Thermo-mechanical loading of intact rock and discontinuities

Coupled thermo-mechanical processes present challenges in a number of areas of geological understanding. Thermal loading can occur due to the natural geothermal gradient or the introduction of an anthropogenic heat source in a rock mass. In this study the thermo-mechanical behaviour of intact rock and discontinuities is investigated through both laboratory experiments and their numerical simulations. A new method was devised for creating synthetic discontinuous specimens with representative discontinuity topography suitable for thermo-mechanical triaxial testing. Different synthetic compositions were trialled to achieve a composition representative of a sedimentary lithology. Whilst synthetic discontinuities could repeatedly be created with representative topography, a synthetic composition with mechanical characteristics of a sedimentary lithology could not be created. Following testing of synthetic specimens, thermo-mechanical triaxial experiments were then performed on intact specimens of sandstone at temperatures up to 100°C. Additionally these experiments were repeated under the same thermo-mechanical conditions on specimens with a single pre-existing discontinuity running through the specimens at 30° from the vertical, allowing shearing to occur on the discontinuity under triaxial conditions. Laboratory results of testing on sandstone showed a reduction of up to 15% peak strength with increasing thermal loading between ambient temperature and 100°C for intact specimens, whereas specimens with discontinuities present an initial increase in discontinuity peak shear strength with increasing thermal loading to 50°C, before a reduction in peak shear strength thereafter. The laboratory experiments were replicated using thermo-mechanically coupled discrete element method grain based models. The grain based models highlight the build up of thermally induced localised stresses within the intact specimens due to grain scale heterogeneity of thermal properties, resulting in the initiation and accumulation of tensile thermal micro-cracks, causing reduced strength. When a discontinuity is introduced to the models, the discontinuity allows room for thermal expansion resulting in thermal closure, until maximum thermal closure is reached. Thermal micro-cracking then occurs as observed in the intact specimens, causing a reduction in strength. These findings offer new contributions to the understanding of the thermo-mechanical behaviour of intact rock and discontinuities.