Research On Influence Of Excavation On Existing Underground Metro Tunnels In Xi’an

With the growth of urban population,traffic congestion is becoming more and more serious.Making full use of underground space can effectively solve the problem of traffic congestion.However,with the continuous development of urban subway lines,new projects adjacent to existing metro tunnels often appear,and are bound to face the engineering dilemma of the influence of excavation on existing metro tunnels.When the excavation project is located above the existing metro tunnel,the surrounding stress is released due to earthwork excavation,which will affect the tunnel structure in different degrees.Therefore,how to accurately estimate the influence of excavation on existing underground tunnels is an urgent problem to be solved.Based on this background,this paper combines theory with practice and conducts a systematic study.The main research contents and methods are summarized as follows:(1)Theoretical analysis: The shield tunnel is simplified into a beam model placed on the Winkler foundation,and the calculation formula of longitudinal displacement,bending moment and shear force of the tunnel under the unloading stress of excavation is derived,and the corresponding program is written by Matlab for calculation.The accuracy of the proposed method is verified by comparing the results of other methods with that of a project close to the subway excavation.Further,on the basis of the engineering example,parameters such as width,length,depth,vertical distance between excavation and tunnel,soil Poisson’s ratio,tunnel bending stiffness,coefficient of foundation bed are changed to calculate and analyze the influence of the tunnel’s vertical displacement,bending moment and shear force under different parameters.(2)Based on the concept of residual stress,taking the measured earth pressure in the passive area of Xi ’an excavation as the boundary condition of Mindlin solution,the initial residual stress coefficient of Xi ’an area is invertively deduced as 0.45.The unloading ratio R,which reflects the influence of unloading quantity,and the residual stress coefficient,which reflects the influence of initial stress level,are defined.The functional relationship between the two is established.Combined with Matlab program,the calculation method of tunnel deformation caused by excavation in Xi ’an area is proposed when considering residual stress.The accuracy of this method is proved by comparing with other methods and measured data of a project in Xi ’an.(3)Taking the excavation under a typical geological condition in Xi ’an area as the excavation,assuming that there is a metro tunnel under the excavation,the finite element software is used to establish the analysis model of the influence of excavation on the tunnel.The numerical simulation is compared with the theoretical analysis results,which verifies the correctness and rationality of the theoretical analysis.By changing the splicing mode of tunnel segments,the insertion ratio of envelope structure and the elastic modulus of surrounding soil,the vertical and horizontal displacement of tunnel under different parameters are studied.