## 4 On-Site Monitoring of Disaster Prevention and Response Measures for Water-Rich Loess Tunnels
### Result Analysis
#### 4.3.3 Analysis of Contact Stress Between Surrounding Rock and Initial Support of the Tunnel  
The analysis of contact stress between the surrounding rock and the primary support near the tunnel is a crucial means to verify the theoretically calculated rock pressure, understand the stress distribution in the primary and secondary support structures, and determine the sharing ratio between them. It provides important guidance for revising the calculation of rock pressure and optimizing the design of support structures. Based on the statistical analysis of data collected from the earth pressure cells (WC1-8) at the monitoring section, the time-history curve of contact pressure between the surrounding rock and the primary support, as well as the distribution map of this contact pressure, are shown in Figures 12 and 13, respectively. As can be seen from these figures, the contact pressure between the surrounding rock and the primary support at various measurement points exhibits the same temporal characteristics, showing a nonlinear increase with time and the expansion of excavation space. After the inverted arch is closed, the pressure increases slowly, and it basically stabilizes after the construction of the secondary lining. The distribution of contact pressure on the entire section roughly forms a symmetrical \"four-corner\" pattern. The contact pressures of the surrounding rock at the left and right arch waists (upper bench arch feet) are 258.2 kPa and 273.4 kPa, respectively, while those at the left and right arch feet are 218.6 kPa and 192.7 kPa, respectively. Overall, the contact pressure is greater at the arch than at the sidewall and the smallest at the inverted arch. The contact pressure of the tunnel passing through soft plastic loess exhibits the following patterns: Under the same burial depth, the contact pressure of soft plastic Class VI surrounding rock is larger than that of general Class IV and V surrounding rocks, and there is unevenness. The contact pressure distribution of soft plastic Class VI surrounding rock forms a \"four-corner\" pattern, while that of Class IV and V surrounding rocks mostly forms a \"cat's ear\" pattern. The existence of soft plastic loess greatly increases the rock pressure borne by the primary support. Considering the moisture content of the surrounding rock near the tunnel and the changes in displacement around the tunnel, after the excavation of the middle bench, the moisture content of the surrounding rock at the arch waist (upper bench arch foot) reaches the maximum, and after the excavation of the lower bench, the moisture content at the arch foot reaches the maximum. Due to the migration of moisture in the surrounding rock towards the free face, water accumulation at the arch foot leads to the softening of the surrounding rock. The surrounding rock at the arch foot exerts extrusion on the tunnel wall, causing convergence deformation of the primary support and resulting in an increase in contact pressure. The deformation of the surrounding rock and the changes in contact pressure further indicate that in tunnel lining and support construction, it is essential to strengthen the support at the arch foot to provide sufficient resistance against the deformation of the surrounding rock. This is crucial for ensuring the stability of the surrounding rock and construction safety and should be given attention. Taking the closure of the inverted arch as the time node, the development of contact pressure on the surrounding rock can be divided into two stages: from the excavation of the upper bench to the closure of the inverted arch and from the closure of the inverted arch to the construction of the secondary lining.