Abstract: The ongoing urban subway construction has led to an increase in foundation pits near existing underground structures. However, there remains a lack of systematic research on the soil instability modes in adjacent foundation pit engineering. This study employs numerical analysis to investigate the instability modes of asymmetric / symmetric limited soil in the adjacent foundation pit project near an existing subway station. It proposes applicable conditions under various instability modes and analyzes the impact of internal support quantity, foundation pit depth, and spatial relationship between the foundation pit and the existing subway station on soil stability and slip surface properties. The research shows that: 1) Adjacent foundation pit engineering exhibits three to six cases of symmetric or asymmetric limited soil instability modes. 2) As the number of internal supports increases, the slip body’s range expands, the starting position shifts downward in the retaining structure and the safety factor of limited soil stability gradually rises. 3) The increase of depth in the foundation pit influences with the sliding body range, initially increasing and then decreasing, leading to a gradual decrease in the safety factor of finite soil stability. 4) Analysis of the asymmetric slip surface in adjacent foundation pit projects indicates that as the overburden soil thickness of the existing subway station increases, the approaching range of the slip surface near the station decreases gradually, and the starting position shifts outward. This research provides a theoretical foundation for calculating limited soil earth pressure, designing foundation pit retaining structures, and selecting safety control measures for existing subway stations.

Key words: foundation pit engineering, adjacent additional construction, limited soil, slip plane, factor of safety, failure mode