Excavating in a deep plastic clay layer: observations at the HADES URF

The geological disposal of radioactive waste has been studied in Belgium since the early seventies at the Belgian Nuclear Research Centre (SCK•CEN) in Mol. The research is focused on the Boom Clay layer: a poorly-indurated clay that is found from a depth of 190 metres under the site in Mol where it has a thickness of about 100 metres. It displays a plastic behaviour which results in self-sealing properties and a relatively high convergence when excavating galleries in it. The hydraulic conductivity is in the order of 10−12 m/s. In 1980 the construction of an underground facility HADES was started. Its purpose was to examine the feasibility to construct an underground disposal site and to provide an underground infrastructure for experimental research on the geological disposal of radioactive waste. The laboratory was extended in 2007 by a 45 m long gallery. The stress redistribution of the clay formation to the excavation was measured by piezometers and flat jacks in the clay around the gallery. This paper presents these measurements and describes the hydromechanical response of the Boom clay to the excavation of the gallery.