| With the mass construction of urban metro,more and more deep foundation pit projects are located above existing metro tunnels.Due to the strict deformation control of metro tunnels,the excavation of foundation pit will inevitably have a certain impact on the deformation of tunnels.Therefore,in order to ensure the safety of metro tunnels,it is necessary to study the influence of foundation pit excavation on existing metro tunnels.Based on the foundation pit project of underlying passage construction in Qingdao Meiling East Road,this paper uses FLAC3D to analyze the influence of foundation pit excavation on the horizontal displacement of the retaining structure of foundation pit and the surface settlement outside the pit,and compares it with this project monitoring data to preliminarily verify the rationality of the parameters used in numerical simulation.Then,the variation law of displacement and stress of lining structure of existing metro tunnel caused by foundation pit excavation pit is analyzed.The correctness of finite element calculation results is verified by comparing the theoretical calculation results of tunnel vertical displacement with the numerical simulation results.Finally,the influence of tunnel depth,distance between tunnel center and foundation pit center,shape of foundation pit excavation,excavation method and foundation reinforcement on tunnel vertical deformation is analyzed by single variable method.The main conclusions are as follows:(1)With the progress of foundation pit construction,the horizontal and vertical displacement of tunnel lining structure increases,and the vertical deformation is more affected by the foundation pit excavation than the horizontal deformation.After the foundation pit excavation,the overall tunnel deformation presents the uplifting deformation trend towards the foundation pit direction,and the cross-section shows the horizontal compression and vertical tension of the "ellipse" plane deformation of the foundation pit opening.Excavation has little effect on the stress of tunnel lining structure,and no obvious phenomenon of additional stress concentration occurs.Lining structure is not easy to be damaged,which meets the strength requirements.(2)The vertical displacement of tunnel lining structure decreases with the increase of tunnel depth and the distance between tunnel center and foundation pit center.Through theoretical calculation formula,the variation law of additional stress produced by excavation of foundation pit is calculated,which changes with the distance between tunnel center axis and foundation pit center and the depth of tunnel vault,and the corresponding fitting curve formula is made,so as to provide guidance for similar construction.(3)Under the same excavation area,the vertical displacement of the tunnel can be reduced by choosing the excavation shape of the foundation pit whose length is perpendicular to the long side of the foundation pit;the maximum vertical displacement of the tunnel can be reduced by about 14.4%compared with that of the conventional layered excavation;the sooner foundation reinforcement is carried out,the smaller the rebound of the foundation pit and the uplift deformation of the tunnel caused by the excavation of the foundation pit will be,and the impact on the underlying tunnel will be smaller. |
