Rock fall case study: Hawkesbury Sandstone

This paper documents a rock fall in the Hawkesbury Sandstone. Two distinct phases, preceding the ultimate rock fall event have been identified and documented; a creep-loading phase and a rock fracture phase. Water is identified as playing a significant role in the creep-loading phase. Over the long term, peak water pressures from intermittent heavy rainfall events have contributed to the slow, creeping movement of the overlying, joint-bounded block resulting in the progressive loading of the underlying rock mass. The rock fracture phase was extremely rapid and overlaps with the rock fall event. The kinematics of the rock fall has been interpreted from broken surfaces, scratches and positions of debris. From the timing of events, it is likely that wetting/saturation of the intact sandstone from sustained rainfall in the weeks preceding the rock fall would have significantly reduced the intact rock strength. Based on the site investigation and reconstruction of the failure mechanism, the rock fall is classified, in the Crunden and Varnes (1996) scheme, as a complex extremely rapid rock fall and rapid dry debris slide.