Research On Stress Path Of Foundation Pit Excavation Considering Rotation Of Principal Stress Axis

The characteristics of "many,large,near and deep" make the surrounding environment of foundation pit more and more strict on the ground deformation caused by excavation.Under the typical unloading condition of foundation pit excavation,the soil produces shear stress and the principal stress axis rotates,which leads to the decrease of soil strength and obvious plastic deformation.If we ignore this important feature and adopt the traditional plastic theory in the design of foundation pit,the actual engineering is relatively unsafe.Although the rotation of principal stress axis of soil has been paid attention to in deep foundation pit project,the theory and method of foundation pit design under the condition of rotation of principal stress axis have not made much progress.In order to make up for the deficiency of the existing theory,it is urgent to carry out in-depth research on the mechanical behavior of foundation pit excavation soil under the condition of principal stress axis rotation,so as to improve the design theory and guide the engineering practice.Based on the project of National Natural Science Foundation of China(51979122),this paper analyzes and studies the excavation stress path considering the rotation of principal stress axis in the super deep foundation pit with typical supporting structure of "diaphragm wall + support".The main contents and achievements are as follows:(1)Based on the typical supporting form of "diaphragm wall + support",a three-dimensional numerical model is established by numerical simulation software to carry out numerical simulation.According to the simulation results,the observation points of stress path are selected.(2)Leading in the direction angle of principal stress axis α,the specific numerical changes of p,q and α are obtained through theoretical analysis.The three-dimensional stress path of foundation pit excavation considering the rotation of principal stress axis is deduced,and its general law is analyzed.(3)By changing the horizontal and vertical arrangement of the support in the foundation pit excavation process,the stress path of the foundation pit excavation under different conditions is compared and analyzed.The influence of the change of the support system in the foundation pit on the stress path of the foundation pit excavation considering the rotation of the principal stress axis is discussed,and then the support system in the foundation pit is optimized.The results show that: When the horizontal layout form of support system is selected,the plastic deformation of soil caused by rotation of principal stress axis is smaller when the arrangement form of "dense at both ends and sparse in the middle" is selected than that of horizontal uniform distribution;On the premise of reasonable economy and convenient construction,the influence of principal stress axis rotation can be reduced by increasing the number of supports in the vertical plane;In the vertical plane of the foundation pit,the closer the first support is to the top of the slope,the smaller the rotation amplitude of the principal stress axis of the soil is;In the design of foundation pit support considering the rotation of principal stress axis,the above ways can be used to reduce the plastic deformation of soil caused by rotation of principal stress axis,so as to improve the stability of foundation pit.