0 Preface The ANSYS finite element analysis program is a product of the well-known CAE supplier ANSYS. It is a large-scale general finite element analysis software that integrates structure, fluid, electric field, magnetic field and sound field analysis. It can interface with most CAD software to realize data sharing and exchange. It is one of the advanced CAD tools in modern product design. ANSYS' civil engineering package ANSYS/Civil FEM is used to study the characteristics of steel structures, reinforced concrete and geotechnical structures, such as steel structures, building construction, bridges, dams, diverticulum and tunnels, underground buildings, etc. Deformation, stability and seismic response, from the aspects of mechanical calculation, combination analysis and specification verification and design, provide a comprehensive solution for building and geotechnical engineers with powerful and convenient analysis. In this paper, the finite element calculation of rigid pile composite foundation requires the program to provide a variety of unit forms (annular elements, three-dimensional elements, etc. for axisymmetric problems), and can perform elastoplastic calculations. ANSYS meets these requirements, while its convenient pre- and post-processing capabilities save a lot of time. It is feasible and effective to calculate the rigid pile composite foundation with the ANSYS program. In this paper, the application method of ANSYS finite element analysis program in CFG pile composite foundation is discussed. On this basis, the working mechanism of CFG pile composite foundation is studied, and the benefits are quite abundant [1]. 1 finite element calculation model [1-2] 1.1 The constitutive model of the soil The stress-strain relationship of soil is very complicated. It has nonlinear, elastoplastic, viscoelastic, rheological and other characteristics, and is also affected by joint fissures. It is very difficult to comprehensively consider various factors in actual calculation. In many indoor simulation experiments, it is found that under too much load, both the PS curve of the pile and the stress-strain relationship of the soil will have obvious nonlinear characteristics. It is unreasonable to describe this relationship with the linear elastic model. . It is not difficult to find out from the existing model research results of soil strength that the elastoplastic model can better reflect the nonlinear characteristics of soil. In this paper, the Drucker-Prager elastic-ideal plastic model is used for the soil as the main simulation object. DP materials can meet this requirement in ANSYS software. In the data sheet of the DP material option, you need to enter 3 values: cohesion C, internal friction angle (p(0), expansion angle h. The expansion angle h is used to control the volume expansion, if the expansion angle is 00 , volume expansion does not occur; if the expansion angle is equal to the internal friction angle, severe volume expansion occurs in the material. In general, the expansion angle of 00 is a conservative method. 1.2 Basic assumptions In order to simplify the problem, the model assumes the following assumptions in the finite element calculation: 1) the same material is homogeneous and isotropic; 2) the pile and the cap are linear elastic bodies, and their deformation conforms to Hooke's law; 3) The soil between the piles is a homogeneous single soil layer, the soil and the cushion are all Drucker-Prager ideal elastoplastic models; 4) the pile-soil interface and the relative slip between the pile cap and the cushion; 5) the soil caused by the pile is not considered The change of the original displacement field and the stress field. 1.3 calculation model For the single pile problem, the section of the pile and the cap can be simplified into a circular shape according to the area, so that the three-dimensional problem is transformed into the axisymmetric problem, which can be calculated according to the plane problem. The pile, soil, cushion and cap unit adopt the calculation range of the 42nd class two-dimensional isoparametric solid element method (ie quadrilateral element) composite foundation provided by ANSYS program. The mesh division is semi-automatically completed by the program, at the top of the pile, The pile end is mesh-encrypted, and 360 grid elements are calculated, and the total number of nodes is 401. The calculation domain is radially extended from the edge of the platform by 20 times the pile diameter, and the vertical direction is calculated to the pile end. The units near the pile are densely divided and are far from the pile. The lower boundary and lateral boundary of the foundation soil are far from the pile body, and the load has little influence. It is regarded as a fixed boundary without displacement. The geometric model and mesh division are shown in Fig. 1. Practice has shown that the geometric and mechanical parameters of the various materials used in the calculation model are shown in Table 1. Only the complete contact and the coupled continuous deformation are considered for the pile-soil interface, and the relative slip phenomenon between them is not considered. After the displacement constraint is defined and loaded, the finite element solution can be solved. Since the analysis process needs to change different loads, pile length to diameter ratio, cushion modulus, etc., the process is computationally intensive. If it is time-consuming and laborious to re-model each time, an executable log file is generated in ANSYS. , define some parameters, each time a new calculation starts, modify the parameters in this file, and then directly call it in the program to execute. 2 calculation results and analysis In this paper, the influence of various influencing factors on composite foundation is analyzed by finite element calculation. In the analysis, the load is taken at 150 kPa. Except for the analysis of the length-to-diameter ratio of the pile, the pile length is taken 5 m and the soil layer is 10 m thick. Figure 2 is a displacement contour plot obtained by the program. 2.1 Influence of pile and soil modulus The stiffness of the pile in the composite foundation is one of the most important factors affecting and calculating the settlement of the composite foundation. When the stiffness of the pile changes, the deformation of the composite foundation will change greatly, and the deformation characteristics will also change greatly. The axial stress of the pile increases with the increase of the pile-soil modulus ratio, which indicates that the softer the soil relative to the pile, the more obvious the stress concentration of the pile, the lower the bearing capacity of the soil, and the load is more borne by the pile; The smaller the modulus, the larger the stress ratio, and increasing the modulus of the pile can also increase the stress ratio. When the modulus of the soil decreases, the settlement of the pile top and the soil increases, and the settlement difference between the soil and the pile top is basically unchanged, indicating that the penetration of the pile to the cushion is not affected by the modulus of the soil, such as Figure 3 shows. When the modulus of the pile is increased, the settlement of both is reduced, and the settlement difference between the soil and the pile top is significantly increased. Figure 4 shows that the upper penetration is mainly affected by the modulus of the pile and the cushion. However, when the modulus of the pile is large, the settlement of the pile top and the soil surface are basically the same, as shown in Fig. 4. This is because when the modulus of the pile is large, the compression deformation of the pile body is small and negligible. The settlement of the pile top can be approximated by the displacement of the pile against the resistance of the soil. Under the same load, the composite modulus and the settlement of the composite foundation are basically the same. 2.2 Influence of replacement rate Among the composite foundation values, the area replacement ratio m is defined as Ap/A, the cross-sectional area of ​​the Ap pile, and the base area of ​​the A-reinforced foundation. Due to the lithology of the foundation soil, the unevenness of the superstructure load and the size of the foundation plane, it is impossible to have equally spaced piles under the entire foundation. For a composite foundation with only piles under the foundation, the sum of the cross-sectional areas of the piles and the area of ​​the composite soil equal to the total area of ​​the foundation is called the average area replacement ratio. It can be seen from Fig. 5 that as the replacement rate of piles and soils increases, the area of ​​piles in the composite foundation increases, so that the contact stress at the top of the piles decreases, and the settlement deformation at the top of the piles and the surface of the soil decreases. However, the load distribution of the piled soil is transferred to the pile body, the load on the pile body is getting larger and larger, the bearing capacity of the whole composite foundation is increased, and the foundation settlement deformation is reduced. This also indicates that with the increase of the replacement rate, the settlement of the pile top, the soil surface and the cap are reduced, but the influence of the two on the settlement is not the same. By increasing the displacement rate to reduce the settlement, the reduction of the settlement when the radius of the cap is reduced is much larger than the decrease of the settlement when the pile diameter is increased. This is because when the uniform load of the substrate is constant, reducing the radius of the cap will reduce the total load, and the settlement is also reduced more. Therefore, when it is necessary to control the settlement, increasing the number of piles (equivalent to reducing the pedestal radius of a single pile) is more effective than increasing the pile diameter. 3 Conclusion ——The modulus of the soil has a great influence on the settlement of the composite foundation. As the modulus of the soil increases, the settlement decreases gradually; when the modulus is the same, the deformation of the pile top and the soil surface are almost equal; ——The modulus of the pile has little effect on the settlement of the composite foundation. When the modulus is the same, the deformation of the soil surface is greater than the settlement of the pile top; ——With the increase of the replacement rate, the settlement of the composite foundation gradually decreases; under the condition of the same replacement rate, the deformation of the soil surface is slightly larger than the deformation of the pile top. 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