London Clay

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ContentsPart 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 Skemptons 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 packages of your choice (Oasys etc) and also present a specimen hand calculation. ................................ ................................ ................................ ............ 10 Initial design ................................ ................................ ................................ ...... 11 2nd design ................................ ................................ ................................ .......... 13 3rd design ................................ ................................ ................................ ........... 16 Part 4 ................................ ................................ ................................ ........................ 19 To suggest an appropriate stabilisation method and evaluate the improvement on the factor of safety................................ ................................ ................................ . 19 Other methods of slope stabilisation ................................ ................................ .. 21 Reference ................................ ................................ ................................ ................. 23 Appendix ................................ ................................ ................................ ................... 24
Part 1Part 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 fr om 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 1
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immediately adjacent to the slip plane is about 35, compared with water content of around 30 in unsoftened clay.Engineering property of London Clay Liquefied Limit (%) Plastic Limit (%) Plasticity index (%) Void ratio Clay franction< 2 m (%) Natural water content (%) Bulk Density ( gm 3) ndrained Shear Strength ( kPa) Effective cohesion (kPa) Effective angle of friction (degrees) Residual Shear strength (degrees) Secant modulus of elasticity ( Nm 2) Coefficient of volume change (m 2 N1)(q) Coefficient of consolidation (m 2yr1) Permeability (ms 1) Effective stress ratio (K 0)
Weathered 66100 2234 3655 (h) 0.691.41 (b)2349 1. 02.00 1001 5 1218 1 23 10.522v
nweathered 50105 2435 4165 (h) 0.600.83 1928 1.922.04 100400 1 252 2029 (t) 9.41 (g) 25141 v 0.010.002 s 0.0940.003 0.36.0 (m) 2.2*10 10 3*108 (p) 3*10 10 3*108 1.12.8
0.50.18
0.22.0
0.54.4
(Bell, 2000 Lists engineering properties of some British clay soils of Tertiary and Mesozoic age. The table only shows information concerning London clay.
b
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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 jointe d, 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 ab out 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 slopes of the London Clay, the strength is near the re sidual strength. This is shown in the 4
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, progressi ve 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 ones, which are caused by tectonic movements, the strength will be at the residual value.
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Part 2Develop a representitative but simplified (metric section (based on figure 22Kensal Green Wall failure of Skemptons paper a retaining wall for your own analysis and design exercise
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Part 3To carry out a stability analysis of your retaining wall, using suitable software packages of your choice (Oasysetc and also present a specimen hand calculation.
The following characteristics were used in the design of the wallnit weight of wall Bulk unit weight of London Clay Effective friction angle (London clay) Effective Cohesion (London clay) Effective residual friction angle Ballast unit weight Effective angle of friction (Ballast) Effective cohesion (Ballast) Ka (London Clay) Kp (London Clay) Ka (Ballast) Kp (Ballast) FOS sliding FOS bearing FOS overturning 24 KN/m3 18 kg/m3 19 14 kPa 1 12 KN/m3 50 10 kPa 0.548 1.83 0.132 .55 1.5 3 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
In part 2 the retaining wall labelled as the initial design (page ) 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 0.0283 5 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.
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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 2 nd 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 0.682 8, Bearing 2. 8 54 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 3 rd 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.
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The factor of safety calculated for sliding this time around is1.52363 , Bearing 10.23 0 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. T he wall will still need to be modified in various ways to increase the factor of safety.
1
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Part 4To 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. odifying 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.
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This is also proven within the software Oaysis Greta. In the 2 nd design of the retaining wall in part 3, the factor of safety against sliding was 0.682 8 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 3 rd 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.rd
sing the Oaysis Greta software, it can be seen
that in the analysis of the 3 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 althou gh the factor of safety for the overturning has increased, the sliding factor of safety decreases dramatically to 1.2 048
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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 insu late 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 a nd 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.
DrainageDrainage 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 conditio ns to the seepage regime in a slope, being about the desired reductions in pore water pressures at depth.
Temperature treatments and grouting for slope stabilisationTemperature 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 free ing of the pore water. 21
Ground free ing 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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ReferenceBell, F. (2000). Engineering properties of soils and rocks. London: Blackwell Science. British Geological Survey. (198 ). Geology of the country around hastings and Dungeness sheet memoir 320/321. Geological memoir. Bromhead, E. (1986). The stability of slopes. Glasgow: Surrey niversity Press. Dixon, N., & Bromhead, E. (2002). Landsliding i n London Clay Coastal Cliffs. Geotechnical Society of London , 32 343. Gourvenec, S. ., air, . J., Bolton, . D., & Soga, K. (2005). Ground Conditions
around an old tunnel in London Clay. Proceedings of the institution of Civil Engineers, 2533. Reeves, G. ., 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, 101.
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Appendix
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