## 5 Sensitivity Analysis of Parameters
### Water Level Drop (\(s_{\rm w}\))  
Five water level drops were studied, which were 10 m, 15 m, 20 m, 25 m and 30 m, for which the corresponding water levels in the well after dewatering were 30 m, 25 m, 20 m, 15 m and 10 m, respectively.  
Figure 12 plots the underground water levels for the five scenarios. From the figure, as the \(s_{\rm w}\) increases, there is a corresponding decrease in the water level within the surrounding strata. When \(s_{\rm w}\) measures 10 m and 15 m, the burial depth of the tunnel axis remains below the groundwater level. However, as \(s_{\rm w}\) exceeds 20 m, the segment of the tunnel closest to the dewatering well location emerges above the water level. In Figure 13, the curves depict the additional stress levels experienced by the tunnel under various \(s_{\rm w}\) conditions. As shown, the additional stress on the tunnel escalates with the increase in \(s_{\rm w}\). This behavior is attributed to the rise in the total effective stress within the soil above the tunnel as \(s_{\rm w}\) increases. Nevertheless, when the water level drops below the tunnel, the additional stress on the tunnel ceases to increase. Figure 13 also highlights that once \(s_{\rm w}\) within the well surpasses 20 m, the maximum value of additional stress on the tunnel remains constant, while the range over which this maximum value is generated expands.  
Figure 11: Curves of the tunnel’s bending moments under different spacing \(d\).  
Figure 12: Curves of water level under different water level drop depths \(s_{\rm w}\).  
The tunnel settlement and bending moment curves corresponding to various \(s_{\rm w}\) are presented in Figures 14 and 15. Figure 14 reveals that as \(s_{\rm w}\) increases, both the vertical settlement of the tunnel and the range of settlement expand. Specifically, as \(s_{\rm w}\) transitions from 10 m to 30 m, the maximum vertical settlement of the tunnel rises from 4.01 mm to 9.12 mm. It is noteworthy that the rate of increase in maximum settlement significantly diminishes once \(s_{\rm w}\) exceeds 20 m. This can be attributed to the enhanced impact of precipitation on the surrounding water level, resulting in an increased additional load on the tunnel and, consequently, greater settlement. Figure 15 illustrates that as \(s_{\rm w}\) increases from 10 m to 20 m, the maximum bending moment experienced by the tunnel increases from 3.37 MN\(\cdot\)m to 5.06 MN\(\cdot\)m. However, with \(s_{\rm w}\) surpassing 20 m and reaching 30 m, the maximum bending moment starts to decline, and the position of the maximum bending moment shifts away from x = 0 to both sides. This shift results in the emergence of two symmetrical bending moment peaks. Notably, these peaks align with the region of maximum additional stress depicted in Figure 13, indicating that they coincide with the intersection between the groundwater level and the tunnel. In engineering applications, it is advisable to closely monitor the tunnel location closest to the water level during dewatering events.  
Figure 14: Curves of tunnel displacement under different water level drop depths \(s_{\rm w}\).  
Figure 13: Curves of additional stress under different water level drop depths \(s_{\rm w}\).