Study On The Influence Of Foundation Pit Excavation On The Structure Stress And Deformation Of Shield Tunnel Shaft

With the rapid development of urban comprehensive pipe corridor,rail transit and underground space in our country,the number of shaft is more and more.The research on the influence of foundation pit excavation on the shaft stress and deformation of adjacent shield tunnel has not been found out.In this paper,the unloading effect of the foundation pit bottom and the side wall and the barrier effect of the bracing system of the foundation pit are considered,and the dimension of the equivalent load on the soil at the bottom of the bracing structure is modified uniformly,the theoretical model and calculation formula of additional stress produced by the shaft of shield tunnel under the unloading effect of excavation of foundation pit are established,the length A and width B of the excavation,the depth h of the excavation,the minimum distance m between the excavation and the shaft,the floating weight γ and the stress loss rate β were changed and Sensitivity analysis curves were drawn based on the control variable method.The displacement,bending moment and shear force at the free end and the joint of the shaft tunnel are solved by the load superposition method and the semi-analytical and semiempirical method.Finally,the 3D finite element Midas GTS NX software is used to simulate the shield tunnel shaft located beside and below the foundation pit,and the simulation results are compared with the theoretical values.The main research contents are as follows:(1)When the foundation pit is located at the side or under the tunnel shaft,the unloading effect of the shaft bottom is greater than that of the side wall.In the horizontal direction,the vertical shaft is subjected to the pulling force near the side of the foundation pit,and there is a turning point below the surface where the vertical stress changes from upward to downward,indicating that under the action of the unloading of the foundation pit,there is a risk of pulling out of joint in shaft joint structure.(2)Under the unloading action of Foundation Pit,the force on the shaft increases along the depth,and the force on the bottom of the shaft is the largest.From the analysis of the horizontal additional stress,with the increase of the distance M between the side wall and the shaft,the horizontal additional stress of the shaft will be smaller,and the direction of the stress points to the pit At the same time,with the increase of excavation depth h,floating weight,excavation length a,excavation width B,and stress release coefficient β,the horizontal additional stress on the shaft increases,and the stress direction also points to the excavation.According to the analysis of vertical additional stress,with the increase of the minimum distance M between the shaft and the side wall of the foundation pit,the vertical additional stress of the shaft will decrease,with the increase of excavation depth h,floating weight,excavation length a,excavation width b and stress release coefficient β,the vertical additional stress will increase.(3)When the tunnel is located at the side of the foundation pit,the distance between the foundation pit and the shaft is the most sensitive to the influence of the excavation unloading on the force of the shaft structure,and the excavation depth is the second.And when the tunnel is located directly below the foundation pit,the horizontal additional stress sensitivity of the shaft is the largest in the depth of the foundation pit excavation,followed by the width of the foundation pit excavation,but the vertical additional stress sensitivity of the shaft is the largest is the spacing,followed by the depth of excavation.(4)Because of the trapezoid distribution of the horizontal additional load along the height,the t-shaped tubular structure composed of the shaft and the horizontal shield tunnel can be equivalent to the cantilever beam structure,the trapezoidal load distribution can be decomposed into a triangular distribution and a rectangular distribution,and their bending moment equations and shear equations can be established.Based on the bending moment equation,an approximate deflection differential equation is established,and the horizontal displacements at the free ends of shaft and tunnel are calculated.(5)Because the vertical additional stress is small,the vertical displacement of the shaft is small and can be neglected.It can be generally considered that the vertical displacement of the shaft joint of the tunnel comes from the vertical displacement of the shield tunnel under the unloading condition of the excavation of the foundation pit.Based on the Mindlin solution and superposition principle,the horizontal and vertical displacements of the external surface of the tunnel(near the foundation pit)caused by the unloading of the bottom of the foundation pit and the four side walls of the foundation pit are calculated,and the maximum vertical displacements are obtained at the center of the shaft(the joint between the shaft and the tunnel),showing the characteristics of a normal distribution.(6)When the foundation pit is located above the tunnel shaft,the shaft is not in the center of the Foundation Pit,which is easy to cause the rotation of the horizontal tunnel and produce the horizontal displacement of the shaft,therefore,the horizontal displacement when the foundation pit is located above the horizontal tunnel shaft is greater than the horizontal displacement when the foundation pit is located beside the horizontal tunnel shaft.