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
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.
2
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
3
(Skempton, 1964)
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.
5
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
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.
10
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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12
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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15
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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17
18
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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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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
21
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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