# Temperature Effect on Stability of Tunnel Face Under Unsaturated Seepage Condition
## 1 Introduction  
The stability analysis of tunnel faces has always been a focal point in the tunnel excavation process, and many scholars have conducted research on this topic. [PERSON] et al. [1] investigated the stability of unsupported tunnel working faces under various geological conditions and depths of cover by analyzing a large number of 3D numerical analysis results. [PERSON] et al. [2] proposed a new discretization technique for 3D tunnel working faces in weak strata with random spatial positions. [PERSON] and [PERSON] [3] employed a streamlined approach to forecast the ground displacement associated with tunneling. A quantitative analysis model for tunnel face stability was established by [PERSON] et al. [4], resulting in a calculation formula for the tunnel face stability coefficient based on the rock mass silo theory, which enabled a quantitative description of face stability. In the past, the influence of temperature on the stability of the tunnel face was often neglected. Currently, with the appearance of numerous tunnels around the worldthat are affected by temperature, the impact of temperature has become an issue that requires careful consideration. Issues related to tunnels affected by temperature have been researched by many scholars. The patterns of temperature variation during the construction of high-temperature tunnels was revealed through field experiments by [PERSON] et al. [5]. [PERSON] et al. [6] conducted a comprehensive study on the initial temperature distribution in high geothermal tunnels by integrating model testing, numerical analysis, and field measurements. [PERSON] et al. [7] conducted experiments to establish the correlation between the high temperature gradient across the inner and outer walls of tunnel lining support structures and the internal temperature-induced stress within these structures; they also compared these experimental findings with numerical simulation outcomes. All these studies emphasize the significant impact of temperature on tunnel stability. However, research on the failure mechanisms of tunnel faces related to temperature is lacking. Temperature's influence on tunnel face stability is predominantly manifested through its effect on soil moisture content, which in turn induces soil matric suction and apparent cohesion, as noted by [PERSON] et al. (2020) [8]. Matric suction of unsaturated soil was proven to decrease monotonically with increasing temperature, as demonstrated through experimental simulation of the undrained behavior of unsaturated soil under high temperatures by [PERSON] and [PERSON] [9]. [PERSON] et al. [10] introduced a temperature-sensitive equation for matric suction and a corresponding soil-water characteristic curve (SWCC) model that accounts for temperature variations. The results indicate that under a given matric suction condition, an increase in temperature will lead to a decrease in saturation degree. The response and behavior of unsaturated soil under different temperatures and matric suction was studied by [PERSON] et al. [11]. [PERSON] and [PERSON] [12] introduced a model that captures the nonlinear interplay among matric suction, soil saturation, and the shear strength of unsaturated soils. This model was expanded to account for temperature's impact on unsaturated soil's shear strength, and a subsequent comparison was made between the predicted and actual data for compacted silt across three distinct temperature levels. A specialized triaxial testing device that controls both suction and temperature, equipped with local strain measurement capabilities, was employed by [PERSON] et al. [13] to determine the secant shear modulus of unsaturated silt under diverse suction levels and temperatures. The experimental results obtained confirmed that the shear modulus of unsaturated soil at low strain levels is significantly increased by increased matric suction and is decreased by rising temperatures.