Study On Calculation Method And Law Of Tunnel Deformation Caused By Old Pile Removal

With the increasing number of new subway tunnel projects,some of which require the removal of old piles at the site before shield excavation,the population of medium-sized cities and cities is densely populated,and urban transportation and buildings continue to move underground.The removal of old piles will inevitably cause the stress release of the soil around the pile foundation,which will cause the soil around the pile foundation to move to the old pile removal area.This kind of soil movement will have a certain impact on the deformation of adjacent existing subway tunnels,and in severe cases,the tunnel segment will crack and cause bolt deformation,which threatens the safe operation of trains,and how to better evaluate the impact of old pile removal on adjacent existing subway tunnels is one of the difficulties in current research.Relying on the old pile removal project of an adjacent existing subway tunnel in Shanghai,this paper uses a combination of on-site measurement,theoretical analysis and numerical simulation to study the calculation method and influence law of the deformation caused by the removal of old pile in the adjacent existing subway tunnel.The research results can provide theoretical support for evaluating the impact of old pile removal on the safe operation of adjacent existing tunnels,and provide reference for similar engineering projects.The main research work carried out in this paper is as follows:（1）In order to derive the influence of old pile removal on adjacent existing tunnels,this paper analyzes according to the two-stage theoretical research method.Firstly,based on the Mintlin solution,the additional load caused by the removal of the pile foundation at the adjacent tunnel is obtained.Secondly,considering the influence of pile side friction resistance and groundwater,the additional stress is calculated and superimposed according to the modified Geddes stress solution and superstatic pore water pressure.Then,considering the shear layer parameters,the existing tunnel is simplified to the Euler-Bernoulli beam on the Pasternak foundation,and finally a simplified calculation model is obtained for the deformation of the adjacent existing tunnel caused by the removal of old piles.（2）Based on the analytical solution of monopiles,the calculation principle under different pile distribution is derived,and the calculation method of deformation of adjacent existing subway tunnels after pile removal is given through the superposition principle.The calculation results are compared with the measured values to verify that the deformation law of the adjacent existing subway tunnel caused by the removal of old piles is obtained,and the parameters such as buried depth,pile diameter and shear stiffness of the tunnel are studied.（3）The static level automatic monitoring system based on magnetostrictive displacement sensing technology can carry out all-weather,high-efficiency and high-precision real-time monitoring of subway tunnels,and use this technology to study the influence of old pile removal on adjacent existing subway tunnels,which provides important measured data for theoretical analysis and numerical simulation.By calibrating and testing the magnetostrictive displacement sensor,the basic parameters such as linearity,basic error,return error and repeatability are obtained.At the same time,the uncertainty of the sensor measurement results is studied,which provides data support for the measurement accuracy of the monitoring system.（4）The FLAC^3D software was used to model and analyze the subway tunnel under the influence of old pile removal construction,and the contact surface unit was set between the pile soil,and the numerical simulation results were compared and analyzed with the measured values,which verified the accuracy of the simulation results.On this basis,the influence of parameters such as pile tunnel distance and tunnel equivalent bending stiffness on tunnel deformation is studied,and reasonable reinforcement measures are proposed to facilitate the construction of the project.