Numerical Analyses Of Stability Of Soil Slopes And Interaction Of Soil And Support Structure In Foundation Pit Engineering

It has been well recognized that the interaction of soil and support structures, nonlinearities of soil materials and contact surface between soil and structures and the stability of foundation pit slope are three main issues in the construction and design of deep foundation pit engineering. These issues have not been well solved in engineering practice. Therefore these topics are involved in this thesis. The research work done includes as following:1. The hyperbolic-type nonlinear model and rigid and perfectly-plastic model are usually adopted to simulate the constitutive behavior in tangent direction of contact surface. in fact, however, the hyperbolic-type nonlinear model can not definitely display the plastic flow deformation after shear failure while rigid-perfectly-plastic model can not describe the nonlinear deformation before shear failure. In order to overcome these drawbacks, elastic and elasto-plastic theories are combined together and a nonlinear elasto-perfectly plastic model is developed to simulate nonlinear and inelastic behavior of interaction between soil and structure. In the model, shear deformation before shear failures is computed by the nonlinear model and the plastic deformation after shear debonding is given by the elasto-plastic model. Therefore, the model can comprehensively show the deformation behavior and failure mechanism of the contact surface. On the basis of this constitutive model for contact surface, the contact-zone element with a certain of thickness for numerical simulation of the contact surface between soil and structure is proposed for FEM-based computations. Based on numerical analyses, it is recognized that the contact-zone element with non-zero thickness, instead of no-thickness contact element, should be employed to simulate behavior of soil and structure interaction.2. Two types of nonlinearity occur usually in geotechnical engineering problems, i.e., physical nonlinearity caused by the nonlinear behavior of geo-materials, and geometrical nonlinearity induced by the nonlinear characteristics of interfaces between soil and structure. For such twice nonlinear problems, the elsto-perfactly-plastic model is used for displaying soil nonlinear behavior and the nonlinear elasto-perfectly plastic model developed in this thesis is employed for describing the nonlinear behavior of contact surfaces. Then an effective nonlinear algorithm which combines the incremental procedure and iterative scheme is developed for solving the resulting nonlinear FEM equations. The proposed numerical method is applied to numerical analyses of foundation pit excavation and bracing and slope stability. A number of numerical examples are given to verify the efficiency and validity of the proposed procedure.3. The slope stability analysis based on the elasto-plastic FEM by using shear strength reduction techniques has been commonly accepted and widely used in slope engineering. Based on the computed displacements or generalized shear strain at a certain state of slope in which convergence of iterations of finite element computations cannot be gained, such a state is usually defined as the instable state. At the moment of instability, the variations and distributions of some physical quantities such as plastic strain and stress level are used to assess the initiation and development of failure zone. Therefore, the conventional procedure for defining the instability state by controlling the maximum iterative number for convergence judgment is not appropriate. Moreover, such a convergence criterion that the ratio of unbalanced residual forces to totalexternally-applied loads or the ratio of incremental displacements to the total displacements cannot exceed 10~3 shows a certain degree of man-made randomness. In fact, the distribution of generalized shear strain at any stage before overall failure occur can be displayed based on the computational results of elasto-plastic finite element analyses by using strength reduction technique. While the distribution zone of generaliz...