Micro-mechanisms of temperature-dependent triaxial strength in Beishan granite and implications for high-level...

Beishan granite, China's candidate host rock for high-level radioactive waste (HLW) disposal, experiences alterations in strength and failure characteristics under prolonged thermo-mechanical coupling conditions, impacting the long-term stability evaluation. In this study, the strength and failure behavior of Beishan granite specimens, which were heated to 25, 200, 300, 400, 500, and 600 ℃, and subsequently cooled to room temperature, were investigated by triaxial tests under confining pressures of 5, 15, and 25 MPa. The results indicated that the triaxial compression strength (TCS) exhibited non-monotonic strength variation with temperature. Below 300 ℃, TCS increases by 10.2-14.7% through crack closure from differential thermal expansion (quartz α = 11 × 10⁻⁶/℃ and feldspar α = 5 × 10⁻⁶/℃) and evaporation-induced effective stress enhancement. Beyond 400 ℃, TCS shows an average decline of 14.27%, primarily governed by intergranular cracking as expansion stresses exceed intergranular bonding forces. Acoustic emission monitoring revealed that crack propagation transitions from distributed micro-fracturing to localized macro-cracking as the temperature exceeds 400 °C. Moreover, the strength degradation rate with increasing temperature of Beishan granite declines from 19.4% to 9% with increasing confining pressure (5 to 25 MPa), demonstrating that elevated in-situ stresses (representing greater disposal depths) effectively suppress thermal damage. These findings establish a critical temperature threshold and depth-compensation principle, providing thermo-mechanical design criteria for HLW repository engineering.