Dependency of the Elastic Modulus of Opalinus Clay on the Confining Pressure

Knowledge of the elastic constants of geomaterials is important for successful numerical simulations and the prediction of the behaviour of underground geological and engineering structures. Triaxial tests on porous rocks have shown that the elastic constants of the material may change with the confining pressure, and in anisotropic materials they also depend on the orientation of the effective load with respect to the bedding. Until now, few such data are known for claystone which is considered as a possible host rock for long-term waste depositories. We present first results of triaxial tests on samples of argillaceus rock from various drilling sites in the sandy facies in Laboratoire souterrain du Mont-Terri (Switzerland), which were subjected to multiple loading and unloading cycles at stepwise varied confining pressures. The elastic moduli have been calculated from stress-strain data. The elastic modulus is found to increase monotonically with the confining pressure. However, the functional dependency of the former on the latter quantity varies with the orientation of the sample with respect to the bedding. A linear dependency is found for samples drilled perpendicular or at a medium angle to the bedding, while those drilled parallel to the bedding exhibit a power-law dependency.