Argyle is mined by the Rio Tinto Argyle Diamond Mines JV and operates as a conventional open cut mine with lamproite and waste rock being drilled and blasted before being loaded out in a truck and shovel operation.
While Argyle is looking at developing an exploratory decline to investigate underground mining, an immediate priority was to drill horizontal depressurisation holes in the open pit walls to intersect the water table to relieve water pressure build-up in perched water tables.
According to Argyle Diamond Mine engineering geologist Paul Gillespie, this is a precautionary measure as increased water pressure during the wet season could lead to slope instability.
A safe and stable slope will prevent rock fall hazards and allow the company’s general fleet to work more productively.
Rock Engineering was contracted for the depressurisation work and chose an Atlas Copco ROC L8 blast hole rig for job.
To allow horizontal drilling to 200m, Atlas Copco modified the ROC L8’s standard carousel, which has 48m of tubes in the rack, in its Perth workshop.
Extra tubes can now be reloaded while the feed is still in the drilling position. The rig uses 102mm and 125mm hammers, 89mm drill pipes and 140-152mm bits.
The rock is soft to medium-hard but is abrasive and in very broken ground, which can make drilling more difficult.
Air velocity can drop in broken ground and lead to problems transporting cuttings to the surface.
However, according to Rock Engineering, the ROC L8 has enough air compressor capacity on board to handle the situation.
Targets
Targeting a rock mass that may be prone to instability, the drill pattern is specific to geological features capable of retaining water.
Targets and their locations vary greatly, with the drilling depths required ranging from 100m to 200m at 10-15º up dip.
“The drilling is difficult as there is a lot of bedded quartzite, which is sometimes hard and abrasive, so it takes a lot to shatter the rocks,” Gillespie says.
“But, so far the rig is drilling 110-140mm diameter holes to 200m without any problems.”
When drilling vertical holes, the ROC L8’s pullback back force is rated at 40kN, with 30kN of feedforce.
Gillespie says Argyle has not trialled the rig in a lot of water yet as it has been working higher up the wall.
“Although, I think that the further we go down the wall it will show that the rig is conservatively rated at 200m,” he says.
Rock Engineering project manager - drilling John Ivanovski says the contract is basically a campaign job.
“We could have used purpose built or specialised rigs, but they don’t have the versatility,” Ivanovski says.
“We are in and out, and we needed flexibility.”
He says the ROC L8 blast hole rig was selected as it was the only rig that the company could get to do 200m depressurisation holes as well as production blast holes.
He says the service from Atlas Copco has been friendly and efficient, and the timeliness of response has been good on the ground.
Working initially with Atlas Copco key customer manager Tony Sorgiovanni and its and surface drilling product manager Lars Senf worked well.
“They wanted to know the specs, how deep we needed to drill, rock conditions and based on that information they told us what the rig could and couldn’t do,” Ivanovski says.
“The contract is versatile, and the L8 has handled all the holes it requires in the varying ground conditions drilling up to 150m in a shift, more usually around 100m.”
Rock Engineering’s fleet also includes a ROC F9 and CH1100, doing smaller diameter holes, cable-bolting, meshing, and pre-split around the site. The operators have found it easy to use as they can hop straight from the F9, which has almost identical controls to the L8, and across the ROC series rigs.