Time dependent behaviour of the Boom clay at great depth - An application to the construction of a waste disposal facility

Belgium is considering a clay formation as a possible disposal medium for the storage of conditioned radioactive waste. The clay rheology at great depth is studied by laboratory tests and in situ experiments performed from an underground laboratory build at 223 m depth in the Boom clay formation present in the N-E part of Belgium under the nuclear site Mol-Dessel. The aim of this research consists in simulating the excavation, in unfrozen clay, of large diameter galleries to be dug for the final disposal site. A finite element program using Vyalov's creep law was chosen as a first approach. The determination of the rheological parameters was made on the basis of the available laboratory experiments. Different numerical simulations were performed in order to reproduce in situ experiments, making possible a first refinement of the parameters. A shaft and gallery excavation are now being simulated, and numerical displacements will be compared to experimental observations.