Design, research and practice of foundation pit of five-story ultra-deep station in soft soil with high water content under complex environmental conditions

Combining the case of Xiawafang Station on Tianjin Metro Line 8—the deepest metro station in soft soil water-rich areas in China at that time—this project fully considers the operational requirements of existing lines and formulates an overall implementation plan for the station foundation pit. Special attention is paid to the adjacent old residential buildings with shallow foundations, leading to the proposal of a micro-deformation control technology scheme for ultra-deep foundation pits. The design, research, and practical implementation of a five-level underground ultra-deep station foundation pit are successfully completed. Research results show that: By deeply integrating temporary air ducts with permanent ancillary structures, the construction of the transfer hub station achieves non-disruptive renovation of existing operational lines; introducing the pit width parameter and adopting a circular arc sliding method considering width for basal heave stability verification significantly optimizes the retaining wall's embedment depth, substantially reduced project costs, and effectively mitigates the adverse effects of deep diaphragm wall trenching on surrounding structures; through the practice of Xiawafang Station's 40-meter ultra-deep foundation pit, despite some deformation monitoring results of the retaining wall and adjacent buildings exceeding standard limits, the overall project implementation remains reasonable and controllable under proper execution and targeted deformation control measures.This study provides valuable references for the design and construction of ultra-deep station foundation pits in soft soil water-rich regions.