Analysis And Research On The Influence Of Metro Deep Foundation Pit Excavation On The Operating Tunnel Underneath

The Urban rail transit is an inevitable product of urban development.It effectively solves the travel problems of urban residents and the intensive development of limited urban space,and provides a safe,comfortable and punctual travel experience.However,as a linear project,urban rail transit crosses a wide range of urban areas.In the central urban area with advanced economic development,it is inevitable to cross,parallel or cross with other buildings.In order to obtain development space in the urban area with dense buildings,the underground space of the city continues to develop"downward and vertical",which will inevitably lead to the excavation and disturbance of soil.Limited to the current situation that the rail transit line and other underground spaces in the city cannot be planned,constructed and operated at the same time,there are more and more cases of excavation of new foundation pit above the operating tunnel.Based on the foundation pit excavation project of hua-shan-he station of Wuhan rail transit,the subway operating tunnel of line 19 is below the subway foundation pit.Using the method of combining on-site monitoring and finite element analysis,this paper systematically analyzes the stress and deformation impact of the excavation and unloading of the deep foundation pit of the upper subway on the operating subway tunnel below.The principal research methods and conclusions are as below:(1)By establishing the three-dimensional solid model of the upper foundation pit-operating subway-soil,the influence of the excavation and unloading process of the subway deep foundation pit on the stress and uplift deformation of the tunnel is studied.The research shows that different from the plane strain assumption,the three-dimensional modeling calculation results show that the stress and strain of the tunnel have certain changes in the longitudinal direction;The influence range of the upper foundation pit excavation on the tunnel stress is mainly concentrated in the excavation range of the foundation pit.The vault stress of the tunnel outside the excavation range of the foundation pit is small,and the vault stress of the tunnel along its longitudinal direction increases with the increase of the excavation depth;The heave of the tunnel is the largest at the midpoint of the foundation pit,and the heave decreases gradually with the increase of distance.The influence range of excavation and unloading on the heave of the tunnel is about 1.0b from the center of the foundation pit(B is the excavation width of the foundation pit);In the cross section direction,the tunnel shows the deformation law of vertical elongation and horizontal compression,and the vault position is easier to crack than other areas.(2)By establishing the three-dimensional solid model of the upper foundation pit-operating subway-soil,the effects of different tunnel embedded depth on the stress and uplift deformation of the tunnel are studied.The research shows that the stress of the tunnel changes greatly within the excavation range of the foundation pit,which is the influence range of the unloading effect of the foundation pit on the stress of the tunnel,while the vault stress of the tunnel outside the foundation pit is small;With the increase of the buried depth of the tunnel,the arch crown stress of the tunnel decreases along its longitudinal direction;As the tunnel depth increases,the influence of the upper excavation and unloading on the tunnel deformation is nonlinearly weakened.It is suggested that in the actual engineering construction,the net distance between the bottom of the foundation pit and the tunnel vault should be guaranteed as much as possible,and the buried depth B should be greater than one time of the tunnel diameter.(3)By establishing the three-dimensional solid model of the upper foundation pit-operating subway-soil,the effects of different length width ratio on the stress and uplift deformation of the tunnel are studied.The research shows that when the length width ratio L/B≥2.0 and the normalized excavation width B/H_e≥2.0,the tunnel uplift caused by the excavation of the deep foundation pit of the upper subway exceeds the early warning value of the tunnel deformation control standard in the national specification technical code for safety protection of urban rail transit by 10mm;When the length width ratio L/B≥4.0 and the normalized excavation width B/H_e≥4.0,the maximum tunnel uplift exceeds the standard control value of tunnel deformation in the national specification technical code for safety protection of urban rail transit by 20mm.This means that the upper foundation pit with smaller length width ratio L/B has less adverse impact on the operating tunnel below,and vice versa.In the design of metro foundation pit,the short side of the upper deep foundation pit shall be arranged in parallel with the operating tunnel as far as possible,and a small length width ratio L/B shall be adopted to reduce the adverse impact of excavation and unloading on the tunnel.(4)The field monitoring shows that due to the excavation unloading,the vertical displacement of the arch crown in the tunnel presents an upward convex distribution.The tunnel uplift at the center of the foundation pit is the largest,and the tunnel uplift at both ends of the foundation pit is the smallest.The tunnel deformation monitoring data is in good agreement with the simulation results.