## 3 Research Progress of High-Temperature Rock Mechanics in Large-Buried Tunnels
### Thermo-Mechanical Properties of Rock Mass  
[PERSON] et al. [38] conducted an experimental study on the mechanical properties of granite at different temperatures, and it is found that the mechanical properties of granite was not obviously influenced within 400 \({}^{\circ}\)C. Beyond that, the peak stress, Poisson ratio, and elastic modulus sharply decreased. [PERSON] et al. [39] compared the mechanical property differences of granite, tuff, and breccia after different high-temperature treatments, and analyzed the changes of peak stress, peak strain, and elastic modulus with temperature. [PERSON] et al. [40; 41] systematically studied the mechanical properties, structural effects, and strength characteristics of high-temperature granite. [PERSON] et al. [42] presented a generalized rule describing the effects of high temperature on marble by an extensive review of Chinese publications. Based on the experimental research on the thermophysical characteristics of borehole surrounding rock in granite under high temperature and pressure, the equations of elastic modulus \(E\) and Poisson ratio \(\mu\) of borehole surrounding rock in the range of normal temperature \(\sim\)600 \({}^{\circ}\)C were fitted, which are shown in Equations (1) and (2) [43]. \[E=78.366\exp(-0.0015T) \tag{1}\]  
\[\mu=0.0699\ln T+0.0028 \tag{2}\]  
[PERSON] et al. [44; 45] and [PERSON] et al. [46] studied strength, fracture characteristics, permeability, and thermal damage of Beishan granite after heat treatment, and pointed out that the relationship between crack number and temperature was accorded with the Gauss curve relationship, i.e.,  
\[n=n_{0}+\frac{A}{w\sqrt{\pi/2}}\exp\left[-2\bigg{(}\frac{T-T_{0}}{w}\bigg{)}^{ 2}\right] \tag{3}\]  
where \(n\) is the number of cracks in hot cracking; \(T\) is the temperature; \(n_{0}\), \(A\), \(w\) and \(T_{0}\) are material constants associated with granite. Wherein, \(T_{0}\) = 260.91011 \({}^{\circ}\)C.  
In addition, [PERSON] et al. [47; 48; 49] and [PERSON] et al. [50] studied the mechanical properties, permeability and thermal fracture of \"Luhui\" granite under high temperature and pressure, as well as the compressive strength, tensile strength, and permeability of granite after rapid cooling. [PERSON] et al. [51; 52; 53] explored microfracture, mechanical property deterioration, and the effects of mineral particle size on the strength of Strathbogie granite under different cooling modes. [PERSON] et al. [54] analyzed the thermophysical properties of Central Aare granite and the irreversibility of thermal fracture. [PERSON] et al. [55] explained the propagation, evolution, and failure characteristics of thermal cracks in the process of rock damage, established a damage model, and revealed the failure mechanism of thermal damage of rock. [PERSON] and [PERSON] [56; 57] derived the thermal damage evolution equation and the one-dimensional thermodynamic coupled elastic-brittle damage constitutive equation (Equation (4)) based on the thermal damage of granite. \[\sigma=E_{0}\varepsilon[1-D(T)][1-D(\varepsilon)] \tag{4}\]  
where \(E_{0}\) is the elastic modulus at 20 \({}^{\circ}\)C, and \(D\) is the damage variable. [PERSON] et al. [58] introduced the concept of thermo-mechanical coupling factor and proposed a one-dimensional nonlinear thermo-mechanical coupling constitutive model to reflect the stress-strain relationship of rocks at different temperatures, which is shown in Equation (5). \[\sigma=\mu E_{0}\varepsilon(1-D_{T})(1-D_{F}) \tag{5}\]  
where \(\mu\) is the function of strain \(\varepsilon\) and temperature \(T\), which characterizes the coupling effect of temperature and force, and is called thermo-mechanical coupling factor; \(D_{T}\) is the thermal damage variable; \(D_{F}\) is the damage variable caused by force. [PERSON] et al. [59] adopted the change of elastic modulus of rock to express the macroscopic damage caused by high temperature, and deduced the total damage expression of rock under the thermo-mechanical coupling effect, which can be expressed by Equation (6).