# Numerical Studies on Rockbursts in Tunnels with High In Situ Stresses and Geothermally Rich Areas
## 1 Introduction  
Rockbursts are a kind of geological disaster that often occur in deep mining and tunnel engineering. They not only threaten the safety of workers but also cause a delay in construction [1; 2; 3]. In the Sichuan-Tibet railway project in China shown in Figure 1, a large number of long and deeply buried tunnels need to be excavated, and rockbursts have occurred many times during tunnel excavation from Nyingchi to Lhasa. According to geological survey data on the Sichuan-Tibet railway project, the in situ stress of the surrounding rock can reach 42 MPa, and the geothermal temperature can be greater than 90 \({}^{\circ}\)C [4]. The high temperature and great in situ stress undoubtedly cause great difficulties for tunnel excavation, and rockbursts are particularly serious. To maintain the stability of the surrounding rock, it is helpful to study the stress evolution and the possibility of rockbursts occurring in the surrounding rock under both high-temperature and high in situ stress conditions during tunnel excavation. In tunnel excavation within a geothermally rich area, the change in temperature after excavation greatly influences the stress distribution and mechanical properties of the surrounding rock [5; 6; 7; 8]. In recent years, many studies have focused on the effect of high temperature on rock damage; however, relatively few studies have focused on the influence of temperature on rockbursts [9; 10; 11]. [PERSON] et al. [12] studied the effect of thermal damage on the rockburst strain of brittle rock under true triaxial loading, in which the specimens were uniformly heated to a specified temperature and then allowed to cool naturally to room temperature, after which the strain burst tests were performed on the cooled samples. The results indicated that thermal damage caused a delay in rockbursts but demonstrated more intense strain burst behavior when the preheating temperature increased from 100 to150 \({}^{\circ}\)C. [PERSON] et al. [13] et al. developed a rockburst device under the thermodynamic coupling condition, and the results indicated that the increase in temperature enhances rockbursts; however, their experiment was performed under a uniform temperature condition, and cement, not rock, was used in the experiments, making the conditions unlike those in practical rock engineering. [PERSON] et al. [14] estimated the temperature distribution of a tunnel at different depths according to the measured data from the Sangzhulin tunnel on the Lhasa-Nyingchi railway, and the results showed that the temperature increases with the depth of the tunnel, resulting in an increase in the possibility of rockbursts. Most studies have indicated that an increase in temperature in rock increases the probability of the occurrence of rockbursts [15; 16]. However, rockburst experiments with preheating then cooling to room temperature (i.e., heating\(\rightarrow\)cooling\(\rightarrow\)testing) are not consistent with the practical situation in tunnel excavation engineering in geothermally rich areas where the tunnel surface exchanges heat with the air (i.e., excavation\(\rightarrow\)cooling) and forms a thermal gradient in the surrounding rock. Such a gradient would lead to thermal stress in rock and change the stress distribution, and a change in conditions would likely impact the occurrence of rockbursts [17]. To date, there have been few studies on the influence of tunnel cooling caused by ventilation on the stress distribution and rockbursts in surrounding rock. To further understand the stress evolution and the rockburst mechanism in geothermally rich areas, this study begins by theoretically studying the influence of the change in temperature on rock failure and then numerically studies the stress evolution in excavated tunnels with high temperatures to analyze the possibility of rockbursts. The related parameters, such as the convective heat transfer coefficient, in situ stress and fault effect, on the stress distribution and rockbursts are discussed in detail.