London Clay

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3rd year soil mechanics coursework.

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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 N-1)(q) Coefficient of consolidation (m 2yr-1) Permeability (ms -1) Effective stress ratio (K 0)

Weathered 66-100 22-34 36-55 (h) 0.69-1.41 (b)23-49 1. 0-2.00 100-1 5 12-18 1 -23 10.5-22v

nweathered 50-105 24-35 41-65 (h) 0.60-0.83 19-28 1.92-2.04 100-400 1 -252 20-29 (t) 9.4-1 (g) 25-141 v 0.01-0.002 s 0.094-0.003 0.3-6.0 (m) 2.2*10 -10 -3*10-8 (p) 3*10 -10 -3*10-8 1.1-2.8

0.5-0.18

0.2-2.0

0.5-4.4

(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 22-Kensal 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 f