# Analysis and Application of Lining Resistance to Water Pressure in Tunnel through Karst Cave
## 2 Failure Mode of Grouting in Tunnels Crossing High-Pressure Karst Caves
### Analysis of Tunnel Safety Thickness for Different Calculation Cases
#### 3.2.2 Analysis of Safety Thickness of the Composite Surrounding Rock  
From the research results in Figure 15, it can be seen that, if 3 MPa water pressure was required to be resisted, the surrounding rock thickness needed to reach 10 m, and, when the surrounding rock thickness was less than 10 m after parallel heading expansion, the cavity filling needed to be grouted and reinforced. At this time, the remaining surrounding rock after excavation and the grouted stone body were considered as a whole, the composite surrounding rock safety distance between the tunnel and the surrounding hidden cavity was studied, and the composite surrounding rock safety thickness was calculated together. The calculation model is shown in Figure 12, where the surrounding rock thickness was taken as 1 m, 2 m, 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, and 10 m. For each surrounding rock thickness, the numerical simulation calculation of grouting range was carried out separately, and the thickness of grouted cobble body was taken as 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, and 10 m, respectively. The comparison of water pressure resistance of different composite rock thicknesses is shown in Figure 16. As can be seen from the figure, a greater thickness of the surrounding rock and a thicker grouted stone body led to a higher corresponding composite surrounding the rock's resistance to water pressure capacity. When the thickness of the grouted stone body was certain, the trend of the composite surrounding rock's resistance as a function of the water pressure capacity increased with the thickness of the surrounding rock, which could be divided into three stages: growth period, stable period, and accelerated period. When the thickness of the surrounding rock was less than 3 m, the composite surrounding rock's resistance to water pressure capacity increased significantly with the thickness of the  
Figure 15: Comparison of water pressure resistance of different surrounding rock thicknesses. surrounding rock. When the thickness of the surrounding rock was 3-4 m, the growth rate of the water pressure resistance curve slowed down; when the thickness of the surrounding rock reached above 4 m, the slope of the water pressure resistance curve gradually increased. After analysis, it can be seen that a greater thickness of the composite surrounding rock led to a stronger water pressure resistance; as the thickness of the composite surrounding rock and the distance between the karst and tunnel increased, the influence of karst became weaker relative to the tunnel. According to the five locations of observation points shown in the model of Figure 12, the calculated results of pressure at these observation points were extracted to analyze the trend of pressure values at different thicknesses of grouting, and the results are shown in Figure 17. From Figure 17a, it can be seen that with the increase in thickness of grouting, the overall trend of water pressure behind the second lining decreased first and then became stable. When the thickness of grouting increased from 3 m to 5 m, the maximum water pressure at the observation point behind the lining was reduced from 0.3 MPa to 0.04 MPa, after which the water pressure no longer decreased significantly with a further increase in the thickness of grouting. From Figure 17b, it can be seen that, for the earth pressure behind the second lining, the effect of the thickness of grouting on the earth pressure was not obvious. With the increase in thickness of grouting from 3 m to 10 m, the earth pressure at all observation points always fluctuated around a stable value, which may be due to the fact that the self-weight load generated by the rock was basically carried by itself after being reinforced, and the self-supporting capacity of the surrounding rock was given full play at this time. To summarize the results, it is recommended that a grouting thickness of 5 m selected for site construction can effectively reduce the pressure behind the second lining.