## 3 Comparative Analysis of Working Performance of Pile-Raft System
### Numerical Modeling
#### 3.1.2 Model Building  
The 3D numerical model of the structure was created using PLAXIS 3D.  
Figure 4: Schematic profile of calculation model.  
Figure 3: Schematic profile of calculation model.  
* Model Size According to the size of the pile-raft foundation and the excavation depth of the basement foundation pit, the length and width of the model were determined to be two times the excavation depth by changing the size and solving the model several times. The stratum depth of the model ranged from the ground to the top of the hard pebbles. The eventual length, width, and height of the model were determined to be 100 m, 80 m, and 30 m, respectively.
* Element Selection The soil layer and cushion were discretized by a 10-node tetrahedral element. The six-node plate element was used to simulate the basement floor, wall, and raft structure. The three-node beam element was used to simulate the basement pillar. The embedded pile element was used to simulate the CFG pile. The interface was set between the cushion and the basement floor to simulate the interaction between the reinforced concrete floor and the unbound aggregate cushion considering the stiffness difference between them. A zero-thickness interface element was used as the interface element in this study [29], and the relative displacement of the joints on both sides of the interface was allowed to occur. The parameters of the interface in PLAXIS are associated with the parameters of the surrounding soil (i.e., the cushion material in this case) by a reduction coefficient \(R_{\text{inter}}\). In this paper, \(R_{\text{inter}}\) was set as 1.0. When the model grid was divided, the distribution of the global element was set to \"very fine\", and the soil mesh around the pile-raft foundation was locally refined. The total number of solid elements was more than 100,000. The model mesh is shown in Figure 5.
* Material Model and Parameters The soil and cushion were considered elastoplastic materials in this study. The hardening soil (HS) mode was used, which is suitable for the analysis of mechanical behavior under loading and unloading. The input parameters of the hardening soil (HS) model include secant stiffness in the drained triaxial test \(E_{50}^{ref}\), tangent stiffness for oedometer loading \(E_{oad}^{ref}\), unloading/reloading stiffness \(E_{ur}^{ref}\), exponential power \(m\), Poisson's ratio \(v\), cohesion \(c\), and frictional angle \(\varphi\). These parameters were determined via the method provided in the finite element program. \(E_{oad}^{ref}\) and \(v\) were estimated empirically, while \(E_{50}^{ref}=E_{oad}^{ref}\), \(E_{ur}^{ref}=3E_{50}^{ref}\), and \(m\) was equal to the default value of 0.5. Other parameters were obtained via the basic geotechnical test, the triaxial test, and the consolidation test [33]. Other settings in PLAXIS 3D were default values of the program. The cushion was simulated by an ideal elastoplastic model following the Mohr-Coulomb criterion. The basement floors, walls, pillars, rafts, and piles were assumed to be in a linear elastic state. Moreover, because this paper focuses on the analysis of  
Figure 5: Three-dimensional (3D) mesh model.  
building settlement, the reinforcement zone of the composite soil nailing wall during the excavation of the basement's foundation pit was treated by the equivalent method of increasing the value of the parameters of overall strength and stiffness. The parameters of soil were determined according to survey reports, geotechnical tests, and engineering experience, and those of the structural material were converted according to the structural cross-section and material type, as shown in Tables 1 and 2.  
The boundary conditions of the model were constrained by conventional static displacement. The upper surface of the model was free along all directions. The lateral side of the model was supported by sliding hinges, which limited horizontal displacement, and it could move freely along the vertical direction. The bottom boundary of the model was fixed along all directions.