Effect of pre-existing cracks on thermal cracking of granitic rocks under confinement

Pre-existing cracks affect the thermal, hydraulic, and mechanical behavior of rocks. However, little effort has been devoted to investigating thermal cracking interactions with pre-existing cracks. In this paper, we propose a novel thermo-mechanical grain-based finite discrete element method approach to investigate the influence of pre-existing cracks on thermal cracking processes under confining pressure in granitic rocks. An intact synthetic granite sample was mechanically and thermally calibrated against experimental data for Stanstead granite, and petrographic thin sections of thermally treated granite up to 380 degrees C were used to qualitatively calibrate the thermal cracking pattern in terms of cracking evolution with respect to mineral grains. Then, we introduced varying levels of pre-existing cracks density to the calibrated synthetic sample to study the emergent thermo-mechanical behavior under unconfined and confined conditions. Additionally, we studied the effect of varying friction coefficients of pre-existing cracks on the thermal cracking process. Results show that thermal cracks preferably initiated from the tips of pre-existing ones due to stress concentration and the number of initiated thermal cracks decreased with increasing pre-existing cracks density. A decreasing trend of thermal cracking with confinement was also observed. Interestingly, increasing pre-existing crack friction increased the number of thermal cracks under different confinement conditions due to increased stress concentration along cracks interfaces. The results of this novel numerical process contribute to a better understanding of failure mechanisms in granitic rocks, particularly in geothermal reservoirs and underground nuclear waste repositories under various thermal and mechanical loading conditions.