A Multi-Scale Study on the Property Degradationof High-Temperature Treated Beishan Granite

Granite is the main host rock for the underground storage of nuclear waste in Beishan, China. Heat is continuously generated during the long-term disposal of nuclear waste; therefore, it is important to investigate the influence of high temperature on the physical and mechanical properties and the constitutive relation of Beishan granite. In this study, laboratory experiments on heat-treated (25 ∘C, 200 ∘C, 400 ∘C, 600 ∘C and 800 ∘C) Beishan granite samples were performed in combination with nuclear magnetic resonance (NMR) analysis and regular physical-mechanical tests. The results show that the elastic modulus tends to decline faster at the temperature ranges of 25–200 ∘C and 600–800 ∘C by approximately 26.767% and 66.996%, respectively. Compared with the results at 25 ∘C, the peak stress decreases by 72.664% at 800 ∘C. The peak strain increases gradually from 25 ∘C to 600 ∘C and abruptly from 600 ∘C to 800 ∘C. The peak strain at 800 ∘C is 2.303× greater than that at 25 ∘C. Based on the damage theory, the Weibull distribution, the rock damage threshold point, and the residual strength, this study corrected the Drucker–Prager (D–P) criterion to consider the damage stress and then to establish the constitutive model of thermally damaged Beishan granite. The parameters required for the model are conventional mechanical parameters that can be calculated from the uniaxial test results, thus making the model convenient to apply. Meanwhile, the mechanical behavior of thermally damaged Beishan granite under uniaxial compression was simulated using the Particle Flow Code (PFC) to explore the development of cracks from the microscopic scale. The research results can provide theoretical support for the calculation and numerical simulation related to the mechanics of high-temperature treated rocks.