Catastrophic Failure of Shanghai Metro Line 4 in July 2003: Postaccident Rehabilitation

The destructive failure of Shanghai Metro line 4 across underneath the Huangpu River on July 1, 2003, led to massive ground subsidence and collapse of pre-existing structures in the proximity; additionally, the failure generated large amounts of human-made rigid obstructions in the strata, which made rehabilitation of the collapsed metro line be very technically challenging. By extensive comparisons in terms of technical difficulties, economic costs, and construction durations, a plan of reconstruction of the collapsed metro line at its original location rather than relocation was selected finally for the postfailure rehabilitation. The majority of the failed metro line was replaced by a $262\text{-}\mathrm{m}\text{-}\mathrm{long}\times 23.7\text{-}\mathrm{m}\text{-}\mathrm{wide}\times 38.5–41.2\text{-}\mathrm{m}$-deep cut-and-cover tunnel bottoming out in the confined aquifer, whose two ends were connected with the existing intact metro line via two 10-m-long tunnels mined within the aquifer by the ground-freezing method. Excavation of the cut-and-cover tunnel was retained by 1.2-m-thick and 65-m-high perimeter diaphragm wall panels braced by 9–10 levels of steel-reinforced concrete struts. To ensure the project safety and mitigate potential adverse influences of field works on the urban environments, extensive innovative construction plans, measures, and techniques were designed and then implemented into the reconstruction, e.g., a lightweight cofferdam and pile-supported steel platform for diaphragm walling through the existing metro line underneath the river; unique protective solutions and construction techniques for execution of vertical and horizontal ground-freezing programs for diaphragm walling and mining; special procedures of cleaning up the existing tunnels previously filled with water, sand, and cement; innovative joints and grouting measures adopted between the reconstructed tunnel and the pre-existing intact metro line, and so on. Furthermore, the initial designs were optimized and adjusted with the progress of field construction works. Lastly, the long-term field performance data demonstrated the success of this rehabilitation project, which affected the urban environments rarely. The construction plans, measures, and techniques outlined in this paper are practical, useful references for professionals worldwide to deal with similar underground projects in the future.