Study On Meshfree-related Methods Used In Seismic Liquefation Problem Of Foundation Soil

As a major form of earthquake disaster, liquefaction of sandy soil often causes uneven subsidence of building foundation and destruction of structure, which bring tremendous damage to the humanity. Large deformation caused by liquefaction is a main phenomenon of saturated sandy soil disruption under seismic excitation; it has an important engineering value to predict the deformation quantitatively of the actual boundary problem. In recent years, finite element method (FEM) has been widely used in geotechnical engineering problems. But the results simulated by FEM strongly depend on mesh quality, and it may not be able to complete calculation normally because of serious mesh distortion caused by non-uniformity spatial distribution of liquefaction process. Meshless method constructs the approximation functions by a set of discrete points; it does not require inter-node connection information, thus it can overcome the calculation interruption caused by mesh distortion, and it is suitable for large deformation simulation of liquefaction. This research is focused on the application of meshless method on liquefiable analysis, the main investigations consist of the following three portions:1. Meshless method is applied to numerical simulation of liquefaction problem, then the merits that meshless method can overcome calculation interruption caused by mesh distortion when using FEM are verified. Meanwhile, as a numerical approximation method, calculation error of meshless method is inevitable, so the adaptive process is programmed. The adaptive program is calculated by T-B stress recovery scheme and z-z error estimation, the error is estimated by energy norm of difference of recovery stress and meshless stress. If the local relative error exceeds the limit relative error, then new nodes will be generated at the high error area. The material parameters of the refined node are transferred to the newly added nodes, and the variables of the new nodes are interpolated from the values of the old nodes. Next calculation step is based on the new node discretization. By numerical simulation of a saturated soil block that suffering uniformly distributed load on the surface, the verification that the adaptive process can improve accuracy is done. Seismic response analysis of an embankment constructed on saturated sandy foundation is carried out, by comparison the results with those of FEM and Centrifuge test, the effectiveness of EFGM is proved. Then, by comparison the results obtained by adaptive method with those obtained by coarse and fine nodal discretization, the effectiveness of the adaptive process is verified and the response rule of the embankment is shown. 2. There are many advantages of meshless method in simulating large deformation, but there are also shortcomings in calculation time. FEM is coupled with meshless method, then, the coupled method can combine the advantages of the two methods and give full play to their advantages. It means that in the position prone to mesh distortion meshless method is used, in other regions FEM is used. In the interface region, transition elements are conducted, by constructing approximation functions of displacement and pore water pressure, the two methods are coupled together. By numerical modeling of liquefiable slope, it is validated that the coupled method can save computing time than the simple meshless method, and be able to achieve a reasonable accuracy. Then, parameter analysis is made to the slope model using the coupled method.3. Analyzing the dynamic response of the structure constructed on liquefiable sand foundation, it is needed to consider soil-structure interaction. It is already very mature for FEM to analyzing interaction analysis, but it is still rare for meshless method when involving interaction analysis. Through programming Fortran procedure, saturated sand foundation which is easy to have a large deformation when liquefaction is discretized by meshless method; structure is discretized by FEM, then, the contact region between foundation and structure is modeled by non-thickness frictional contact elements, the meshless-finite element coupled contact algorithm for soil-structure interaction is implemented. The reliability of programming is verified through pile-soil interaction analysis, and then, the caisson model is simulated by the method obtaining the law of dynamic response, influence factors analysis is also made.