# Study on the Bearing Characteristics of Composite Foundations with Permeable Concrete Piles
## 3 Numerical Simulation of Composite Foundations with Permeable Concrete Piles
### Parameter Selection  
(1) Displacement boundary conditions: The bottom boundary is fully fixed. The sides are subjected to horizontal constraints, and the top is set as a free boundary condition. (2) Fluid boundary conditions: The bottom surface is defined as an impermeable boundary. The top surface of the foundation soil and the pile body are set as permeable boundaries. The \"fluid model assign null\" command defines impermeable materials, and the initial pore water pressure in the soil layers is defined using the \"initialize pore-pressure\" command. (3) Constitutive model selection: The pile body and cap are modeled using an elastic constitutive model, and the soil and cushion are modeled using the Mohr-Coulomb constitutive model. Since the pile body typically remains in the elastic stage when the surrounding soil fails, the elastic model is chosen to better reflect the pile's behavior. (4) Fluid-solid coupling calculation: Soil is defaulted to a porous medium in FLAC3D flow-solid coupling analysis. The fluid flow in a porous medium conforms to [PERSON]'s law. Also, it conforms to the Biot consolidation theory, which considers the pore structure of soil and the seepage characteristics of porous water based on the concept of porous medium. In FLAC3D fluid-solid coupling, changes in pore water pressure directly affect the soil's stress state and mechanical response. At the same time, soil deformation will also lead to changes in the pore water pressure, forming a dynamic process of mutual cyclic feedback. FLAC3D stream-solid coupling solution supports a variety of calculation methods, such as the manual calculation method, the master-slave process method, and the direct solution method. In the manual calculation method, the user can independently set the time step of single seepage and single mechanics operation; in the master-slave process method, the  
Figure 5: Finite difference modeling and meshing. running process is divided into two types: the primary process and the child process, and the two states are coordinated and unified; therefore, the calculation efficiency is improved. In the direct solution method, each seepage time step corresponds to a mechanical time step. Although the calculation is complex, it is simpler and can meet specific engineering needs. The model in this study uses an entirely flow-solid coupling method to simulate the composite foundation of permeable concrete piles under load and sets fluid as the primary process. Mechanics is to solve the model from the process. (5) Calculation parameter selection: The porosity and permeability coefficient of the pile body are selected based on the optimal mix proportion obtained from the material tests described in Section 2.3. The soil parameters are chosen according to the data in the study \"Discussion on the Zoning of Chinese Soft Soil and Its Parameter Statistical Characteristics\" [24]. The elastic modulus \(E\) is set within the range of compression modulus (2.5-3.5) \(E_{s}\). Model parameters include the friction angle \(\varphi\), cohesion \(c\), elastic modulus \(E\), Poisson's ratio \(\
u\), density \(\rho\), permeability coefficient \(k\), and porosity \(n\). Specific values are listed in Table 4. Since the relative displacement between the pile body and the surrounding soil is relatively large, the assumption of displacement continuity does not hold. Therefore, contact interfaces are defined between the cap and cushion, cushion and pile body, cushion and soil, and pile body and soil. The pile-soil contact surface is set as a permeable boundary for the permeable concrete pile composite foundation. The values of the normal stiffness coefficient \(k_{n}\) and the tangential stiffness coefficient \(k_{s}\) of the contact surface are ten times the maximum stiffness at the junction of the contact surface. The value can be calculated through Equation (4), and the calculation results are shown in Table 5[25]. Friction angle \(\varphi\) and cohesion \(c\), with values set at 70% of the \(\varphi\) and \(c\) values of the adjacent soil layers.