Site works have commenced for a 4.15m diameter, 108m deep shaft to be used for ventilation and a second escapeway.

 

The ventilation shaft will intersect the mine workings adjacent to the end of the 2300m long tunnel and is being constructed parallel to the tunnel.

 

McConnell Dowell has been contracted to construct the ventilation shaft using raise-bore equipment.

 

The base case for the McConnell Dowell contract was previously using an Alimak system where the ventilation 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 inherent safety and programming risks.

 

However, drilling at the ventilation shaft site showed ground conditions were not good enough for Alimak and raise-boring will be required.

 

Pike River said the new system will require allocation of $NZ5 million of the existing project contingency.

 

Utilising the raise-bore method of construction, a pilot hole of approximately 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 will be supported by a combination of resin anchored bolts, mesh and shotcrete.

 

Pike River said shaft construction would take about seven months, with completion due October 2008.

 

Once the shaft is completed 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 towards the end of 2008, the mine will continue to be ventilated via the tunnel under the current ventilation system with first production on track for July 2008.

 

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

 

The ventilation shaft has been located on 650 square metres of an existing cleared drill site to minimise the surface vegetation which has to be removed. The shaft site will include the cut slopes and walls needed to support the surface pad, on which construction equipment, helicopter landing pad and fan will be located.

 

The surface pad will be formed by a mix of cut and fill with the lower pad fill supported by a “mass-block” wall.

 

A lay-down area and the helicopter landing pad will be constructed as an elevated deck to minimise the excavation and installation of mass blocks and the overall disturbance of the site. The elevated decks are able to be installed suspended over the existing ground cover with minimal disturbance.