Study On Deformation Control Guideline Of Large-long-deep Foundation Pit Based On Spatial Effect

Deep foundation pit could be regarded as a three-dimensional structure with spatial effect.With regard to current design methods and academic research of the deep foundation pit,however,the spatial effect has seldom been taken into account.Instead,it is always considered as two dimensional plane strain problem in order to simplify calculation process,which fails to reflect the actual soil pressure and displacement,moreover,the design and research implementations would thus achieve conservative results,especially for the deep foundation pit with complex geometry or with complex supporting system.This is not consistent with the principle of economical and rational control of foundation pit deformation during construction process.Combined with the "Wuhan Greenland Center" project,this paper aims to study the deformation control guideline of large-long-deep foundation pit based on relevant spatial effect theories and three dimensional finite element(3D FEM)modeling and monitoring data.The main contents are as follows:(1)Through the actual monitoring data of the deep foundation pit in Wuhan Greenland Center the spatial effect is studied.Space form and geometrical dimensions of the deep foundation pit and its supporting system are discussed from three perspectives,viz.,lateral displacement of the wall,horizontal displacement of the soil outside the pit and the post-wall subsidence.Properties and deformation of the supporting structure and the strata in addition are analyzed,based on that some relationships of spatial effect between the foundation pit and the supporting system are explored and summarized.(2)The three-dimensional failure modes are divided into axisymmetric parabolic and intermediate linear curves.Based on limit analysis upper bound method and the limit equilibrium analysis theory,study on the spatial effect of large-long-deep deep foundation pit with cohesive soil is carried out.The upper limit of the total active earth pressure and the inclination of the critical slip surface in three-dimensional state and two-dimensional state are obtained.Hence,on this basis,the influence coefficient of spatial effect of deep foundation pit of any section is obtained.In addition,the influence of depth ratio of foundation pit and cohesion on the spatial effect coefficient is illustrated by an example.(3)The three dimensional finite element model of deep foundation pit of Wuhan Greenland Center is established by means of geological survey data,actual construction data and similar engineering experience.According to the actual monitoring data during prophase of construction process,the model and calculation parameters are adjusted.The deformation of the diaphragm wall and ground settlement behind the diaphragm wall are simulated and analyzed,and compared with the actual monitoring data during the later stage of construction.(4)Based on the 3D finite element model of the deep foundation pit of Wuhan Greenland Center,excavation sequence,horizontal support stiffness,angle stiffness and the thickness of the envelope are studied for purpose of deformation control of the super-large area excavation by changing the corresponding parameters.Some guidelines of economical and reasonable for construction of large-long-deep foundation pit are suggested.