## 3 Results and Discussion
### Shear Mechanical Properties
#### 3.2.1 Effect of Heat Treatment Temperature on Shear Mechanical Properties  
As demonstrated in Figure 11, the variation in shear strength of different granite types with varying heat treatment temperatures is evident. It is observed that, for a constant shear angle, both coarse- and fine-grained granites demonstrate an initial increase followed by a subsequent decrease in shear strength as the heat treatment temperature is increased. In addressing the implications of high temperature on granite strength, the scientific community has put forward two predominant hypotheses [18, 39, 40, 41]. Firstly, the thermal expansion of mineral grains has been suggested to fill the original pores in the rock, thereby enhancing its integrity. This is accompanied by the evaporation of internal moisture, thereby increasing the friction between the mineral grains. Conversely, the presence of different thermal expansion coefficients among minerals leads to the generation of thermal  
Figure 10: Granite peak load and peak displacement versus temperature curves: (**a**) peak load of coarse-grained granite, (**b**) peak displacement of coarse-grained granite, (**c**) peak load of fine-grained granite, (**d**) peak displacement of fine-grained granite.  
stresses within the rock, ultimately resulting in thermally induced cracking. The former process is designated as thermal hardening, while the latter is termed thermal damage. The net effect of temperature on the strength of granite is the result of a competition between these two processes. When thermal hardening is predominant, the shear strength of granite exhibits a substantial increase compared to its strength at room temperature (e.g., at 200 \({}^{\circ}\)C, where the shear strength of granite reaches its maximum). Conversely, when thermal damage is dominant, the shear strength of granite is significantly lower than its room temperature shear strength, as observed at 500 \({}^{\circ}\)C.  
As demonstrated in Figure 12, the variation in cohesion and internal friction angle of coarse- and fine-grained granite with heat treatment temperature is observed. It can be deduced from the figure that the change in cohesion follows a trend consistent with that of shear strength, while the internal friction angle initially decreases and then increases with increasing heat treatment temperature. This is in contrast to the trends previously observed for shear strength and cohesion, and is primarily attributed to the thermal hardening effect, which enhances the cohesion of the granite and mitigates the influence of internal friction. Conversely, thermal damage generates a substantial number of thermal cracks in the granite, thereby weakening its cohesion and increasing the internal friction, resulting in an increase in the internal friction angle.  
Figure 11: Variation in shear strength with temperature for different types of granite: (**a**) coarse-grained granite, (**b**) fine-grained granite.  
Figure 12: Variation in cohesion and internal friction angle with temperature for different types of granite: (**a**) coarse-grained granite, (**b**) fine-grained granite.