Study On Float Deformation And Segment Damage Evolution Of Existing Shield Tunnel Underneath Excavation

With the proposal of the 14th Five-Year Plan and the draft prospect goal outline of 2035 for the new operating mileage of urban rail transit and the total investment,the construction of urban rail transit in China has set off a new wave of construction climax,and the subway as a system of the highest proportion in rail transit,also occupies the largest proportion in the new rail transit project.Urban subway lines are often located in the core area of the city,some new subway or ground buildings are just above the existing lines.These new subways or ground buildings are involved in foundation pit excavation in the construction process.Therefore,the engineering cases of foundation pit excavation above the existing subway tunnel often appear in major cities.The excavation of the upper foundation pit may induce the local floating of the existing shield tunnel underneath the tunnel.The additional stress caused by the local floating lead to diseases such as segment dislocation and crack,which not only adversely affects the service performance of the existing tunnel,but also threatens the safety of the subway train.Therefore,it is of great theoretical significance and engineering practical value to study the floating deformation characteristics of existing shield tunnel induced by foundation pit excavation and the consequent damage law of segment,and reveal the floating deformation law and segment damage mechanism of existing shield tunnel induced by foundation pit excavation.Based on the National Natural Science Foundation of China(51878074),using numerical simulation technology,this paper systematically studies the mechanism of floating deformation and the distribution characteristics of segment damage in existing shield tunnels underneath excavation.The main research contents are as follows:(1)A three-dimensional finite element model of the newly built foundation pit excavation on the shield tunnel is constructed based on the project of Guangzhou Rail Transit Line 18 under the Panyu Avenue Station of Shenzhen Metro.By simulating the excavation unloading and supporting structure construction of the foundation pit,the settlement of the surrounding soil,the pore water pressure around the tunnel,the floating deformation of the shield segment and the damage deformation of the segment are obtained.(2)Based on the three-dimensional fine modeling technology,the three-dimensional fine numerical analysis model of the foundation pit excavation above the existing shield tunnel is constructed.By simulating the excavation unloading and supporting structure construction of the foundation pit,the deformation characteristics of the shield tunnel segment,the distribution law of the damage zone and the stress characteristics of the steel bar in the segment in the fine three-dimensional model are obtained.(3)Five influences of five factors,namely,the relative position between the foundation pit and the existing tunnel is orthogonal or oblique,the distance between the foundation pit and the existing tunnel,the excavation process of the foundation pit,the spatial position of the foundation pit and the tunnel,and the change of groundwater level,on the damage and deformation of the segment of the existing shield tunnel induced by the foundation pit excavation is studied.By analyzing the distribution characteristics of the floating deformation of the shield tunnel and the damage of the segment under the change of the relevant parameters in the five factors,the evolution law of the damage of the segment of the shield tunnel under the action of the above factors is obtained.(4)A three-dimensional numerical model of the deformation of the existing shield tunnel induced by the excavation of the tunnel is constructed considering the dewatering of the foundation pit,the pile load under the foundation pit and the local application of the bolt.By comparing the deformation and the distribution characteristics of the segment damage of the existing shield tunnel induced by the excavation of the foundation pit under different measures and parameters,the control effect of the relevant measures on the floating and segment damage of the existing shield tunnel induced by the excavation of the foundation pit is obtained,which provides an effective reference for the prevention and control of the floating of the existing shield tunnel induced by the excavation of the foundation pit.