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Coal miner Pike River says it will have to spend an extra $5 million because poor ground conditions have forced it to change the method of drilling a ventilation shaft.

The 4.15m diameter and 108m deep shaft will ventilate the West Coast mine and be an emergency exit. It will intersect at the end of the 2300m long mine tunnel.

McConnell Dowell will use raise-bore equipment to construct the shaft, which is a change from using an Alimak system. Under the Alimak system the shaft is excavated from the base up to the surface by installing a track system up the wall of the shaft from which workers drill and blast the roof of the shaft.

The Alimak system relies on good ground conditions and carries higher risks.

"Drilling at the ventilation shaft site shows that ground conditions are not good enough for Alimak and raise-boring will be required," the company said.

Under the raise-bore method, a pilot hole of about 300mm diameter is drilled from the surface to intersect the tunnel. The raise-borer is then located on the surface and a 4.15m diameter reaming head is attached to the reaming rods at the bottom of the hole.

The head is then rotated and drawn up through the ground to excavate the shaft, with all the drill cuttings falling back down the shaft for removal from the mine. To maintain stability the shaft sidewall are supported by a combination of resin anchored bolts, mesh and shotcrete.

The shaft should take seven months to drill. When it is finished an exhaust fan will be located over the shaft to draw air from the mine and it will be the mine's backup ventilation fan. Until the fan is established toward the end of 2008, the mine will continue to be ventilated via the tunnel.

The main fans for the mine will ultimately be sited underground and exhaust up the ventilation shaft.

The extra cost of the shaft will be covered by a contingency reserve. The mine is expected to begin production in July.

The ventilation shaft exit has been located at an existing cleared drill site to minimise the surface vegetation which has to be removed. The mine is inside a national park.

About 650 sq m is required for the shaft site, including the cut slopes and walls needed to support the surface pad, and for a camp of up to eight people.

The surface pad is where the construction equipment, helicopter landing pad and fan will be located.