Forensic Diagnosis of a Leaking Accident during Excavation

Construction of a subway station in Shanghai called for a 31.4–33.1–m-deep excavation, which extended through upper impermeable clayey strata into underlying erodible silt and sand aquifer layers. When the north section of this pit nearly bottomed out on July 23, 2013, a sudden outburst of groundwater took place and the pit base was flooded quickly. In the subsequent two days, the adjacent ground and building successively underwent 30–42-mm settlements in a plunging manner. To prevent aggravation of the situation, a contingency plan was initiated quickly in an attempt to seal off the water flow path. By constructing mixed-in-place and jet-grouting piles respectively along the outer and inner perimeters of the diaphragm wall where leakage occurred, flow channels were successfully blocked. Despite this, the building adjacent to the outburst of groundwater suffered long-term postexcavation settlement. Forensic diagnoses show that this through-wall leaking accident was related to two major factors: (1) ignorance of minor leakages emerging two weeks before without adopting any remedial measure; and (2) a flawed slab connector embedded in the confined aquifer layer. Based on visual inspection and retrospective analysis of field monitoring data, the possible leakage flow path and associated ground subsidence zone were mapped and a settlement history of the affected building was reconstructed. Both contractual and technical lessons learned from this accident will be useful for professionals to mitigate or prevent future incidence of through-wall leakage during excavation.