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.


Clay shales are amongst the most difficult rock types in terms of characterizing their effective strength and stiffness properties and modeling their short- and long-term deformation associated with stress perturbation due to drilling or tunneling. Due to their often extremely low permeability, the response of clay shales to short-term loading or unloading (e.g. tunnel excavation) is essentially undrained and excess pore pressure (positive or negative) may develop with the tendency of the material to contract or dilate.

A series of undrained triaxial tests were conducted on Opalinus Clay samples to determine the pore pressure evolution during the elastic and inelastic behavior. This contribution explores the effect of confining stress on poroelastic properties, the confining stress dependent dilatancy and related pore pressure effect, and the influence of stress path on the pore pressure response.

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