Monitoring and modelling of thermo-hydro-mechanical processes - main results of a heater experiment at the Mont Terri underground rock laboratory

The long-term safety of permanent underground repositories relies on a combination of engineered and geological barriers, so that the interactions between the barriers in response to conditions expected in a high-level waste repository need to be identified and fully understood. Co-financed by the European Community, a heater experiment was realized on a pilot plant scale at the underground laboratory in Mont Terri, Switzerland. The experiment was accompanied by an extensive programme of continuous monitoring, experimental investigations on-site as well as in laboratories, and numerical modelling of the coupled thermo-hydro-mechanical processes. Heat-producing waste was simulated by a heater element of 10 cm diameter, held at a constant surface temperature of 100 C. The heater element (length 2 m) operated in a vertical borehole of 7 m depth at 4 to 6 m depth. It was embedded in a geotechnical barrier of pre-compacted bentonite blocks (outer diameter 30 cm) that were irrigated for 35 months before the heating phase (duration 18 months) began. The host rock is a highly consolidated stiff Jurassic clay stone (Opalinus Clay). After the heating phase, the vicinity of the heater element was explored by seismic, hydraulic, and geotechnical tests to investigate if the heating had induced changes in the Opalinus Clay. Additionally, rock mechanic specimens were tested in the laboratory. Finally, the experiment was dismantled to provide laboratory specimens of post - heating buffer and host rock material. The bentonite blocks were thoroughly wetted at the time of the dismantling. The volume increase amounted to 5 to 9% and was thus below the bentonite potential. Geo-electrical measurements showed no decrease of the water content in the vicinity of the heater during the heating phase. Decreasing energy input to the heater element over time suggests hence, that the bentonite dried leading to a decrease of its thermal conductivity. Gas release during the heating period occurred most pronouncedly in a borehole closest to the heater (0.5 m), where after an incubation period of about 6 months after the beginning of heating bell-shaped release curves of carbon dioxide and hydrogen sulphide developed over 10 months indicating that chemical reactions in the Opalinus Clay are restricted. Metal corrosion caused by Opalinus Clay (as it occurred in the rock mechanic laboratory) is probably restricted as well by the supply of a reactant such as oxygen. Gas release data also suggest that the gas permeability of the Opalinus Clay may be inhomogeneous. Bentonite and Opalinus Clay show only very weak modifications induced by the heater experiment. Numerical calculations were done with axisymmetric as well as with anisotropic models. The input data for the anisotropic properties of the Opalinus Clay were provided by the rock mechanic tests. Data of anisotropic creep properties were determined in the laboratory and integrated in a Burgers model (but have not yet been used in the Finite Element modeling). Mineralogical analysis shows that the mechanical properties of the Opalinus Clay depend on microstructure rather than mineralogy. The match between monitored and calculated time series of temperature and pore pressure is good. The pore pressure development shows the impact of transition processes in the Opalinus Clay caused b y watering of the bentonite, heating, and cooling. The hydraulic conductivity has considerable influence on the magnitude of the pore pressure values. The long-term steady stress field does not appear to be affected by the heating and cooling. The bentonite blocks insulate the heater element so that the host rock was only moderately heated (about 65 C at the interface buffer-rock). The temperature field extended to a maximum radial distance of about 5 m only. (authors)