Experimental study of the hydro-mechanical response of Opalinus Clay – Part 2: Influence of the stress path on the pore pressure response

Triaxial tests on back-saturated specimens using different controlled stress paths were conducted to study the influence of the stress path on the hydro-mechanical behavior of a clay shale. Opalinus Clay, a clay shale chosen for the disposal of high-level nuclear waste in Switzerland, was utilized. Three different stress paths approximating a tunnel excavation were applied: a two-dimensional stress path with isotropic initial stress conditions (i.e. pure shear compression), a two-dimensional stress path with anisotropic initial stress conditions, and a three-dimensional stress path. The influence of the stress path was investigated with respect to the specimens' geometry. Two end members were tested: specimens where the axial load was applied parallel to bedding (P-specimens) and specimens where the load was applied normal to bedding (S-specimens). Differences in the hydro-mechanical response were identified with respect to the specimens' geometry but also to the stress paths. When subjected to the two-dimensional stress paths, positive excess pore pressure was measured for S-specimens at low differential stresses whereas P-specimens showed negative excess pore pressure. The magnitudes of excess pore pressure were higher for the anisotropic two-dimensional stress path. Dilation associated with yielding was observed for all specimens. The application of the three-dimensional stress path revealed pore pressure responses that were conceptually comparable to the pore pressure response observed in situ. Pore pressure increase in front of the tunnel face can be related to poroelastic response of the rock mass to a change in differential and mean stress. Again, differences between P- and S-specimens were identified. Decrease in pore pressure before the tunnel face can be related to the dilation associated with yielding. No difference in peak strength between P- and S-specimens and the different stress paths was found. Nevertheless, it was shown that the state of failure is affected by the transversal isotropy as S-specimens fail at lower effective stresses compared to P-specimens.