Innovative design of slender rock pillar formed within large span road tunnels and cavern Y-junction in Hawkesbury Sandstone

Abstract

This paper presents an innovative design method for a slender rock pillar formed in a road tunnel Y-junction. This method has been successfully implemented in two of Sydney's widest-span road tunnel projects. The pillar strength is estimated based on synthetic or virtual uniaxial compression testing results on pillar models with anticipated adverse geological features and conditions encountered in Hawkesbury Sandstone of Sydney using distinct element numerical modelling. The proposed pillar design comprises two main steps. Firstly, the Factor of Safety against rock pillar failure is calculated based on the estimated peak pillar strength with and without rock bolt stitching and induced stresses. If the target Factor of Safety is not achieved, a set of longer rock/cable bolts to install adjacent to the rock pillar as a new type of pillar treatment is proposed to minimise the support contribution of the rock pillar. The required minimum support pressure the rock pillar provides is calculated using a ground reaction curve concept. Then the safety factor against the Y-junction's global failure is calculated based on the rock pillar's minimum required support pressure and residual strength. The proposed design method is demonstrated in the context of a recent large road tunnel project in Sydney, Australia. The proposed design method can provide confidence in rock pillar stability, potentially preventing costly and carbon-intensive full pillar replacement with concrete.