## 3 Results and Discussion
### Influence of Two-Sided Surcharge/Unloading on the Tunnel Below Using Wet Sand
#### Influence on Additional Soil Pressure at the Bottom of the Tunnel  
The variation in the additional soil pressure at the bottom of the tunnel under different surcharge conditions is shown in Figure 24, where the positive values of the additional soil pressure represent compression and the negative values represent tension.  
Figure 23: Stress \(\sigma\) contour of wet sand at surcharge on two sides.  
Figure 22: Stress \(\sigma\) contour of dry sand at surcharge on two sides.  
Analysis of Figure 24 shows the following:  
1. Unloading can reduce the additional soil pressure of the tunnel subsoil to a certain extent, from the process of surcharge to 172 kg on two sides to the completion of unloading on both sides; the maximum reduction in the additional soil pressure of the tunnel subsoil is 0.7094 kPa, which occurs at measurement point 2 at \(-40\) cm from the tunnel center; the maximum reduction in additional soil pressure of the tunnel subsoil is 61.0%, which occurs at measurement point 3 at \(-20\) cm from the tunnel center. The maximum reduction in soil pressure is 61.0%, which occurs at measurement point 3 at \(-20\) cm from the center of the tunnel.
2. During the process from surcharge to 172 kg on two sides to complete unloading to 0 kg on the left side (i.e., surcharge is still 172 kg on the right side), the additional soil pressure at measurement point 6 at 40 cm from the tunnel center increases instead of decreasing, and the increase reaches 0.2134 kPa, which is an increase of 18.35%. The local unloading of the surcharge will lead to an increase in the accumulated additional soil pressure of the tunnel subsoil, and the unloading sequence will affect the distribution of the additional soil pressure of the tunnel subsoil.
3. When the two-sided surcharge is 172 kg, the surcharge on both sides is symmetrical along the longitudinal axis of the tunnel, but the accumulated additional soil pressure of the tunnel's subgrade is not completely symmetrical, which indicates that the surcharge order will affect the distribution of the additional soil pressure of the tunnel subgrade.
4. When the two-sided surcharge is 172 kg, the additional soil pressure of the tunnel subgrade shows three peaks, one of which occurs at measurement point 4 at the bottom of the tunnel, probably because the stress concentration occurs here, resulting in a high additional soil pressure; two of these peaks occurred at measurement points 2 and 6 directly below the surcharge, and the range of the two-sided surcharge was 0.4 m \(\times\) 0.4 m, but the difference between the accumulated additional soil pressure of the tunnel subsoil directly below the surcharge and the additional soil pressure of the tunnel subsoil below the edge of the surcharge was great, which was related to the characteristics of the sand soil itself--under the action of loading, the sand particles at the edge of the substrate (i.e., bearing plate) are very easy to extrude toward the lateral direction, so the middle of the substrate (i.e., bearing plate) must provide a greater balance of the reaction force, so that the reaction force distribution is inverted \"convex\"; specifically, refer to Figure 25.  
Figure 24: Variation in additional soil pressure at the bottom of the tunnel under different surcharges in Case 4.