Study on Anti-Heave Stability and Deformation Characteristics of Narrow Foundation Pits in Ningbo Metro

［Objective］ Compared with wide foundation pits, the stress and displacement field changes of the soil at pit bottom and outside caused by narrow foundation pit excavation are smaller. The relevant calculation methods specified in current specifications rarely consider the influence of the foundation pit width, easily leading to conservative design of the retaining wall, causing waste. It is necessary to study the basal anti-heave stability and deformation characteristics of narrow foundation pits. ［Method］ Aiming at three kinds of typical subway foundation pits in Ningbo, the method specified in Shanghai′s Technical Codes for Foundation Pit Engineering and the improved method are used to calculate the basal anti-heave stability. Based on a comparison of the calculation results, optimization suggestions for the retaining wall depth are given. According to the foundation pit information of Ningbo Metro stations, the relationship between the foundation pit deformation and the basal anti-heave safety stability coefficient, and the influence of the foundation pit width on the foundation pit deformation are systematically analyzed. ［Result & Conclusion］ The curve of the basal anti-heave stability coefficient varies with the embedded depth in a segmented manner, with the turning point at the boundary between soft and hard soil layers; the wall depth optimization space is related to the embedded ratio, soil shear strength index and whether considering the foundation reinforcement; the average lateral displacement of the retaining wall and the average land subsidence outside the pit are relatively discrete; with the increase of the basal anti-heave stability coefficient, the upper limit of the foundation pit deformation tends to approach the lower limit, which is mainly related to the engineering geological conditions; the soil deformation value and the influence range decrease with the narrowing of foundation pit width, while showing little correlation with the embedded depth.