The Study For Finite Element Analysis Of Wall-anchored Support System Of Deep Excavation

The rapidly development of high-rise building and the full use of underground space are two trends in the city construction, which will cause a large -scale number of excavation and construction of the deep foundation pits. However it is very difficult to analyze the internal force of support system of foundation pit because of the complex interaction between the surrounding soil and support structure when the foundation pit is excavated. It is also very difficult to analyze the site in-situ soil test completely because of the complexity of support system. So, it is significative to simulate and analyze the support system of deep foundation pit. On the one hand, it can reduce the cost of braced cut structure by making an economical and reasonable programme of support structure in different soil throught simulation and analysis of stress and deformation of support structure. on the other hand, it can choose parameters and make suggestions to the foundation braced cut engineering design, according to analysis of working performance and the factors of support structure.The traditional method can't concern the interaction between the support structure and surrounding soil and analyze well the displacement of the foundation and the slope when we analyze support structure of foundation pit and concern security of foundation pit. This paper will use the FEM software, Ansys, to simulate the support structure of foundation pit and the soil operating performances. The main tasks and research results are listed as follow.1. Finite element model is found using the Ansys software to simulate wall-anchored support system of deep excavation.2. The ground settlement characteristics which caused by foundation pit excavation in different soil are analyzed; Analyzing the influences of retaining wall, soil, the anchors to support structure performance. Some suggestions are given to the design of support structure of deep foundation pit excavation, and fix on several design parameters as a reference. A FEM is proposed to simulate foundation pit braced cut structures and the excavating procession.(1) With the increasingly number of the distance between the point on the floor and the boundary of the foundation pit and settlement, the settlement decreases from the top of the anchor sticks and will not be steady until reach three depths of the foundation pit.(2)Although increasing the thickness of walls can probably reduce its displacement, it will make the bending moment much bigger and the Engineering Cost much higher. Therefore, it is unreasonable to reduce the displacement in that way.(3)With the increasing of the soil shear strength index, the displacement of the support structures gradually reduced, active earth pressure and the passive earth pressure are increased; when the cohesion and the angle of the friction reach a certain value, the increasing of the soil shear strength index will not make a significant influence on the support structure.(4) It will reduce the displacement of the support structures and the positive bending moment of the wall by increasing the length of anchor arms appropriate, and it also would reduce the quantities of reinforcing steel bar in the wall. But anchor arms will not influence much on internal force and the deformation of the support structures, when the length surpass the twice of the depth of foundation pit.(5) In the construction of the foundation pit engineering, we should as possible as to reduce the ground overload in the outside of the foundation pit. If we can not avoid the overload, we should make design of the braced cut structure according to the increase number of the braced cut structure displacement, internal force and the earth pressures for each additional 20 KPa.(6) The tangential stiffness on the contact surface have great influence on the deformation and the internal force of the structure of the braced cut, obviously, we should take the tangential contact stiffness factor for 0.3～0.5,(7) For the spring shear strength between soil and the anchor arms has great influence on displacement of the support structures and the internal force. The shear strength between the anchor arms and the soil in hard clay can be define as 5～10 MPa/m.(8) The result of the considered foundation pit excavation process about the displacement and the bending moment of walls is same as the actual situation. So, we should as far as possible to calculate by the way of the considered excavation process in deep foundation pit.3.Throught the analysis of examples, the model which consider the interaction between soil and support structures in the model is more reasonable and realistic.