Research On Stability Analysis Of Soil Structures Based On The Finite Element Limit Equilibrium Method

In soil mechanics, stability analysis is a classic problem which engineers often encounter. For example, stability of earth dam, gravity dam and embankment; bearing capacity of foundation and footing stability; active and passive earth pressure and retaining wall stability; failure of tunnel and wall rock stability; and so on. In theory, many researchers think slope stability, earth pressure and bearing capacity of foundation are belong to the same theoretical system and suggest to establish a general numerical method.In all the numerical stability methods, the finite element limit equilibrium method (FELEM) is a representative method and will be introduced into general use. There exists great potential and broad prospects for application.Beased on the summary and review of research achievements on the stability analysis methods, this paper introduces the basic theory of the FELEM and adopts the method to analyze earth pressure, bearing capacity of foundation and slope stability. Then, for the hardening/softening behavior of wall rock, excavation damaged zone of a tunnel is simulated by this method. Finally, it is suggested that the FELEM could be used to analyze the stability of geotechnical structures as a general method. . This paper reflects the following results:(1) A 2D optimization search method for the critical silp surface is proposed, where Particle Swarm Optimization is combined with the Hooke-Jeeves pattern search, and is used to analyze the stability of soil structures. This search method increase the efficiency and precision of the calculation procedure.(2) The impacts of the parameters of soil material on the results obtained from the FELEM and stability of a couple of relative complex slopes are analyzed. Specifically, explanations of the effect of Poisson's ratio and dilation angle on the method are given. In the application of the FELEM, directions of of shear stresses at any point along a critical slip surface may not follow the same trend towards the slide. A solution is proposed to solve the problem. And some representative examples are selected from the literature to validate the efficiency of the search method. Then multi-stage slope stability is analyzed by the FELEM.(3) The Rankine and Coulomb theories are widely accepted by engineers because of their simplicity. It is well known that the assumption of a plane slip surface is not reasonable for rough walls. Therefore, a curved slip surface must be considered. The FELEM is developed to address this problem. And Terzaghi log-spiral passive earth pressure theory is extended and a log-spiral active earth pressure method is proposed. The comparisons between the FELEM and log-spiral show that the location and shape of the critical slip surfaces by the two methods are the same.(4) Considered the width of footing, unit weight of foundation and friction between footing and foundation, the bearing capacity of foundation is determined by the FELEM. The comparison between the FELEM and other methods from the literature reveals that the shape of the critical slip surface is similar to Hill mechanism for a smooth footing and Prandtl mechanism for a rough footing.(5) For the FELEM, definition of a factor of safety evaluating slope stability is proposed and the softening strain behavior of the soil material is considered. A comparison with limit equilibrium method (LEM) based on the peak strength and residual strength is performed. And search method for local shear failure surface is proposed. Progressive failure process of soil structure during loading is analyzed in order to explain its failure mechanism.(6) Based on the researches on Boom clay selected as a potential host rock formation for deep geological disposal of high-level waste, the focus is made on simulation of the excavation damaged zone (EDZ) in hollow cylinder experiment and in-situ connecting gallery excavation through finite element computations with a modified Drucker-Prager model considering hardening and softening plasticity and elasto-plastic anisotropy. Furthermore, it presents the FELEM applied to predict fractures in the EDZ. The modelling results agree with experiments quantitatively and confirm the feasibility of predicting the EDZ.