SHORT-TERM ANALYSIS; CASE STUDIES Ref: Skempton, A.W. and H.Q.Golder, Practical Examples of = 0 Analysis of Stability of Clays, Proc. of the 2nd International Conference on Soil Mechanics and Foundation

Engineering , Vol. 2, pp 63-70. Case Record (1): Critical Height of a Vertical Cut:

1.5ft

10ft8.5ft

51

6ft 10ft Full-scale Test by Wayne Edwards (1938)

Hcrit = 8.5ft = 112.5 pcf C = 280 psf = 12o

Using Terzaghis Equation,

Hc = (4C)/ - Zo Hc = (4*280)/112.5 1.5 = 8.45 ft But angle of failure surface was not 45o; but 45 + (/2) = 45 + 6 = 51o

Case Record 2: Failure in Eau Brink Cut

Ref: Skempton, A.W. (1945), A slip in the West Bank of the Eau Brink Cut, Journal of Institute of Civil Engineers, Vol. 24, p.267.

Failure occurred due to erosion near the toe. Strengths and location of actual shear surface was established from soil borings. F.O.S. for the actual failure surface = 1.3 F.O.S. (min) = 1.02

Conclusion: = 0 analysis, while giving the correct factor of safety, does not yield correct slip surface.

Case Record 3: Chingford Reservoir Embankment Failure (During Construction Failure)

Ref: Cooling, L.F. and H.Q.Golder (1942), The analysis of the Failure of an Earth Dam During Construction, Jour. Inst. Civil Engrs., London, Vol. 19, p.38. Analysis performed on actual slip surface. Soil shear strengths on actual slip surface. Factor of Safety Calculated = 1.05 True angle of friction, was very small. Probably, using actual slip surface wasnt a bad assumption. Case Record 4: Slip in the Sea Wall at Brightling sea. Ref: Skemption and Golder .. same as in case Record 1.

Beach level lowered Row of concrete blocks placed triggered failure Actual Slip surface was not established with accuracy. Significant variation in shear strength along the slip surface(7 samples for shear

strength determination which was not adequate) F.O.S. calculated = 0.99(min)

Case Record 5:Failure of Earth Mound at Waltham Abbey Ref : Skempton and Golder . Same as in case Record 1

Rolled clay fill Failure immediately after construction Minimum F.O.S (by trial) = 1.06 Clay was compacted at natural water content which was higher than proctor optimum. (Therefore = 0 was a reasonable assumption)

Case Record 6: Huntspill River Cut Ref : Skempton and Golder . Same as in case Record 1

Excavated Earth slope Large variation in shear strength Depth of soft clay unavailable

only an approximate value was used based on nearby piling operations. Minimum F.O.S. (by trial) = 0.90 ( = 0 analysis)

USE OF = 0 ANALYSIS IN O.C. CLAYS Case Record 1: Ref: Ireland, H.O. (1954), Stability Analysis of the Congress Street open in Chicago, Geotechnique 04, pp.163 168).

Approximate location of the slip surface was established from the field evidence Strength of Clays from borings.

For the actual Slip surface F.O.S. = 1.51 For min. (Trial and error) F.O.S. = 1.11

Author determined that = 0 method did not give good results for the O.C. clay. Possible reason Progressive Failure leading to Residual Factor

Case Record 2: Bradwell Nuclear Power Station Ref: Skemption A.W. and LaRochelle, P(1965), The Bradwell Slip: A Short Term Failure in London Clay, Geotechnique, pp. 221-242.

Slip 1: Occurred 5 days after excavation was complete Average Cu measured along failure surface = 2000 psf Factor of safety (for Cu = 2000) =1.8 For F.O.S. = 1.0 Cu =1120 psf

56.0.)()( =

measCreqdC

u

u

Slip 2: Occurred 19 days after excavation

52.0.)()( =

measCreqdC

u

u

Question: Is the method of analysis incorrect? OR Are the strength measurements incorrect?

Observation1: Is the loading Rate fast enough for the undrained assumption to be true?

To test, perform 15min triaxial compression versus 7 day triaxial compression

32.0

30.9

31.0

14.5psi 17psi

14 psi 9 psi

31.0

15 min C=2100 psf 7 day 1700 psf Under slow loading rates; O.C. clay has dilated under shear and shear zone drew more water in resulting in lower strength. Observation 2: Lab sample Intact clay Field soil Fissured (especially excavation causes fissures to open) In-situ test showed a further 30% reduction in strength. (Cu)15 min 2100 psf * 0.8 (loading rate) (Cu)7day 1700 psf * 0.7 (fissures) (Cu)in-situ 1190 psf

Case Record 2: Failure in Eau Brink CutCase Record 6: Huntspill River CutCase Record 2: Bradwell Nuclear Power Station