Experimental study of the hydro-mechanical response of Opalinus Clay - Part 1: Pore pressure response and effective geomechanical properties under consideration of confinement and anisotropy

Consolidated drained and undrained tests with pore pressure measurements were conducted on back-saturated specimens of a clay shale to characterize the influence of confinement and anisotropy on the pore pressure response and effective geomechanical properties (i.e. first-loading E-modulus, stress at onset of dilation and peak strength). Opalinus Clay, a clay shale chosen as host rock for high level nuclear waste in Switzerland was utilized. The result showed that there is a dependency on the confinement of Skempton's pore pressure parameter A and B, the stress at the onset of dilation, and the first-loading E-modulus. Additionally, a change in behavior of the material was observed at effective consolidation stresses between 5 and 8 MPa. The specimens at lower effective consolidation stresses (i.e. heavily overconsolidated specimens) showed a dilatant behavior in the pre-peak region and a significant post-failure stress drop. Specimens consolidated at higher effective consolidation stresses (i.e. slightly overconsolidated to normally consolidated specimens) showed compaction from initial loading until post-peak and a brittle-ductile post-failure behavior. These observations were manifested in pore pressure response curves, effective stress paths and stress-strain curves. Furthermore, they could be correlated to a non-linear appearance of the peak strength failure envelope. An explanation for the non-linear failure envelope related to the dilatant structure of the material is suggested.