The influence of device stiffness on swelling pressure in clayey rocks

This study investigates the influence of device stiffness on swelling pressure measurements in partially saturated, anisotropic clayey rocks by combining laboratory experiments with a newly developed hydro-mechanical model. A custom high-pressure oedometer, allowing simultaneous measurement of axial and radial stresses, was employed to test specimens of Opalinus Clay Shale, the host rock for geological disposal of nuclear waste in Switzerland. In the experiments, the axial swelling pressure was found to be several MPa lower than the radial swelling pressure, contrary to expectations based on material anisotropy for specimens with bedding planes oriented orthogonally to the sample axis. To investigate this discrepancy, a hydro-mechanical model was developed to explicitly account for the specimen–device coupling, using the device stiffness as input parameter. The numerical analyses showed that the lower axial swelling pressure can be explained by the influence of device stiffness on the measured response; the analysis was corroborated by a detailed experimental calibration of the testing set-up which highlighted a marked difference between the axial and radial stiffnesses of the apparatus for stress levels up to a few MPa. In addition, the model allowed the identification of critical ranges of relative specimen–device stiffness governing swelling pressure measurements, indicating that the effect of the testing apparatus becomes significant for geomaterials exhibiting both high swelling capacity and relatively high stiffness, such as clayey rocks. These findings highlight the need to account for device stiffness to ensure consistent and reliable swelling pressure measurements across different laboratory setups.