Large deformation characteristics and controlling measures of steeply inclined and layered soft rock of tunnels in plate suture zones

Abstract

To clarify the inducing factors and mechanisms of geological disasters, such as large deformation and tunnel collapse, tunnels with layered and soft rock in the Luang Prabang suture zone were examined in this paper. Based on an investigation and summary of the disaster characteristics of tunnel deformation, the excavation loosening zone and geo-stress conditions were field-tested by using a sonic detector and a hollow inclusion strain gauge to analyse the correlation between the surrounding rock deformation and the extent of the excavation loosening zone and geostress distribution. The results show that, influenced by the Luang Prabang geological suture zone, underground water and the mechanical characteristics and occurrence of the rock mass, the surrounding rock mass deformation presents obvious asymmetric features. A large deformation of the rock mass more easily occurs at the tunnel crown and right spandrel (waist), and the main deformation phenomena are the spalling and cracking of shotcrete, buckling and bending of the steel arch and large horizontal convergent deformation. According to the sonic velocity monitoring of the rock mass, the scope of the excavation loosening zone of the rock mass is approximately 5.5 m to 7 m, which is beyond 60% of the tunnel width. Based on the field geostress test, the tunnels in plate suture zones bear intensively horizontal tectonic action, and the maximum lateral pressure coefficient of three tunnels is more than 1.5 (σ1/σ3). Thus, the horizontal convergence deformation and supporting structure failure of the tunnel are serious. Some controlling measures were adopted to release and control rock mass deformation, such as improving the advance support structure, enhancing the steel arch strength and increasing the allowed deformation of the rock mass. Moreover, asymmetric support design, such as local borehole grouting and temporary horizontal support structures, also be implemented. Additionally, the optimal design of rock bolt installation and grouting reinforcement depth should be given more attention during tunnelling.