Study On Deformation Mechanism And Grouting Control Of Bridge Pile Foundations Induced By Adjacent Shield Tunnelling

With the remarkable improvement of the domestic economic level,people’s requirements for improving urban traffic service function is increasing.A growing demand on infrastructure is competing for the urban surface area,but also the subsurface,which makes shield tunneling activities inevitably pass through lots of pile foundations at close distance in densely urban area.Even the most advanced shield technology,it will easily cause surface settlement,pile inclination,and discontinuous deformation in the construction section,which may affect adjacent pile foundations,even threaten the safe of superstructure.Therefore,an accurate assessment of potential risk and corresponding protection measures of tunneling-induced pile displacement has emerged as a significant technical concern in shield tunneling.Two cases were found in the project of interval tunnels from Wanjiali Square Station to Furong District Government Station and from South Gaoqiao Station to Guitang Station of Changsha Metro Line 5.A Two-Stage Analysis Method(TSAM)was adopted to improve the shortcomings of the existing theory.The analytical solutions of three-dimensional soil and pile displacement were established.Based on the fluid-soil coupling theory,the displacement and stress variation process of adjacent pile induced by shield tunneling were analyzed.The optimal grouting depth and optimized grouting form to control the adjacent pile displacement were proposed.The main contents include the following aspects:(1)The working principle of shield tunneling machine and the stratum disturbance zone induced by shield tunneling were illustrated.The main factors affecting the displacement of surrounding soil during shield tunneling were also analyzed.In the first stage,based on the Mindlin’s solution in the semi-infinite space,the analytical solution of the three-dimensional soil displacement field under the action of multiple construction factors was derived.The analytical solution was verified by comparing with the field measured data.(2)In the second stage,by comparing the advantages and disadvantages of various foundation models,based on the variational method and the principle of minimum potential energy,the longitudinal and horizontal lateral displacement control equations of single long pile in Pasternak’s foundation were derived.The finite difference form was used to extend the analytical solution of three-dimensional pile displacement to layered foundation,and the pile stiffness matrixs under different boundary conditions were derived.It is verified that the Two-Stage Analysis Method proposed in this paper is in good agreement with the finite element results.(3)Three-dimensional finite element models were established by Midas GTS NX to analyze the trend of bridge pier settlement,seepage velocity of groundwater,ground surface settlement trough,longitudinal,horizontal and vertical displacement of the pile and pile stress under fluid-soil coupling.By comparing with the finite element calculation results and field measured data under different working conditions,it is found that the results based on fluid-soil coupling in water-rich stratum are closer to the actual situation than those without groundwater and with stable groundwater level.(4)The results indicate that grouting to 3 m below the tunnel bottom is the most suitable grouting depth.The inadequate grouting depth may aggravate the displacement of pile foundation.Excessive increase of grouting depth can only improve the displacement control effect of pile foundation,which will increase the cost of grouting and aggravate the initial stratum disturbance.On this basis,an "L" type optimized grouting form was proposed,which can effectively reduce the influence of shield thrust force on pile displacement.This paper presents a mechanism study of pile-soil-tunnel interaction.The deformation mechanism and grouting control of pile foundations induced by shield tunneling are studied by theoretical derivation,numerical analysis and field measurement,which provides reference for similar engineering construction and has broad engineering application prospect.