london clay

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Contents Part 1....................................................... 1 Part To crtically study and comment on the engineering geology of London Clay and make a reasoned/referenced summary of geotechnical properties for London Clay.................1 Part 2....................................................... 6 Develop a representitative but simplified (metric) section (based on figure 22-Kensal Green Wall failure of Skempton’s paper) a retaining wall for your own analysis and design exercise................................................... 6 Part 3...................................................... 10 To carry out a stability analysis of your retaining wall, using suitable software package’s of your choice (Oasys etc) and also present a specimen hand calculation..............10 Initial design...........................................11 2 nd design................................................13 3 rd design................................................16 Part 4...................................................... 20 To suggest an appropriate stabilisation method and evaluate the improvement on the factor of safety...................20 Other methods of slope stabilisation.....................21 Reference................................................... 23

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3rd year soil mechanics coursework.

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Page 1: London Clay

Contents

Part 1...........................................................................................................................1

Part To crtically study and comment on the engineering geology of London Clay

and make a reasoned/referenced summary of geotechnical properties for London

Clay.........................................................................................................................1

Part 2...........................................................................................................................6

Develop a representitative but simplified (metric) section (based on figure 22-

Kensal Green Wall failure of Skempton’s paper) a retaining wall for your own

analysis and design exercise...................................................................................6

Part 3.........................................................................................................................10

To carry out a stability analysis of your retaining wall, using suitable software

package’s of your choice (Oasys etc) and also present a specimen hand

calculation..............................................................................................................10

Initial design.......................................................................................................11

2nd design...........................................................................................................13

3rd design............................................................................................................16

Part 4.........................................................................................................................20

To suggest an appropriate stabilisation method and evaluate the improvement on

the factor of safety.................................................................................................20

Other methods of slope stabilisation..................................................................21

Reference..................................................................................................................23

Appendix...................................................................................................................24

Page 2: London Clay

Part 1

Part To critically study and comment on the engineering geology of London Clay and make a reasoned/referenced summary of

geotechnical properties for London Clay

London clay is a type of clay which appears in the southeast of England. It is of

Eocene age and has been consolidation according to (Skempton, 1964) under a

thickness of sediments which have been removed by erosion and vary from 500 ft in

the eastern parts of Essex up to 1000 ft in the region of west London.

(British Geological Survey)

(Dixon & Bromhead, 2002) Also confirm with (Skempton, 1964) in their article

published in Geotechnique, London Clay in coastal cliffs. (Dixon & Bromhead, 2002)

mentions that London Clay is a very stiff heavily overconsildated fissured silty clay

deposit of Neogene (Eocene) age. (Reeves, Sims, & Cripps, 2006) also adds that it

is more sandy at the base and top Parts are laminated and it contains nodular

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claystones and rare sandy partings. The formation is commonly weathered to brown

clay to depths of 5 to 10m.

(Skempton,1964) Typical Profile of London Clay

When looking through the full length of London Clay, fissure and joints can be found.

The fissures and joints though are far more obvious when it comes to weathered

zones which are usually 30ft to 40ft deep as can be seen from the figure above.

Upon inspection on the weathering of London clay, (Skempton, 1964) mentions that

the brown colour of the clay is the indication to look out for rather than the blue

colour of the unweathered material. Geothile and limonite are also minerals that give

indication of weathering. These can be found in some of the fissures and joints.

London clay contains illites which are the commonest clay minerals; formed by the

decomposition of some micas and feldspars; predominant in marine clays and shale.

(Skempton, 1964) Mentions that the peak strength varies can be taken as C’=320

lb/sq.ft and Ф’=20⁰. Residual strength (Фr’) has been measured according to

(Skempton, 1964) about 16⁰.

(Skempton, 1964) Also notes that tests were conducted on block samples from a

deep shaft at Ashford Common, near Staines and they show that the values of c’ and

Ф’ in the unweathered London Clay are considerably greater than those in the

weathered zone.

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The peak strength though is measured on specimens which are considerably small

specimens and relate to the intact material according to (Skempton, 1964). Tests

that are conducted on larger specimens show lower strengths which are due to the

inclusion of fissures.

According to an article regarding the ground conditions around an old tunnel in

London Clay by S.M Gourvenec, the strength of London Clay is strongly dependent

on the specimen density with higher strength in denser samples. There exists a

relationship between the depth at which the London clay samples are collected and

the density. The deeper the sample is retrieved, the higher the strength is observed

which means the samples are denser (lower void ratio). It is also worth noting that

the denser the sample of London Clay is, the more stiffness can it is.

(Gourvenec, Mair, Bolton, & Soga, 2005) performed a borehole investigation at a

greenfield site in Kennington, South London which provided visual evidence of the

changing nature of London Clay with depth, identifying an increasing portion of silt

and sand particularly as its base was approached. There was an increase in the

sandiness of the clay which brought a reduction in natural moisture content, reducing

plasticity, higher permeability, and higher shear strength and stiffness with depth.

(Reeves, Sims, & Cripps, 2006) indicates that the weathered clay may have a very

high moisture content, however, in the dry periods, the material which is in the upper

few metres may be desiccated resulting in high strength. The depth of desiccation

may be greater if trees are present. The weathered material also becomes more

fissured. Below the depth of season cariation the moisture content of London Clay

caries by only a few per cent with depth.

The bulk density of London Clay varies

between 1.70 and 2.05 Mg/m3 depending

on weathering grade and location.

A characteristic of London Clay is the

zone of softened clay which extends

roughly an inch on either side of the slip plane. The diagram to the left show three

examples of London Clay. What is interesting to note is that is that the water content

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(Skempton, 1964)

Page 5: London Clay

immediately adjacent to the slip plane is about 35, compared with water content of

around 30 in unsoftened clay.

Engineering property of London Clay Weathered Unweathered

Liquefied Limit (%) 66-100 50-105Plastic Limit (%) 22-34 24-35Plasticity index (%) 36-55 41-65Void ratioClay franction < 2μm (%)

(h) 0.69-1.41 (h) 0.60-0.83

Natural water content (%) (b)23-49 19-28Bulk Density (Mgm-3) 1.70-2.00 1.92-2.04Undrained Shear Strength (kPa) 100-175 100-400Effective cohesion (kPa) 12-18 17-252Effective angle of friction (degrees) 17-23 20-29Residual Shear strength (degrees) 10.5-22 (t) 9.4-17Secant modulus of elasticity (MNm-2) (g) 25-141Coefficient of volume change (m2MN-1)(q)

Mv=0.5-0.18 Mv=0.01-0.002Ms=0.094-0.003

Coefficient of consolidation (m2yr-1) 0.2-2.0 0.3-6.0Permeability (ms-1) (m) 2.2*10-10 -3*10-8

(p) 3*10-10 -3*10-8

Effective stress ratio (K0) 0.5-4.4 1.1-2.8(Bell, 2000) Lists engineering properties of some British clay soils of Tertiary and Mesozoic age. The table only shows information concerning London clay.

b= Upper limit value for mudflow

g= Depth up to 46m

h= Calculated from SG, w and yb values

m= Laboratory test

p= In situ test

t= Ring shear test

Site investigations were conducted by (Skempton, 1964) at Northolt and Kensal

Green where failures have occurred and it was found that in weathered London clay,

which is heavily fissured and jointed, there wil be some decrease in the shear

strength parameters, below the peak values, even during the process of excavation.

There is also evidence that in slips that have taken place after 20 or 30 years, the

average strength of the clay has fallen to about 60% of the way from peak to

residual. A site called Sudbury hill was also investigated after a slip which occurred

after 50 years, and the average strength of the clay has fallen by 80%. In natural

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slopes of the London Clay, the strength is near the residual strength. This is shown

in the Jackfield landslide which is a natural slope on weathered Fissured clay which

also shows strength nearly equal to the residual value.

It can be said then that when fissures and joints are present in the clay, progressive

failure can be expected and this process will continue until the residual strength is

reached. In the case of clays which are not fissured or jointed, the decrease in

strength from peak strength is actually so small that it can be considered as

negligible.

No matter what type of clay is involved (Skempton, 1964) mentions that once a

failure has already occurred, the residual strength is the factor that controls any

subsequent movements on the existing slope surface. (Skempton, 1964) Also notes

that in shear zones, which are caused by tectonic movements, the strength will be at

the residual value.

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Page 7: London Clay

Part 2

Develop a representitative but simplified (metric) section (based on figure 22-Kensal Green Wall failure of Skempton’s paper) a

retaining wall for your own analysis and design exercise

6

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Part 3

To carry out a stability analysis of your retaining wall, using suitable software package’s of your choice (Oasys etc) and also

present a specimen hand calculation.

The following characteristics were used in the design of the wall

Unit weight of wall 24 KN/m3

Bulk unit weight of London Clay 18 kg/m3

Effective friction angle (London clay) 19⁰Effective Cohesion (London clay) 14 kPaEffective residual friction angle 17⁰Ballast unit weight 12 KN/m3

Effective angle of friction (Ballast) 50⁰Effective cohesion (Ballast) 10 kPaKa (London Clay) 0.548Kp (London Clay) 1.83Ka (Ballast) 0.132Kp (Ballast) 7.55FOS sliding 1.5FOS bearing 3FOS overturning 2

Note: Although the minimum factor of safety is mathematically 1 it is still preferable

to aim for a factor of safety that is larger than this. The reason as to why the factor of

safety should be more than 1 is because this is the absolute minimum factor of

safety required to make a structure stand due to the stabilising forces being equal to

the destabilising forces.

Also, the ballast was placed in this design to act as a stabilising force for the

retaining wall.

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Initial design 1

In part 2 the retaining wall labelled as the “initial design” (page 7) was used to test

how this retaining would cope. The results show that the retaining wall fails in sliding,

bearing and overturning.

The factor of safety calculated for sliding was 0.36222, Bearing 0.02835 and

overturning 1.00049. This is clearly unacceptable and the wall will need to be

modified in various ways to increase the factor of safety.

1 Print outs from the Oasys software will be provided for the initial design and can be found in the appendix

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2 nd design

The retaining wall was modified and the drawing and dimensions can be seen in part

2 labelled 2nd design (page 8). In summary, the thickness of the wall increased as

well as the front angle. Also, the thickness of the base increased and the length of

the base behind the ball also increased.

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The factor of safety calculated for sliding this time around is 0.68278, Bearing

2.78754 and overturning 2.84130. This is still clearly unacceptable although there

has been an improvement upon all of the FOS. The only FOS that passes is the

overturning FOS. The wall will still need to be modified in various ways to increase

the factor of safety.

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3 rd design

The retaining wall was modified and the drawing and dimensions can be seen in part

2 labelled 3rd design (page 9). The angle on the back of the retaining wall has slightly

increased and the thickness of the base slab has also decreased. The length of the

base from the back wall however has had a 1.4 meter increase to help against

sliding since sliding was becoming problematic.

The factor of safety calculated for sliding this time around is 1.52363, Bearing

10.23707 and overturning 8.62049. Sliding, bearing and overturning now all pass.

The overturning and bearing have exceptionally high FOS whilst the sliding has a

FOS of 1.52363. The only FOS that passes is the overturning FOS. The wall will still

need to be modified in various ways to increase the factor of safety.

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Part 4

To suggest an appropriate stabilisation method and evaluate the improvement on the factor of safety

There are a number ways that stabilisation techniques can be used so that the factor

of safety is improved. Modifying the retaining wall itself will improve the factor of

safety in a number of ways.

If the intention is to increase the factor of safety against sliding, then the Breadth of

the cantilever wall will need to be increased. This is shown by the formula below. It

can be seen that the only to increase the factor of safety in this case is to increase

Breadth.

FS (sliding )=( (∑ V ) tan(K1∗∅ 2))+B∗K 2∗C 2+Pp ¿/(Pa∗Cosα )

This is also proven within the software “Oaysis Greta”. In the 2nd design of the

retaining wall in part 3, the factor of safety against sliding was 0.68278 with a

breadth of 4.6 meters according to the “oaysis Greta” software. Adjusting the Breath

to 6.3 meters, as done in part 3 for the 3rd design, increases this factor of safety to

1.52363.

To increase the factor of safety against the overturning moment, the addition of a

shear key may be a viable solution. Using the Oaysis Greta software, it can be seen

that in the analysis of the 3rd design, from part 3, the factor of safety against

overturning is 8.62049. Adding a shear key to this retaining wall at a distance of 4

meters with a height and width of 1 meters increases this factor of safety to 9.65154.

What is interesting to note is that although the factor of safety for the overturning has

increased, the sliding factor of safety decreases dramatically to 1.27048

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Page 19: London Clay

Other methods of slope stabilisation

(Bromhead, 1986) proposes several methods to stabilise slopes. If a slope is too

high or steep then lower or flatten it, if the materials are too weak then strengthen or

replace them; if the porewater pressures are too high then lower them; and finally, if

the slope is subject to undesirable external influences, then insulate it from them.

Such solutions will always be the cheapest, and technically will be the easiest, if the

slope can be considered in isolation from its surroundings.

Soil anchor

(Bromhead, 1986) mentions that anchors in soil and rock slopes can be of two types:

they can be unstressed, and rely purely on a dowelling action to increase the

resistance to sliding; or they may be stressed. In this latter type, the axial load in the

anchor increases the effective stresses at depth in the soil or rock, improving the

strength. A vector component of the anchor force may also act to reduce

destabilising forces and moments.

Drainage

Drainage is a viable and effective slope stabilisation method however, as a long term

solution it suffers greatly because the drains must be maintained if they are to

continue to function according to (Bromhead, 1986). Often, the design is such that

maintenance is impossible. Proper maintenance is rarely planned, and even more

rarely practised. With this in mind, it is possible to see the role of drainage more

clearly in the wider picture.

The main objective of using drainage is to control the movement of surface water,

and through their influence on the hydraulic boundary conditions to the seepage

regime in a slope, being about the desired reductions in pore water pressures at

depth.

Temperature treatments and grouting for slope stabilisation

Temperature treatment may be a possible means of stabilising a slope due to the

effect of elevated temperatures on clays, first driving off excess moisture and then

baking the material. Strength can even be improved due to freezing of the pore

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water. Ground freezing is most likely to be a useful in soils such as silts or fine sands

where temporary control of slope stability is all that is required. For a more

permanent nature, high temperature treatments are used.

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Reference

Bell, F. (2000). Engineering properties of soils and rocks. London: Blackwell

Science.

British Geological Survey. (1987). Geology of the country around hastings and

Dungeness sheet memoir 320/321. Geological memoir.

Bromhead, E. (1986). The stability of slopes. Glasgow: Surrey University Press.

Dixon, N., & Bromhead, E. (2002). Landsliding in London Clay Coastal Cliffs.

Geotechnical Society of London, 327-343.

Gourvenec, S. M., Mair, M. J., Bolton, M. D., & Soga, K. (2005). Ground Conditions

around an old tunnel in London Clay. Proceedings of the institution of Civil

Engineers, 25-33.

Reeves, G. M., Sims, I., & Cripps, J. C. (2006). Clay Materials Used in Construction.

Bath: The Geological Society.

Skempton, A. (1964). Long term stability of clay slopes. Geotechnique, 77-101.

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Appendix

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