Excavation induced fractures in a plastic clay formation: Observations at the HADES URF

The Belgian research programme for geological disposal of radioactive waste focuses on the Boom Clay as the potential host rock formation. To examine the feasibility of constructing an underground repository in this clay layer, an underground research facility HADES has been constructed in several stages since 1980. The two galleries most recently excavated, the Connecting Gallery in 2002 and the Praclay Gallery in 2007, were constructed by means of an industrial method using a tunnelling machine. During these excavations the hydro-mechanical response of the clay was characterised. A fracture pattern was observed consistently during the excavation of both galleries. The extent of this fractured zone was determined for the Connecting Gallery, but requires some further study. A strong hydro-mechanical coupling and a clear time dependency were noticed, even at an unexpectedly large distance from the excavation. Furthermore the Boom Clay responds in an anisotropic manner to the excavation due to anisotropy in the in situ stress state and the Boom Clay characteristics. Self-sealing processes were observed and appear to occur relatively fast.