3-dimensional Fluid-solid Coupling Numerical Simulation For The Whole Process Of Deep Foundation Pit Excavation Of Diaphragm Wall

For deep excavation in a crowded urban city, an accurate and reliable design calculation method is indispensable to protect the surrounding environment and keep the facilities safe.Among the many foundation engineering design methods, finite element is the most widely used. Past experience has shown that it is good and relatively accurate to predict the deformation of retaining structures and their internal forces, but the predicted surface settlement outside the excavation zone has still room for improvement.In order to improve the accuracy of analysis of deep excavation modeled by the finite element method, a 3-dimensional element of 8 nodes with 6planes is taken to proceed for further perfection, through in-depth study of major influencing factors as analysis of fully coupling by the stress and seepage fields, the constitutive model of soil, seepage of underground water,the whole process of construction, the collective interaction between soil and retaining structures, and by using self written Visual C++ calculation program for analysis, an relatively close analysis results has been reached while in comparing a practical measured case and the one then under calculated. The main research work of this paper is summarized as follows:1. Proceed analysis of the fully coupling action of the stress and seepage fields by selection of elastro-plastic soil constitutive model and the expressed strain’s coefficient of permeability;2. A new elastro-plastic model is formed by combining the expressed characteristic elastic deformation of the Duncan-Chang hyperbolic E-B model and in modifying the characteristic elastic parameter ? of the modified Cam-Clay model, which needs without preset E and μ, and is possible toreflect the state of variance between the confining pressure σ3 and the elastic characteristic of soil mass.3. Using the relation between the variation of water buoyancy caused by the change of the element nodal water head, a conclusion is forward that the bulk density of soil mass in the seepage field is no longer a constant value and a method of calculating element bulk density has been deducted.4. Aiming to resolve the 2 difficult computational problems, i.e taking a3-dimensional integration of the boundary formed by the free surface and element plane surface while the groundwater free surface penetrates the element, and a 2-dimensional integration by the boundary of the free surface,it is proposed to adopt parameters such as coordinate transformation and isoparametric transformation technology to converse the integration boundary and integrand, making the upper limit and lower limits of the integration boundary in line with the formatting requirements of Gaussian Numercial Integration, from which a more exact numerical solution could be obtained and it helps to improve the oscillation resulted from the process of solving the equations.5. By introducing vigorously the rigid-plastic strength material theory, it has been deduced after failure, a 3-dimensional space interface model with thickness, and no longer consider the functioning of all stresses coupled mutually; and by presenting “comprehensive failure point” concept to express the first appeared failure stress position. Finally and under simulation of large-scale simple shear testes, it has been tested and verified in finding the local of appeared failure between the soil and structure by using the numerical methods.6. By using a simulation analysis of the entire process from forming the trench to pouring concrete in each trench segment in the diaphragm wall and in addition to have a more realistic “initial” stress state, it also examines the influences to the ground settlement outside excavation zone caused by theconstruction of the trench segments, it has been found a larger ground settlement close to the trench segment, this phenomenon and the pattern of surface subsidence caused by ground excavation are different; but with their superposition, it is more in line with the results measured.7. Through an iterative linearization in elasto-plastic material and with reference to the practice of calculating midpoint stress in the midpoint stiffness method, a tentative stress is obtained by using the elasto-plastic stiffness matrix at the midpoint increment. This method can lower greatly the value of the unbalanced stress and reduce the number of iterations.8. Aiming to cope with the asymmetric problem brought by the coupling matrix for overall consolidation coefficient matrix in finite element equation,the iterative method is proposed, to calculate firstly equations of the stress field, then the other part of seepage field, and by using the “1 Norm amend non-primary element double precondition” combined with ICCG method to calculate the linear equations of these two parts.