Deformation Law and Spatial Effect of Deep Foundation Pits for Subway Construction in Soil-Rock Composite Strata in Seasonally Frozen Areas

The stability and safety of metro deep foundation pit in the soil-rock composite stratum in the seasonally frozen area are the key issues in the design and construction of the foundation pit. In order to ensure the soil-rock composite stratum in the seasonally frozen area, the deformation of the supporting structure is within the safe range when the deep foundation pit is excavated. At the same time, it will reduce the impact of the construction of metro deep foundation pit on the surrounding buildings. During the construction of foundation pit, close monitoring shall be carried out and the actual monitoring data shall be studied and analyzed. During the excavation of each layer of the foundation pit, monitor the displacement change of the foundation pit support structure and the settlement change of the surrounding ground surface, study the deformation law of the subway deep foundation pit in the soil-rock composite layer, and discuss the spatial effect of the foundation pit excavation in the seasonally frozen area. This paper mainly takes the deep foundation pit project of Anxin Road Station of Changchun Metro Line 5 as the research object, and analyzes the actual monitoring results. The results show that: (1) With the long side of the foundation pit, the deformation of the retaining structure of the foundation pit increases with the distance from the pit corner, and the middle of the long side of the foundation pit is less constrained by the retaining structure. (2) During the excavation of soil-rock composite foundation pit in seasonally frozen soil area, the pile displacement, axial force of support and uplift of support structure caused by rock excavation are relatively small, while the lateral displacement of foundation pit below the rock surface and thawing settlement deformation of soil are basically unchanged. The maximum lateral displacement of the foundation pit at the upper part of the rock surface moves upward. During the construction of deep foundation pit, there is a large displacement before the erection of the third layer of steel support. Attention should be paid to the soil properties of the deep silty clay layer, as well as to the rapid construction and synchronous maintenance.