The underground research facility at Mol, Belgium

We have at intervals over the last 15 years made visits to the SCK-CEN facility at Mol, Belgium, to carry out pressuremeter testing in the underground research facility HADES. This is a system of shafts and tunnels some 224 metres below ground level in a zone of Boom Clay in a highly plastic condition. The clay has interesting self healing properties when fractured, displays extremely low levels of permeability and offers a possible solution to the problem of the disposal of high level nuclear waste. Since 2000 the facility has been run by an expert group called EURIDICE and pressuremeter testing has been used during the construction of the facility and after to examine the engineering properties of the clay. We ourselves began work there in 1999 with a self boring pressuremeter. We were not allowed to introduce water into the formation and so drilled using air from a modified drill rig to implement the self boring process. Special casing and drilling parts were designed by us with some help from the drilling contractor to give the ideal flow path for delivering the air and returning the cuttings. The bulk of the testing has been horizontal. Speed is important in this material – it must be bored and tested as rapidly as possible because after one hour the material will close onto the probe with sufficient force to make extracting the equipment almost impossible.

Successful pre-bored tests have also been carried out with a 95mm HPD. This allowed a larger pressure to be applied and a greater cavity expansion achieved than is possible with a self bored probe.