# Numerical Simulation of Tunnel Face Support Pressure in Layered Soft Ground
## 3 Numerical Simulation Results and Discussion  
### Influence of Shear Strength Parameters on the Support Pressure  
The objective of this section is to investigate the impact of variations in shear strength parameters (friction and cohesion) within the top and bottom layers on the support pressure exerted on the excavation face, considering different soil layers. The study is categorized into five sections, wherein each section explores and analyzes how changes in bottom-layer parameters affect the support pressure when keeping the shear strength parameter of the upper part constant. Subsequently, under unchanged lower-part parameters, we examine and analyze how variations in upper-part parameters influence the support pressure on the excavation face. In the first case (Figure 4a), the strata boundary is at the top of the tunnel. For the change in friction angle, with the change in \(\Delta\varphi_{0}\), the support pressure is reduced from 26.5 kPa to 6 kPa, a total reduction of 20.5 kPa. With the change in \(\Delta\varphi_{1}\), the support pressure is reduced from 19.5 kPa to 15 kPa, a total reduction of 4.5 kPa. For the change in cohesion, with the change in \(\Delta c_{0}\), the support pressure is reduced from 25 kPa to 11.5 kPa, a total reduction of 13.5 kPa, and with the change in \(\Delta c_{1}\), the support pressure is reduced from 20 kPa to 16 kPa, a total reduction of 4 kPa. It can be seen that in the case of group a, the parameter change in the bottom layer is more sensitive than that of the top layer, that is, the change in the shear strength parameter of the bottom layer has a greater influence on the support pressure of the excavation face. In the second case (Figure 4b), the strata boundary is D/4 from the top of the tunnel. Under the influence of \(\varphi\) and \(c\), the variation law of destructive force is similar to that of Group a. The parameter variation in the lower layer has a greater influence on the support pressure of the excavation face than that of the upper layer. However, the difference is that the influence of the upper-soil parameter change on the supporting force is increasing, while the influence of the lower-soil parameter change is decreasing. In the third case (Figure 4c), the formation boundary intersects the tunnel axis. For the parameter changes studied, although the influence of changes in the upper stratum on the support pressure of the excavation face is slightly greater, the amplitude is small. Therefore, it can be considered that both the change in shear strength parameters in the lower stratum and the change in parameters in the upper stratum have roughly the same influence on the support pressure of the excavation face, that is, in this case, it is not important whether the shear strength parameter changes in the upper or lower strata. In the fourth case (Figure 4d), the formation boundary is 3D/4 from the top of the tunnel. For the friction angle, the supporting pressure decreases by 7.5 kPa with the change in \(\Delta\varphi_{0}\), while the supporting pressure decreases by 17 kPa with the change in \(\Delta\varphi_{1}\). For cohesion, the supporting pressure decreases by 3.5 kPa with the change in \(\Delta c_{0}\), while the supporting pressure decreases by 13.5 kPa with the change in \(\Delta c_{1}\). It is not difficult to see that for the studied parameter changes, the parameter changes in the upper layer have a \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Physical Model**} & \multicolumn{5}{c}{**Model Properties**} & \multicolumn{2}{c}{\(P_{\eta}/\gamma D\)} \\ \cline{2-7}  & **D (m)** & **C/D** & \(\gamma\) **(kN/m3)** & **c (kPa)** & \(\varphi\) (\({}^{\circ}\))** & **Physical Model** & **Numerical Model** \\ \hline [PERSON] (1994) [18] & 5 & 1 & 15.3 & 5 & 42 & 0.072 & 0.078 \\ [PERSON] (2006) [31] & 4 & 2 & 15.3 & 0 & 42 & 0.053 & 0.051 \\ [PERSON] et al. (2011) [17] & 5 & 1.5 & 14.7 & 0 & 34 & 0.095 & 0.092 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Validation of Numerical Models.