The making of Pike River

Located 45 kilometres northeast of Greymouth, near the top of the Paparoa Range, Pike River holds New Zealand’s largest deposit of high-quality hard coking coal. The coal’s low ash content (one percent compared to eight percent in premium Australian coking coals) and high-fluidity are particularly valued by international coke and steel producers, who use it as a fuel and a catalyst.

The field has two seams – the Brunner coal measures and the deeper Paparoa measures. The Brunner seam is estimated to contain just over 58.5 million tonnes of recoverable coal. An additional eight million tonnes may be recoverable from the Paparoa seam.

To access the Brunner seam, Pike River Coal had to build a 2.3-kilometre tunnel through metamorphic rock – the largest and longest tunnel built in New Zealand since the second tunnel at the Manapouri hydro plant was completed in 2001. As a mining company Pike River could have developed its own adit, but instead opted to let a fast-track design-build civil contract for the job to McConnell Dowell Constructors (NZ).

The first full face blast in mid September 2006 revealed poorer than expected rock conditions and it was apparent that, instead of being largely self-supported, the tunnel would require a lot of reinforcing. The 5.5 metre-wide, 4.5 metre-high tunnel was excavated at various inclined grades to pass under located surface features and chemically anchored rock bolts were used to hold reinforcing mesh to the excavated D-profile.

A robotic shotcreting machine, brought in from Australia, was used to apply shotcrete fibre-reinforced using polyolefin structural fibres, as used on the Northern Gateway Project in Auckland.

Before striking coal, the tunnel had to pass through a major fracture zone—the Hawera fault. This proved slow going, and it took crews more than five weeks to drill and blast their way through the 60 metre zone of crushed rock. The possibility of methane gas infiltration meant explosion-proof equipment had to be brought in.

The tunnellers struck coal at the predicted location on the 17th of October 2008, when the tunnel came up under the Brunner seam, adjacent to an existing borehole. A roadway was driven northwards and the excavation completed at the base of what would become a ventilation shaft.

Because of faulting and geological movements, coal seams on the West Coast do not run in straight lines and it is essential to map seams before they are mined. A contractor from Australia used a track-mounted in-seam drill to create a series of holes, each several hundred metres long, to sample the coal in the immediate vicinity of the shaft and the other roadways to be driven in the area. The information was used to fine-tune the geological model of the western pit-bottom area and plot the best road map for the machines to cut coal. Limited coal production using a conveyor began in October 2008.

The mine then suffered a setback when a ventilation shaft partially collapsed, delaying production. There have been further production issues since then. A graben was encountered immediately to the west and north of the pit bottom and Pike had to drill and blast through non-productive rock to get to the coal seam.

If the graben had been further away from the pit bottom, the mine development could have gone around it. Having to drill and blast delayed production - but it is not expected to be material in terms of the total coal resource or the life-of-mine performance. The first coal shipment is now scheduled for the first quarter of this year.  

Trouble in the ventilation shaft

Work began on the 108 metre-deep, 4.15metre-diameter ventilation shaft in 2008. As planned, the shaft was to be created using an Alimak system – this involves excavating a shaft base-up by installing a track system up the wall of the shaft from which workers drill and blast the roof of the growing shaft. But again, ground conditions weren’t suitable and a raise-boring system was deployed at the cost of an additional $5 million.

A pilot hole of about 300mm diameter was drilled from the surface to intersect the tunnel and then enlarged by drawing a 4.15 metre diameter reaming head up the shaft, with all the drill cuttings falling back down the shaft for removal from the mine.

Difficult rock conditions were expected and seven months allowed for the job; however, the job was completed several weeks ahead of schedule. But before reinforcement of the shaft could be completed, the walls of the lower shaft collapsed.

Meantime, there were capital requirements to be met which needed funding to keep cashflow positive. In March/April 2009, Pike River raised $45 million to cover the estimated $7 million cost to fix the shaft and bridge the funding gap caused by delayed production.

The rockfall was plugged by pouring concrete down from the top of the shaft and a 600mm ‘slimline’ vent drilled out to get air to the pit bottom. The $800,000 slimline vent was completed in mid May 2009, providing enough air for coal cutting to resume. An Alimak team from Australia, working around the clock, completed the main shaft bypass the following month and the shaft-top fan was able to start exhausting air up the shaft. With flow-through ventilation restored, final work installing the water-fed flumes and coal crusher at the pit bottom could be completed.

Two heavy cutting machines, a roadheader and a continuous mining machine, have been in use since June last year. Production will be significantly boosted by installation of hydro monitor equipment (high pressure water cannons). Two ‘guzzlers’ will guzzle up the coal cut by the hydro monitors, crush it to less than 200mm, and feed it into steel flumes.

Water and gravity will carry the crushed coal to the pit bottom coal handling facilities where it will be further crushed to 35mm and pumped via the water-fed slurry pipeline 10 kilometres to the coal preparation plant at the bottom of the Pike River valley. When fully commissioned, the hydro monitors will cut coal at an average rate of more than 2000 tonnes a day. First hydro coal is expected in the April-June 2010 quarter.

Pike River Coal has 14,000 tonnes of coal stockpiled and the company was expecting to ship 20,000 tonnes to 30,000 tonnes in its first coal shipment in the January to March quarter of this year.

Environmental costs

A lot of  Pike River Coal mine and its infrastructure sits on (or under) Crown land administered by the Department of Conservation. Establishing the mine involved years of opposition and delay.

In 1996, Pike River Coal, a NZOG subsidiary at the time (NZOG retains a shareholding of just under 30 percent), decided to mine the Brunner seam.

It took eight years to obtain resource consent. Part of the “price” exacted for approval was a significant contribution to the local environment and conservation programmes.

Pike River Coal has set up a pest control programme in the Pike Stream catchment area which is heavily infested by pests including rats, stoats, possums, goats and deer. The company has also put an environmental monitoring programme in place, and implemented ecological and biodiversity studies. The company estimates it spends $200,000 per year on these efforts.