Kimberlite pipes are mined open cast for diamonds at Orapa and Letlhakane in Botswana. The kimberlites are very low permeability
even when weathered but are saturated due to high ground water levels. The country rocks of basalts overlying sandstones contain
significant quantities of water at various piezometric pressures.
A very large kimberlite pipe is mined at Orapa and 2 small ones at Letlhakane. Investigations showed that dewatering was necessary
at the latter and a perimeter deep well system was installed in 1982. The piezometric pressures in the country rocks have
been successfully maintained below the pit floor, though some modifications will be required in future as mining goes deeper.
The Orapa kimberlite was shown to be too large and too low permeability to be successfully dewatered with wells or a gallery.
Recommendations were given to reduce the problems created by wet kimberlite, in terms of day to day mining operations.
Steffen Robertson and Kirsten Inc