Rock fall mitigation for an open pit mine experiencing substantial rock fall from overblasting

Abstract

Rock fall is a common hazard in open pit mines causing safety concerns for workers, transportation routes and may adversely affect mine production. To reduce and mitigate rock fall hazards, a benched slope design is normally used and, occasionally, additional mitigation may be necessary in areas with excessive rock fall hazards. Newmont's Boddington Gold Mine in Australia is experiencing substantial rock fall in the south portion of their open pit mine due to rock damage from blasting resulting in highly fractured wall rock. This has increased the volume of rock fall experienced in the mine, and the rock fall has caused accumulation of debris on benches and crest loss, reduced bench width, in some parts of the mine by as much as 40 percent. The loss of bench width has reduced the effectiveness of the benches for rock fall mitigation and allowed rock fall to travel much farther down slope than desired for safety. Because of these conditions, there is a 20 meter exclusion zone near the rock walls in the mine, reducing production and impairing safety conditions. The mine is currently mitigating the rock fall by draped mesh, spot bolts, cable lashing and benches originally excavated at 8 meters in width. As the open pit is deepened, a more permanent rock fall mitigation method is desired. Newmont proposed several modified bench designs to be evaluated. The new bench designs were evaluated by modeling rock fall on the existing and proposed bench designs using the Colorado Rock Fall Simulation Program (CRSP) and RocFall. Newmont provided rock fall data for modeling that included the normal coefficient of restitution and rock fall video from the open pit. Rock fall kinetic energies, bounce heights and velocities on existing benches and proposed benches were examined on five cross-sections from the open pit to assess which bench design had the most efficient rock fall mitigation. The importance of testing the normal coefficient of restitution and use of rock fall video for rock fall modeling were also examined, as well as the slope steepness versus rock fall run out with the new bench designs.