Tunnel support for stress induced failures in Hawkesbury Sandstone

Underlying much of Sydney, Australia is a composite rock formation known as the Hawkesbury Sandstone. This unit is composed of clastic layers of variable competency including a number of thick and strong layers. The presence of very high horizontal stresses within these layers is widely accepted. Deep excavations such as basements or open cuts in Hawkesbury Sandstone often experience moderate horizontal movements in excess of 1 mm per metre of rock excavation. These movements can result in various scales of damage and excess loads on supporting elements. Stress induced failures in tunnels and underground excavations have also been observed in a number of projects and include crushing, spalling and or slabbing of intact rock blocks or shear failures associated with planes of weakness. While most design approaches in ground engineering account for shear failure mechanisms, the assessment of brittle failure is less common and less well understood. Conventional models and failure criteria do not appropriately describe such behaviour and consequently the impacts on ground support may not always be appropriately addressed. This paper presents some discussions on the modelling and assessment of brittle failure in Hawkesbury Sandstone and some of the impacts of high in-situ stresses on tunnel support design.