# Safety Analysis of Secondary Lining of Yulinzi Tunnel Based on Field Monitoring
## 1 Introduction  
With the improvement of China's infrastructure, the transportation network in the northwest region has also developed rapidly. However, due to the widespread distribution of loess in the northwest region, the number of loess tunnels is increasing. Due to its unique structural characteristics such as particle morphology and arrangement, high porosity, soluble salt content, and water sensitivity, loess undergoes significant changes in strength and deformation as its moisture content increases [1; 2; 3; 4]. Due to the softening of surrounding rock in loess tunnels, some water-rich loess tunnels may experience excessive deformation, water leakage, cracking, and collapse in practical engineering [5; 6; 7; 8; 9]. The secondary lining is always in an unstable state [10], which will have a negative impact on the long-term stability of the structure and seriously affect the safe operation of the tunnel. In order to ensure the safe operation of loess tunnels in water-rich areas, it is particularly important to understand the internal force law of loess tunnels in water-rich areas and to study the safety evaluation of secondary lining structures in water-rich areas. Most studies on the deformation law of tunnels have adopted field monitoring methods [11; 12]. The same approach has also been used to study the surrounding rock pressure and deformation of loess tunnels. Many scholars have proposed revised for-mulas for calculating the surrounding rock pressure of loess tunnels, based on extensive monitoring data and statistical patterns of stratum damage [13; 14; 15], in order to guide the structural design of loess tunnels. However, some secondary lining structures of loess tunnels still crack over time, indicating that the surrounding rock pressure of loess tunnels has certain time-dependent characteristics. In response to this problem, scholars have studied the deformation law of the surrounding rock of loess tunnels and summarized the deformation law of the tunnel surrounding rock based on a large number of monitoring and measurement data. It is generally believed that after the surrounding rock is excavated,the deformation of the surrounding rock changes with time. In general, there are three stages: the rapid deformation stage, continuous deformation stage, and slow deformation stage (creep deformation stage) [16; 17; 18; 19; 20]. It can be seen that the slow deformation of the surrounding rock of the loess tunnel over time is its most important feature, indicating that the loess has obvious creep characteristics, which is the main reason for the time-varying characteristics of the pressure of the surrounding rock of the tunnel. The influence of the creep of loess surrounding rock on the tunnel structure must be considered in the safety evaluation of the secondary lining structure of loess tunnels. On this basis, scholars have carried out significant research on the secondary lining deformation of loess tunnel structures. Through the field investigation of an existing loess tunnel structure, [PERSON] and [PERSON] [21] found that the tunnel vault cracks were more serious than the side wall cracks. Due to the infiltration of farmland irrigation and rainwater, the surrounding rock pressure gradually increased, and the cracks were constantly in development. [PERSON] [22] systematically monitored the construction and operation period of the loess tunnel in a water-rich area and pointed out that the disturbance of tunnel construction, irrigation of farmland, and heavy rainfall significantly affected the mechanical properties of the support structure through concrete. Through data analysis of the surface strain gauge, the authors drew the conclusion that the stress of the secondary lining structure was continuously increasing. He [23; 24] used the model test method test to study the gradual deterioration process of the secondary lining under the action of creep and obtained the four stages of deterioration: the elastic stage, initial failure stage, ultimate failure stage, and unstable stage.