## 3 Results and Discussion  
### The Impact of Dry Sand in Two-Sided Surcharge/Unloading on the Tunnel Below  
#### 3.1.1 The Effect on Lateral Convergence  
The lateral convergence on the left side of the tunnel with an eccentric surcharge of 172 kg and on the right side without surcharge is shown in Figure 5; the lateral convergence of the tunnel under the two-sided eccentric surcharge of 172 kg is shown in Figure 6. The lateral convergence deformation of each measurement point of the tunnel in each surcharge stage in Case 1 is shown in Figure 7. The transverse convergence of each measurement point with the surcharge is shown in Figure 8. A positive value of the convergence represents a lateral increase in the tunnel section, while a negative value represents a lateral decrease in the tunnel section.  
Figure 5: Change in the lateral convergence of each measurement point of the tunnel when the surcharge (172 kg) is only on the left side.  
Figure 6: Change in the lateral convergence of each measurement point of the tunnel when the surcharge (both 172 kg) is on two sides.  
According to Figures 5 and 6, the following can be seen:  
1. In Cases 1-3, the tunnel's maximum transverse convergence values occur when the eccentric surcharge is 172 kg on two sides of the tunnel, but the maximum transverse convergence value for Case 1 occurs at measurement point 1 (right in the middle of the tunnel), while Cases 2 and 3 occur at measurement point 2, due to the different locations of the loads on the two sides of the tunnel, resulting in different combined forces of the loads acting on the tunnel on two sides.
2. The maximum lateral convergence value of the tunnel in Case 1 is 0.2914 mm, the maximum lateral convergence value of the tunnel in Case 2 is 0.5562 mm, and the maximum lateral convergence value of the tunnel in Case 3 is 0.0368 mm, indicating that when the surcharge is applied on two sides of the tunnel, as the position of the left surcharge's offset increases from 0.2 m to 0.6 m, the lateral convergence value first increases and then becomes smaller, and decreases rapidly after the surcharge offset reaches a certain degree.
3. Case 2 is a symmetric surcharge on two sides; after the eccentric surcharge on the left side is 172 kg, when the right side continues to surcharge to 172 kg, it is equivalent to the total load increasing by a factor of 1, but the local lateral convergence value of the tunnel increases by several times, which is extremely harmful to the tunnel. According to Figures 7 and 8, the following can be seen:  
1. When the surcharge is on two sides under dry sand soil, unloading on either side can effectively reduce the lateral convergence value of the tunnel. Taking measurement point 1 as an example, after unloading the surcharge on two sides, the lateral convergence value decreases from the maximum 0.2914 mm to 0.2017 mm, representing a reduction of 30.6%.  
Figure 8: Transverse convergence variation of each measurement point with surcharge in Case 1.  
Figure 7: Transverse convergence deformation of each measurement point in each surcharge stage in Case 1.  
In the process of unloading a two-sided surcharge under dry sand soil, the lateral convergence value of the tunnel is negative, indicating that the tunnel section becomes laterally smaller at this time, which may be due to the redistribution of soil stresses during the unloading process, making the tunnel's lateral convergence recover \"excessively\".
3. The transverse convergence of measurement point 5 is less regular, because measurement point 5 is the farthest from the surcharge, and the transverse convergence value is small, while there is an obvious \"boundary effect\" near the edge of the model box.  
The transverse convergence deformation of measurement point 1 (in the middle of the tunnel) at each surcharge stage in Case 1 can be seen in Figure 9, where \(\theta_{\text{sand}}\) represents the pressure diffusion angle.