## 3 Finite Element Analysis and Field Monitoring
### Distribution of Monitoring Points  
During the excavation of the pilot tunnel, a number of settlement monitoring points are placed on the surface, one every 15 m in the longitudinal direction, and the horizontal distribution monitoring is shown in Figure 6.  
The soil settlement monitoring points should penetrate the hard shell layer, and should be separated from the hard shell layer and in close contact with the original soil under the hard shell layer, and it is strictly forbidden to take the shallow way to set up the monitoring points. The monitoring point is installed by drilling through the hard crustal layer with a 150 mm diameter water auger and then burying the measuring rod in the soil layer, protected to the ground by a casing filled with medium coarse sand and cement mortar between the casing and the borehole. The measuring rod was not less than \(\Phi\)16, length of not less than 800 mm. the top of the measuring point from the bottom of the protective  
Figure 5: Pile element contact model diagram: (**a**) horizontal direction; (**b**) cutaway drawing. Figure 8: (**a**) \(\mu\) = 0.22; (**b**) \(\mu\) = 0.25; (**c**) \(\mu\) = 0.28; (**d**) \(\mu\) = 0.31; (**e**) \(\mu\) = 0.34; (**f**) \(\mu\) = 0.37; (**g**) Cross-sectional field monitoring; (**h**) Max settlement values for each rise–span ratios. The pile-beam-arch method with six kinds of rise-span ratios can be solved to obtain the displacement variation in each construction stage after finite element calculation. Since the settlement hardly changes in the beam-pile construction phase, the settlement values of the beam-column construction are not shown in the Figure 8. The variation in settlement is shown in Figure 8.  
Different rise-span ratios have a significant effect on surface settlement, and when the rise-span ratio is 0.22, the maximum value of surface settlement is approximately 79 mm. When the rise-span ratio is 0.28, the surface settlement is reduced 51 mm, and the difference between the two is 28 mm, a reduction of 35%. The actual rise-span ratio of the station is between 0.28 and 0.31. From Figure 8g, it can be concluded that the change law of the surface settlement of the simulated value and the monitoring value is approximately the same, which confirms the accuracy of the finite element numerical simulation from the side. Due to the effect of stress redistribution, it leads to the displacement of the soil in the borehole and eventually forms loosening. From the analysis of the measured data, the surface settlement caused by the excavation of the cave pile method and the settlement trough theory proposed by the Peck curve have some agreement. The width of the settlement trough decreases with increasing depth of burial, so that the settlement of the vault caused by the excavation of the station is not completely transferred to the surface, and the self-stability and stress redistribution of the soil will absorb part of the deformation, reflecting the process that the deep soil disturbance is gradually transferred to the surface in an expanding trend. From Figure 8 it can be seen that the settlement is greater when the rise-span ratio is relatively small. However, this does not mean that the settlement becomes smaller as the rise-span ratio increases. When the rise-span ratio is 0.28, the settlement reaches the minimum value. As the rise-span ratio increases, the settlement increases and the overall trend is first to decrease and then to increase. However, after the rise-span ratio of 0.28, although the settlement volume becomes larger, the increase is more moderate and almost no change occurs, and the settlement volume tends to a stable state. The amount of surface settlement caused by the four main construction stages of the pile-beam-arch method is shown in Table 2.  
It can also be seen from Table 2 that each rise-span ratio accounts for more than 37-59% of the total settlement in the pilot tunnel construction stage, about 5% for the pile-beam column, 20-37% for the top arch construction, and 12-20% for the main construction.