A CU triaxial test (axial compression) on a specimen of saturated over-consolidated clay was carried out under a
cell pressure of 507 kN/m2. Consolidation took place against a back pressure of 200 kN/m2. The principal stress difference at failure was 538 kPa and the pore pressure reading at failure was 120 kPa. A companion drained test (CD) performed on an identical sample of the same clay at a consolidation pressure of 151 kPa (back pressure was zero) shows that the friction angle of the sample is 19 degrees.
(a) How much is the undrained cohesion (Cu) of the sample? (kPa)
(b) What maximum principal stress difference would you expect at failure for the companion CD test? (hint: the cohesion of the sample is NOT zero!)(kPa)
(c) What maximum principal stress difference would you expect if the second test was performed by keeping σ1 constant and reducing σ3 (ie. lateral extension test)? (kPa)
(d) How much is the Skempton parameter A at failure (Af) for the first (undrained) test?
(e) Assuming Af is independent of the applied confining pressure for this sample, what maximum principal stress difference would you expect if the second test was performed undrained (CU) (assume zero back pressure)? (kPa)
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