Numerical study of rock pillars for large span tunnels in Hawkesbury sandstone

Stability assessment is often required where rock pillars are formed between twin tunnels or within Y-junctions. These pillars may in some cases be relatively slender with width to height ratios, W/H, less than 0.5 due to alignment optimisation for minimum tunnel cross-passage lengths or widening in Y-junctions. Despite the slenderness, being stiff support elements, such pillars will still attract significant loading resulting from stress redistribution due to the excavation. Inadequate pillar strength could lead to severe instabilities and failure of the adjacent rock mass with potentially catastrophic consequences to the associated underground openings. From a design viewpoint, understanding the pillar strength and failure mechanisms is important to provide adequate pillar treatment and ground support for the associated tunnels and caverns. This paper presents a numerical investigation on rock pillar stability for excavations in Hawkesbury Sandstone with a focus on slender pillars in large span tunnels and caverns. The pillar strength is based on the results of virtual uniaxial compression testing on pillar models using discrete element numerical modelling. Ground and in-situ stress conditions typically encountered in Sydney are assumed. The mechanical behaviour of the rock is simulated assuming an elastic-plastic strain-softening constitutive model that captures brittle failure and progressive pillar failure mechanisms. In addition, rock pillar treatments and ground support strategies are also discussed.