Benchmark study of undrained triaxial testing of Opalinus Clay shale: Results and implications for robust testing

Triaxial testing of argillaceous rocks and shales is significantly more challenging than conventional rock mechanical testing. The challenges are mainly related to the very low permeability these geomaterials, their sensitivity exposure atmosphere brines, which induces variations water content, suction effective stress. There currently no international standards guide service laboratories for robust procedures shales. A benchmark study undrained triaxial was therefore initiated with three leading in shale testing, performing 13 tests using two different methods establishing sample saturation prior deformation. Both paid particular attention minimize volume expansion specimens during saturation, loading rate shear phase all selected based on intrinsic properties drainage configurations ensure pore fluid pressure equilibration across specimen. Opalinus Clay core material from Mont Terri underground research laboratory used specimens, intervals cores were pre-selected basis computer tomography heterogeneity. detailed diagnostic analysis performed, a comparison results presented. Good reproducibility stress paths achieved by at identical or near-identical initial conditions. In particular, test over larger range stresses indicate similar evolution consistent picture derivation global properties. On example Clay, demonstrates that can be if some key adequately addressed.