## 4 Comparison between the Monitoring Data and the Results of 3D FEM Analysis  
### Three-Dimensional Finite Element Analysis of the Excavation and Retaining Piles  
The assumptions of the 3D finite element analysis included the following: (1) The initial stress was isotropic, (2) the gravel layer was homogeneous and independent of time, and (3) the influence of groundwater was not considered. Since deep-well pumping was implemented in the work area and the pre-precipitation water level was lower than the excavation surface, the pore water pressure was not considered in this study. Regarding the setting of the boundary of the 3D finite element analysis, the movement direction of the boundary was limited by the roller, and the XYZ range was 50 m in each direction. The 3D finite element mesh used a total of 9 element groups, 528 hexahedral MTH20 elements, and 2977 nodes, as shown in Figure 10a-c. The mesh diagrams of the first-stage stratum excavation to G.L.-3.5 m and the second-stage stratum excavation to G.L.-6.75 m are shown in Figure 10d,e, respectively. Figure 9: Results obtained with the 2D FEM analysis of the second-stage excavation: (**a**) iso-value of the total displacement; (**b**) iso-value of the second deviatoric stress; (**c**) vertical surface displacement along the AB line; (**d**) horizontal displacement of the retaining pile along the BC line. The numerical simulation and analysis of this research adopted a 3D finite element program that was developed in the laboratory, and its main calculation content and steps included three main parts: (1) the pre-processing program, (2) main calculation program, and (3) post-processing program. The calculation steps and contents were as follows: (1) pre-processing program: preparing calculation data, including the calculation range, automatically generated 3D excavation and support meshes, calculation matrix optimization and re-node numbering, inputting parameters according to the selected composition mode, and defining each module group, boundary conditions, the use of the initial stress method, the number of iterative operations, and the allowable accuracy; (2) main calculation program: reading data and related calculations, including the parameters of each group of elements, the total domain coordinate value of each node, the parameters of the constitutive law of each module, the degree of freedom of each node, the state of force, the number of repeated operations, the allowable accuracy value, etc. ; (3) post-processing programs: visual interface processing and drawing output, including the iso-value of the displacement distribution map of the x, y, and z directions and total displacement, the iso-value of the stress distribution map of the principal stress and deviatoric stress, the distribution map of the stress field and displacement field, etc. In the 3D finite element program, to simulate the construction sequence for the retaining piles during the excavation, for the numerical simulation of the excavation of the gravel layer, the elements in the excavation area were first removed, and then the equivalent  
Figure 10: Mesh of the 3D finite element analysis: (**a**) borders and dimensions; (**b**) gravel layer and retaining piles; (**c**) retaining piles; (**d**) first-stage stratum excavation; (**e**) second-stage stratum excavation. reverse was applied via nodal forces on the excavation surface.