A consolidated undrained (CU) triaxial test was conducted on a normally consolidated clay sample with a confining stress of 100 kPa. The sample failed when the principal stress difference (deviator stress) was 80 kPa. The pore water pressure at failure was measured to be 60 kPa. Assume the cohesion intercepts c=c' = 0 kPa. a. Find the major and minor effective principal stresses at failure (sigma1f and sigma3f).
b. Determine the effective internal angle of friction (phi').
c. Determine the normal effective stress (sigma'f) and the shear stress (sigma’f) on the plane of failure.
d. Find the expected shear strength at failure if a direct shear test is conducted on the same clay which is subjected to an effective normal stress of 150 kPa.