Numerical Study of the Effects of Interbedded Layers on Rock Burst in Tunnel Engineering

Abstract

Rock burst is one of the most hazardous problems that is encountered during tunnel excavation. However, less research concerns tunnel excavation in an interbedded layer with alternating strong and soft layers. In this paper, the mechanism of an intense rock burst that occurred in the headrace tunnels of the Neelum–Jhelum (NJ) Hydropower Plant in Pakistan is analyzed based on field investigations. In contrast to a tunnel in homogenous rock, the interbedded layer could create additional factors that trigger or enhance the risk of rock burst. The interface slippage, stress redistribution that is caused by the elastic modulus contrast and ultra deformation of the soft layer are considered three adverse factors that trigger or enhance rock bursts in a layered rock mass. Numerical analyses are carried out on triggering or enhancing rock bursts on the associated abrupt interface slippage and crack propagation that use a boundary element method. For the stress redistribution and transferred elastic strain energy, it is based on a finite-element method (FEM). The numerical results are presented for these three triggering or enhancing mechanisms. The slippage of the layer interfaces could trigger a rock burst and assist in the fracture growth of the flaws between the interface and the tunnel. The enhancement of the stress in the strong layer could generate a stronger stress concentration to cause compressive failure of the tunnel. The soft layer deformation could increase the stress in the strong layer and transfer more energy to the burst elements. Although the three mechanisms are addressed separately, they could come into play at the same time. The results could be helpful in finding a reliable means to reduce rock burst risk in a layered rock mass.