Deformation characteristics of foundation pit of five-story ultra-deep subway station in soft soil area

With the in-depth development of urban rail transit, although there has been extensive research on the deformation of foundation pits in soft soil areas, there are relatively few studies on the deformation law of foundation pits under the ultra-deep reverse construction method. Based on a case of a 38.3 m ultra-deep metro foundation pit project in Tianjin, this paper provides an in-depth analysis of various deformation characteristics. The conclusions are as follows: ① Compared with ordinary foundation pits, the lateral displacement of the retaining walls of ultra-deep foundation pits in complex environments presents multiple patterns. The long side walls all present an "inwardly convex" deformation. The short side walls present various deformation modes such as "cantilever shape" and "S shape", while the wall adjacent to the existing foundation pit presents an "outward protruding type", simultaneously generating a torsional deformation of 2.167‰. ② The vertical displacement range of the top of the wall is-0.022% to 0.037%H_e (H_e is the excavation depth). During the shallow excavation stage, affected by its own weight, the wall shows a sinking trend. During the deep excavation stage, the rebound of soil during excavation unloading plays a dominant role, resulting in the floating of the soil on the wall. ③ The average rebound of the column is 3.48 mm, ranging from-0.001% to 0.030%H_e, which is 55% to 80% smaller than that of the traditional forward method. ④ The construction of the surface settlement model of the ultra-deep foundation pit shows that the maximum settlement location is 1/3 H_e away from the wall. The main and secondary affected areas are 3H_e and 5H_e away from the foundation pit respectively. Moreover, the settlement of shallow foundation buildings is greater than that of deep foundation buildings due to the greater flexibility of the base plate. The research results of this paper can provide important reference for similar ultra-deep projects.