Abstract: The construction of China's first deep underground research laboratory (URL) for high-level radioactive waste (HLW) geological disposal has commenced in the Beishan area, Gansu Province, and the surrounding rock of the URL is mainly granite. Accurate characterization of the mechanical properties of deep Beishan granite is crucial for ensuring the long-term safety and structural stability of the repository. Traditional macroscopic mechanical testing methods, such as uniaxial compression tests, yield scattered data and entail high costs and logistical challenges associated with deep borehole coring. To address these limitations, this study integrates nanoindentation experiments with finite-element numerical simulations to precisely characterize the mechanical behavior of Beishan granite under complex thermo-stress coupling conditions. Nanoindentation results were employed to validate the finite-element model, which was subsequently applied to investigate the response of granite under varying temperature and stress fields. The key findings are as follows: (1) The elastic modulus measured by nanoindentation reached 70.66 GPa, higher than the 62.39 GPa obtained from conventional uniaxial compression tests, highlighting the ability of this method to capture localized micromechanical properties. (2) The validated finite-element model reliably simulates the thermo-mechanical responses observed in nanoindentation experiments. (3) With increasing temperature, the elastic modulus, hardness, and fracture toughness of granite exhibit progressive decreases, attributable to thermally induced damage. (4) Under varying stress conditions, elastic modulus and fracture toughness remain relatively stable, whereas hardness shows a significant decline with increasing equivalent stress. (5) Effective mitigation strategies for thermo-mechanical coupling effects, especially at elevated temperatures, are recommended to maintain the mechanical integrity of granite during the construction and operation of the Beishan URL and future HLW repositories. These results provide a solid scientific basis for the safe geological disposal of HLW in China, contributing to the sustainable development of nuclear energy and environmental protection.

Key words: nanoindentation, finite element simulation, thermo-mechanical coupling, mechanical properties, Beishan deep granite, high-level radioactive waste disposal, underground research laboratory