Dilatancy of clay shales and its impact on pore pressure evolution and effective stress for different triaxial stress paths

A series of laboratory tests on back-saturated specimens of Opalinus Clay was conducted to investigate the pore pressure response during undrained isotropic compression and elastic/inelastic shearing. Pore pressure measurements conducted during undrained loading utilizing a standard triaxial stress path suggest that pore pressure changes are primarily controlled by the transversal isotropic elastic behavior at low compressive loads, and the tendency of the clay shale to dilate as the differential stress exceeds the dilatancy threshold. In addition, both, poroelastic properties (i.e., Skempton’s pore pressure parameter B) and the tendency of the clay shale to dilate depend strongly on the confining stress. The stress path utilized in standard triaxial tests may substantially differ from the stress path associated with drilling or tunneling, and its influence on the effective strength is to date poorly understood. An alternative stress path used in this study, however, does not reveal significant differences with respect to strength or pore pressure evolution during shearing.