Study On The Influence Of The Deformation Of The Underlying Shield Tunnel Caused By The Unloading Of The Foundation Pit And The Comparison Of The Effect Of Control Measures

At present,due to the continuous development of urban underground space,the existing underground shield tunnel is facing more and more security risks,and the construction of foundation pit above will inevitably have a certain impact on it.Therefore,we should not only meet the stability requirements of the foundation pit itself,but also focus on the deformation of the segment structure to prevent the risk of cracking and failure.Especially for the deformation sensitive soft soil area,it is of great practical significance to accurately predict the additional internal force and deformation of shield tunnel caused by foundation pit unloading and analyze the effect of different engineering measures to control deformation.In order to solve this problem,the following works are carried out by theoretical analysis and numerical simulation:(1)Under the unloading action of foundation pit,the influence of incomplete unloading at the bottom of foundation pit and retaining structure on unloading stress release of soil around foundation pit is comprehensively analyzed.The differential equation of tunnel deflection curve is established by using Mindlin basic solution and Pasternak model.Through the finite difference method and MATLAB programming,the displacement and internal force of each section of the tunnel are obtained.Compared with the results of the finite element case and the actual engineering monitoring,the reliability of the method is verified,and the sensitive factors affecting the tunnel deformation are analyzed.(2)Based on the modified Mohr-Coulomb(hardening soil)constitutive model,a threedimensional finite element model of the interaction between the stratum,the foundation pit support system and the tunnel structure is established.The loading and unloading modulus of the soil can be defined respectively,and the double hardening behavior of the soil in shear and compression can be reflected.The surface displacement outside the pit,horizontal displacement of retaining structure and longitudinal deformation of tunnel caused by foundation pit excavation are analyzed,and the model calculation results are compared with the empirical curve and engineering monitoring data.The results show that the modified Mohr-Coulomb model is suitable for analyzing the influence of foundation pit excavation on tunnel in soft soil area,and can provide model basis for similar projects.(3)Relying on the foundation pit project on the South Bank of Cross Gate in Hengqin District,Zhuhai City,this paper further studies the law and control effect of tunnel uplift deformation under different engineering measures,such as soil reinforcement,setting uplift piles and surcharge back pressure,through 39 finite element models.The results show that the reinforcement of soil at the bottom and outside of the foundation pit strengthens the connection with the retaining structure,which makes the retaining structure bear more soil unloading stress,so as to restrain the rebound at the bottom of the foundation pit and the uplift deformation of the tunnel;the reinforcement of both sides of the tunnel can increase the foundation stiffness and reduce the horizontal displacement of the tunnel;the setting of uplift piles can use the pile side friction to limit the soil deformation,The pile length and pile spacing should be selected according to different geological conditions and tunnel deformation requirements,and the increase of tunnel uplift deformation can be effectively controlled by surcharge back pressure.