Geotechnical services company Geomech Africa, a subsidiary of Geogroup, imported its 20 t, SDC 450-24 sonic drill rig from Canada at a cost of $1-million specifically to handle deep borehole work in mineral sands at multinational diversified miner Rio Tinto’s Richards Bay Minerals (RBM) sands project near Richards Bay, in KwaZulu-Natal.
“Sonic drilling is most suited to softer ground formations because of the high frequency vibrations generated in the sonic head of the drill rig. This causes the liquefaction of the ground at the drill bit and facilitates good core recoveries in ground in which conventional drilling would not succeed,” explains Geogroup marketing manager Caroline Kruger.
She says, owing to the challenging drilling conditions, Geomech Africa hired the services of one of the world’s most eminent sonic drillers from Canada for a six-week period to assist the local drilling crews to become accustomed to the new geological conditions found in the dunes of the RBM site.
“Six weeks into the project, it became evident that the ground in the dunes in this area is difficult to drill, even with sonic drilling, especially at depths below 120 m.”
Kruger explains that an opinion of the specialist sonic driller was that traditional sonic-style drilling is not able to achieve more than 25 m a day in the sand conditions and, as a result, a new approach had to be investigated.
“There is currently no ‘wireline system’ available on the market which enables the user to quickly retrieve the inner tube of the core barrel without the need to remove the rods from the ground in these conditions.”
She indicates that the sonic drilling at the RMB site had to be managed in a more conventional drilling method requiring the rods and core barrel to be removed from the borehole each time the core barrel is filled with sample.
“The tough conditions have meant that the friction welded rods and casings imported from Germany are continually breaking. Knowledge of the impact of the conditions on currently available materials has led to the development and manufacture of rods and casings using local expertise and local Richards Bay artisans,” Kruger explains.
“It is to be noted that throughout this difficult project, positive teamwork between client and contractor has resulted in zero safety incidents or accidents in over two years of drilling on the site.”
Kruger notes that the safety record and the site-specific knowledge gained will be invaluable to the company in bidding for the next three-year phase of the contract.