Surface Deformation Characteristics and Influencing Factors in Deep Foundation Pit Excavations for Subway Projects in Ningbo’s Soft Soil Area

The development of underground spaces is crucial for modern urban environments, particularly in coastal cities with prevalent soft soil conditions. Deep foundation excavation works in such areas present technical challenges due to complex deformation phenomena including soil settlement and the lateral displacement of supporting structures. This study analyzes deformation patterns associated with deep foundation pit excavations in Ningbo’s soft soil areas by examining 10 cases of subway station projects. This study evaluated the relationship between the maximum surface settlement and various engineering parameters using statistical and comparative analyses and also compared the results of each relationship with those of other regional studies. The results indicate that multiple coupled parameters—the excavation depth, diaphragm-wall-embedded depth ratio, support system stiffness, and pit aspect ratio—significantly shape the deformation patterns. The average ratio of the maximum surface settlement to the excavation depth is 0.64%, notably higher than in regions such as Hangzhou and Shanghai. The maximum lateral displacement in this study averaged 0.37% of the excavation depth. The maximum lateral displacement of the diaphragm walls in this study averaged 0.37% of the depth of excavation and, in addition, the average positive correlation between the depth at which the maximum lateral displacement occurred and the depth of pit excavation was hδhmax=He + 1.46. A positive correlation also emerged between the maximum ground settlement and lateral displacement of the diaphragm walls. But the influence of the shape of the pit on the deformation will show different types of relationships depending on the area and geotechnical conditions, which need to be further investigated.