The behaviour of normally consolidated clay as observed in undrained direct shear tests

The direct shear test as conventionally used for measuring the strength parameters of soil for use in classical stability analyses has the major disadvantage that the stress conditions in the specimen during the test are not known. The shear stress and normal stress acting on the central (horizontal) plane of the specimen are deduced from the observations made on the boundary; the values measured at failure are assumed to lie on the Mohr-Coulomb failure envelope from which the strength parameters are derived. Two special types of apparatus that apply simple shear allow measurements to be made from which the complete stress system (assumed to be uniform) within the specimen can be deduced. The effective stress paths obtained in shear tests on two normally consolidated clays reveal complex patterns of behaviour. In particular, they show that the conventional interpretation of the direct shear test is wrong; the stress point associated with the horizontal plane at failure does not lie on the failure envelope...