Drained triaxial testing of shales: insight from the Opalinus Clay

Abstract

The investigation of the mechanical behaviour of swelling claystones and shales is challenging because of their very low permeability and of their high sensitivity to changes in water content. The former makes it difficult to carry out triaxial tests with controlled homogeneous water pore pressure fields, and the latter results in some possible effects of swelling when unsaturated extracted specimens are re-saturated prior to being tested. This work presents some data from drained triaxial tests performed on specimens of Opalinus Clay, designated as host rock for radioactive waste disposal in Switzerland, extracted at shallow depth close to the city of Lausen. The data are compared to those recently published from undrained triaxial tests on specimens from the same place, and also to two sets of recent data independently obtained on Opalinus Clay specimens sourced from greater depth at the Mt Terri Underground Research Laboratory. By comparing the data of our drained tests on Lausen specimens to those from specific undrained tests in which swelling has been prevented, quite a good comparability in shear strength is observed at confining effective stresses larger than 5–6 MPa. This is related to the small magnitude of the swelling occurring above this stress during the specimen hydration in drained tests. The question of the possible linearity of the shear strength criterion at low stress is also discussed with respect to both our data and other published ones. It is suggested that undrained tests be also carried out at low confining stresses to investigate whether some perturbations due to hydration swelling occur in this area that could result in a nonlinear shape of the criterion. The change in Young modulus and Poisson coefficient with stress is also discussed, and the data on Opalinus Clay are compared to those of the Callovo–Oxfordian claystone from France, evidencing interesting similarities in terms of shear strength properties and Young’s moduli.