Evaluating the effect of earthquakes on open pit mine slopes

Abstract

Among a few earthquake-induced failure instances in open pit mine slopes, none were significant enough to be recorded as causing operational interruptions or injuries. However, there have been a significant number of intense earthquake-triggered natural slope and tailing dam failures which caused serious damages, injuries and death. This research addresses this dichotomy through studies of earthquakes and associated geological parameters on 177 open pit mines, 95 natural slope failures and 37 tailing dam failures, all over the world. The earthquake parameters include magnitude, epi-central distance, focal depth and intensity, while the geological characteristics considered include failure type, water condition, rock type and topography. After analyzing these parameters, it was concluded that the natural slope failures are triggered mainly by earthquakes with depths and epi-central distances of less than 40 and 100 kilometers, respectively. Only 7 open pit mines in this study were subjected to such earthquakes. Studying the earthquake intensity shows that only 20 mines were subjected to shaking intensities of more than 6.0 in the last 25 years, while for natural slopes and tailing dam failures almost 90 percent were subjected to this range of intensities. Among the geological characteristics, water condition, failure type and topographical effect were found to separate open pit mines from the failed natural slopes and tailing dams due to earthquakes. Particularly, water condition plays a role in nearly all of the natural slope failures and obviously in all tailing dam failures, while open pit mines are relatively less affected due to natural or mechanical dewatering. Most recorded natural slope failures are shallow, which are not considered reportable in open pit mines, unless they cause injury or intense property loss. Natural slope failures are commonly triggered on the hill tops and ridges as a result of a topographical amplification phenomenon, while the absence of this effect due to one-face or valley-shape pit slopes increases the pit slope stability. The top soil layer of natural slopes and the unconsolidated layer of tailing dams are widely subjected to stiffness contrast amplification, whereas pit slope surfaces are frequently cut which reduces this effect. Numerical analyses performed in this research show that the topographical amplification on hill-shaped natural slopes is up to 4 times that of pit slopes. The coupled effect of topography and stiffness contrast may increase the amplification factor to 8 for the top soil layer of natural slopes compared to competent pit slopes. This research shows that the effect of earthquakes on open pit mines cannot be ignored due to the limited number of endured open pit mines in the vicinity of strong earthquakes. However, site characteristics of pit slopes make them less likely to fail compared to natural slopes and tailing dams.