Analysis Of Deformation And Stability Of Nail Supported Deep Mixed Wall In Excavation

This study presents an investigation on the behavior of deformation and stability of nail-supported deep mixed (DM) wall in excavation. The research is started from a field case of monitored lateral deformation of nail-supported DM retaining wall of the foundation pit of the Oriental Art & Music Center in Shanghai. Finite element method (FEM) is employed to analyze the deformation and stability. The behavior of a nailed retaining wall is analyzed using two dimensional plain strain (PS) FEM. Strength reduction method in FEM is adopted to analyze the stability behavior of nail-supported DM wall during the pit excavation. The factors affect the behavior and stability of the foundation pit is discussed. Based the on the research work of this study, the following primary findings are obtained:1) Simulation of 3D effect of nail in PS. In the nail-soil composite material, the nail endures the tensile forces, the soil endures the pressure, and the interaction between nail and soil is through skin frictions around the nail. Consequently, the behavior of the nailed-soil under loading is three dimensionally. In order to obtain the correct results in PS analysis, interface elements is employed to express the 3D effect of each nail in 2D FEM simulation. A coefficient called periphery regulation is employed to reduce the skin resistance of nail in 2D PS analysis. The calculated results are compared with the field measured values. The results show that the proposed numerical scheme can simulate the field performance fairly well.2) Distribution of tensile forces in nail. The FEM calculated results show that the internal force in nails is related to the locations of nail. The forces in nails at the top and bottom layer are small. Axis stress in each layer of nails occurs during the excavation of soil layer under the nail location and the axis stress increases with the increase of thedepth of excavation. For the nail-supported DM wall system, the tensile force in nails decreases along the length of the nail and the maximum tensile force is at the connection between DM wall and nail. This phenomenon is much different from the simple soil-nailing system without DM wall, in which the maximum force happens in the middle of the nail.3) The factors affecting the deformation behavior of nail-supported DM wall system. The following factors are investigated and discussed in details: the length, stiffness, spacing, and angle of nail. The results show that these factors of nail have a great influence on the lateral displacement of the nail-supported DM wall. The displacement of the nail supported DM wall decreases with the increase of the length of nail, which indicates that it is very effective to control the displacement of retaining wall through increasing the length of nails. The displacement of the retaining wall decreases with the increase of the stiffness of nail, however, the displacement will not increase more after the stiffness of nail reaches to a certain value. The displacement of the wall increases linearly with the increase of the nail-spacing. The displacement is small when the inclination angle of the nail (the angle of the nail to horizontal direction) is ranged from 5°to 20°, however, displacement increases much when the inclination angle is larger than 30°.4) The factor of safety of the retaining wall during excavation decreases with the increase of excavated depth. However, the factor of safety will increase greatly after nails are constructed. Nails should be installed after the excavation work is finished as soon as possible.5) Although the stiffness and the spacing of nail have much influence on the lateral displacement, they have almost no effect on the factor of safety. Therefore, the stiffness and the spacing of nail can be designed based on controllable value of the lateral displacement. The factor of safety increases till to 20°with the increase of the inclination angle of the nail and then decrease with the increase of the inclination angle of nail. The factor of safety increases greatly with the increase of the length of nail.