Over a ten-year period, there have been three instances where the author could not determine the cause of slope failure from the well-known conventional methods. Slope stability analyses conducted on the slope design showed that the Factor of Safety (FoS) was above the recommended minimum value. Analyses of the orientations of geological structures did not point to any kinematic admissibility. Nevertheless, the slope had failed, and recommending remedial measures was difficult because the cause of failure had not been established.
Close observation, however, indicated that the ground profile was not typical and that there was weak material embedded within strong ones (such as completely weathered layers in otherwise unweathered rock) below the toe elevation of the slope. The weak foundation material appeared to be over-stressed, evidenced by the heaving of the ground at the toe of the slope failure as encountered in general geotechnical shear failure (1). Reassessment of the cause of instability from a bearing capacity failure perspective made it easier to see why the slope would fail when conventional methods did not predict such behaviour. Two such failures are presented in the sections below.
Footwall Slope Failure at a Zinc Mine in India
In July 2007, a multi-bench slope failure occurred in the footwall slope of an open pit zinc mine over a strike length of about 100m. The weathered zone of this slope was from 390 to 360mRL. Below 360mRL, the footwall rock was generally unweathered.
The predominant geological structure in the footwall was foliation which ran sub-parallel to the slope face and often caused bench-scale instability. The general dip of the foliation planes was about 85º, though flatter in some sections of the slope. The rock types were Garnet, Biotite, Sillimanite, and Gneiss (GBSG) which made up about 70% of the country's rock. Other rock types in order of decreasing presence were Graphite, Mica, Amphibolites, Pegmatites, and Mylonites (2).
Limit Equilibrium analyses conducted on the overall design slope height of 250m (390 – 140mRL) gave a minimum Factor of Safety of 1.2 (under static conditions). Kinematic stability analyses on the slope using pit wall mapping data did not provide evidence of multi-bench structurally controlled instability. The footwall slope was mined with 10-m high benches, 6.5-m wide berms, 70º batter angles, and an inter-ramp slope angle of 44.6° when a failure occurred (3).
The footwall slope generally had a North-South trend, and the failure was bordered on the North end by a well-defined fault. The Southern border of the collapse appeared like broken cantilever support, with shearing through the failed rock mass at that end. The failure began with the appearance of cracks on the slope as mining progressed. All efforts to determine the cause of the sudden impending multi-bench failure proved futile until the 320-310m bench was being mined when a weak zone of weathered Biotite of soft clay consistency, about 25m wide, was found adjacent to the fault in fresh rock. The mining of the weak zone resulted in acceleration of slope movement and ended in a tilted, failed rock mass which slumped towards the weathered Biotite zone and truncated at the fault like a broken cantilever. These observations suggested that the failure was likely caused by settlement within the weathered Biotite zone. At least 3m of settlement was observed within the weathered Biotite material near the fault, and significant movement occurred adjacent to the fault. On each berm, the portion adjacent to the fault was depressed more than areas further away. The differential settlement caused by this zone of weathered Biotite in a fresh rock material must have caused tensile stresses in the surface of the affected rock mass, resulting in cracks in the slope that led to failure. It is noteworthy that the failure stabilized below 310mRL once mining exited the weathered Biotite zone (4).
