The Redundancy of Systematic Rock Bolts in Cavern Construction

Abstract

The general design approach for the permanent supporting system of caverns and drained tunnels with more than half a span of rock cover follows a 3-Stages Approach, which is 1) deriving the initial supporting types based on the NGI Q-system (2015), 2) checking of the initial supporting types by Reinforced Rock Arch (RRA, Bischoff and Smart, 1977) theory and 3) verifying the design by finite element analyses to confirm the support requirements arrived from 1) and 2) above. The development and use of the Q-system for the temporary and permanent design of rock bolts and shotcretes are routinely used in practice, which would not require any deliberations. For the RRA approach, the concept is to form a reinforced rock arch from the systematic rock bolts around the opening by improving the confining pressure to increase the surrounding rock mass strength, which, in theory, provides a better utilization of the rock mass strength than that of the rock bolts. In this study, 3D and 2D finite element analyses have been carried out to investigate the soil-structure interaction and load transfer mechanism from the rock stress to the rock bolts at each stage of excavation. Typical rock mass properties commonly adopted in Hong Kong are used, and a large cavern span of 32 m and height of 36 m is used. With the use of the same material properties and geometry, finite element simulations are carried out using the 3D software RS3 to generate the ground convergence-support reaction curve and compare it with the longitudinal displacement profile (Vlachopoulos and Diederichs, 2009) for predicting the amount of ground relaxation before the supporting bolts are activated. Based on the ground convergence-support reaction curve from the RS3, another 2D finite element program, RS2, compares the load developed in the systematic rock bolts in a 2D plane strain vs a 3D stress environment. The paper discusses the stress distribution, plastic zones, convergence in the rock mass and the development of rock bolt loads before, during and after each stage of excavation. With different ranges of Q-value tested, it can be concluded that the ground behaves nearly in an elastic behavior due to the low stress-to-strength ratio of the rock mass, and the load mobilized in the rock bolts has an insignificant contribution to the cavern convergence, stress redistribution due to the free-moving boundary conditions at the cavern walls, as demonstrated by a low mobilized working force less than 20% of the tensile strength of the working forces developed in both the 2D and 3D analyses. The systematic rock bolts in a homogeneous isotropic rock mass can be considered a redundant and prescriptive measure. Its main contribution will only be developed in a jointed rock mass by increasing the confining stresses across the rock joints and mobilizing the shearing resistance of the jointing materials.