Application of Liquid Nitrogen Freezing to Recovery of a Collapsed Shield Tunnel

Abstract

An accident involving the collapse of a running tunnel along the Nanjing Metro (Jiangsu Province, P. R. China) occurred as the tunneling shield arrived at the receiving shaft. The brine freezing method was used for the recovery of the collapsed tunnel. During the recovery project, another collapse resulting in the leakage of water and sand occurred. By considering the tight construction schedule, the freezing method was also used for the second recovery. Liquid nitrogen freezing was adopted as the primary process and supplemented by brine freezing. On the basis of the real-time variation of the frozen soil temperature, which was obtained through an informatized construction system, this study analyzes the freezing effect at each construction stage of the second recovery and identifies the advantages of liquid nitrogen freezing over brine freezing. It also evaluates the safety and rationality of the freezing scheme. The successful completion of each construction stage and reconnection of the collapsed section of the metro tunnel are described. Compared with brine freezing, liquid nitrogen freezing is shown to have the advantages of faster cooling, lower temperature, and wider frozen range. The freezing construction scheme of the second recovery project was safe, but it should be more economical. To reduce the engineering cost, it is necessary to fully utilize the advantages of liquid nitrogen in other freezing projects.