Enhancement Effect of Confined Pressure and Formation Mechanism in Jointed Rock Tunnel with High Geostress

Large squeezing deformation is currently becoming one of the most remarkable problems of tunnel construction in Western China. During the tunnel excavation, some geotechnical problems were encountered in the surrounding rock, such as excessive overbreak and intense deformation by asymmetric squeezing, mainly because of intense anisotropic of structural strength. The large deformation led in severe damage of the primary support, even splitting of secondary support, which have a significant influence on construction and security of tunnels. The complex failure phenomena and a series of experimental results in tunnels are summarized in this paper. According to this, the paper proposed a synthetic interpretation, namely enhancement effect of confined pressure (EECP), which resides in different types of jointed rockmass under high in-situ stress. Considering different mechanic parameters of rockmass, sensitivity analysis on EECP has been carried out through numerical simulation (FLAC3D). It is indicated that EECP is remarkably influenced by Poisson's ratio of rockmass. Combined statistic mechanics analysis of rockmass with geological record of Zhifang Tunnel and Liangshui Tunnel of Lanzhou-Chongqing railway in China. The calculated pressure between surrounding rock and lining according to present standard exhibits relatively smaller than that exists in tunnels. It is revealed that the formation of EECP can be interpreted by large Poisson's ratio effect, which resides in layered rockmass under compress condition.