Multi-scale triaxial direct shear creep behaviors of granite under thermal–mechanical coupled conditions in the context of geological disposal of radioactive waste

The long-term strength of rocks under high-temperature and high-pressure conditions is important for assessing the stability of high-level radioactive waste disposal facilities. This study examines the long-term strength, creep behavior, and deformation of granite under thermal–mechanical coupling triaxial direct shear conditions. Utilizing a self-developed apparatus, experiments were performed on deep-borehole granite across five temperatures from 30 to 110 °C and three confining pressures (9, 15, and 22 MPa), incorporating both instantaneous and multi-stage creep tests. TDS experimental results reveal that within the temperature range of 30 to110 °C, the long-term strength of granite decreases with increasing temperature, with 70 °C identified as the critical threshold for thermal crack initiation. The cohesion parameter initially increases and then decreases with temperature, while the internal friction angle shows a slight increase. Nanoindentation tests demonstrate that within the temperature range of 30 to110 °C, the hardness of quartz, feldspar, and biotite decreases by 10.78%, 36.37%, and 45.48%, respectively; the elastic modulus decreases by 26.78%, 27.16%, and 47.29%; and the fracture toughness decreases by 15.70%, 33.78%, and 31.95%. These results provide a theoretical basis for the design and safety evaluation of HLW repositories and offer critical insights into ensuring the long-term stability of host rocks in various underground engineering applications.