Forensic Investigation of a Water Inrush Accident during Ground Freezing Recovery Work

An accident involving the collapse of a running tunnel along Nanjing metro occurred when the tunneling shield arrived at the receiving shaft. Then vertical ground freezing was adopted to recover the collapsed tunnel. However, another water inrush accident happened during the freezing work, leading to a sinkhole of area $15\times 20\mathrm{m}$. This kind of accident that occurred during a recovery process was unusual, so a forensic investigation was conducted. The results showed that a variety of factors contributed to the second accident, including a flawed layout of freezing pipes, position deviation and unreasonable material change of the bulkhead, premature removal of the frozen soil next to the bulkhead, and the absence of thermal insulation measures. These causes can be summarized as the lack of appreciation of the position and extent of the weak area of frozen soil wall at the connection between the collapsed and the intact tunnel. This weak area as well as the possible permeable path were mapped by visual inspection and retrospective analysis of field temperature monitoring data. The lessons learnt from this accident will be helpful to reduce the risk of such accidents in later similar projects for both design and construction sides.