Mine Radar, Geotechnical Engineering, Drilling And Blasting, Rope Access, Rockfall Protection

Drill rig solves problems

Bringing a mine back on stream after a 15-year lay-off is usually not a daunting prospect, particularly if the subject is an open-cut mine. But when the mine walls have been submerged in 100 metres of shark-infested seawater for the whole of that period, it’s a very different proposition.

This was the challenge facing ground support and drilling contractor, ROCK Australia Mining + Civil , when tendering for a key role in bringing Main Pit at Mt Gibson’s Koolan Island iron ore mine back into production. When BHP closed the mine, in 1993, part of the mine rehabilitation program was to breach a cliff and fill Main Pit with seawater.

Mt Gibson Mining acquired the property in 2007. The deposits in the satellite Acacia, Mullet and Barramundi pits produced an immediate revenue stream, but the jewel in the crown was always to be Main Pit, with its rich 60+% hematite resource.

ROCK won the contract on the basis of a comprehensive operational plan that required, in essence, for seawater to be pumped from the pit while the walls were stabilised, and safety measures taken to protect workers from rock falls.

The plan developed by ROCK’s  Mike Moore was approved by the Mines Department in 2007, and the crew was mobilised in March 2008.

At the planning stage, the key challenge was to source a drill rig capable of handling all the ground support functions while operating from a floating base. The rig also had to be capable of remote operation whenever the condition of the wall posed a direct threat to the operator.

ROCK opted for the Sandvik DX800R hydraulic, self-propelled surface drilling rig.

Mike Moore explained that the orientation of the rig to its crawler tracks enabled it to be secured to the barge parallel to the mine wall, for maximum stability, while its reach and unique 90o slew capability provided the biggest operating envelope available from a drill capable of being operated from a floating barge.

“In effect, we chose the biggest small drill in the business,” said Mike Moore. The remote control facility was a prerequisite for the chosen drill, so that the rig could be

 operated from a second barge, well away from the footwall, in areas where the wall face showed signed of crumbling.

ROCK ordered two DX 800R rigs, which were slightly 

modified in ROCK’s Perth workshop before being sent to Koolan.

“We set up each rig’s hydraulics to drive the winches that keep the barge snug against the footwall, and also to drive the grout mixer,” he said.

“We also designed a manual rod handler for the use of self-drilling anchors and, because we would be operating from floating platforms, we removed the dust collector system to reduce weight.

The drills were initially used as conventional crawler rigs, setting up road barriers and pins along the crest of the mine to support the steel mesh that would be used to seal the face of the wall to provide protection from falls of small rocks.

They were then lifted by crane on to two custom-built barges, from which they have been operating ever since.

The basic methodology is that water is pumped from the mine for a two-week period, lowering the water level in that time by four to five metres. 

At that point the rigs come into play, drilling horizontal holes for deep-seated ground support to stabilise the footwall.

Once the water level has dropped to reveal an area of the wall to a depth of 30 metres, and the face has been stabilised, steel Tecco mesh imported from Geobrugg in Switzerland is rolled down the sheer face of the wall, tugged into position by the barges, and vertical seams are then secured by riggers from rope access specialists, Perth-based Vertech Group, who abseil down the face of the wall.

The current footwall rehabilitation phase, which involves stablising and meshing 1500 linear metres of footwall to a height of 120 metres, will be completed in late 2010.

Mike explained that the dewatering and ground support program required for Koolan’s Main Pit  included a number of mining landmarks.

“We believe this is the first use of remote control rigs in Australia, and operating those rigs from floating barges is almost certainly a mining world first,” he said.

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