Mechanical Behaviors Of Existing Large Diameter Shield Tunnel Due To New Tunnels Construction Below

In the process of rapid development of urban underground space,the integrated rail transit form dominated by rail transit construction has become an important part of underground space development in the future.With the progressive development of urban underground space,the phenomenon of new tunnel crossing the existing underground structure is widespread,making the mechanical response of the existing structure become the focus and difficulty of engineering construction research.In the crossing project,the intersection between the new tunnel and the existing tunnel is a major source of risk,and construction and excavation may lead to excessive deformation of the existing line,regional stress concentration,and affect the safety of the existing tunnel structure.Based on the shield tunneling project of Beijing Metro Line 12 under Tsinghua Park Tunnel,this paper conducts an in-depth study on the deformation prediction and internal force response of existing structures by theoretical analysis,numerical calculation and field measurement.The main results are as follows:(1)According to the field measured data,the internal forces and deformation rules of existing large-diameter shield tunnels under the influence of new tunnel construction are expounded.By analyzing the vertical time-history deformation characteristics of shield tunnel,the deformation curve of large-diameter shield tunnel can be roughly summarized into four stages: "advance settlement","grouting uplift","underpass construction" and "deformation stability".The joint displacement and ring bolt stress are analyzed and summarized,and it is found that the joint displacement and bolt stress are basically consistent.(2)Based on the principle of energy conservation,a calculation method is proposed to predict the formation settlement caused by new tunnel construction,and the influence of different parameters on surface settlement and plastic zone radius is studied.Compared with the traditional method,the proposed method can predict the formation deformation more accurately by considering the plastic area of soil layer and the different soil mechanics parameters of composite stratum.On this basis,the influence of underpass engineering on the existing structure is considered,and a theoretical calculation method for predicting the deformation of the existing structure is proposed.The results show that the deformation of the existing structure increases with the increase of the volume loss rate and decreases with the increase of the buried depth of the new tunnel and the increase of the stiffness and size of the existing structure.(3)Using refined simulation technology,three-dimensional numerical simulation analysis is carried out for large-diameter shield tunnels,revealing the internal forces and deformation rules of existing large-diameter shield tunnels under the influence of the undercrossing of newly-built tunnels.The settlement of the tunnel,the deformation of the joint between the connecting bolt and the pipe piece,and the variation of the force of the bolt under different construction stages are studied.The results show that the new tunnel has the greatest influence on the existing structure when it is constructed directly under the existing tunnel.