Discussion of a large scale failure on the footwall of the Nchanga Open Pit, Zambia

A large scale failure involving over 1 000 m of slope has occurred through the soft rock mass of the footwall of the Nchanga Open Pit, Chingóla, Zambia. The overall height of the slope in the failure region is 184 m and the overall face angle 30°. Rock types in the slide area were comprised of banded sandstones, banded shales and felspathic quartzite. The failure has occurred as a result of the undercutting of the underlying felspathic quartzite ore bearing horizon and the development of high water pressures within the slope. A simple monitoring programme consisting of tension crack measurements and precise levelling was instituted. Results from the monitoring enabled mining to continue below the failure zone up until the day before the failure took place with the prevention of damage to equipment or personnel in the area. The paper discusses the monitoring and possible causes of failure of the layered sequence of the footwall in the open pit.