More O'Ferrall, GC & Simbile, NS 2020, 'Addressing pit wall instabilities in Africa’s largest open pit copper mine', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering
, Australian Centre for Geomechanics, Perth, pp. 1507-1520, https://doi.org/10.36487/ACG_repo/2025_104
Kansanshi Mine is currently Africa’s largest open pit copper mine. Initial slope geometry was determined with limited geotechnical data, assuming dry wall conditions. As the mine developed, the assumptions made for the initial design proved to be rather optimistic. The depth, and degree, of weathering was greater than assumed, and groundwater influence was soon realised. Phyllite is present fairly close to the original surface in some portions of the pit, and the clay content of this material appears to have been overlooked during the design phase. After approximately three to five years of exposure to the climatic conditions, the ‘fresh’ phyllite has degraded and wall instability in this material type is starting to occur. In addition to this issue, several large fault zones traverse the pit. These zones are deeply weathered, and issues are being experienced due to blast-induced failure of the weathered fault gouge.
The focus of the Geotechnical Engineering team on Kansanshi Mine is on obtaining pertinent geotechnical data to assess the instability risk of the final walls of the pit (the depth is currently approximately 200 m, and one of the final walls comprises the weathered phyllite, with no option to pushback), assessing the presplit blast design to minimise back-break into the wall and the fault zones, and develop a saprolite depressurisation program in an attempt to maintain the stability of the uppermost benches close to the pit crest.
Instabilities experienced and addressed during the development of the pit, remedial actions taken to minimise the risk associated with these occurrences, and findings of the geotechnical data assessments are presented.
Keywords: design, instability, depressurisation, presplit, weathering