Research On The Lateral Deformation Of The Soil And Shield Tunnel Outside The Foundation Pit Controlled By Isolation Pile

As the role of isolation piles in the project is widely recognized,a large number of tunnels adjacent to the foundation pit project began to use isolation piles to protect the subway tunnel outside the pit.In order to reduce the impact of the isolation pile construction on the subway tunnel,most of the current projects adopt the layout of isolation piles near the foundation pit,of which the gantry type isolation piles are the main ones.In recent years,the research on isolation piles has focused on isolation piles arranged close to the tunnel.The study found that when the isolation piles are placed close to the tunnel side,the effect is significant.And the action mechanism of the isolation piles near the tunnel is revealed through traction and blocking.At present,there is still a lack of research on isolation piles near the foundation pit.Therefore,it is necessary to carry out in-depth research on isolation piles near the foundation pit to reveal the mechanism of controlling the deformation of the tunnel outside the pit.Based on the research background of a tunnel with isolation piles approaching the foundation pit in Ningbo area,this paper studies the isolation piles near the foundation pit based on the PLAXIS platform and using the finite element method which can consider the small strain characteristics of the soil.In the study,the empirical method and the back analysis method were used to calibrate the parameters of the soil constitutive model,and the finite element calculation and analysis of the engineering prototype were carried out.The reliability of the calculation model is verified by comparing the calculation results with the monitoring data.hen,based on the simplified model of the project,the action mechanism of the isolation pile near the foundation pit is revealed through the displacement field.After that,the design parameters of the isolation pile are analyzed in a calculation analysis model using a single soil layer Finally,combined with the relevant requirements of the foundation pit environmental protection,the control effect of the isolation pile under different deformation conditions of the foundation pit is studied.In the research,the influence of the full life cycle of the tunnel is considered,and the control effect of the isolation pile on the lateral deformation of the tunnel under different operating conditions was studied.Based on the above research,this paper has obtained the following main results:(1)Based on the research results of existing scholars on the calibration of the HS-Small model parameters,combined with the characteristics of the marine soft clay in Ningbo area,and using the parameter back analysis method,the calibration relies on the engineering soil HS-Small model parameters.The reliability of the calculation model and model parameters used in this study was verified by conducting a three-dimensional finite element simulation of the supporting project and comparing the calculation results with the monitoring data.At the same time,it also proves the applicability of the HS-Small model in the analysis of the tunnel adjacent to the foundation pit project.(2)This paper innovatively proposes a built-in isolation pile layout form,and on the basis of relying on the project,using the finite element method to verify the protective effect of this layout form of isolation pile on the subway tunnel outside the pit.At the same time,by comparing with the original gantry type isolation pile layout of the project,it is found that the control effect of the built-in isolation pile is significantly better than the portal-type isolation pile.(3)The study found that after the isolation piles are installed,the isolation piles will share the unloading effect borne by the envelope structure,produce a certain additional deformation,and form an affected area with increased displacement.As for the envelope structure,the unloading effect it bears is reduced and the deformation is reduced,it will form a displacement-reduced influence area.On the side of the isolation pile,it will be dominated by the affected area of increased displacement,resulting in the expansion of the affected area of that side.On the side of the envelope structure,it will be dominated by the area affected by the displacement reduction,resulting in a narrower area of influence on that side.At the same time,the addition of isolation piles will cause a certain change in the original displacement field.There is a fixed line of displacement,which can separate the areas of increase and decrease of displacement.The position of this line in space can be obtained by calculation.This line can be used to judge the influence of isolation piles on the lateral deformation of the subway tunnel outside the pit.(4)Through the study of two kinds of isolation piles near the foundation pit,it is found that the built-in isolation piles have a certain control effect on the lateral deformation of the soil outside the pit,but due to the spatial effect,it cannot affect the soil farther away from the foundation pit.The gantry type isolation pile is better than the built-in isolation pile in controlling the lateral deformation of the shallow soil near the foundation pit,but as the depth H and the distance D from the foundation pit increase,its control effect will gradually be lower than that of the built-in isolation pile,and even negative effects.(5)The design parameters of the built-in isolation pile length L,pile diameter D,pile spacing S,and the layout range I along the Y direction have a significant influence on the control effect of the built-in isolation pile.To control the lateral deformation of the soil outside the pit,the design parameters of the built-in isolation pile must be selected reasonably.In this study,the reasonable value range of each design parameter is determined through calculation,which can provide some guidance for the actual project in the future.(6)The environmental protection level of the foundation pit and the operating state of the tunnel will affect the effect of the isolation piles to control the lateral deformation of the subway tunnel outside the pit.The research found that in most cases,with the improvement of the environmental protection level of the foundation pit,the control efficiency will gradually increase.That is,the control efficiency corresponding to the isolation pile when the foundation pit is deformed is relatively high.And as the damage degree of the subway tunnel increases,the stiffness of the tunnel will continue to decrease,and the isolation pile has a more significant control effect on the lateral deformation of the subway tunnel with the more serious damage.