## 3 Results and Discussion
### The Impact of Dry Sand in Two-Sided Surcharge/Unloading on the Tunnel Below
#### 3.1.2 Effect on Ground Settlement  
The variations in ground shift under different surcharges for Cases 1-3 are shown in Figures 10-12, where a positive value of the ground shift represents augmentation and a negative value represents settlement.  
Combining Figures 10-12, we can see that the following:  
1. As the position of the left surcharge's offset increases from 0.2 m to 0.6 m, the peak of the left ground settlement also shifts, and the peak of the left settlement decreases from 0.9815 mm to 0.5925 mm in the process of the offset, which is mainly due to the change in the position of the left surcharge's offset, resulting in the boundary effect becoming more and more obvious.
2. The amount of settlement at each measurement point of the ground first increases and then decreases during the unloading process, and the effect of unloading on the  
Figure 9: Schematic diagram of the change in the central section of the tunnel at each surcharge stage of Case 1: (**a**) No surcharge. (**b**) Left-sided surcharge 172 kg. (**c**) Two-sided surcharge to 172 kg. (**d**) Unloading to 0 kg on the left side. (**e**) Unloading to 0 kg on two sides.  
mitigation of ground settlement is not obvious, because the soil is still consolidating during the unloading process, causing the maximum amount of ground settlement. This is because during the unloading process, the soil is still consolidating, which leads to the maximum amount of ground settlement lagging behind.
3. The peak of the ground settlement on the left and right sides always occurs when the right surcharge is 172 kg and the left surcharge is finished unloading; the final peak of the ground settlement on the right side during the whole surcharge/unloading process is always larger than the final peak of the ground settlement on the left side. This may be related to the sequence of surcharge/unloading.  
Figure 11: Variation in ground shift under different surcharges for Case 2.  
Figure 12: Variation in ground shift under different surcharges for Case 3.  
Figure 10: Variation in ground shift under different surcharges for Case 1.  
Combining Figures 13-15, the following can be seen:  
1. As the position of the left surcharge offset increases from 0.2 m to 0.6 m, the additional surrounding pressure of the tunnel decreases sharply. Taking point 1 as an example, with a left surcharge of 172 kg, the additional surrounding pressure decreases from 1.9003 kPa to 0.2410 kPa when the left surcharge's offset position increases from 0.2 m to 0.6 m, which is 87.3%; with the left and right surcharge of 172 kg, the additional surrounding pressure decreases from 3.2576 kPa to 0.3545 kPa when the left surcharge's offset position increases from 0.2 m to 0.6 m, representing a decrease of 89.1%.
2. After surcharge on one side of the tunnel, if the other side continues to surcharge, it will cause the surrounding pressure to continue to increase, and when the surcharge on two sides reaches 172 kg, the local additional surrounding pressure of the tunnel increases by more than 100%, which is extremely harmful to the tunnel.
3. In dry sand, unloading can effectively reduce the additional surrounding pressure of the tunnel, but it will not eliminate the additional surrounding pressure immediately; after unloading is completed, the additional surrounding pressure will still exist in most of the tunnel to some extent. The more distant the surcharge location, the higher the residual proportion of additional pressure.  
The changes in additional pressure in the center of the tunnel in each surcharge stage in Case 1 are shown in Figure 9.