Open Pit Water Control Safety: A Case Of Nchanga Open Pit Mine, Zambia

摘要

Abstract: Mining in Chingola, Zambia, started underground in 1931, and was catastrophically flooded and closed. The present Nchanga Underground Mine (NUG) started in 1937. The Nchanga Open Pit (NOP) mine started in 1955, situated to the west of NUG, and partially overlying it. Open pit water control safety operations in the Nchanga-Chingola area, have successfully enabled the safe extraction of millions of tonnes of copper ore annually over the past 60 years, from NUG mining as well as the NOP. At the start, Nchanga mining license surface already had NUG, and many watershed divides, with the Nchanga and Chingola streams being the main streams feeding into Zambia's second largest river, Kafue river, and 42% of the year was characterised by heavy rains ranging between 800mm to 1300mm per annum. In this paper, the presence of very significant amounts of seasonal rain and subsurface water in the mining area was identified as both a curse and a blessing. An excess in seasonal rain and subsurface water would disrupt both open pit and underground mining operations. In order for NOP to be operated successfully, stable and free from flooding, coping water management tactics were adopted from 1955 to 2015, including: 1. Underground mine pump chamber pumping system; 2. Piezometer instrumented boreholes; 3. Underground mine 1500-ft sub-haulage east borehole dewatering, beneath the open pit; 4. Nchanga and Chingola stream diversionary tunnel and open drains; 5. Nchanga stream causeway and embankment dam in the Matero School ' Golf Club area; 6. Pit perimeter borehole pumping; 7. Outer and inner pit perimeter drains and bund walls; 8. In-pit ramp side drains; 9. In-pit sub-horizontal borehole geo-drains and water; and 10. Pit bottom sump pumps. Application of grout curtains along the Vistula River, Poland, was noted as a possibility in the right circumstances, although it had never been used at Nchanga Open Pit. An additional conclusion was that forward health, safety and environmental end-of-life planning was required for the extensive district-wide infrastructure of the open pit water control system, for public safety after life of mine.