In-situ Shear Modulus Determination by Pressuremeter Tests in Opalinus Clay and Reconciliation with Laboratory Tests

Opalinus Clay is the designated host rock for a deep geological repository of radioactive waste in Switzerland. The determination of its geo-mechanical properties relies heavily on laboratory tests on small specimens. To assess the in-situ elastic stiffness at a larger scale, pressuremeter tests were performed in Opalinus Clay at the Mont Terri Rock Laboratory, with the testing probe oriented both perpendicular and parallel to the bedding planes. The shear modulus of the Opalinus Clay is determined using the unload data of the pressuremeter tests in different lithofacies and at multiple expansion pressure levels. The measured shear modulus is dependent on the expansion pressure at the initial stage of the test but approaches a relatively constant value when a pressure magnitude of about 5 MPa is reached. The stiffness anisotropy of the Opalinus Clay, the rock mass disturbance, and the local fractures at test intervals can affect the measured moduli. In this test program, the impact of lithofacies was not evident at low expansion pressures and could not be evaluated at a greater expansion pressure. The shear modulus of the Opalinus Clay exhibits a nonlinear dependence on strain increment, which can be interpreted using a power-law stress–strain relationship. The small-strain nonlinearity is also dependent on the expansion pressures for the Opalinus Clay. At expansion pressures greater than 5 MPa, the strain-dependent shear moduli obtained from pressuremeter tests are comparable with those determined by triaxial tests on intact core specimens. At the shear strain increment of 0.1%, a secant shear modulus (parallel to bedding) of approximately 3 GPa for the intact Opalinus Clay can be concluded from both the pressuremeter and triaxial tests. Highlights High-quality pressuremeter tests are performed in Opalinus Clay to assess its in-situ static elastic stiffness using the unloading steps at multiple expansion pressure levels. Assuming axisymmetric borehole deformation, the shear modulus determined in the borehole oriented parallel to bedding is significantly lower than that determined in the borehole oriented perpendicular to bedding. The measured shear modulus can also be affected by the rock mass disturbance and the local fractures at the test interval. The interpreted secant shear modulus is nonlinearly dependent on the small strain increment during unloading, which can be characterized by a power-law stress-strain relationship. An agreement on the measured shear moduli (parallel to bedding) can be established between the pressuremeter and laboratory triaxial tests on intact Opalinus Clay when the strain dependency of the modulus is considered for both tests. This study presents a framework for bridging the elastic stiffness measurements of weak argillaceous rocks using high-quality laboratory and in-situ tests.