Lessons and Mitigation Measures Learned from One Deep Excavation Failure Case

Deep excavations, most of those are temporary retaining structures, have not attracted enough attention from all parties involved in projects. Nowadays there have been numerous failures of deep excavations, and mostly have significant impacts on the safety of the excavation and the surrounding environment, especially in congested urban areas with poor hydrogeology and geology conditions. However, many failure cases remained unpublished and poorly explored due to complex reasons. The reported failure cases were too scarce to provide enough lessons to improve corresponding guidelines. Therefore the database needs more supplement cases. This paper briefly presents the failed case of a deep excavation in soft soil area in downtown city of Shanghai, China, which had an excavation area of about 25,000 m² and two-story basements. According to the field investigation, forensic studies and preliminary analysis, the main reasons for the failure was pointed out. Furthermore the re-checking calculations and mitigation measures were described briefly. The direct cost of repairing and rebuilding the failed excavation was more than 20 million RMB (about 2.8 million dollars), much more expensive than savings. Preserving some redundancy in design and construction is essential. This failed case report is expected to help deep excavations attract more attention and avoid similar failures.