Squeezing failure of tunnels: A case study

The Laodongshan Tunnel, as a part of Guangtong-Kunming railway in China is one of the most important connections in the whole railway line. The large deformation and failure of primary support occurred in the early tunneling stage, and deformation characteristics and failure causes of primary support were obtained by means of field investigation and analysis. There have been many factors that result in the occurrence of squeezing failure of tunnel during construction, such as high in situ stress, soft-weak rock, groundwater seepage, weak support, delay closure of primary support, etc. The lessons to be learned is that for tunnels construction in weak rock mass under high in situ stress, the controlling principle of tunnel deformation ought to be based on combination of stress releasing and support resistance. The support type and method during tunnel excavation needed to be adjusted dynamically based on the pre-excavation geological forecast (e.g. tunnel seismic prediction (TSP), ground penetration radar (GPR), and horizontal pre-boreholes, etc.). Besides, many countermeasures were proposed during construction, including strengthened pre-support, improvement of support stiffness, grouting reinforcement, installing additional supports, closing the tunnel ring early, and timely installation of secondary lining. Finally, from feedbacks of monitoring results, squeezing failure of Laodongshan tunnel was effectively controlled. Thus these countermeasures can guarantee safety construction in weak rock mass under high in situ stress was achieved.