Micro-Mechanical Study on the Failure Process of Clay Formations During Excavation of Galleries

Sound evaluation of the failure process around galleries is an important item for the stability and safety of underground excavations. In many European Countries, clay geological formations are selected as potential host rock for nuclear waste disposal, e.g., Boom Clay (a plastic clay) and Opalinus clay (an indurated clay) are promising formations for the deposit of nuclear wastes in Belgium and Switzerland, respectively. This paper aims at understanding the failure mechanism around excavations in these two different clay formations. Recently, results from laboratory tests performed by LMR-EPFL have pointed out that different failure patterns develop during mechanical esting of hollow cylinder samples cored in these two media. However, the reason why two different failure patterns develop is still not fully understood. In order to answer this question, a micro mechanics study is performed based on the Distinct Lattice Spring Model (DLSM). The numerical modeling allows reproducing qualitatively the displacement field observed in the experiments by choosing different micro failure laws for the micro structure. From the modeling results, it comes out that a micro tensile failure between particles may explain the failure process observed in Opalinus Clay, whereas a micro shear failure could explain the one in Boom Clay. The anisotropic convergence near the central hole of the hollow cylinders is as well analyzed.