Master Drilling to use new tunnelling method at Petra mine

JSE-listed Master Drilling has been given the go-ahead by Petra Diamonds to use an innovative method at its new Cullinan mine to develop tunnels at twice the speed of traditional blasting methods.

Fochville-based Master Drilling is using its vertical raisebore drilling system in a horizontal application at the block-cave Cullinan mine, where the kimberlite is an ancient carrot-shaped pipe of volcanic material bearing diamonds.

The method of extracting this pipe is to construct a tunnel ringing the kimberlite and then punch tunnels across the pipe in a grid pattern to collapse the kimberlite above the tunnel into draw points to be carried away and hauled to the surface.

The development of the grid tunnels is what this technology tackles and Master Drilling CE Danie Pretorius said the doubling or tripling of advance rates from 2m per day with drilling-and-blasting methods to 4m or 6m a day with the horizontal raise bore system would bring benefits to companies that could deploy the system.

He said this was one of the “quick wins” Master Drilling had identified in its push to automate a number of time-consuming and costly mine development processes. Pretorius said between 60% and 80% of all mine development was horizontal and nearly all of it was done by drilling and blasting.

Master Drilling was looking for tie-ups with one of four or five companies developing machines that did horizontal tunnel drilling, either in a joint venture or buying the machines to roll out in SA and elsewhere, Pretorius said.

Raise-bore drilling entails drilling a pilot hole between two points, attaching a large reaming head at one end and pulling it to the other, creating a smooth tunnel. Traditionally, this has been used for vertical ventilation and ore pass shafts.

At Cullinan, the test work entailed drilling a 180m-long hole, attaching a 4.5m-diameter reamer and pulling it horizontally to create a tunnel.

Petra’s Ben Swarts said the tunnel was smooth and perfectly round, which decreased ventilation drag and gave the best stability. The accuracy of the drilling method meant no underground areas would be lost to human error.