The large-scale in situ PRACLAY heater test: First observations on the in situ thermo-hydro-mechanical behaviour of Boom Clay

In the frame of RD&D on the geological disposal of radioactive waste in Belgium, the large-scale PRACLAY heater test is being carried out in the HADES underground research facility (URF) in Mol (Belgium). The thermo-hydro- mechanical (THM) behaviour of Boom Clay has been studied over several decades through small-scale laboratory tests and intermediate scale in situ experiments. The intermediate-scale in situ experiment ATLAS allowed to observe the impact of heat on the Boom Clay beyond the zone affected by the drilling of the borehole hosting the heater. On November 3, 2014, the in situ large-scale PRACLAY heater test was started, aiming at studying the THM behaviour of the clay in the near field, i.e. within the excavation damaged zone, and in the far-field of a heated gallery. An extensive monitoring network installed around the PRACLAY gallery allows to observe the evolution of temperature and pore water pressure in the Boom Clay. In this paper, the first observations that have been made since the start of the PRACLAY heater test are reported. These observations generally confirm the knowledge gained previously from experiments performed at smaller scales. The PRACLAY heater test provides a realistic estimate of what the thermal pressurisation of the Boom Clay would be around a geological disposal facility.