Modelling the swelling behaviour of Opalinus Clay shale using a double-structured model

This study investigates the application of a hydro-mechanical model to reproduce the swelling behavior of Opalinus Clay Shale (OPA), a geomaterial proposed as a geological barrier for deep nuclear waste storage in Switzerland. Due to its significant content of active clay minerals, OPA undergoes substantial swelling upon transitioning from a partially saturated to a fully saturated state. Owing to its geological history and depth, OPA exhibits low porosity and a certain degree of fissility. Consequently, hydration, particularly under low confining stresses, may lead to the development of induced porosity along the bedding planes. In this paper, we present a constitutive model designed to capture the swelling behavior of OPA, incorporating the evolution of both claymatrix (micro-) porosity and induced (macro-) porosity. The mathematical formulation is built upon the BExM framework, a widely used constitutive model for unsaturated double-structured geomaterials, here adapted to the specific fabric of OPA. The model is calibrated and validated using experimental tests, demonstrating its capability to reproduce the swelling response of OPA under hydro-mechanical loading conditions.