## 3 Finite Element Analysis and Field Monitoring
### Force Characteristics of Each Rise-Span Ratios
#### 4.2.1 Steel Pipe Pile Axial Force Analysis  
The article uses the finite element analysis method to calculate the mechanical properties of the vault at different each rise-span ratio, and the two-dimensional finite element model to analyse the internal force characteristics of the steel pipe piles. The relationship between the maximum axial force and the rise-span ratio of steel pipe piles is shown in Figure 10. The axial force accumulation of different rise-span ratios in the main stages of the pile is shown in Figure 11.  
Figure 9: Comparison of measured and simulated values.  
The axial force of the steel pipe pile is plotted from the finite element model. It can be seen from Figure 10 that the maximum axial force of the centre pile for each rise-span ratio, as a whole, tends to first decrease and then increase with the increase in rise-span ratio, at a rise-span ratio of 0.22, the axial force of the centre pile is 4430 kN, and at a rise-span ratio of 0.31, the maximum axial force of the centre pile is 3860 kN, a decrease of 13% of the peak value. From the longitudinal comparison in Figure 11, it can be seen that in the top arch stage, as the top arch construction progresses, the excavation area of the soil between the pilot tunnels will increase and the internal cavity area will increase, the overburden pressure will be transferred to the centre pile and the side piles, at this time, the centre pile and the construction of the top and bottom longitudinal beams will form a stable frame structure to jointly bear the force, and the force of the centre pile will start to be prominent, so the axial force of the centre pile will increase sharply. The process of mechanical transformation of the cavity pile method is achieved. The sudden increase in axial force of the centre pile is the result of the excavation of the soil between the pilot tunnels, and the overburden pressure is transferred to the centre pile. After partial removal of the initial support in the pilot tunnels, the axial force in the centre pile was slightly reduced by the addition of cross bracing to ensure the stability of the structure as required by the design, taking into account the stiffness of the structure. However, as construction progressed, the axial force in the centre pile continued to increase until it reached its maximum value.  
Figure 11: Steel pipe pile axial force diagram.  
Figure 10: Max axial force of steel pipe piles with each the rise–span ratio.  
rise-span ratio increases, and the rate of decrease is also decreasing, tending to converge. The maximum axial force value of the arch structure decreases from 2303.6 kN at \(\mu=0.22\) to 1982.1 kN at \(\mu=0.37\), a reduction of 14% of the maximum axial force value. An excerpt from the axial force clouds for a rise-span ratio of 0.22 and a rise-span ratio of 0.37 shows that the maximum axial force in the arch occurs at the foot of the arch and the minimum axial force at the top of the arch, and that the axial force in the central arch is slightly less than that in the two side arches. The axial force cloud of the overall structure (Figure 14) shows that the maximum axial force is at the junction of the steel pipe piles and the base plate, and the axial force of the steel pipe piles is greater than that of the side piles.