Study Of Influence On Deformation Of The Subway Tunnel Induced By Foundation Excavation

With the development and utilization of unban underground space on a large scale in our country, more and more foundation pits located above the existing subway tunnel.The construction and excavation of foundation pit would change the stress field and displacement field of the surrounding soil, witch will bring additional internal force and deformation to the underlying subway tunnel. What’s more, it may affect the operation safety of subway system. Thus, the impact and control on the deformation of the underlying subway tunnel caused by foundation pit excavation have alreadly become important issues encountered in the current development of underground space. In order to solve these problems, methods of engineering analogy and investigation, theoretical and analytical analysis, numerical simulation analysis are adopted in this paper. The main contents and conclusions of this paper are shown as follow:1. The deformation mechanism of the underlying subway tunnel caused by foundation pit excavation has been analyzed theoretically.15 cases of the foundation pit instance located above the existing subway tunnel are collected to make a statistical analysis of the deformation features of tunnels. The results show that the vertical displacements of the tunnel are all uplift, all the statistical cases have monitored the vertical displacements of the tunnel, and more than 50% of the uplift values reach or exceed the alarm value (10 mm). The horizontal displacements of the tunnel are have been monitored in rare cases, the monitoring data are all small and change between positive and negative numbers in disorder, and they have no obvious regular changes. The convergence deformations of the tunnel are also have been monitored in rare cases, the convergence deformations of the tunnel show that horizontal compression and vertical stretching, the cross section of the tunnel shows to be vertical ellipse.2. Two-stage method is adopted to analyze the longitudinal deformation of the underlying tunnel caused by foundation pit excavation. Not only the soil unloading effects of the pit bottom and the surrounding walls induced by the foundation pit excavation, but also the interaction between the tunnel and surrounding soil can be considered by this method. This method is in fairly good agreement with the practical engineering. Firstly, the additional stress induced by the foundation pit excavation is calculated by Mindlin fundamental solution. Then the tunnel is simplified as an infinite beam on a elastic foundation, and the governing differential equation of the tunnel is formulated based on Winkler model, vertical displacements and internal forces along the longitudinal of the tunnel are obtained.In addition, some influencing factors of the tunnel’s deformation and internal force is analyzed, such as, excavation area, excavation depth, soil condition, the diatance between the pit axis and the tunnel axis, buried depth of tunnel, tunnel stiffness and the outer diameter of the tunnel. Finally, the method is applied in ptactical engineering, the analysis result is compared with that from 3D numerical simulation and field monitoring data, and a good agreement is obtained.3. Numerical simulation method is adopted to analyze the cross section deformation of the underlying tunnel caused by foundation pit excavation. The results show that the foundation pit excavation induce the cross section of the tunnel generates uplift displacement, horizontal diaplacement and convergence deformation. The convergence deformation of the cross section shows that horizontal compression and vertical stretching, the cross section of the tunnel shows to be vertical ellipse.4. Based on the rapid transformation engineering of Guimiao Road in Shenzhen, the foundation pits of the Road Tunnel located above the existing tunnel of Shenzhen Metro line 11.3D numerical simulation method is adopted to analyze some construction measures and their control effects of the tunnel deformation induced by the excavation above, such as soil reinforcement, uplift pile and heaped load.The results show that soil reinforcement has no obvious effect on the control of the tunnel deformation, uplift pile and heaped load have comparatively remarkable effect on the control of the tunnel deformation. When using various construction measures, the deformation of the tunnel is reduced obviously.