## 3 Field Test of Water Pressure around the Tunnel
### Analysis of Test Results  
After completing the secondary lining, the water pressure sensors were connected to the power supply to monitor the water pressure around the lining structure. The water pressure distribution around the lining for test sections LK51 + 610 and LK51 + 640 in the left hole, and RK51 + 590 and RK51 + 620 in the right hole, is depicted in Figure 8.  
As shown in Figure 8, the water pressure around the tunnel is highest at the inverted arch and arch foot, moderate at the vault and spanded, and lowest at the arch waist. The water pressure at the inverted arch in sections LK51 + 640 and RK51 + 590 is significantly higher than in sections LK51 + 610 and RK51 + 620. The maximum water pressure at RK51 + 620 is 21 kPa, whereas at RK51 + 590, it reaches 49 kPa. This is because sections LK51 + 640 and RK51 + 590 are near the ends of the fault fracture zone. Groundwater from other locations collects at the inverted arch through rock fissures, resulting in high water pressure. Field research indicates that the initial support at the inverted arch is often inadequate. High water pressure during the rainy season can lead to seepage, leakage, or even water disasters. Therefore, timely reinforcement and improved water plugging and drainage measures are recommended based on monitored water pressure values. Additionally, the water pressure at the inverted arch behind the initial support in sections LK51 + 610 and RK51 + 620 is higher than at 5 m inside the borehole. In contrast, in sections LK51 + 640 and RK51 + 590, despite being at the fault zone ends with more developed fissures, the surrounding rock integrity near the arch is better, resulting in lower water pressure.  
Contour plots of water pressure distribution around the tunnel lining, based on data from on-site measurements, were generated using Surfer 15 software, as illustrated in Figure 9.  
Figure 7: Test process. (**a**) Boreholes. (**b**) Placement of water pressure sensors at the inverted arch. (**c**) Water pressure sensor fixation. (**d**) Placement of water pressure sensors in boreholes. (**e**) Completion of equipment deployment. (**f**) Installation of flashing. (**g**) Application of secondary lining. (**h**) Power on and read data.  
Figure 8: Water pressure test results of each section. (**a**) LK51 + 610. (**b**) LK51 + 640. (**c**) RK51 + 590. (**d**) RK51 + 620.  
Figure 9: Distribution of water pressure contours around each section. (**a**) LK51 + 610. (**b**) LK51 + 640. (**c**) RK51 + 590. (**d**) RK51 + 620.  
Figure 9 illustrates a consistent water pressure distribution pattern across the four sections, showing that pore water pressure decreases toward the tunnel center with the highest pressures recorded at the inverted arch and arch foot. In sections LK51 + 610 and RK51 + 620, the water pressure at the vault and arch waist is notably high. Although the inverted arch in sections LK51 + 610 and RK51 + 620 shows lower water pressure, significant pressure impacts the tunnel lining at the vault and arch waist due to fissures in the surrounding rock. It is crucial to reinforce these areas based on the monitored water pressure values promptly.