Three-dimension Coupling Numerical Simulation Of Deep Foundation Pit Dewatering And Land Subsidence In Quaternary Loose Sediment Stratum With Great Thickness

There are Quaternary loose sediment with great thickness in the Changjiang River delta. In this area with high groundwater level, the drop of groundwater level is very large during foundation pit excavation. The problem of land subsidence owing to dewatering of foundation pit is notable. A full coupling-model of seepage and stress is established on the basis of Biot's consolidation theory. The finite element equation of 8-node hexahedron isoparametric element is educed by using finite element method. This model considers free surface boundary, flux boundary, horizontal flow barrier, well boundary, non-linear characteristics of soils and dynamic change of permeability coefficient with stress state. The process of model solution is programmed into computer program and then the computer model is built.These results are applied in SHANGHAI HUANQIU finance center foundation pit dewatering and the reconstructed foundation pit dewatering of Dong Jia-du subway part of four line in Shanghai. The forecasting results are proved to be correct and reliable by follow-up project. The following conclusions are obtained by the way of the seepage-stress coupling method.(1) The coupling model of deep foundation dewatering and land subsidence can obtain groundwater level and displacement of soil distortion. It can be used in the practical project.(2) Considering the non-line characteristic of soils and the interaction between penetrability and stress state of soils, the model applies increment tangent stiffness matrix method to solving elasto-plastic problems. The model correctly describes the process of soil distortion and evidently improves calculative precision and convergent speed.(3) On the basis of the method using invariable mesh and variable seepage matrix in the compound element, the improved variable seepage matrix method of the compound element was presented for the seepage problem with free surface. The element of free surface was divided into dry element, wet element and transition element. Then the disadvantage of unstable surging solution around gauss points was resolved by constructing penalty function with two controlling penalty parameters.(4) On the basis of numerical simulation of well, the integrated hydraulic model was used to simulate seepage and land subsidence of foundation pit dewatering. It could realize the high efficient, precise and quantitative analysis of the complex seepage with densely distributed wells of foundation pit dewatering.