Assessing the Risks and Technical Challenges of Reckless Pre-Excavation Dewatering and Through-Wall Leaking in Thick Aquifers

Till now, practitioners are not fully aware of the risks and technical challenges associated with either pre-excavation dewatering or through-wall leaking in thick aquifers; thus, relevant incidents happened frequently during excavation. The recent construction of one subway station in the city of Nantong, China, provided an opportunity for this investigation, which involved both pre-excavation dewatering and through-wall leaking in the thick silty and sandy formations with its groundwater level near the ground surface. Postaccident diagnoses revealed that arbitrary pumping before soil removal caused rapid lateral kicking-out deflection of the retaining wall, followed by pronounced ground subsidence; during the subsequent excavation, those routine countermeasures failed in blocking off the through-wall leakage. Worse still, the leaking wall displaced dramatically with implementation of the countermeasures, aggravating the leakage even further. Considering the high risk of a catastrophe approaching, an expensive, time-consuming artificial ground-freezing (AGF) program had to be executed as the final attempt to stop the growing through-wall leakage, which was achieved successfully one month later. These incidents taking place pre-excavation and during excavation were related in part with the contractor’s unawareness of the permeable, erodible characteristics of silty and sandy aquifers; thus, the pre-excavation pilot pumping tests followed an inappropriate dewatering procedure and a series of ineffective countermeasures were adopted to stop the through-wall leakage encountered during soil removal. For excavations to be conducted in thick aquifers, groundwater levels within their enclosures should be lowered gradually, defects in waterproof curtains should be detected and fixed in advance of soil removal, and through-wall leaking during excavation should be stopped with appropriate countermeasures in a timely manner; otherwise, severe incidents triggered by waterflow would take place promptly.