Thermal-mechanical behaviour of the rock mass is an important consideration in assessing concepts for deep underground disposal of used nuclear fuel waste. To investigate the effects of thermal loading on the progressive failure of massive brittle granite, an in situ thermal experiment was conducted at the Underground Research Laboratory (URL). Testing was carried out in four stages to assess the effects of drilling/heating sequence (i.e., loading path), borehole interaction and confining pressure on the development of breakouts and excavation damage in the rock mass around a series of 600-mm-diameter observation boreholes. Although the experiment was designed to be principally observational in nature, thermistors/thermocouples, acoustic emission (AE) sensors and extensometers were used to monitor the response of the rock mass. Preliminary results suggest that the drilling/heating sequence, the interaction of adjacent boreholes and confining pressure each affect the development of breakouts. Acoustic emission results show that excavation damage development occurs primarily during periods of drilling, heating or cooling, and tends to decrease during isothermal periods. Patterns of AE events indicate significant damage development ahead of the advancing borehole face during drilling, and extension of initial breakouts during the heating phases of each stage.
In assessing the concept of deep underground disposal of used nuclear fuel waste, Atomic Energy of Canada Limited (AECL) has conducted extensive geomechanics research at its Underground Research Laboratory (URL). The URL (Figure 1) is located approximately 120 km northeast of Winnipeg, Manitoba in the Lac du Bonnet granite batholith, and represents a well-characterized site in a previously undisturbed rock mass for experiments that address issues related to the Canadian Nuclear Fuel Waste Management Program (CNFWMP). One of the primary geomechanics issues being studied at the URL is the development of an excavation damage zone (EDZ) around underground openings, and the potential for increased permeability in the near-field resulting from the EDZ. Experience at the URL has shown that, in large granitic plutons like the Lac du Bonnet batholith, the rock mass at depths of several hundred metres may be subjected to high in situ stresses and highly anisotropic stress ratios. The characteristics of the damaged zone, however, were found to be highly variable around the tunnel, and were dependent on the nature of the stress concentrations (i.e., compressive versus tensile), geology, stress magnitudes and orientations, the excavation method, and confining pressure. The emplacement of nuclear fuel waste underground will cause an increase in temperature within and around a disposal vault (Simmons and Baumgartner 1994). After a 10 year cooling period, the heat output from spent fuel bundles from Canadian reactors is expected to result in a maximum surface temperature of 95ºC on disposal containers, and a temperature of 85ºC at the wall of a typical emplacement borehole in the reference disposal concept. Under these elevated temperature conditions, the thermal- mechanical behaviour of the rock mass will be affected by increased stresses and pore pressures, which may contribute to progressive failure and the development of excavation damage.
