Theory And Application Study On The Non-linear FEM For Soil Stability Analysis

The subject of soil stability analysis which include slop stability analysis, soil pressure calculation and calculation of ground base carrying capacity is the one that classical soil mechanic tried to solve first but has not hitherto solved yet. The early methods of soil stability analysis mostly focused on limit equilibrium method, slip line method and limit analysis method. As the development of the calculation technology, the FEM has become to one important method of soil stability analysis gradually. Comparing to other methods, the FEM has more rigorous theory system. In this paper, taking the soil stability analysis as the object, the correlative question consisting in the FEM of soil stability analysis has been studied weightily. The mainly content include the FEM base theory of geotechnical engineering calculation, the judge criterion of the critical state, the method to assess the critical slip surface, influencing factors of soil stability analysis, theory of gradient-dependent plasticity and its application in soil stability analysis including softening and the application development technology of ABAQUS to develop geotechnical engineering calculation software. The paper has seven chapters, and the contents are arranged as follows:Chapter one: The application of FEM in geotechnical engineering numerical calculation is summarized, all kinds of soil stability including limit equilibrium method, slip line method, limit analysis method and FEM are systematically summarized, the strongpoint and shortcomings of these method are compared and their relation and difference are analysed. Finally, the study contents are confirmed.Chapter two: Some questions relating to the base theory of geotechnical engineering FEM are studied. The work focus on the follow aspects: definition of the initial stress, excavation simulation and calculation of the excavation load, difference of the analysis considering of small and large deformation, simulation of the contact between different materials and calculation of FEM nonlinear equation. This work can be consider as the preparation of analysis soil stability using FEM. The correlative questions of the two usually used constitutive models in geotechnical FEM, Duncan-Chang model and Drucker-Prager model, have been discussed. An extended Drucker-Prager model is introduced. The extended Drucker-Prager model has linear, hyperbolic or general exponent yield surface, can reflect the non-linear feature of the strength of soil; can model materials in which the compressive yield strength is greater than the tensile yield strength; generally allow for volume change with inelastic behavior: the flow rule, defining the inelastic straining, allows simultaneous inelastic dilation (volume increase) and inelastic shearing; can include creep if the material exhibits long-term inelastic deformations; can be defined to be sensitive to the rate of straining, as is often the case in polymeric materials. The FEM calculation format taking the plastic multiplier as independent variables has been derived.Chapter three: The analysis system of FEM in soil stability has been set up.At first, the advantages of FEM comparing with other methods and its state of the art in soil stability are summarized, and some questions are pointed out.A dynamic method used to judge the critical state is proposed. There are two kindsof common methods, one is based on the convergence and the other is based on the distribution of plastic area. Evidently, the difference of the convergence criterion can directly induced the difference of safety factor, thus this kind of methods have some man-made factor. Additionally, although the instability of slop can induce the failure of the convergence of equation solving, the bugs in the FEM program and the questions of the algorithmic can also induce the failure of the convergence of equation solving. So the failure of the convergence of equation solving cannot always expresses the happening of the physical phenomena of losing stability. The plasticity yield is the resul...