Protection of Low-Strength Shallow-Founded Buildings Around Deep Excavation: A Case Study in the Yangtze River Soft Soil Area

The extensive development of urban underground space increases the risk of deformation to adjacent structures during deep excavations. This study investigates the response of three low-strength strip-foundation buildings (#4, #8, and #11 of the Ninggong Apartment) in Nanjing, China, affected by the excavation of an adjacent super-long, narrow subway station. The site is located in a typical soft alluvial area of the Yangtze River, characterized by highly compressible and sensitive soil, which poses substantial challenges. Pre-construction ground improvement was implemented to mitigate the impacts of diaphragm wall trenching; however, monitoring data indicated that buildings’ settlements of this stage still reached 28.2%, 24.8%, and 27.2% of their final values, with extensive influence zones. Subsequent excavation of the eastern and middle sections induced further cumulative and differential settlements, raising safety concerns and necessitating structural strengthening before adjacent western excavation. An integrated underpinning system, combining anchor static pressure steel pipe piles with a raft foundation, was adopted. Although short-term settlement increased during pile and raft installation, post-strengthening settlement rates decreased significantly. The adjacent western excavation caused only 13.3% of the settlement to be observed during the middle section’s excavation. All buildings were ultimately protected from excessive deformation. The protective strategies and lessons learned provide practical guidance for similar projects.