Research and Prospects Regarding the Effect of Foundation Pit Excavation on an Adjacent Existing Tunnel in Soft Soil

With the rapid development of construction on subway tunnels, a large number of foundation pit projects adjacent to existing metro lines have emerged. Excavation of a foundation pit would inevitably change the original stress field and displacement field of the soil body, causing additional deformation to and internal force on an adjacent existing subway tunnel. To fully understand the research progress on the impact of foundation pit excavation on existing tunnels in soft soil, the current research status of adjacent construction problems of foundation pits in soft soil is explained through four aspects: theoretical research, model testing, numerical simulation, and actual results. The results show that the theoretical research at this stage primarily begins from the two-stage method, taking into account the different emphases on research factors and simplified conditions. Model tests include centrifugal model tests and constant gravity model tests, which can be used as an auxiliary research method to verify other research methods. Numerical simulation, which has the characteristics of comprehensive analyses and intuitive results, has been widely used in the design and evaluation of engineering projects. According to the relative position relationship between the foundation pit and the tunnel, the actual measured data of various projects are sorted and analyzed. It is proposed that a three-dimensional unloading coefficient, considering the unloading volume of the foundation pit, shape factor, tunnel depth, and horizontal clearance, can better represent the regularity of the deformation of the adjacent tunnel caused by excavation of the foundation pit. There is a certain linkage relationship between the deep displacement of soil around the foundation pit, deformation of the retaining structure, and deformation of the adjacent subway tunnel. At the same time, it summarizes and analyzes the risk evaluation and impact zoning system are summarized and analyzed, and application examples of control technology and monitoring methods are explored, which can utilize construction experience and provide a practical basis for risk control of on-site engineering. The shortcomings of existing research and the areas that require further discussion are mentioned. It is suggested that in-depth research on the space-time effects of foundation pit excavation in multiple dimensions adjacent to existing tunnel facilities, the applicability of constitutive models, the diversified and refined modeling of structures, the effects of dewatering of the foundation pit, and the seepage of groundwater should be taken into account. This will lead to the promotion of research on safety risk assessment of dynamic construction and impact zoning, development of innovative control and protection technologies, and establishment of a new type of monitoring technology system that is linked and shared. © 2021, Editorial Department of China Journal of Highway and Transport. All right reserved.