Forecasting potential rock slope failure in open pit mines is necessary to maintain safety and mine productivity. Laboratory testing of rain induced landslides in Japan over 20 years ago led to an inversevelocity approach for estimating the time of a slope failure. This approach appears to have been largely overlooked in mining, but has been successfully applied in aiding the prediction of three large slope failures and control of another large instability. The predictions were forecasted five days to three months in advance of failure. This paper provides an overview of the development of inverse-velocity as a tool for rock slope failure prediction and presents four case examples of the successful application of this approach. Forecasting rock slope failure is a complex problem, involving observations, analysis and experienced judgement
Assessment of rock slope failure mechanisms requires an understanding of structural geology, groundwater, rock mass strength and insitu stress conditions. Stress relief associated with mining excavation leads to elastic rebound and ground relaxation displacements that typically dissipate with time, a process that is often referred to as timedependent deformation (Martin 1993, Zavodni 2000). With continuing excavation, regressive slope displacements may occur in a cyclical accelerating/decelerating fashion, but may not lead to slope failure. As strain levels increase, or in the case of weak or altered rock masses, strain softening may lead to plastic (non-recoverable) deformation and progressive failure development. The inverse-velocity method, developed by Fukuzono (1985), provides a useful tool for predicting slope failure time. Astonishingly, even though this approach was developed based on laboratory tests more than 20 years ago, it does not appear to have been applied for real-time slope failure prediction in the mining industry until 2001, when it was used to predict the first of three largescale slope failures presented in this paper.