the laboratory tests was inspirational and for that I would like

to thank Mr Trevor Rhoden as well.

Last but not least I would like to thank all of my friends on my course, especially

Yosef Andom who from the foundation year shared the good and bad times with

me. Without the encouragement and inspiration from extraordinary friends like

Yosef Andom, Hassan Skaiky and Prajee Embogama as well as many others in

my class, this course would never have been as enjoyable. I feel honoured and

privileged to have had the chance to share this journey with them.

Thank you all.

Decleration

I confirm that no part of this coursework, except where clearly quoted and

referenced, has been copied from material belonging to other person.

Contents

List of figures ................................................................................................... 1

List of Table ..................................................................................................... 6

Equations ........................................................................................................ 8

Preface ............................................................................................................ 9

CHAPTER 1 - INTRODUCTION ....................................................................... 10

CHAPTER 2 – LITERATURE REVIEW ............................................................ 12

2.1 Discontinuities ......................................................................................... 12

2.2 Joints and Faults ..................................................................................... 14

2.3 Orientation ............................................................................................... 18

2.4 Stereographic analysis ............................................................................ 20

2.5 Slope instability mode identification ......................................................... 27

2.5.1 Wedge failure .................................................................................... 28

2.5.2 Plane failure ...................................................................................... 29

2.5.3 Toppling failure ................................................................................. 30

2.5.4 Circular failure ................................................................................... 31

2.6 Rock instability causes ............................................................................ 33

2.6.1 Weathering ....................................................................................... 33

2.6.2 Erosion and deposition ..................................................................... 34

2.6.3 Earthquake ....................................................................................... 36

2.7 Properties of the rock .............................................................................. 38

Driving force and Restoring force .................................................................. 40

2.8 Rock laboratory tests ............................................................................... 41

2.8.1 Point load test ................................................................................... 41

2.8.2 Slake durability test ........................................................................... 42

2.8.3 Pundit test ......................................................................................... 43

2.8.4 Undrained Triaxial test ...................................................................... 44

2.8.5 Consistency limit – penetrometer method ......................................... 45

2.9 Stabilisation of rock slope ........................................................................ 46

2.9.1 Rock bolt ........................................................................................... 47

2.9.2 Shotcrete .......................................................................................... 49

2.9.3 Anchored Wired mesh ...................................................................... 51

2.10 Site selection ......................................................................................... 52

2.11 Geology of Cliff End .............................................................................. 59

2.12 Travel log ............................................................................................... 69

2.12.1 November 14th 2010 ....................................................................... 69

2.12.2 November 15th 2010 ....................................................................... 70

2.12.3 November 18th 2010 ....................................................................... 71

2.13 Petrology ............................................................................................... 78

CHAPTER 3 – LABORATORY/FIELD RESULTS ............................................. 79

3.1 Point load test .......................................................................................... 79

3.1.1 Results .............................................................................................. 84

3.1.2 Formulas used for calculations ......................................................... 84

3.2 Pundit test ............................................................................................... 85


3.3 Slake durability ........................................................................................ 86

3.3.1 Results .............................................................................................. 87


3.4 Consistency limit ..................................................................................... 88

3.4.1 Results .............................................................................................. 92


3.5 Undrained Triaxial test............................................................................. 93

3.6 Goodman and Bray Chart ........................................................................ 94

CHAPTER 4 – STABILITY OF THE SITE ......................................................... 96

4.1 Stereographic projection.......................................................................... 97

CHAPTER 5 - Discussion ............................................................................... 104

5.1 Laboratory results .................................................................................. 104

5.2 Field results ........................................................................................... 105

5.3 Analysis of Stereographic projection. .................................................... 106

5.4 Comments on stability ........................................................................... 109

5.5 Slope stabilisation ................................................................................. 109

CHAPTER 6 - Conclusion ............................................................................... 111

Bibliography .................................................................................................... 112

CHAPTER 8 – APPENDIX .............................................................................. 115

Field data..................................................................................................... 115

Lab Data ...................................................................................................... 126

1

List of figures

Figure 1.1 - Greece Fatal Rockfall ........................................................... 10

Figure 1.2 - Rockfall at Pennington Point ................................................. 11

Figure 2.1 – Main discontinuity according to size..................................... 13

Figure 2.2 – Joints ................................................................................... 14

Figure 2.3 – Joint sets at St Mary’s Chapel ............................................. 15

Figure 2.4 – Joint example ....................................................................... 17



Figure 2.7 – Diagram showing discontinuity orientation ........................... 19

Figure 2.8 – Compass .............................................................................. 19

Figure 2.9 – Inclinometer ......................................................................... 19

Figure 2.10 – equatorial and polar projections ......................................... 20

Figure 2.11 – Polar Stereonet .................................................................. 21

Figure 2.12 – Equatorial Stereonet .......................................................... 22

Figure 2.13 – Geological data on tracing paper ....................................... 23

Figure 2.14 – Polar Stereonet example ................................................... 23

Figure 2.15 – Polar Stereonet example ................................................... 24

Figure 2.16 – Equatorial stereonet example ............................................ 24

Figure 2.17 – Stereonet ........................................................................... 25

Figure 2.18 – Stereonet with great circle ................................................. 26

Figure 2.19 – Stereonet with 2 great circles ............................................. 26

2

Figure 2.20 – Diagram of wedge failure ................................................... 28

Figure 2.21 – Wedge failure on stereonet ................................................ 28

Figure 2.22 – Diagram of Plane failure .................................................... 29

Figure 2.23 – Plane failure on stereonet .................................................. 29

Figure 2.24 – Diagram of toppling failure ................................................. 30

Figure 2.25 – Diagram of circular failure .................................................. 31

Figure 2.26 – Circular failure on stereonet ............................................... 31

Figure 2.27 – Stereonet with great circles and angle of friction ............... 32

Figure 2.28 – Coastal chemical weathering ............................................. 33

Figure 2.29 – Mechanical weathering ...................................................... 33

Figure 2.30 – Wave erosion ..................................................................... 34

Figure 2.31 – Mushroom rock pinnacle .................................................... 35

Figure 2.32 – Earthquake ........................................................................ 36

Figure 2.33 – Formation of mountain range ............................................. 36

Figure 2.34 – Formation of a fault ............................................................ 37

Figure 2.35 – Shear displacement vs shear stress ................................. 38

Figure 2.36 – Mohr plot of peak strength ................................................. 39

Figure 2.37 – Driving and Resisting force ................................................ 40

Figure 2.38 – Point load test .................................................................... 41

Figure 2.39 – Slake durability test ........................................................... 42

Figure 2.40 – PUNDIT test ....................................................................... 43

Figure 2.41 – Triaxial test ........................................................................ 44

Figure 2.42 – Sample for Triaxial test ...................................................... 44

3

Figure 2.43 – Cone penetrometer ............................................................ 45

Figure 2.44 – Rebound Hammer .............................................................. 45

Figure 2.45 – Rockfall in Canada ............................................................. 46

Figure 2.46 – Typical rock bolt configuration ........................................... 47

Figure 2.47 – Application of rock bolts and anchoring ............................. 48

Figure 2.48 – Shotcrete example ............................................................. 50

Figure 2.49 – Shotcrete/fibrecrete and rockbolt ....................................... 50

Figure 2.50 – Anchored wire mesh .......................................................... 51

Figure 2.51 – Map of site ......................................................................... 53

Figure 2.52 – Photos of Hastings ............................................................. 54

Figure 2.53 – Map of Fairlight .................................................................. 55

Figure 2.54 – Access to Cliff End site ...................................................... 55

Figure 2.55 – Cliff End site ....................................................................... 56



Figure 2.58 – Satellite view of Cliff End site ............................................. 58

Figure 2.59 – Sketch of Cliff End site ....................................................... 59

Figure 2.60 – Submerged forest .............................................................. 60

Figure 2.61 – Submerged forest .............................................................. 60

Figure 2.62 – Topographical features of Hastings area ........................... 61

Figure 2.63 – Structural geology of Hastings area ................................... 62

Figure 2.64 – Sketch of Cliff section ........................................................ 63


4



Figure 2.68 – Edina Digimap.................................................................... 67

Figure 2.69 – Stratigraphical column ....................................................... 68

Figure 2.70 – First day at Cliff End site .................................................... 69

Figure 2.71 – Second day at Cliff End site ............................................... 70

Figure 2.72 – Third day at Cliff End site ................................................... 71

Figure 2.73 – Topographical survey ......................................................... 72

Figure 2.74 – Satellite imagery of Cliff End site ....................................... 72

Figure 2.75 – Taking the angle of friction ................................................. 73

Figure 2.76 – Schmidt hammer chart ...................................................... 74

Figure 2.77 – Bed layers .......................................................................... 76

Figure 2.78 – Geological strength index for jointed rocks ........................ 77

Figure 2.79 – Hard Sandstone ................................................................. 78

Figure 2.80 – Rock mass with layers of Sandstone and Clay .................. 78

Figure 3.1 – Liquid Limit ........................................................................... 90

Figure 3.2 – Soil classification.................................................................. 91

Figure 3.3 – Mohr’s Circles ...................................................................... 93

Figure 3.4 – Clay sample failure .............................................................. 93

Figure 3.5 – Goodman and Bray chart ..................................................... 95

Figure 4.1 – Topographical Survey .......................................................... 96

Figure 4.2 – Satellite view of Cliff End ..................................................... 96

Figure 4.3 – Stereonet with every discontinuity data plotted .................... 97

5

Figure 4.4 – Stereonet without face ......................................................... 98

Figure 4.5 – Analysis of Face 1 ................................................................ 99

Figure 4.6 – Analysis of Face 2 ................................................................ 99

Figure 4.7 – Analysis of Face 3 .............................................................. 100



Figure 4.10 – Analysis of Face 6 ............................................................ 101




Figure 4.14 – Analysis of Face 10 .......................................................... 103

Figure 5.1 – Soil and bits of rock on the base of the cliff ........................ 109

Figure 5.2 – Rock mass ......................................................................... 110

Figure 5.3 – Bits of rock on the shore .................................................... 110

Figure 6.1 – Blocks of rock on cliff base ................................................ 111

6

List of Table

Table 2.1 – Topographical Survey ........................................................... 71

Table 2.2 – Angle of friction readings ....................................................... 73

Table 2.3 – Schmidt Hammer readings taken on site .............................. 74

Table 2.4 – Dip and Dip direction data ..................................................... 75

Table 3.1 – Raw data for Hard Sandstone ............................................... 79

Table 3.2 – Raw data for rock mass with layers of Sandstone and Clay . 79

Table 3.3 – Calculated point load index ................................................... 80

Table 3.4 – Calculated point load index ................................................... 81

Table 3.5 – Point load strength index ....................................................... 82

Table 3.6 – Classification of rock by strength .......................................... 83

Table 3.7 – Raw results for the PUNDIT test ........................................... 84

Table 3.8 – PUNDIT test calculated results ............................................. 85

Table 3.9 – Cycle 1 raw results ................................................................ 86

Table 3.10 – Cycle 2 raw results .............................................................. 86

Table 3.11 – Hardsandstone calculations for slake durability index ......... 86

Table 3.12 – Rock mass with layers of Sandstone and Clay calculations for

slake durability index ................................................................................ 87

Table 3.13 – Slake durability scale .......................................................... 87

Table 3.14 – Raw plastic limit test results ................................................ 88

Table 3.15 – Liquid limit raw results ......................................................... 88

Table 3.16 – Plastic limit test results ........................................................ 89

Table 3.17 – Liquid limit test results ......................................................... 89

7

Table 3.18 – Data for Goodman and Bray Chart...................................... 94

Table 4.1 – Discontinuity set from plot ..................................................... 97

Table 4.2 – Angle of Friction .................................................................... 98

8

Equations

Point load test

Is = P

De2 =

Area for square = Length x width

A = Cross sectional failure area

Is =

Is(50) = F x Is

Size correction factor= (de/502)0.45

σ = F/A

C = 24 Is(50)

Average (Mean) = Total values/number of items

Average σ = Total values/number of items

Slake durability test

Slake durability index =

PUNDIT Test

Vp = D/t ms-1

Average Length =

Average time =

9

Preface

The aim of this report is to investigate the rock slope stability of Cliff End.

A literature review is conducted which includes the geology of the site,

conditions that can initiate rock slope stability and various methods to stabilise

the rock slope. Numerous site visits were made to collect data for further testing

and the findings can be found in this report. All of the data are analysed and

discussed to determine the stability of the site. Methods to stabilise the rock

slope are also discussed and their merits questioned.

10

CHAPTER 1 - INTRODUCTION

Rock slope engineering is a branch of Geomechanical engineering and is an

integral topic within it. The application of structural geology and rock mechanics

principles form the topic of rock slope engineering these principles lie in the

stability of a slope cut into rock as (Kliche, 1999). The topic of rock slope

engineering includes a wide range of analysis that is normally conducted and

these include, groundwater analysis, geological data collection, slope

stabilisation methods, kinematic and kinetic analysis.

Further, rock slope stability analysis is also an integral topic within Civil

Engineering. Its use and application can according to (Kliche, 1999) be found in

the following areas:

1) Buildings, dam sites or foundations

2) Road cuts

3) Cut and cover tunnelling

4) Irrigation channels

5) Tailing dams

6) Mine dumps

Wyllie 2004, generally agrees with Kliche 1999, and adds further to the list of

activities which require the excavation of rocks. These include

1) Projects involved in

transportation system such as

railways and highways

2) Dams for power

production and water supply

3) Industrial and urban

development

It is therefore necessary to

analyse rock slopes Figure 1.1 - (Greece Fatal Rockfall picture and

photos, 2009)

11

effectively so that the proper measures can be undertaken in order to stabilise

them if necessary.

Failure to analyse the stability of a rock mass can be catastrophic. Figure 1

shows a rock fall in Greece on the main highway linking north and southern

Greece on December 17th 2009 which took the life of an Italian engineer.

Figure 1.2 – (British Geological Survey, 2010)

The above pictures show a rock fall occurring at Pennington Point. What can be

seen in the pictures is the development of the actual rock fall and also the

amount of material involved.

12

CHAPTER 2 – LITERATURE REVIEW

2.1 Discontinuities

The factors that control most rock slopes are joints, faults and fractures which

are otherwise termed discontinuities. Discontinuities represent planes of

weakness (Kliche, 1999). It is these planes of weaknesses that control the

engineering properties of the rock mass by way of splitting the rock mass into

numerous blocks.

(Simons, Menzies, & Matthews, 2001) also agrees with Kliche, in respect of

discontinuities being a major factor when it comes to slope failures. To

determine whether or not a rock slope is stable, one must take into account the

pattern, the extent and the type of discontinuity that are present within the rock

mass.

Looking at BS 5930:1999, the types of discontinuities included for site

investigations are:

Joints

- A joint is formed in compression or tension and is structurally of small

dimension. They lack substantial shear strength in the plane of the

joint. (Palmstrom & Stille, 2010)

Fault

- Faults are defined by (Kliche, 1999) as essentially fractures which

have caused displacements due to tectonic activity. Characteristics of

a fault include crushed and sheared rock. This fracture allows the

water to flow freely which increases weathering.

Bedding fracture

- These are fractures which coincide along the bedding.

Induced fracture

- This is a discontinuity which has no geological origin. They have been

brought about by blasting, coring etc…

13

Incipient fracture

-incipient fractures tend to be found along bedding or cleavage. These

are defined by (Simons, Menzies, & Matthews, 2001) as discontinuities

which retains some tensile strength which may not be fully developed or

which may be partially cemented. Incipient failures are common along

bedding or cleavage.

All of these different types of discontinuities can change the structural or

geological feature or alter the homogeneity of a rock mass as (Palmstrom &

Stille, 2010) mentioned in Rock Engineering 2010. These discontinuities vary

tremendously in length from millimetres to thousands of meters.

Figure 2.1 - Above are the main types of discontinuities according to size. (Palmstrom & Stille, 2010)

In the earth’s crust, there are numerous variations of joints and faults and

(Palmstrom & Stille, 2010) mentions that it is for this reason that it is so difficult

to apply common observation and description of rocks.

14

2.2 Joints and Faults

The most common type of

geological structure found in

rocks is joints. (Jaeger, Cook, &

Zimmerman, 2007) provide a

simple yet detailed explanation of

Joints in rock. Joints are defined

as fractures or cracks in the

rocks along which minimal or no

transverse displacement has

taken place. The spacing in

between joints is parallel or sub

parallel and regularly spaced.

Within a rock mass exist several

set which are oriented in different

ways which breaks up the rock

mass into smaller and blockier

structures. As (Jaeger, Cook, &

Zimmerman, 2007) mentions, this is why joints are very important in rock

mechanics. As the joints divides rock mass into different parts sliding can occur

along the joint surfaces. Another crucial factor is their influence on the paths

they provide for fluids to flow through the rock mass.

Joints exist in a variety of scales. (Blyth, 2005) mentions that well defined joints

are termed as Major joints whereas smaller breaks are minor joints. (Jaeger,

Cook, & Zimmerman, 2007) expands on this by terming the major joints as the

most important set and can be traced for tens or hundreds of meters. The minor

joints are not as important and can be seen usually intersecting the major joints

which is why they are also known as cross joints.

Figure 2.2 – Joint (S.Aber, 2003)

15

This is still not applicable to all cases though as two sets of joints have the

potential to be equally as important as each other.

An interesting point that

(Simons, Menzies, &

Matthews, 2001) that other

authors do not mention is

that even though there is

abundant literature on this

subject and even though

joints are common, they are

one of the most difficult

structures to analyse. The

reason for this is due to the

fundamental characteristics

that are inherent to these

rock masses.

Faults are described by

(Jaeger, Cook, &

Zimmerman, 2007) as

fracture surfaces along

which relative displacement

has transpired transverse to the nominal plane of the fracture. Major faults can

have a thickness ranging from several meters to hundreds of meters. Minor

faults have a thickness ranging from a decimetre to a meter. They can usually

be seen to be approximately planar, and due to this, they provide the crucial

planes along which sliding can occur. (Palmstrom & Stille, 2010) also adds that

the result of most fault zones is of the numerous ruptures which occur during

geological time and have a correlation with other parallel discontinuities that

decrease with size and frequency with distance.

(Villaverde, 2009) notes that the existence of faults at some location indicates

that a relative motion took place between its two sides at some time in the past.

Figure 2.3 - Well-developed joint sets at St Mary’s Chapel, Caithness, Scotland (Norton, 2008)

16

All of the authors agree that the most important aspect in relation to

discontinuities is their spacing and orientation. (Simons, Menzies, & Matthews,

2001) gives a useful list of important characteristics of discontinuities. Their list

is as follows:

Orientation

Spacing (one dimension)

Block size and shape

Persistence

Roughness

Wall strength

Wall coating

Aperture and infilling

Seepage

Discontinuity sets

Both the initial and main concerns in regards to rock slope stability is the

orientation and spacing of the discontinuities. (Wyllie, Mah, & Hoek, 2004)

states that whilst orientation is the number one characteristic that influences

stability, there are other properties such as spacing and persistence that also

have an effect. Three examples from (Wyllie, Mah, & Hoek, 2004) are shown on

figure 2.4-2.6:

17

Figure 2.4 - The persistent J1 joints can be seen dipping out of the face. This forms the

possibility of unstable sliding blocks

Figure 2.5 - The joints here are closely spaced. The low persistence joints cause the

ravelling of small blocks.

Figure 2.6 - Potential toppling slabs are caused through persistent J2 joints dipping

into face.

J1 can be seen to be widely

spaced and the persistence is

greater than the slope height of

the cut.

J1 and J2 can be seen to be

closely spaced and have low

persistence. There is no overall

slope failure.

A series of small thin slabs are

produced due to J2 being

persistent and closely spaced

which dip into the face. This

creates toppling failure.

18

2.3 Orientation

It is essential that the orientation of the discontinuities in a rock mass are

measured and analysed when it comes to rock slope engineering. Since the

vast majority of discontinuities encountered are irregular, data gathered over a

small area will appear scattered. (Simons, Menzies, & Matthews, 2001)

suggests a way to reduce this scatter is to place a 200mm diameter aluminium

measuring plate on the discontinuity surface before measurements are made.

Dip and dip direction are the terminology used to record orientation. They are

defined by (Wyllie, Mah, & Hoek, 2004) as follows:

1) Dip –The dip is measured normal to the strike direction and is the

inclination angle of the plane.

2) Dip direction – this is the horizontal trace of the line of dip, which is

measured clockwise from north. (Kliche, 1999) further adds that the dip

direction is measured from 0⁰ to 360⁰. 0⁰ and 360⁰ = North, 90⁰ = East,

180⁰ = South, 270⁰ = West.

To measure the dip and dip direction, the strike is also needed. This is defined

by (Wyllie, Mah, & Hoek, 2004) as the trace of the intersection of an inclined

plane with a horizontal reference plane. A diagram is shown below by (Wyllie,

Mah, & Hoek, 2004) to illustrate the relationship between strike, dip and dip

direction.

19

Figure 2.7 - Diagram showing discontinuity orientation. Diagram on left showing isometric view and on the right showing the plan view. (Wyllie, Mah, & Hoek,

2004)

To take dip and dip directions, a compass and inclinometer will be required.

(Simons, Menzies, & Matthews, 2001) recommends the use

of a common type of combination between a compass and

inclinometer. These include the Silva compass and the Clar

type compass. They allow for both dip and dip direction to

be taken using the same instrument.

Figure 2.9 - Inclinometer

Figure 2.8 - Compass

20

2.4 Stereographic analysis

When the data has been collected in the field it can be expected that there will

be scatter in the data. To be able to efficiently analyse this, it is vital that a

technique is used to deal with such scatter. Stereographic projection is a

technique that allows for such data to be analysed efficiently. Several textbooks

also term stereographic projection as “Hemispherical projection” but for the

sake of simplicity, it will be referred to as Stereographical projection here.

(Kliche, 1999) mentions that the term stereographic projection literally means

the projection of solid or three dimensional drawings. Stereographical projection

is a method which is often used in rock mechanics for the analysis of planar

discontinuities such as bedding planes, faults, shear planes, and joints. Since

this technique allows data to be analysed visually rather than numerically, it is

considered a valuable technique in rock mechanics due to its simplicity.

In Geomechanics, there are two

types of stereographic

projections that can be used as

(Wyllie, Mah, & Hoek, 2004)

mention. These include both the

polar and equatorial stereonet

as shown in figure 2.10.The two

stereonets, polar and equatorial,

are used for different purposes.


explains that the polar stereonet

is used to plot poles whereas

the equatorial stereonet is used

to plot planes and poles.

Figure 2.10 - Equatorial and polar projections of a sphere (Wyllie, Mah,

& Hoek, 2004)

21

Both stereonets can be seen below on figures 2.11 and 2.12:

Figure 2.11 - Polar Stereonet (Hoek & Bray, 2001)

22

Figure 2.12 - Equatorial stereonet (Hoek & Bray, 2001)

Both stereonets shown above are a common type of stereonet called an equal

area or Lambert (Schmidt) net. All of the areas of an equal area stereonet on

the surfaces of the reference sphere is represented as an equal area. This is

particularly useful as this allows the contouring of pole plots. This in turn will

lead to concentrations of poles which define preferred orientations and sets of

discontinuities.

23

(Wyllie, Mah, & Hoek, 2004) and (Kliche, 1999) both provide methods to plotting

the data onto the stereo nets and provide similar instructions.

Figure 2.13 - The figure above shows geological data and analysed on a tracing paper courtesy of (Wyllie, Mah, & Hoek, 2004)

Data collected from the field are first plotted onto a polar stereonet. This can

either be carried out by hand or computer. The dip direction is marked from 0⁰

to 360⁰. 0⁰ and 360⁰ start from the bottom of the stereonet and 180⁰ is located

on the top of the stereonet.

As can be seen on the left, a

polar stereonet is shown with a

discontinuity plotted. The plot

orientation is 50⁰/130⁰ (dip and

dip direction).

The dip direction is first located

on the outer edge of the stereonet.

In this case, the value is 130⁰.

The dip is then located. The outer

edge indicates 90⁰ and the centre

of the stereonet represents 0⁰ dip.

This process is carried out for

Figure 2.14 - Polar Stereonet example (Wyllie, Mah, & Hoek, 2004)

24

every dip and dip direction collected. Eventually, clusters will form and each

cluster will represent a discontinuity. An example of this is shown on the figure

below.

As can be seen on the left, a cluster

has been formed. A boundary is

drawn around the cluster and the

centre of that area is established.

This then forms the orientation of the

discontinuity set. In this case, the

centre of the circle is 57⁰/199⁰ dip and

dip direction respectively.

The next step requires the use of an

equatorial stereonet. (Wyllie, Mah, & Hoek, 2004) explains this procedure which

involves the plotting of great circles for each of the discontinuity set orientations,

along with the orientation of the face. The purpose of this is to show on a single

diagram, the orientation of every surface with the rock mass that has an

influence on its stability.

(Simons, Menzies, & Matthews, 2001) and (Wyllie, Mah, & Hoek, 2004) contain

worked visual examples of the plotting of great circles on the equatorial

stereonet. The best and most clear step by step example is shown by (Wyllie,

Mah, & Hoek, 2004) on figure 2.16:

Figure 2.16 - Equatorial stereonet example (Wyllie, Mah, & Hoek, 2004)

Figure 2.15 - Polar Stereonet example (Kliche, 1999)

25

Two discontinuity set orientations are plotted on figure 2.16. They are 50/130⁰

and 30/250⁰.

Further examples are shown below.

To plot these two discontinuity sets,

a pin, tracing paper and an

equatorial stereonet is needed. A pin

in pricked through the centre of the

stereonet and the tracing paper is

placed on top. North is marked on

the tracing paper so that the initial

orientation of the tracing paper will

always be known. A dip direction of

130⁰ will also be recorded clockwise

from North.

The tracing paper is rotated to either 90⁰ or 270 from the dip direction recorded.

The dip is then counted from the outer edge, representing 0⁰, to the centre,

representing 90⁰. In this case, a dip of 50⁰ is reached and the great circle is

traced that corresponds to 50⁰.

Figure 2.17 - Stereonet

26

Figure 2.18 – stereonet with great circle

The tracing paper is rotated again so that the mark “N”, returns to its original

position. The entire procedure is repeated again for 30/250 giving a diagram as

shown in figure 2.19.

Figure 2.19 – Stereonet with 2 great circles

27

2.5 Slope instability mode identification

From the completed stereonets, it is possible to identify different types of slope

failure that may occur. This identification of potential stability problems is

paramount as (Wyllie, Mah, & Hoek, 2004) mentions during the early stages of

any project.

(Wyllie, Mah, & Hoek, 2004) and (Simons, Menzies, & Matthews, 2001) provide

patterns to look out for with respect to specific types of failures and both provide

explanations for those different failures. (Kliche, 1999) only goes as far as

showing the steps to making a equatorial stereonet but does not delve into what

the analysis necessarily represents.

There are four types of failure that are of main concern according to (Wyllie,

Mah, & Hoek, 2004). These include Plane failure, Wedge failure, Toppling

failure and Circular failure. (Simons, Menzies, & Matthews, 2001) agrees with

the main types of failures but also adds two more, Flexural toppling and Rock

falls. Since Plane failure, Wedge failure, toppling failure and circular failure are

the main type of failures which can be analysed using a stereonet, these will

form the main focus of this project.

28

2.5.1 Wedge failure

(Simons, Menzies, & Matthews, 2001)

explains that when the orientation is such that

two discontinuities intersect, a wedge failure

will occur if the dip direction is similar to that

of the face and if the dip is greater than the

angle of friction. This is shown on the

stereonet below. As there is no release

surfaces required, for this type of failure it is

considered the most dangerous mode.

Figure 2.20 - Diagram of wedge failure (Wyllie, Mah, & Hoek,

2004)

Figure 2.21 - Wedge failure on stereonet


Pole concentrations

Line intersection

dip direction and

direction of sliding

Dip direction of the

face

29

2.5.2 Plane failure

A plane failure will occur if the dip direction of

the discontinuity has a dip direction similar to

that of the face and if the dip if greater than

the angle of friction.

There must be lateral release surfaces for

plane failure to occur, which will then allow a

block of finite size to slide out of the face

(Simons, Menzies, & Matthews, 2001). A

plane failure is one of the simplest modes of failure.

Figure 2.22 - Diagram of Plane failure (Wyllie, Mah, & Hoek,

2004)

Pole concentrations

Dip direction of

face and direction

of sliding

Figure 2.23 - Plane failure on stereonet


30

2.5.3 Toppling failure

This is a type of failure where long slender rock

blocks dip into the face at angles which are steep

and rest on a basal discontinuity which dips out of

the face with an angle that is less than the angle of

friction for that discontinuity.

Dip direction of

face and direction

of toppling

Figure 2.25 - Toppling failure on stereonet


Figure 2.24 – Diagram of toppling failure (Wyllie,

Mah, & Hoek, 2004)

31

2.5.4 Circular failure

The failure is likely to be circular if the rock

mass is heavily broken or jointed. If on a

stereonet the pattern of discontinuity appears

to be random, then circular failure is a

possibility that should be considered.

One parameter that is left out in these stereonet examples is the angle of

friction as this will have an influence on the stability. (Wyllie, Mah, & Hoek,

2004) does not give a clear explanation as to how to plot the angle of friction

whereas (Simons, Menzies, & Matthews, 2001) does provide this.

To draw an angle of friction of 50⁰ on an equatorial stereonet, a dip of 50⁰ is

counted from the outer edge to the centre. A mark is left on 50⁰ and a compass

is then required to draw the circumference from the centre of the stereonet to

the 50⁰ mark. An example of this is shown on the next page.

Randomly oriented

discontinuities.

Figure 2.26 - Circular failure on stereonet (Wyllie,

Mah, & Hoek, 2004)

Figure 2.25 – Diagram of circular failure

32

Figure 2.27 – Stereonet with great circles and Angle of Friction.

Angle of Friction

at 50⁰

33

2.6 Rock instability causes

In different parts of the world, there are a wide range of conditions that

represent the wide variety of natural processes

that are taking place to shape the earth’s surface.

(Blyth, 2005) agrees that land areas are

constantly being reduced and reshaped and this

is the cause of weathering and erosion. This

process is known as denudation. Rain and frost

are responsible for a process called “weathering”

on rocks that are exposed to the atmosphere.

2.6.1 Weathering

The process of breaking down of the minerals into new compounds is called

chemical weathering. Chemical weathering takes place by action of chemical

agents such as acids in the

air, river water and in rain.

(Blyth, 2005) notes that

although the process of

chemical weathering is slow,

they produce noticeable

effects in soluble rocks.

Chemical weathering can

decrease the amount of

inflilling in discontinuities

which can result in a

decrease of shear stress of

the discontinuity. This can

cause premature slope failures due to the decrease in the angle of friction.

Figure 2.28 - Coastal chemical weathering (Chinese International School)

Figure 2.29 - Mechanical weathering (De Groot, 2005)

34

Mechanical weathering is essentially the breakdown of rock into small particles.

This is achieved by abrasion from mineral particles carried in the wind, constant

temperature changes and by impact from raindrops. In dry areas the land are

shaped by the sand constantly blasting against them during storms. Flaking of

exposed rock surfaces are produced in very hot and very cold climates where

temperature constantly changes.

2.6.2 Erosion and deposition

Figure 2.30 - Wave erosion on Portland Cliff (Chadwick, Wave erosion on a Portland Cliff, 2006)

The agents of erosion are rivers, wind, water waves and moving ice. This is due

to their capabilities which include loosening, carrying particles of soil and

dislodging large pieces of rock and sediment.

As the toe of the rock is eroded away over geological time, this leaves the rock

on top of the toe to overhang above the sea which is known as undercutting.

When the discontinuities control the rock slope stability, undercutting of the

35

slope will cause daylighting of the discontinuities resulting in plane, wedge or

toppling failure as well as other more complex failure mechanisms.

The rate of erosion however, depends on a number of parameters. They include

wind speed, rock type, its permeability as well as porosity and whether the rock

are folded, faulted or weathered.

In climates where little rain is seen, wind is the main source of erosion. The

wind contributes to erosion through the following methods; wind carries small

particles and essentially moves it to another region. The other effect is erosion

as the suspended particles

impact on solid objects. This over

geological time, erodes rock from

the bottom which causes

overhanging of rocks which will

eventually topple.

Valleys are widened and

deepened due to the work of

erosion by the river. (Blyth, 2005)

also adds that the rate of erosion

is increased in times of flood. Rivers also act as agents of transport. They carry

many materials in suspension which eventually leads to the sea.

Figure 2.31 - Mushroom rock pinnacle – wind and sand erosion (Byrd, 2010)

36

2.6.3 Earthquake

Sharp movements along

fractures cause numerous

shocks which continually take

place and relieve stress in the

crustal rocks. Various reasons

cause stress to accumulate

locally until the stress exceeds

the strength of the rock.

When this happens, failure

and slip will occur along the fracture and this followed by a smaller rebound.

(Blyth, 2005) notes that it only takes a few centimetres of movement or less to

create a significant shock due to the amount of energy involved. Earthquake

can have devastating effects and have the potential to send severe shocks

capable of opening fissures on the ground, initiating landslides, and fault scarps.

Weak ground produces the worst effects especially in young deposits such as

sand, silt and clay.

Figure 2.33 - Formation of mountain range due to the convergence of two continental plates. (Villaverde, 2009)

The figure above shows two plate boundary colliding and given enough time,

they will eventually fold up in very much the same manner as an accordion. It is

Figure 2.32 – Earthquake (Man Made Earth, 2010)

37

this process that has creates the world’s mightiest mountain ranges, such as

the Alps and the Himalayas.

Figure 2.34 - Formation of a fault by plates sliding past each other. (Villaverde, 2009)

When the edges of the plate slide past each other, crust is neither created nor

destroyed, nor are there any changes on the Earth’s surface. This type of action

occurs on boundaries which are called faults.

Pressure released during earthquakes widens discontinuities which causes rock

slopes to fail due to the decrease in the angle of friction and/or alteration of the

dip or dip direction of the face,

38

2.7 Properties of the rock

(Hoek & Bray, 2001) mentions that when analysing rock slopes, the most

important factor that needs to be considered is the geometry of the rock mass

behind the slope face. (Kliche, 1999) and (Wyllie, Mah, & Hoek, 2004) also

echo these sentiments. Another factor, which is the next important factor that

needs to be considered, is the shear strength of the potential failure surface.

There are many factors that contribute to shear strength including angle of

friction, cohesion, rock mass density, surface roughness and joint continuity.

Cohesion is defined by the Oxford dictionary of Earth Sciences as the ability of

particles to stick together without dependence on interparticle friction.

Angle of friction is defined simply by the Oxford dictionary of Earth Sciences as

the angle (Ф) which is measured between the normal force (N) and resultant

force (R). This is attained when failure ensues in response to a shearing stress

(S).

The peak shear strength is

influenced by the angle of

friction, discontinuities and

any infilling that may be

present. For a test which is

conducted with continuous

normal stress, a plot of

Shear stress again Shear

displacement is shown.

Figure 2.35 - Plot of shear displacement vs shear stress (Wyllie, Mah, & Hoek, 2004)

39

If the test is carried out in different normal stress levels and the peak shear

strength value is gained from each of the tests, then a plot of Shear stress

against Normal stress can be plotted. This is called the Mohr diagram.

As can be seen on the diagram above, the features of the Mohr diagram is that

it is approximately linear and the slope of the line is equivalent to the peak

friction angle Фp of the rock surface. The point at which the line intercepts the

shear stress axis is the cohesive strength c.

The peak shear strength is defined by:

(2.1)

If no cohesion is present, the equation can be shortened to

(2.2)

Figure 2.36 - Mohr plot of peak strength (Wyllie, Mah, & Hoek, 2004)

40

Driving force and Restoring force

A slope that is stable has a Restoring force that is greater than the driving force.

The driving force is influenced greatly by gravity which creates a downward

movement.

A slope that is unstable has a Restoring force that is less than the driving force.

When the dip of the beds is greater than the angle of friction, driving force is

increased thus destabilising the rock mass.

The cohesion of any infilling material can decrease due to the groundwater

pressure increasing the uplifting pressure. This results in a reduction of

Restoring force as can be seen in the figure below.

Figure 2.36 – Driving force and resisting force (Freeman, 2010)

Infill material

41

2.8 Rock laboratory tests

A variety of laboratory tests need to be conducted to determine what the

properties of the rocks are.

2.8.1 Point load test

The point load test is a reliable test as

this involved the process of actually

breaking the rock between two point as

(Freeman, 2010) states. To conduct

this experiment, a sample is required to

be approximately 50 mm across with

approximately parallel sides.

The point load test is a way of

classifying the rock that can be

performed either in the laboratory or on

site.

The experiment consists of a loading

frame, platens, hydraulic ram and

pump. The necessary force can then be applied onto the specimen so that it

breaks.

Figure 2.38 – Point load test

42

2.8.2 Slake durability test

Figure 2.39 – Slake durability test

Rock materials are susceptible to degradation when exposed to the processes

of weathering including wetting and drying and freezing cycles. Rock types that

are susceptible to degradation are rocks that usually have high clay content

such as mudstone and other rock types such as shale.

The test involves the rock sample being put in a drum and then partially

submerged in water. The drum is then rotated at 20 revolutions per minute for a

period of 10 minutes. The drum is dried and the weight loss is then recorded.

The test cycle is then repeated one more time.

43

2.8.3 Pundit test

Figure 2.40 – PUNDIT test

This is a non-destructive method for determining certain elastic properties of a

rock mass. This test is based on finding the velocity at which an elastic wave

travels through the rock. This will give an indication of the internal structure of

the rock or object.

A PUNDIT (Portable Ultrasonic Non Destructive Index Tester) is used for this

test. At one end of the specimen a compressive stress pulse is generated and

the PUNDIT test records the time taken for the resulting P wave to reach the

other end.

44

2.8.4 Undrained Triaxial test

This test is the most popular test for

testing shear strength. It is suitable for

all types of soil except for very

sensitive clays (Whitley, 2001).

A cylindrical specimen of soil is

required having a diameter/height

ratio of 2:1. (Whitley, 2001) lists

typical sizes as being 76 x 38 mm and

100 x 50 mm.

The specimen is tested under

different cell pressures which then

creates a Mohr circle for each peak or

ultimate failure stress. A common

tangent is then drawn and then this

maybe be taken as the strength

envelope for the soil from which the

angle of friction and cohesion values

can be scaled.

Figure 2.42 – Sample for Triaxial test

Figure 2.41 – Triaxial test

45

2.8.5 Consistency limit – penetrometer method

Consistency limit is used as a basis for

the classification of fine soils. The cone

penetrometer method for liquid limit is

reliant on the relationship between the

moisture content and the penetration of

the soil sample by the cone.

The test consists of a Cone

penetrometer and soil samples with the

appropriate water content which allows

for certain penetration to occur. The

sample is then placed in the oven after

being weighed and the relationship

between the wet and dry mass is

recorded.

To gain the plastic limit, a sample is

rolled until cracks appear. If cracks do

not appear, the sample needs to be

more dry and then rolled again until

cracks appear. The sample is weighed, placed in the oven for 25 hours and

then weighed again.

Schmidt hammer

The Schmidt hammer is a tool used to

give an indication of the strength of the

rock. It is also known as the rebound

hammer.

The hammer is placed on the rock mass

and a spring loaded mass is released

onto the rock. The rebound is dependent on the hardness of the rock.

Figure 2.44 – Rebound Hammer (Poyeshyar Co. Ltd, 2011)

Figure 2.43 – Cone penetrometer

46

2.9 Stabilisation of rock slope

Stabilisation of slopes is vital as the operation of highways and railways,

transmission facilities, power generation and the safety of commercial and

residential development in rocky terrain depend on the surrounding rock to be

stable.

(Wyllie, Mah, & Hoek, 2004) mentions that stabilisation programs are often very

economical as the failure of a slope could bring around high costs. An example

of this can be seen on highways as even a minor rock fall can bring about

damage to vehicles as well as injury or even death to the passengers. The

failure of stabilising a slope can also bring severe traffic and indirect economic

loss. (Wyllie, Mah, & Hoek, 2004) adds that a closure of railroad and toll

highways will result in a direct loss of revenue.

Due to these

reasons, stabilising

a rock face is of

extreme importance.

Below will be a list of

stabilisation

techniques which

are commonly used

to stabilise slope.

Figure 2.45 - Rockfall in Canada caused by heavy rain on

November 9, 1990 (Canada, 2007)

47

2.9.1 Rock bolt

Rock bolting is a technique that is very common due to its flexibility. (Palmstrom

& Stille, 2010) mentions that it is often used for the initial support at the tunnel

sides and can often be used for the final support.

They are regarded as short, low capacity reinforcement which comprises of a

bar or tube fixed into the rock and tensioned as illustrated by (Simons, Menzies,

& Matthews, 2001) in the figure below.

Figure 2.46 - Typical rock bolt configuration (Simons, Menzies, & Matthews, 2001)

Rock bolts are used to prevent toppling by tying together the blocks of rock so

that the effective base width is increased. They can also increase the resistance

to sliding on discontinuity surfaces. Another use for rock bolts is its use as an

anchor structure such as retaining walls and catch nets.

48

Figure 2.47 - Applications of rock bolts and anchoring. (Simons, Menzies, & Matthews, 2001)

Rock bolts being used to

prevent toppling failure

Rock bolts being used to

increase resistance to

sliding

Rock bolts being used as

an anchor structure

49

2.9.2 Shotcrete

Spraying concrete is advantageous when a slope is prone to rock falls, spalling

and sliding of small amount of rock. (Palmstrom & Stille, 2010) also notes that

shotcrete has been in use for several decades. It is a popular method due to its

favourable properties together with flexibility and high capacity.

The term shotcrete is used for a sprayed concrete which comprises of mortar

and aggregate which can be as large as 20mm thick. (Simons, Menzies, &

Matthews, 2001) also notes that the term gunite is also used to describe a

similar sort of material but with smaller aggregate.

Three different shotcrete methods are in use today as (Palmstrom & Stille,

2010) mentions:

1) Wet-mix, dry-mix or ordinary shotcrete sprayed in layers up to 100 mm

thick.

2) Net reinforced shotcrete. This process involves first spraying a layer of

concrete and then installing the net. A second layer is then is sprayed to

eventually cover up the net.

3) Fibre-reinforced shotcrete. This involves 3-5 cm long thin needles of fibre

steel which are mixed in to the wet concrete. (Hoek & Bray, 2001) also

notes that this is the method of choice in Scandinavia and have

completely replaced new reinforced shotcrete.

50

Figure 2.49 - A combination of shotcrete/fibrecrete and rock bolts. (Palmstrom & Stille, 2010)

Figure 2.48 – Shotcrete used for reinforcing of unstable fragments

and small blocks

51

2.9.3 Anchored Wired mesh

If small blocks are prone to falling

then the most economical and

versatile material to prevent the fall is

the wire mesh. It is not uncommon to

see layers of mesh pinned onto the

surface of the rock as a means of

stopping small loose blocks or rock

becoming dislodged. The other

advantage of a wire mesh is in the

case of the small blocks of rock that

eventually do fall down can be guided

into the ditch at the base of the slope.

Figure 2.50 - Anchored wire mesh to prevent small blocks from

becoming dislodged. (Simons, Menzies, & Matthews, 2001)

52

2.10 Site selection

For this research project, a site needed to be selected to assess its rock slope

stability. The selection of the site was based on a number of different

requirements which had to be agreed on with the supervisor. The following were

the requirements:

The site had to be easily accessible

The site had to pose no immediate danger to the public.

The site had to be unstable

The site had to be safe to work on and safety equipment used at all times

The site had to be agreed with the supervisor

Based on these requirements, an investigation was made into which sites

should be shortlisted. The following were the initial short listed sites:

1) Hastings

2) Fairlight Cove

3) Cliff End

4) Pex Hill quarry

5) Derbyshire quarry

6) Paragon beach

7) Tenby beach

Although pictures on Google maps show that Tenby and Paragon beach clearly

have unstable rocks to analyse, to get to the site would take approximately 5

hours by car. This was deemed too far. Pex Hill quarry and Derbyshire quarry

were also far away with approximately a 4 hour car journey to get there. It was

not clear there was easy access to the site either so a site visit to the quarries

would be a last resort. Out of all of the sites listed above, the Hastings area was

the closest with a 2 hour drive which had Fairlight Cove and Cliff End close to

its proximity.

From satellite imagery provided by Bing Maps, the coast along Hastings to Cliff

End were potential sites.

53

The first site visited was the coast along Hasting on October 2nd 2010. There were amples of parking space and the site was very easily

accessible.

Figure 2.51 – map of site

54

As can be seen on the

pictures taken, the site is

not easy to work on.

Although there is

evidence of plane failure

from the rubble at the

bottom, it would not be

comfortable or safe to

work on this site.

Furthermore, a warning

sign on the front of the

entrance also gave

indication as to the level

of safety on the site as

can be seen on the

photograph taken to the

left.

A friendly local then gave

advise on potential sites

mentioning that Fairlight

Cove was also not safe

to work on due to

Landslides and falling

rocks. Cliff End then

seemed to form the best

chance of finding a

suitable site.

Figure 2.52 – Photos of Hastings

55

Figure 2.53 – Map of Fairlight

Driving along the coast of Hastings, Cliff End was eventually reached. Cliff End

provided very easy access to the site from the car park as can be seen below.

Figure 2.54 – Access to Cliff End site.

56

After roughly a 5 minute walk, the Cliffs of Cliff End could be seen.

Figure 2.55 – Cliff End site


57

Walking along the coast was not possible on this day due to the high tides but

this site showed some evidence of instability due to the orientation of the

discontinuities present.

After showing pictures of the site to the supervisor, it was eventually agreed that

the Cliff End site would form the subject for this research project.


58

Car Park

Site

Figure 2.58 – Satellite view of Cliff End site

59

2.11 Geology of Cliff End

The Cliff End site at East Sussex which starts from Haddock’s reversed fault to

just after the Cliff End faults consist mainly of layers of Ashdown Beds, Cliff End

Sandstone and Wadhurst Clay.

The Cliffs have reached their current state by constant wave erosion over

geological time and it is estimated by (Villagenet, 2011) to have eroded at a rate

of 0.6 metres per year. This would indicate that during 1066 the cliffs were

approximately a further 550 metres out.

Figure 2.59 – Sketch of Cliff End site. (British Geological Survey, 1987) A= Ashdown beds CE= Cliff End Sandstone W= Wadhurst Clay.

Approximately 6000 years ago, after the last Ice Age, the sea level was

approximately 45 meters lower than it is at present due to the Polar Regions

having more ice. At that time, a forest grew when England was still joined by a

land bridge to the continent.

The sea level began to rise as the climate became warmer. This caused the

Polar Regions to have their ice slowly melt away causing sea levels to rise

above the level of the forest. The consequences were that the forest drowned

but the wood is still preserved today in salt water and mud.

Pictures taken of Cliff End on the next page illustrate the remains of the

submerged forest.

60

Figure 2.60 – Submerged forest. (Chadwick, Submerged Forest, Cliff End,

2010)

Figure 2.61 – submerged forest (Chadwick, Submerged Forest, Cliff End, 2010)

61

Cliff End

Figure 2.62 – Topographical features of the Hastings area (British Geological Survey, 1987)

62

Figure 2.63 – Structural geology of the Hastings area (British Geological Survey, 1987)

63

Figure 2.64 - Sketch of Cliff section between Haddock’s Reversed Fault and Cliff End (British Geological Survey, 1987)

64

Looking beyond

Haddocks reverse fault

on page 54, the cliff

line has a height of

approximately 20-30

meters which goes as

far as Cliff End. Soft

Wadhurst Clay shales

cut into the upper part

of the cliff which gives

rise to a strip of densely wooded, slipped terrain from which material sometimes

falls to the beach. Within these shales is where the Cliff End Bone Bed occurs

which is a few meters above the top of the Cliff End Sandstone in the top of the

cliff.

Beneath the base of the Cliff End sandstone, a 1m band of shales with a 0.1 m

bed of clay-ironstone gives rise to a notch in the cliff which marks the

intersection of the Wadhurst Clay and the Ashdown Beds. Up to 15 m of

sandstones lie beneath the notch with thin silty mudstone bands which are

exposed above the beach.

As can be seen from

figure 2.63, the rocks

form a gentle anticline

which can be seen along

this stretch of coast and

this causes the base of

the Cliff End Sandstone

to fall almost to beach

level at the Cliff End Fault.



65

There are scattered exposures at the cliff top in the wadhurst clay which

comprises of 16 m of shales and subordinate siltstones with clay ironstone


nodules. There are also fish and plant debris and bivalve moulds at some

horizons.

There are faunal remains at the site which include fish and teeth and some

reptilean bone fragments and teeth. These remains give valuable data as to the

evolutionary lineages within their groups.

Ashdown Beds

The Ashdown beds consists of sandstones, siltstones and mudstones with

subordinate lenticular beds of lignite, sideritic mudstone and spaerosiderite

nodules.

They are from the early Cretaceous period, specifically the Berriasian age which

ranges from 140 million to 145.5 million years ago.

66

Wadhurst Clay

The wadhurst clay mainly consists of grey mudstone which weathers at the

surface to heavy, orcreous mottled, greenish grey and khaki clays. Other

lithologies include sandstone, siltstone, conglomerate, clay ironstone and shelly

limestone. Thin beds of shelly limestone, rich in Neomiodon and Viviparus, are

also present throughout. The top metre of the Wadhurst Clay contains stiff clay

stained red by penecontemporaneous weathering.

Wadhurst Clay are from the early Cretaceous period, specifically the

Valanginian age which ranges from 136 million to 140 million years ago.

Cliff End Sandstone

The cliff end sandstone is a sizable 10 m thick sandstone which is exposed in

the cliffs at Cliff End. The sandstone was thought to be the top part of the

Ashdown beds however the discovery of Wadhurst Clay in the underlying

shales with ironstone has shown that sandstone does form a part of the

Wadhurst Clay formation.

67

Figure 2.68 - Edina Digimap 2011 showing bedrock on the Cliff End area.

Normal Inferred fault

Wadhurst Clay

Ashdown formation

Normal observed

fault

68

Figure 2.69 – Stratigraphical column (British Geological Survey, 1987)

69

2.12 Travel log

2.12.1 November 14th 2010

Arrived on the Cliff End site early morning at 8:00 am where the tide was low.

This allowed 6 hours of investigation before the tide returned. This day was

spent observing the site, selecting the faces to analyse and to take dip and dip

directions. The aim was to take as many dip and dip directions as possible.

Figure 2.70 – First day at Cliff End site.

Oxfo

rdia

n

Lo

wer

Kim

me

rid

gia

n

70


Arrived on the Cliff End site early morning at 9:00 am where the tide was low.

The main focus of this day was again to take as many dip and dip directions as

is possible. By the time the high tide started to arrive, 722 dip and dip direction

readings were taken.

Figure 2.71 – Second day at Cliff End site.

71


Having arrived at Cliff End at 10 am, and having taken enough dip and dip

directions, the focus of attention could be placed elsewhere. The task for this

day was to do a topographical survey, take Schmidt hammer readings, take the

angle of friction and to also take back rock samples to the Laboratory for testing.

A topographical survey was taken with the below readings

Dip Direction (⁰) Length of face (mm)

136 2800

032 720

138 980

065 2850

022 740

138 2400

042 650

132 20500

028 400

121 1250

Figure 2.72 – Third day at Cliff End site

Table 2.1 – Topographical survey

72

The readings were then used in AutoCad to create a topographical survey of

the faces to be analysed.

Face 1

Face 2

Face 3

Face 4

Face 5

Face 6 Face 7

Face 8

Face 9

Face 10

Face 1 Face 10

Figure 2.73 – Topographical survey

Figure 2.74 – Satellite imagery of Cliff End site

73

Angle of friction

It was important to get the angle of

friction so that an analysis could be

done for the stereonet. To get the

angle of friction, two rocks were

places on top of each other on a

clipboard. The board was tilted slowly

until the rock on top starts to slide

away. The dip is then measured

using the inclinometer. To get the

angle of friction of the clay, a sample of the clay needs to be taken to the

laboratory and a tri-axial test needs to be performed.

Description Angle of Friction ⁰

Hard Sandstone 37

Rock mass with layers of Sandstone and Clay 42

Figure 2.75 – Taking the angle of friction

Table 2.2 – Angle of friction readings

74

Schmidt Hammer test

Rock type Angle Reading Average Reading

MPa

Hard Sandstone 90 34, 32, 34 33 30

Rock with mixture of mainly Sandstone and Clay

90 28, 28, 24 27 20

Table 2.3 – Schmidt Hammer readings taken on site

Figure 2.76 – Schmidt hammer chart

75

Dip and dip direction data – sample. All 722 points can be found in the appendix

No Dip Dip Direction

Comments No Dip Dip Direction


Comments

1 02 302 Discontinuity 32 81 063 63 88 130

2 05 078 33 87 130 64 88 130

3 03 138 34 90 122 65 88 124

4 05 120 35 89 124 66 05 005

5 04 120 36 89 132 67 03 082

6 06 98 37 89 042 68 10 052

7 03 98 38 08 089 69 13 112

8 02 30 39 02 120 70 15 118

9 03 120 40 08 122 71 30 070

10 04 50 41 10 100 72 31 082

11 03 132 42 89 132 73 70 110

12 02 58 43 81 112 74 85 132

13 01 120 44 85 124 75 88 132

14 16 118 45 84 028 76 90 110

15 11 130 46 90 030 77 01 188

16 86 120 47 84 040 78 71 100

17 85 116 48 88 130 79 78 122

18 90 114 49 86 134 80 00 178

19 86 118 50 90 124 81 59 092

20 88 122 51 85 128 82 89 132

21 89 118 52 08 180 83 74 128

22 84 120 53 07 230 84 88 126

23 89 124 54 05 238 85 82 068

24 66 072 55 80 120 86 76 042

25 62 091 56 85 122 87 88 140

26 78 102 57 89 042 88 88 130

27 02 112 58 84 038 89 70 118

28 77 034 59 88 042 90 75 112

29 88 124 60 82 052 91 81 022

30 88 126 61 08 018 92 66 136

31 90 138 62 82 108 93 70 154

Table 2.4 – Dip and Dip direction data

76

Discontinuity description

Figure 2.77 – Bed layers

Discontinuities have a typical height of 1100 mm, width of 960 mm and a length

of 2000 mm. Cliff End consists of the same discontinuities throughout the entire

site.

77

Geological Strength Index

Figure 2.78 – Geological strength index for jointed rocks

The rock was blocky and had a moderately weathered surface. Using the

Geological strength index, the rock mass was in the range of 40-50 which is a

moderately strong rock.

78

2.13 Petrology1

Hard Sandstone

Hard sandstone, dark greyish blue, fine course

grained sand particles, cemented, strong on

the surface but weaker in the middle, large

block sizes, 5 major discontinuity sets, does

not fizz with acid.

Rock mass with layers of sandstone

and clay

Rock mass with layers of sandstone

and clay, light yellowish grey, weak on

the surface, large block sizes, 5 major

discontinuity sets, does not fizz with

acid.

1 Rock Mass Description sheets for both rock types can be found in the appendix

Figure 2.79 – Hard Sandstone

Figure 2.80 – Rock mass with layers of sandstone and clay

79

CHAPTER 3 – LABORATORY/FIELD RESULTS

3.1 Point load test

To gain the strength characteristics of the rock mass, a point load test needs to

be performed. The pundit test will help gain the uniaxial compressive index

which will then be used to check against a uniaxial compressive index chart.

The table below are the raw results gained from the experiment.

Hard Sandstone

Force at Failure (N) Length of Axial Loading (mm) Area (mm)2

Height Average length

10280 33.2 55.34 1837.29

6400 19.91 36.14 719.55

10600 25.2 64.14 1616.33

12260 32.69 25.17 822.81

7190 42.28 34.18 1445.13

Table 3.1 – Raw data for Hard Sandstone

Mixture of Sandstone and Clay

Force at Failure (N) Length of Axial Loading (mm) Area (mm)2


2700 23.09 63.46 1465.29

890 21.82 41.14 897.67

560 25.48 47.28 1204.69

9600 21.73 38.34 833.13

8200 16.59 31.42 521.26

Table 3.2 – Raw data for rock mass with layers of Sandstone and Clay

The table on the next page shows the calculations to get the Uniaxial

compressive index

80

Hard Sandstone

Table 3.3 – Calculated point load index

Is = 5.62 N/mm2

The size corrected PLS index Is(50) = Is x F

Average F = 0.90

Is(50) = 5.05 N/mm2 = 5.05 Mpa

Force at

Failure (N)

Length of Axial Loading (mm)

Area (mm)2

De2=4A/π

(mm2)

De (mm) Is=P/Dee F= (De/50)0.45 Is(50)=F x Is Average Is


10280 33.2 55.34 1837.29 2339.31 48.37 4.39 0.99 4.33

5.62

6400 19.91 36.14 719.55 916.16 60.27 6.99 0.80 5.57

10600 25.2 64.14 1616.33 2057.97 45.36 5.15 0.96 4.93

12260 32.69 25.17 822.81 1047.63 32.37 11.70 0.82 9.62

7190 42.28 34.18 1445.13 1840.00 42.90 3.91 0.93 3.65

81


Table 3.4 – Calculated point load index

Is = 3.79 N/mm2

The size corrected PLS index Is(50) = Is x F

Average F = 0.85

Is(50) = 3.21 N/mm2 = 3.21 Mpa

Force at

Failure (N)

Length of Axial Loading (mm)

Area (mm)2

De2=4A/π

(mm2)

De (mm) Is=P/Dee F= (De/50)0.45 Is(50)=F x Is Average Is


2700 23.09 63.46 1465.29 1865.67 43.19 1.45 0.94 1.35

3.79

890 21.82 41.14 897.67 1142.96 33.81 0.78 0.84 0.65

560 25.48 47.28 1204.69 1533.86 39.16 0.37 0.90 0.33

9600 21.73 38.34 833.13 1060.77 32.57 9.05 0.82 7.46

8200 16.59 31.42 521.26 663.69 25.76 12.36 0.74 9.17

82

Description Point Load Strength Index (MPa)

Equivilant Uniaxial Compressive Strength (MPa)

Extremely high strength >10 >160

Very High strength 3 – 10 50 – 160

High strength 1 – 3 15-60

Medium strength 0.3 – 1 5 – 16

Low strength 0.1 – 0.3 1.6 – 5.0

Very low strength 0.03 – 0.1 0.5 – 1.6

Extremely low strength <0.03 <0.5

Table 3.5 – Point load strength index (Franklin and Brox, 1972)

The Hard Sandstone has a point load index of 5.05 MPa which indicates very

high strength with a Equivilant Uniaxial Compressive Strength of 50 – 160. The

rock mass with the mixture of Sandstone and Clay has a point load index of

3.21 MPa which also represents very high strength with a Equivilant Uniaxial

Compressive Strength of 50 – 160. Both the Hard Sandstone and the rock mass

with the mixture of Sandstone and Clay can be classified by Hoek’s rock

classification table, seen on table 3.6, as being strong to very strong.

83

Table 3.6 Classification of rock by strength (Hoek, E, & Marinos, P, 2000)

84

3.1.1 Results

The Hard Sandstone had a point load strength index of 5.05 MPa and the

mixture of Sandstone and Clay had a point load strength index of 3.21 MPa.

3.1.2 Formulas used for calculations

Is = P

Area = length x width

De2 = 4A/π

Uncorrected Point Load Strength (Is) = P/De2

F= (De/50)0.45

Is(50)=F x Is

85

3.2 Pundit test

A PUNDIT (Portable Ultrasonic Non Destructive Index Tester) is used for this

test. At one end of the specimen a compressive stress pulse is generated and

the PUNDIT test records the time taken for the resulting P wave to reach the

other end. The table below shows the raw results for the PUNDIT test

Description of rock Time (ms x 10-6) Length (mm)

Hard Sandstone

43.2 230

22.4 100

45.3 230


63.5 145

30.7 130

23.6 40

The table on the next page shows the calculations to get the Average Velocity

(ms-1)

Description Time (ms x 10-6)

Average time (ms x 10-6)

Length (mm)

Length (m)

Average Length (m)

VP ms-1 Average Velocity ms-1

Hard Sandstone

43.2

36.97

230.00 0.23

0.19

5324.07

4955.21 22.4 100.00 0.10 4464.29

45.3 230.00 0.23 5077.26


63.50

39.27

145.00 0.15

0.11

2283.46

2737.64 30.70 130.00 0.13 4234.53

23.60 40.00 0.04 1694.92

Table 3.8 – PUNDIT test calculated results


VP = D/t

Table 3.7 – Raw results for the PUNDIT test

86

3.3 Slake durability

The test involves the rock sample being put in a drum and then partially

submerged in water. The drum is then rotated at 20 revolutions per minute for a

period of 10 minutes. The drum is dried and the weight loss is then recorded.

The test cycle is then repeated one more time. This is all to test how prone a

rock mass is to weathering.

The table below are the raw results gained from the slake durability test

Sample Initial mass (g) After Cycle 1 mass (g)

Dry mass (g)

Mix of clay and sandstone

310.9 269.4 250.0

Hard Sand Stone 301.9 302.9 299.7

Table 3.9 – Cycle 1 raw results

Sample Initial mass (g) After Cycle 2 mass (g)

Dry mass (g)

Mix of clay and sandstone

250.0 251.3 235

Hard Sand Stone 299.7 300.6 299

Table 3.10 – Cycle 2 raw results

Hard Sandstone

Test number

Initial weight (g)

Final weight (g) Slake durability index (%)

Average slake durability index (%)

1 30.1.90 299.70 99.27 99.52

2 299.70 299.00 99.77

Table 3.11 – Hard Sandstone calculations for average slake durability index

87

Mix of Sandstone and Clay

Test number

Initial weight (g)

Final weight (g) Slake durability index (%)

Average slake durability index (%)

1 310.90 250.00 80.41 87.21

2 250.00 235.00 94.00

Table 3.12 – Rock mass with layers of Sandstone and Clay calculations for

slake durability index

Group Percentage retained after one 10 – minute cycle (dry weight basis)

Percentage retained after two 10 minute cycle (dry weight basis)

Very high durability >99 >98

High durability 98-99 95-98

Medium high durability 95-98 85-95

Medium durability 85-95 60-85

Low durability 60-85 30-60

Very low durability <60 <30

Table 3.13 – Slake durability scale

3.3.1 Results

Using the table above, the Hard Sandstone has an average slake durability

index of 99.52 % which means that it has very high durability. The first and

second cycles were both slightly above 99% which was expected.

The Rock mass which contained a mix of Sandstone and Clay had a slake

durability index of 80.41% on the first cycle and 94% on the second cycle.


Slake durability index = (initial weight/final weight)*100

88

3.4 Consistency limit

The cone penetrometer method for liquid limit is reliant on the relationship

between the moisture content and the penetration of the soil sample by the

cone.

This test must be performed due to the layer of Clay that runs along the site.

Tin number Mass of empty tin (g)

Mass of tin plus “wet” soil (g)

Mass of tin plus “dry” soil (g)

5.9 5.3 16.3 14.7

Table 3.14 – Raw Plastic limit test results

Test number Penetration (mm)

Tin number

Mass of empty tin

(g)

Mass of tin plus “wet”

soil (g)

Mass of tin plus “dry”

soil (g)

1 27.8, 25.8, 27.6

Ave = 27.1

30 3.8 12.6 10.4

2 3.6, 3.6, 4.1

Ave = 3.8

139 3.7 16 27.1

3 9.7, 10.2, 10.5

Ave = 10.1

99 3.5 24.4 20.3

4 15.5, 15.0, 15.0

Ave = 15.2

136 3.5 33.8 14

Table 3.15 – Liquid limit test results: raw results gained from the Liquid limit test

The table on the next page shows the calculations necessary to classify the soil.

89

Mass of tin (g) Mass of tin plus wet soil (g) Mass of tin plus dry soil (g)

5.3 16.3 14.7

Table 3.16 – Plastic limit test results

Plasticity index = 17.02%

Penetration Tin weight (g) Mass of tin + wet soil (g)

Mass of tin + dry soil (g)

Water content %

27.1 3.8 12.6 10.4 33.33

15.2 3.5 16 14 28.63

10.1 3.5 24.4 20.3 24.40

3.8 3.7 33.8 27.1 19.05

Table 3.17 – Liquid limit test results.

A penetration against water content graph is plotted on the next page.

90

0

5

10

15

20

25

30

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00

Pe

ne

trat

ion

Water content %

Linear (Penetration)

Figure 3.1 - Liquid Limit = 30 %

91

Figure 3.2 – Soil classified as clay with low plasticity.

92

3.4.1 Results

This test has confirmed that the soil is clay with low plasticity.


Plasticity index = water content = (wet-dry/dry-tin) x 100

93

3.5 Undrained Triaxial test

The specimen for the triaxial test was prepared and the computer attached to

the triaxial test produced the necessary Mohr Circle so that the angle of friction

of the clay could be gained. The figure below shows the Mohr circle created.

For the rest of the results, please refer to the appendix.

Figure 3.3 – Mohr’s Circles

Results

With the aid of computer software, Mohr’s circle’s were created for the clay

sample. The angle of friction for the Clay was measured at 15⁰.

The sample failed by sliding as

can be seen on figure 3.4 which

gives an indication as to how the

clay will fail.

15⁰

Figure 3.4 – Clay sample failed in

Tri-Axial test

94

3.6 Goodman and Bray Chart

Table 3.18 – Data for Goodman and Bray Chart

From figure 3.4 on the next page, DS1, 3 and 5 are stable. DS2, 4 and 5 fall into

the toppling and sliding to toppling zones.

Obs No Base (m) Height

(m)

Angle of Friction ( ) Dip of main

discontinuity

Comments:

H.S Rock with layers of

Sandstone and Clay

1 3.78 1.07 37 42 04 DS 1

2 0.62 0.95 37 42 64 DS 2

3 1.103 1.14 37 42 09 DS 3

4 0.58 1.40 37 42 73 DS 4

5 4.36 1.40 37 42 10 DS 5

6 2.21 1.25 37 42 88 DS 6

Average

b/h

2.11/1.20

95

Figure 3.5 – Goodman and Bray chart

DS1

DS3 DS5

DS2 DS4 DS6 Zone C: Toppling only

96

CHAPTER 4 – STABILITY OF THE SITE

This part of the project will investigate the stability of the Cliff End site. A

topographical survey was taken which contains 10 faces and all 10 faces were

analysed with the aid of a stereonet software called “Stereonet 32”.

Face 1 Face 2

Face 3

Face 4

Face 6

Face 5

Face 7 Face 8

Face 9

Face 10

Face 1

Face 10

Figure 4.1 – Topographical survey

Figure 4.2 – Satellite view of Cliff End

97

4.1 Stereographic projection

A total of 722 Dip and Dip directions were taken of the Cliff End site. All 722

were typed into the stereonet 32 software and the programme determined 6

discontinuities with the value of both their dip and dip direction marked with a ▲

on the polar stereonet.

Discontinuity set Dip Dip direction

DS1 4 308

DS2 64 035

DS3 09 023

DS4 73 123

DS5 10 079

DS6 88 100

Figure 4.3 – Stereonet with every discontinuity data plotted

Table 4.1 – Discontinuity set from plot

98

Figure 4.4 – Stereonet without face.

The figure above shows all 6 discontinuity sets plotted with 3 angles of friction.

For a Plane failure to occur the dip of the face must be greater than the dip of

the discontinuity and the dip direction of both the face and discontinuity must be

similar, +-20⁰. The angle of friction must also be less than the discontinuity set.

For a Wedge failure to occur, the intersection of two discontinuities must have a

dip direction similar to that of the face (+-20⁰) as well as having a dip less than

that of the face. The intersection must also be less than the intersection.

Description Angle of Friction (⁰)

Hard Sandstone 37

Mixture of Sandstone and Clay 42

Clay 15

Clay

Mixture of Sandstone

and clay

Hard

Sandstone

Table 4.2 – Angle of Friction

99

Plane failure along discontinuity set 4. All other discontinuities sets are stable.

Face 1

Face 1 = 84/136

Face 2 = 89/032

Wedge failure along DS2 and DS6. Plane failure along DS2. Possible

wedge failure along DS2 and DS4.

Figure 4.5 – Analysis of Face 1


100

Plane failure along DS4. All other discontinuities are stable.

Wedge failure along DS2 and DS4. All other discontinuities are stable.

Face 3 = 89/138

Face 4 = 82/065



101

Wedge failure along DS2 and DS6. Plane failure along DS2. All other

discontinuities are stable.


Face 5 = 81/022

Face 6 = 84/138



102

Plane failure along DS2. Possible wedge failure along DS2 and DS6. Possible

wedge failure along DS4 and DS2. All other discontinuities are stable.


Face 7 = 83/042

Face 8 = 89/132



103

Plane failure along DS2. Wedge failure along DS2 and DS6. Possible wedge

failure along DS2 and DS4. All other discontinuities are stable.


Face 9 = 88/028

Face 10 = 85/121



104

CHAPTER 5 - Discussion

5.1 Laboratory results

Hard sandstone and the rock containing a mixture of mainly Sandstone and

Clay, found on the site, were tested in the Slake durability test. Hard sandstone

was found to have a consistent slake durability index on both cycle 1 and cycle

2 with a slake durability index of 99.52 %. This confirms that the Hard

sandstone is highly resistant to weathering which suggests that the particles

within the Hard sandstone are very well cemented.

The rock mass that contained a mixture of mainly Sandstone and Clay did not

have consistent results through both cycles. The first cycle had a slake

durability index of 80.41% and the second had a slake durability index of 94%.

Using Gamble’s Slake durability scale, this would mean that the durability range

lies from low durability to medium high durability which is a significant difference.

The reason for this is that the rock mass initially contained a layer of clay which

disintegrated in the first cycle. In the second cycle, the rock mass no longer has

the clay inside so the difference in weight from the initial weight to the final

weight is far less than the first cycle’s initial and final weight.

It is important to note though that the slake durability test does not take into

account other mechanical or physical process such as fractures as a result of

heat expansion or contraction and freeze thaw cycles.

The Pundit test was used to test the pulse velocity of the Hard sandstone and

the rock mass containing mainly a mixture of Sandstone and Clay. As expected,

the pundit test showed that the Hard sandstone was more dense. The Hard

sandstone had a pulse velocity of 4955.21 ms-1 and the rock mass with a

mixture of Sandstone and Clay had a pulse velocity of 2737.64 ms-1. This is due

to the higher density of the Hard Sandstone which reduces air voids and

imperfections.

The point load test was used to give an indication as to what the strength of the

rock samples were. The Hard Sandstone had a point load index of 5.05 MPa

which indicates very high strength with a Equivilant Uniaxial Compressive

105

Strength of 50 – 160 MPa. The rock mass with the mixture of Sandstone and

Clay has a point load index of 3.21 MPa which also represents very high

strength with a Equivilant Uniaxial Compressive Strength of 50 – 160 MPa.

Before the point load test, it was predicted that the hard sandstone and the

mixture of Sandstone and Clay would be classified as rocks with high to very

high strength due to the Schmidt hammer readings that were taken on the site.

The laboratory test results confirmed these readings with the Hard Sandstone

having a point load strength index of 5.05 MPa and the mixture of Sandstone

and Clay having a point load strength index of 3.21 MPa.

To classify the soil found on site, the consistency limit test had to be performed.

This is a test that is reliant on the relationship between the water content and

the plastic limit of the sample. The liquid limit was 30% and the plastic lndex

was 17%. Using the soil classification graph, the soil was found to be clay with

low plasticity. Before the test, it was expected that the laboratory test would

show that the soil sample was clay. This is due to various sources that

discussed Wadhurst Clay in great detail as well as geological maps that gave

indication of Wadhurst Clay on the site. By touch and feel, it was also expected

for the Clay to be of low plasticity. This test has confirmed that the soil is indeed

Clay with low plasticity.

5.2 Field results

As an early indication of the rock strength, the Schmidt hammer reading was an

quick and easy test to carry out. The Hard Sandstone was stronger with an

average reading of 33 whilst the rock with mixture of mainly Sandstone and

Clay had a reading of 27. It must be noted that this is only a measure of the

exterior strength of the rock mass and not an indication of the strength that is

inside the rock. Based on these initial readings, there was an expectation to the

behaviour of these rocks in the laboratory tests.

106

5.3 Analysis of Stereographic projection.2

Face 1

The stereonet only shows a plane failure which runs along discontinuity set 4

(DS4). The face can be seen to have a dip which is less than DS4. Face 1

having a dip of 84⁰ and DS4 123⁰ respectively. The face also has a similar dip

direction to discontinuity set 4 with 136⁰ and 123⁰ respectively which is within +-

020⁰. This means that DS4 is within the danger zone, therefore the stereonet

shows a plane failure for this face under all angles of friction.

Face 2

The stereonet shows a wedge failure occurring along discontinuity set 2 (DS2)

and discontinuity set 6 (DS6). The point at which DS2 and DS6 intersect is

greater than all three angles of friction. The dip direction of the intersection is

015⁰ and the dip direction of the face is 032⁰ which indicates that the

intersection is in the danger zone. The dip of the face is also greater than the

intersection which all add up to indicate a wedge failure along DS2 and DS6.

DS4 and DS2 also intersect but with a dip direction of 66⁰, it clearly does not fall

into the danger zone.

There is also a plane failure along DS2. This is due to the dip of DS2 being

greater than all three of the angles of friction as well as having a dip direction of

035⁰ which is very similar to that of the face’s 032⁰. The dip of the face (89⁰) is

also greater than DS2 (64⁰) which meets the conditions for a plane failure to

occur.

Face 3

On this stereonet, a plane failure is existent along DS4. The dip of the face (89⁰)

is greater than that of DS4 (73⁰) and the dip direction of the face (138⁰) is within

2 When the phrase “entire angles of friction” is page 98 for the values of each angle of friction.

107

+- 20⁰ of DS4 (123⁰). DS4 also has a dip which is greater than all three angles

of friction. This places DS4 in the danger zone and due to this, plane failure

occurs along this discontinuity set

Face 4

On this stereonet, a wedge failure occurs along DS2 and DS4. As can be seen

on the stereonet, DS2 and DS4 intersect at a point where its dip direction (66⁰)

is almost identical to that of the dip direction of the face (65⁰). The dip of the

intersection (60⁰) is less than the dip of the face (82⁰) and the intersection’s dip

is greater than all three angles of friction. This places the intersection in the

danger zone which indicates a wedge failure.

Face 5

This stereonet shows a wedge failure along DS2 and DS6. The point at which

DS2 and DS6 intersect has a dip direction of 15⁰ of and the face has a dip

direction of 022⁰ which are similar. The intersection has a dip (15⁰)lower than

that of the face (81⁰) but greater than that of all angles of friction which places

the intersection in the danger area.

There is also a plane failure on this stereonet. DS2 has a dip direction (035⁰)

which is similar to that of the face (022⁰). The dip of DS2 (64⁰) is also less than

the dip of the face (81⁰) but greater than all three angles of friction which places

DS2 in the danger area. This results in a plane failure along DS2.

Face 6

This stereonet shows a plane failure along DS4. The dip direction of the face is

138⁰ which is similar to DS4’s dip direction of 123⁰. The dip of DS4 (73⁰) is also

less than that of the face’s dip of 84⁰ but greater than all of the angles of friction

which means that DS4 lies within the danger zone thus, resulting in a plane

failure.

108

Face 7

This stereonet shows a plane failure along DS2. The dip direction of the face

(042⁰) is similar to that of DS2 (035⁰). The dip of DS2 (035⁰) is also greater than

all three angles of friction as well as being lower than that of the dip of the face

(83⁰). This places DS2 in the danger zone thus resulting in a plane failure.

Face 8

This stereonet shows another plane failure along DS4. DS4 has a dip direction

of 123⁰ which is similar to the dip direction of the face (132⁰). The dip of DS4 is

73⁰ which is less than the dip of the face which is 89⁰ but is greater than all

three angles of friction. This means that DS4 lies within the danger zone thus

resulting in a plane failure.

Face 9

On this stereonet a wedge failure exists along DS2 and DS6. The point at which

they intersect is 15⁰ which is similar to the dip direction of the face which is 028⁰.

The dip of the intersection (60⁰) is also less than the dip of the face (88⁰) but

greater than all three angles of friction. This means that the intersection lies

within the danger zone thus forming a wedge failure.

There is also a plane failure along DS2 due to its dip direction (035⁰) being

similar to that of the dip direction of the face (028⁰), its dip (035⁰) being less

than that of the face’s dip (88⁰) but greater than all three angles of friction. This

places DS2 in the danger zone thus resulting in a plane failure along DS2.

Face 10

This stereonet shows a plane failure only along DS4. The dip direction of the

face (121⁰) is very similar to that of the dip direction of DS4 (123⁰). The dip of

DS4 (73⁰) is less than that of the dip of the face (85⁰) but greater than that of all

three angles of friction. This places DS4 in the danger zone thus resulting in a

plane failure along DS4.

109

5.4 Comments on stability

Discontinuity set 1, 3 and 5 took no part in any failure of the face since the dip

was on all three discontinuity sets were less than all three of the angles of

friction. The main discontinuity sets were 2, 4 and 6. When the orientation of the

face was SE°, only a plane failure would occur along discontinuity set 4. When

the orientation of the face was towards NE°, NNE° and ENE° there were plenty

of instances where both wedge and plane failures would occur. The only

exception to that was face 7 which had a dip direction not similar to the two

intersections. It is then reasonable to assume that a wedge and plane failure is

to be expected if the orientation of the face is towards NE°, NNE° and ENE°.

The Goodman Bray Chart, Figure 3.4 on page 84, confirms that there are no

failure along Discontinuity set 1, 3 and 5. Discontinuity set 2, 4 and 6 are prone

to Sliding and toppling failure according to the Goodman Bray Chart.

5.5 Slope stabilisation

Along the Cliff End site, it has been identified that the most common type of

failures are plain failure and wedge failure depending on the orientation of the

face. There was also a more immediate concern due to the debris and small

rocks that would occasionally fall from the top of the cliff. Putting on a hard hat

is enough to avoid injuries but it is not recommended for the general public to

walk along the cliff face without a hard hat.

To avoid any

immediate danger, it is

highly recommended

that anchored wire

mesh is used on the

site. This will divert the

small blocks of rock

that could fall off the

cliff, onto the base of

the slope safely.

Figure 5.1 – soil and bits of rock on the base of the cliffs

110

Shotcrete is then recommended due to the ability to decrease the chance of

rockfalls and sliding of small rocks. Local residents could find this to be not as

aesthetically pleasing.

Before applying

any of these

methods, it is

important to take

the time to check

the tide times as

the tides only allow

a 5-6 hour window

to work before the

high tide comes in.

This also does not allow for labourers to work on any day as only specific times

and days will provide daylight and low tides.

It remains questionable whether it is worth the time and effort to use methods

such as rock bolting to stabilise the 0.5 km of cliffs as this does not pose any

immediate danger to the

public or to properties as

there are none at the

base of the cliff. Warning

signs are recommended

to be placed near the cliff

to urge people who want

to walk along this area to

stay clear from the cliff.

Tourists often come to Cliff End to walk along the coast and take in the sites

that Cliff End has to offer. Since there is no immediate danger to them as long

as they keep a distance from the cliff, then there should be no need to tarnish

the site which many come to see.

Figure 5.2 – Rock mass

Figure 5.3 – Bits of rock on the shore

111

CHAPTER 6 - Conclusion

The Cliffs have reached their current state by constant wave erosion over

geological time and it is estimated by (Villagenet, 2011) to have eroded at a rate

of 0.6 metres per year. This would indicate that during 1066 the cliffs were

approximately a further 550 metres out.

The site consisted of Hard Sandstone, Wadhurst Clay and Cliff End sandstone

which were taken to the lab and tested upon. The sandstones were proven to

be highly durable, strong rock.

The clay is a major factor when it comes to the instability of the cliffs as the

angle of friction is 15⁰ and is more prone to erosion than the Sandstones.

Through stereographic projection, it was proven that wedge failures would only

occur if the face’s orientation was facing towards NE°, NNE° and ENE°. Plane

failure could occur anywhere from NE° to SE°. Failures would tend to occur

along discontinuity sets 2, 4 and 6. Discontinuity sets 1, 3 and 5 had a dip less

than the entire angle of friction hence no failure would occur along those

discontinuity sets.

There are a number of remedial actions that have been proposed in the short

term such as shotcrete and using anchored wired mesh but this could make the

site less aesthetically pleasing. It is recommended to hold a consultation with

the local residents to decide on what remedial actions should take place.

Figure 6.1 – Blocks of rock on cliff base

112

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115

CHAPTER 8 – APPENDIX

Field data

116

Discontinuity data

No Dip Dip Direction



Comments

1 02 302 Discontinuity 32 81 063 63 88 130

2 05 078 33 87 130 64 88 130

3 03 138 34 90 122 65 88 124

4 05 120 35 89 124 66 05 005

5 04 120 36 89 132 67 03 082

6 06 98 37 89 042 68 10 052

7 03 98 38 08 089 69 13 112

8 02 30 39 02 120 70 15 118

9 03 120 40 08 122 71 30 070

10 04 50 41 10 100 72 31 082

11 03 132 42 89 132 73 70 110

12 02 58 43 81 112 74 85 132

13 01 120 44 85 124 75 88 132

14 16 118 45 84 028 76 90 110

15 11 130 46 90 030 77 01 188

16 86 120 47 84 040 78 71 100

17 85 116 48 88 130 79 78 122

18 90 114 49 86 134 80 00 178

19 86 118 50 90 124 81 59 092

20 88 122 51 85 128 82 89 132

21 89 118 52 08 180 83 74 128

22 84 120 53 07 230 84 88 126

23 89 124 54 05 238 85 82 068

24 66 072 55 80 120 86 76 042

25 62 091 56 85 122 87 88 140

26 78 102 57 89 042 88 88 130

27 02 112 58 84 038 89 70 118

28 77 034 59 88 042 90 75 112

29 88 124 60 82 052 91 81 022

30 88 126 61 08 018 92 66 136

31 90 138 62 82 108 93 70 154

117

94 70 043 126 70 131 158 74 120

95 81 036 127 85 150 159 75 120

96 13 320 128 71 044 160 76 122

97 87 328 129 68 064 161 74 128

98 86 322 130 88 120 162 76 122

99 90 322 131 89 130 163 74 022

100 86 063 132 73 060 164 74 018

101 15 122 133 70 080 165 75 010

102 06 130 134 55 040 166 44 052

103 00 133 135 60 062 167 85 110

104 45 040 136 26 108 168 72 118

105 46 042 137 88 130 169 87 110

106 41 040 138 25 108 170 88 108

107 85 311 139 68 058 171 84 130

108 85 281 140 85 048 172 71 016

109 06 323 141 88 130 173 80 030

110 00 140 142 88 122 174 85 024

111 00 019 143 68 112 175 85 120

112 05 039 144 81 118 176 84 150

113 41 042 145 88 160 177 86 112

114 85 083 146 86 140 178 88 102

115 78 029 147 90 132 179 86 130

116 80 050 148 88 118 180 82 140

117 11 229 149 88 120 181 85 116

118 80 118 150 86 140 182 88 178

119 89 130 151 90 132 183 69 132

120 40 140 152 86 112 184 83 122

121 10 020 153 90 114 185 89 122

122 80 125 154 78 002 186 90 040

123 75 058 155 82 030 187 81 053

124 88 128 156 55 033 188 88 120

125 85 128 157 73 112 189 71 040

118

190 74 104 222 80 134 254 05 184

191 88 122 223 89 126 255 08 190

192 86 125 224 85 126 256 03 050

193 70 108 225 78 122 257 84 125

194 88 126 226 90 130 258 81 130

195 78 116 227 88 122 259 78 124

196 88 062 228 88 068 260 90 130

197 89 038 229 83 068 261 65 140

198 90 132 230 84 120 262 15 123

199 84 088 231 85 128 263 72 138

200 88 122 232 81 128 264 88 060

201 87 129 233 78 024 265 80 140

202 85 123 234 84 074 266 62 080

203 88 142 235 81 114 267 66 090

204 85 130 236 80 109 268 84 130

205 86 126 237 80 118 269 55 038

206 86 128 238 89 128 270 82 127

207 85 124 239 85 118 271 64 132

208 73 088 240 84 038 272 73 089

209 06 040 241 90 120 273 83 124

210 02 075 242 88 040 274 81 126

211 08 090 243 86 130 275 90 128

212 04 042 244 82 128 276 04 038

213 09 054 245 83 078 277 06 042

214 90 153 246 75 128 278 09 090

215 86 128 247 88 150 279 89 130

216 89 118 248 80 124 280 84 080

217 88 048 249 76 110 281 82 140

218 85 074 250 65 130 282 90 142

219 85 058 251 88 120 283 79 110

220 83 126 252 90 124 284 83 120

221 87 122 253 74 028 285 88 129

286 88 129 318 58 300 350 63 072

119

287 81 120 319 45 071 351 90 102

288 85 115 320 47 073 352 88 280

289 86 120 321 90 151 353 10 141

290 90 130 322 89 148 354 00 140

291 83 125 323 40 130 355 11 140

292 78 030 324 41 132 356 65 064

293 00 116 325 15 042 357 63 064

294 03 120 326 45 036 358 86 130

295 55 021 327 47 034 359 85 127

296 52 023 328 85 160 360 22 072

297 50 352 329 84 015 361 20 072

298 52 354 330 76 116 362 00 065

299 84 130 331 78 115 363 02 310

300 86 128 332 67 030 364 03 312

301 40 050 333 03 072 365 00 088

302 44 051 334 05 072 366 01 090

303 86 104 335 06 020 367 08 080

304 86 102 336 06 020 368 10 082

305 81 130 337 64 072 369 62 120

306 85 128 338 66 072 370 60 123

307 05 110 339 04 322 371 55 186

308 04 110 340 06 324 372 90 130

309 45 029 341 05 166 373 90 129

310 48 028 342 74 110 374 88 080

311 60 050 343 88 130 375 75 120

312 56 053 344 86 130 376 65 096

313 60 060 345 84 128 377 70 118

314 60 061 346 80 140 378 77 140

315 12 032 347 80 092 379 75 138

316 13 032 348 82 091 380 80 066

317 55 296 349 65 070 381 62 130

382 50 102 414 05 306 446 10 310

120

383 60 104 415 04 310 447 01 302

384 85 104 416 05 322 448 03 300

385 80 120 417 08 324 449 01 298

386 85 100 418 05 322 450 02 302

387 80 120 419 00 324 451 08 297

388 81 112 420 05 310 452 05 300

389 75 120 421 03 308 453 00 298

390 77 118 422 02 312 454 01 302

391 87 112 423 05 298 455 08 290

392 85 120 424 09 310 456 03 004

393 64 120 425 06 302 457 02 008

394 77 112 426 12 316 458 02 020

395 80 122 427 08 302 459 02 018

396 84 124 428 07 330 460 00 022

397 88 115 429 01 300 461 06 290

398 85 108 430 20 200 462 12 292

399 75 110 431 14 322 463 00 304

400 83 139 432 10 312 464 01 298

401 980 122 433 09 325 465 03 300

402 88 120 434 08 326 466 05 308

403 67 128 435 12 316 467 06 310

404 60 126 436 08 300 468 04 306

405 79 118 437 11 320 469 08 132

406 75 110 438 05 306 470 08 302

407 10 320 439 07 310 471 03 308

408 12 312 440 08 302 472 05 298

409 09 304 441 05 302 473 06 310

410 08 308 442 08 310 474 04 000

411 12 322 443 09 310 475 08 322

412 02 304 444 06 308 476 08 325

413 08 000 445 07 298 477 03 316

121

478 08 300 510 80 120 542 85 114

479 05 305 511 87 116 543 90 111

480 06 307 512 81 190 544 85 111

481 02 313 513 88 305 545 90 111

482 15 301 514 85 102 546 87 120

483 07 297 515 87 295 547 86 110

484 05 298 516 85 280 548 87 108

485 06 292 517 85 300 549 85 121

486 12 304 518 85 298 550 85 123

487 09 310 519 88 112 551 80 120

488 08 302 520 90 296 552 90 120

489 15 301 521 85 116 553 88 130

490 08 306 522 86 108 554 80 120

491 05 310 523 85 116 555 75 116

492 04 322 524 90 120 556 72 112

493 06 316 525 90 152 557 79 119

494 12 302 526 80 123 558 80 118

495 02 297 527 89 123 559 81 120

496 06 264 528 80 125 560 83 123

497 12 328 529 70 125 561 85 120

498 02 318 530 85 121 562 75 121

499 06 324 531 85 114 563 80 120

500 03 319 532 85 115 564 77 121

501 04 321 533 89 116 565 90 120

502 09 323 534 82 103 566 87 124

503 02 320 535 90 110 567 85 124

504 06 316 536 74 110 568 88 124

505 89 102 537 85 108 569 76 011

506 80 164 538 83 110 570 69 005

507 77 118 539 82 102 571 65 020

508 78 116 540 89 100 572 74 016

509 79 115 541 81 160 573 73 015

122

574 87 008 606 14 032 638 60 010

575 76 006 607 85 082 639 80 050

576 72 003 608 81 074 640 80 024

577 67 010 609 88 072 641 80 042

578 88 000 610 80 072 642 75 038

579 77 358 611 88 068 643 52 020

580 80 004 612 66 058 644 52 020

581 80 008 613 87 082 645 50 019

582 78 005 614 81 079 646 70 038

583 85 002 615 82 078 647 82 040

584 73 006 616 81 081 648 88 041

585 75 002 617 75 078 649 86 052

586 15 038 618 63 202 650 65 042

587 18 020 619 65 208 651 89 044

588 15 018 620 64 206 652 50 040

589 18 012 621 66 208 653 48 063

590 19 008 622 74 205 654 44 038

591 71 020 623 73 206 655 62 040

592 19 018 624 72 200 656 60 022

593 21 019 625 72 210 657 79 024

594 15 023 626 57 198 658 80 020

595 19 023 627 75 200 659 78 020

596 18 023 628 71 203 660 59 024

597 14 023 629 78 222 661 70 024

598 18 032 630 64 215 662 70 036

599 28 032 631 68 185 663 72 028

600 14 018 632 80 190 664 75 028

601 12 018 633 72 208 665 78 040

602 14 024 634 80 200 666 75 038

603 18 038 635 65 210 667 70 040

604 18 030 636 78 208 668 71 042

605 18 024 637 79 208 669 78 042

123

670 85 038 702 57 026

671 72 010 703 59 028

672 75 180 704 54 028

673 70 178 705 56 027

674 65 175 706 76 052

675 62 180 707 76 058

676 88 012 708 85 058

677 85 009 709 80 048

678 82 005 710 85 050

679 80 010 711 82 046

680 86 008 712 72 042

681 88 010 713 74 052

682 88 010 714 78 052

683 84 006 715 78 082

684 82 012 716 72 028

685 80 010 717 75 038

686 85 063 718 70 003

687 85 022 719 72 036

688 70 012 720 71 005

689 700 013 721 74 008

690 68 013 722 75 012

691 62 015

692 62 010

693 64 015

694 65 016

695 63 018

696 76 038

697 72 038

698 64 036

699 60 030

700 64 031

701 58 032

126

Lab Data

127

Undrained Triaxial compression test

Rock Slope Stability of Cliff End

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