Study On The Influence Of Adjacent CFG Pile Composite Foundation Of Shield Tunnel Excavation

In recent years,with the rapid development of urban construction and the arrival of the upsurge of Metro construction,the number of engineering cases of shield tunnel passing through CFG pile composite foundation is increasing.How to accurately predict the deformation of composite foundation and take effective technical measures to control the deformation in the safe range is the core issue.However,the mechanism of the influence of shield tunnel excavation on adjacent CFG pile composite foundation is still unclear,and further research is urgently needed.Therefore,relying on the actual crossing section,this paper studies the settlement and deformation of shield tunneling through CFG pile composite foundation and the change of pile-soil internal force by using theoretical calculation method and finite element method,and puts forward corresponding prevention and reinforcement measures.The research conclusion is expected to provide theoretical basis and technical support for composite foundation design and shield tunneling control.This paper uses Midas GTS/NX software to carry out numerical simulation.After verifying its reliability through two-stage theoretical calculation,the mechanism of pilesoil deformation of CFG pile composite foundation caused by additional soil action caused by shield tunnel excavation is studied.This paper discusses the influence of shield tunnel excavation on CFG pile composite foundation from two aspects of CFG pile design parameters and shield excavation construction parameters,and puts forward corresponding solutions.The main conclusions are as follows:(1)The influence of shield tunneling soil on the internal force of composite foundation is small,and the stress of pile and surrounding soil decreases with a change rate of 3%.The stress of surrounding soil around tunnel lining increases with a change rate of16%.The above data show that the load transfer of the superstructure after shield tunneling transfers to the surrounding and the level of the tunnel lining.For the rigid piles without reinforcement,which need not be considered in the design stage,the shear resistance of the piles is weak,and the shear force at the end of the piles increased to 139 kN when shieldtunneling passes through the construction,so it is very likely that the shear failure of the piles will occur.Therefore,in the process of tunnel construction,technical measures such as soil grouting reinforcement and strengthening design parameters of lining segments can be taken to ensure the normal use of existing and existing structures.(2)The settlement deformation of CFG pile composite foundation mainly occurs after shield tunnel excavation.The horizontal settlement of pile body is W-shaped.The vertical settlement process of pile and soil can be divided into two stages: one is the instantaneous relative movement of pile and soil,the relative displacement is 1.5 mm;the other is the consolidation settlement process of composite foundation,which is consistent with the load transfer path before excavation.(3)In the study of design parameters of different composite foundation in shield tunnel excavation,it is found that: the diameter of pile has obvious influence on settlement of composite foundation;the change of pile length causes the change of relative position of tunnel excavation surface,which results in the sudden change of deformation and stress of composite foundation.When the length of pile exceeds 13 m,the change of pile stress is no longer obvious because of the effective length of pile;the change of pile spacing has significant influence on each evaluation index of composite foundation.When the pile spacing is more than 3 m,the ability of CFG pile composite foundation to resist additional deformation is basically lost;The cushion layer plays a key role in the coordination of stress and deformation before and after shield excavation,and the modulus of soil around piles has a significant response to the additional action of composite foundation,showing a smaller overall settlement deformation.The essence of the change of modulus of soil around piles in construction is the reinforcement measures of the soil above the tunnel.(4)By changing different excavation modes to establish different numerical models and doing relevant analysis,the following conclusions are drawn: with the decrease of net spacing between piles and tunnels,pile,soil settlement and pile end shear increase;the larger the diameter of tunnel excavation surface,the greater the stress release of soil,thegreater the impact on the stress and deformation of composite foundation;The settlement of CFG pile composite foundation under shield tunnels(net spacing less than 0.5 D)and large diameter(11 m)will exceed the allowable limit value.compared with other shield tunnels,side-crossing has less impact on composite foundation;The double-track excavation of composite foundation has more uniform settlement and stress than the excavation of one side at the same time,but the influence depth is greater.In view of the above adverse deformation effects,shield tunneling parameters can be adjusted appropriately in the actual construction process and corresponding engineering reinforcement measures can be taken to reduce the damage of shield tunneling to composite foundation and buildings.