## 1 Introduction  
Tunnel hazards caused by extremely high/low temperatures, such as freeze-thaw damage in cold regions [1, 2] and heat disasters in high-temperature zones [3, 4, 5], have always been an overwhelming concern [6, 7, 8]. Installing a reasonable thermal insulation layer (TIL) is widely accepted as an effective method to weaken the heat exchange between the tunnel wall and the airflow [9, 10, 11, 12]. Thus, studying the variation temperature in the tunnel before and after paving TIL is helpful in designing the tunnel TIL to prevent rock failure disasters induced by the temperature variation [13, 14]. When the airflow goes through a tunnel, heat exchange occurs due to the temperature difference, leading to their temperatures being simultaneously varied before reaching a balance [1, 3, 4, 15]. In cold regions, the tunnel rock is cooled and frozen in the winter by cold airflow but heated and melted in the spring as the airflow temperature increases [7, 16]. Such freezing-thawing cycles induce freeze-thaw damage, weakening the tunnel rock strength and reducing the tunnel safety [17]. Such damage is effectively prevented by installing TIL on the tunnel wall, which decreases the heat flux between tunnel rock and airflow due to low thermal conductivity [8, 18]. Numerous studies have focused on the temperature variation before and after TIL installation by analytical [6, 19], numerical [7, 20],and experimental methods [21]. They found that the heat exchange is intense at the tunnel entrance and becomes weak with the increased ventilation distance, resulting in the freeze-thaw damage always appearing near the tunnel entrance [22]. Paving TIL near tunnel entrance can efficiently eliminate freeze-thaw damage, which is verified by combing numerical simulation and engineering data in the Yuximolegait tunnel [9], Nan Shan tunnel [2], and other tunnels [7; 8; 10; 23]. In high-temperature tunnels, TIL is paved to prevent the heat flux from the tunnel rock to the airflow, ensuring that the airflow temperature is acceptable for the safety of tunnel excavation and the physical health of workers [4; 24]. Thus, TIL needs to be installed on the whole tunnel wall rather than on the entrance area in the cold area [3]. Meanwhile, the TIL insulation capacity is varied with changing the TIL properties and airflow parameters, such as TIL thickness [10; 25], TIL thermal conductivity [16; 24; 26], initial airflow temperature [3; 19; 20], and airflow speed [19], which are conducted for TIL design optimization to balance the tunnel cost [23; 27]. Previous studies play vital roles in the temperature variation in cold region tunnels, ensuring such tunnels avoid freezing-thawing disasters. However, only a limited number of studies have focused on the airflow temperature variation in a high-temperature tunnel. Moreover, the variation in mechanical properties of the tunnel rock with changing temperature induced by the ventilation is rarely studied, whether in cold tunnels or high-temperature tunnels [10]. In some high-temperature tunnels, the maximal rock temperature exceeds 200 \({}^{\circ}\)C, such as 208 \({}^{\circ}\)C in the Sichuan-Tibet Railway and more than 200 \({}^{\circ}\)C in the EGS-E (Enhanced Geothermal System-Based Excavation) tunnel (Figure 1), resulting in considerable temperature difference between the airflow and the tunnel rock [3; 5]. The rock temperature remarkably decreases once being ventilated under the cold airflow [3], shrinking the tunnel rock and inducing associated thermal stress. That stress possibly damages the tunnel rock and reduces the tunnel instability if the thermal stress exceeds its tensile strength [28; 29; 30; 31]. Therefore, it is key to ensure the effective support and safe operation of high-temperature tunnels to investigate the variation in the temperature and stress field and the damage mechanism of tunnel rock induced by the cold airflow.