Stability analysis of the north-eastern slope of Daralou copper open pit mine against a secondary toppling failure

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Secondary toppling failures have complicated mechanisms. In these instabilities, some natural or manmade external factors motivate and overturn a rock mass. One of the most common types of secondary toppling failures is slide-toe-toppling mode. When a blocky or layered rock mass is overlaid by a soil or weak rock mass, the combination of a toppling failure in the rock at the toe and a circular or non-circular sliding in the soil or weak rock in the top leads to this hybrid failure. In 2015, such failure occurred in north-eastern slope of Daralou copper open pit mine. During this instability, the slope consisted mainly of two 10-m benches and one 4-m semi-bench. Since it was supposed to excavate the slope to the depth of 54 m, experts were worried about the project facing a similar larger failure in the future. Hence, a widespread investigation was carried out on the failure. Firstly, the mechanism of this instability was clarified with geological and geotechnical surface and subsurface investigations. Next, existing analytical solution which had been presented for analysis of static slide-toe-toppling failure was modified and some new equations were obtained for analysis of the failure under dynamic condition. Then, a computer code was developed based on the new equations for evaluation of the instability. In the next step, the failed slope was analyzed using the code and finite element method, and outcomes of these analyses were compared with real conditions. The comparison showed that there was satisfactory agreement between the analytical, numerical and actual results. Therefore, these methods were used to predict the behavior of the final slope in static and dynamic conditions. These evaluations indicated that if displaced materials are completely excavated and removed from the mine and overall dip of the slope is reduced, the final slope will be stable against slide-toe-toppling failure. This paper summarizes the results and concludes that both the modified analytical approach and finite element method are able to analyze a slope against the secondary toppling mode. Furthermore, the research proves that re-sloping and unloading are two adequate methods which can be used for stabilization of the failure.