Study On Construction Mechanical Effect Of Overlapping Metro Tunnel With Small Interval

With the vast construction of urban rail transit,there is more and more construction of overlapping metro tunnel with small interval in urban prosperous areas because of limit of narrow topography,complex environment and other factors.Two holes of overlapping metro tunnel interact with each other greatly with the change of interval and spatial position.Construction of tunnel disturbs surrounding rock many times and change law of surrounding rock stress and deformation is different from that of separated tunnel bringing engineering design and construction much risk.So ti is necessary to study construction mechanical effect of overlapping metro tunnel with small interval.Er Lang station to Hua Long station metro tunnel of loop line in Chongqing rail transit is separated tunnel firstly and gradually becomes overlapping tunnel with overlapping period 98 meters long in total.Overlapping tunnel is divided into two periods according to interval,one period is type BI whose interval is 3.6 meters to 6.5 meters,the other period is type BⅡwhose interval is 0.8 meter to 3.6 meters.The paper is based on practical engineering and applies theoretical calculation,numerical simulation analysis and monitoring measurement to deeply study construction mechanical effect of overlapping metro tunnel with small interval.The research results can be used for reference of design and construction by similar projects.Choose typical cross section of type BI tunnel and simplify it to double circular deep-buried holes in half-infinite plane for theoretical calculating.Use Schwarz alternate method to transfer multiply connected domain to simple connected domain.Cauchy integral method can be applied to gain two complex stress function through 1.5 times iterative calculation and solve theoretical solution σ_x 、σ_y and τ_xy of feature points on edge of double circular holes.Establish two-dimensional numerical model of cross section same as theoretical calculating,then extract numerical solution σ_x 、σ_y and τ_xy of feature points on edge of double circular holes.Finally contrast the theoretical and numerical solution to obtain conclusion.The conclusion shows that theoretical and numerical solution matches well and their change rule almost fit.Choose cross section which the interval is the smallest of BⅡtunnel and simplify it to single tilted elliptic tunnel in half-infinite plane to perform theoretical calculation.Two complex stress function of the plane can be obtained through conformal transformation and Cauchy integral method and theoretical solution σ_x 、σ_y and τ_xy of feature points on edge of single tilted elliptic tunnel can be solved out according to the complex stress function.Establish two-dimensional numerical model of cross section same as theoretical calculating and extract numerical solutionσ _x 、σ_y and τ_xy of feature points on edge of single tilted elliptic tunnel.Finally contrast the theoretical and numerical solution to obtain conclusion.It shows that theoretical and numerical solution matches well and varies periodically with the cycle of π.Establish three-dimensional numerical model through finite element software Midas GTS to study construction sequence of overlapping tunnel with small interval.Comprehensively analyze and contrast surrounding rock stress,displacement and plastic zone of two construction sequence numerical simulation calculation to select safe and reasonable construction sequence.The conclusion shows that it is reasonable to excavated left hole firstly.Establish three-dimensional finite element model to study excavation step distance of left and right hole.Comprehensively analyze and contrast surrounding rock stress,displacement and plastic zone of four excavation step distance 0.5m,1m,1.5m and 2m to select economic and safe excavation step distance.Excavation step distance of Left and right hole is 2 meters for type BI tunnel.Excavation step distance of Left and right hole is 1.5 meters and 1.0 meter separately for type BⅡtunnel.Establish three-dimensional finite element model to study excavating dynamic effect of overlapping metro tunnel with small interval.Excavating dynamic effect can be obtained after analyzing surrounding rock stress and displacement changing law of four selected cross section of left hole.Finally contrast numerical and monitoring results to verify the conclusion and propose advice for practical construction.