Excavation and kinematic analysis of a shallow large-span tunnel in an up-soft/low-hard rock stratum

Abstract

The excavation of a shallow large-span metro tunnel in an up-soft/low-hard rock stratum restricted by a rock mass requires knowledge of the stratum kinematic mechanism to adopt an appropriate construction method to control the surface settlement and ensure structural safety. The purpose of a kinematic analysis is to predict the state of stratum collapse based on the nonlinear Hoek-Brown failure criterion and the upper bound theorem. The rock mass deformation and the supporting structural mechanics characteristics of the tunnel under different excavation methods are studied using numerical simulations. The Xinggongjie Station tunnel on the No. 1 Line of the Dalian Metro is used as an example. Furthermore, the ground surface settlement, the internal displacement of the surrounding rock mass and the stress on the supporting structure are monitored during the construction period. The accuracy of the theoretical kinematic model and the rationality of the selected excavation method are verified by comparing the theoretical results with the field monitoring results. The study shows that the slip failure surface and ground settlement curve can be accurately determined using the kinematic model. Different excavation sequences result in differences in the surface subsidence, and the most favourable sequence for restricting the subsidence is to excavate the arch-side pilot first and the middle pilot last. The rock mass deformation and the surface subsidence are acceptable according to field monitoring under this excavation sequence. The load ratios of the surrounding rock mass borne by the initial support, which is the primary bearing structure of the tunnel, at different monitoring points are greater than 80%. The proportion of the secondary lining is approximately 20%, which is the safe reserve of the tunnel. The tunnel has been demonstrated to be safe; it has been placed into service and has operated well.