Driving one of the longest tunnels in New Zealand history, Pike River Coal and its contractor overcame a series of ground support problems to reach 60 million-plus metric tons of high-quality coking coal By Alistair Mackenzie

When it reaches full production, the Pike River coal mine, deep inside the Paparoa National Park, will be New Zealand’s largest-output underground coal mine, producing up to 1 million mt/y over its projected 18-year life. Bringing the mine into production has been a saga of rockfalls, big engineering, and sheer dogged persistence.

Located 45 km northeast of Greymouth, on the west coast of the South Island, Pike River holds New Zealand’s largest deposit of high-quality hard coking coal. The coal’s low ash content (1% compared with 8% 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 coalfield has two seams: the Brunner Measures and the deeper Paparoa Measures. The Brunner seam is estimated to contain just over 58.5 million mt of recoverable coal. It is possible that an additional 8 million mt may be recoverable from the Paparoa seam.

Pike River Coal Co. (PRC) was floated on the New Zealand and Australian Stock Exchanges in July 2007. The company has three major shareholders: New Zealand Oil & Gas (29%), Gujarat NRE Ltd. (7%) and Saurashtra World Holdings Private Ltd. (6%). Gujarat and Saurashtra are Indian investors and customers.

A detailed mine design, undertaken in conjunction with the consent process, was completed in June 2005, and three months later the PRC board gave its formal approval to proceed with the project.

To access the Brunner seam, Pike River had to drive a 2.3-km 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).

Because the site is in a mountainous national park, it wasn’t possible to conduct exploratory drilling along the path of the proposed tunnel route. Instead, geological data had to be gathered by mapping stream beds above the proposed drive and exposed cliff faces in the area. These investigations indicated most of the rock would be of high quality with minimal fracture planes, and likely to be self supporting, with relatively minor areas (less than 10%) requiring supplemental supporting systems. But that didn’t prove to be the case.

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-m-wide, 4.5-m-high tunnel was excavated at various inclined grades to pass under located surface features and chemically anchored rock bolts used to hold reinforcing mesh to the excavated D-profile.

Due to restrictions in setting up a concrete batching plant on Department of Conservation-controlled land, wet-mix shotcrete was trucked in from Grey-
mouth, 1.5 hours away. The shotcrete was retarded for 13 hours and varying doses of accelerator were added at the nozzle. A robotic shotcreting machine, brought in from Australia, was used to apply the fiber-reinforced shotcrete.

Because the tunnel had to get through as quickly as possible to start earning return on capital and a continuous conveyor would allow coal production to start as soon as the deposit was reached, Pike River asked McConnell Dowell to install a continuous conveyor mucking system, instead of installing a conveyor once the tunnel excavation was complete.

As it turned out, the continuous conveyor brought several benefits. It reduced movement up and down the unsealed surface of the tunnel and allowed McConnell Dowell to increase productivity by removing larger loads of drill and blast muck. From the continuous conveyor, muck was delivered into a large muck bin at the portal, then into dump trucks for onward disposal.

Before striking coal, the tunnel had to pass through a major fracture zone—the Hawera fault. The team reached the pit-bottom area adjacent to the fault in April 2008. On either side of the tunnel in this location, McConnell Dowell excavated 471 m of large roadway cavities, up to 8 m wide and 11 m high, to allow the installation of water and coal slurry tanks and pumping systems.

Tunnelling through the Hawera fault was slow going, taking crews more than five weeks to blast and drill their way through the 60-m zone of crushed rock. The possibility of methane gas infiltration meant explosion-proof equipment had to be brought in.

On the western side of the Hawera fault, the Brunner coal seam and surrounding strata have been curved upward by the thrusting action of the eastern rock mass. The tunnellers struck coal at the predicted location on October 17, 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 the seams before they are mined. In December 2008, a contractor from Australia began using a track-mounted in-seam drill to gather information 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.

Work then began to raise-bore the 108-m-deep, 4.15-m-diameter ventilation shaft that would provide sufficient ventilation for full-scale mining. The upper 30 m of the bore was injected with concrete grout to stabilize the ground ahead of shaft excavation. Difficult rock conditions were expected and seven months were allowed for the job, however, things went well and the job was completed several weeks ahead of schedule. Work then began on supporting the shaft walls with steel mesh and bolts, and preparations were made to install a surface exhaust fan over the shaft to draw air from the mine.

But before the bolt and mesh reinforcement was completed, the walls of the lower shaft collapsed. The first exports of coal had been expected in April, but the collapse forced Pike River to go back to the market to raise NZ$45 million to cover the estimated NZ$7-million cost to fix the shaft and bridge the funding gap caused by delayed production.

The rockfall was plugged by pouring concrete down the shaft and a 600-mm “slimline” vent was drilled to get some air to the pit bottom, while the shaft was restored. A team from Australia worked around the clock to cut a bypass shaft around the blockage. The NZ$800,000 slimline vent was completed in mid-May, providing enough air for coal cutting to resume. The main shaft bypass was completed the following month and the shaft-top fan was able to start exhausting air up the shaft. As the shaft was created, rockbolts, mesh and shotcrete were used to secure the shaft walls and maintain stability to a “life-of-mine” support standard.

Two heavy cutting machines, a roadheader and a continuous mining machine, were deployed June 2009. More than 17,000 mt of coal has been stockpiled so far, and will be part of the inaugural export shipment of 20,000 to 30,000 mt of hard coking coal to be sent to Pike River’s Indian customers, due in the January to March quarter of 2010.

Production will be significantly boosted by installation of hydro monitor equipment (high-pressure water cannons). Two “guzzlers” will swallow up the coal cut by the hydro monitors, crush it to less than 200 mm, 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 35 mm and pumped via the water-fed slurry pipeline 10 km 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 2,000 mt/d. The first hydro coal is expected in the April-June 2010 quarter.

Alistair Mackenzie is a Christchurch, New Zealand-based journalist specializing in engineering and technology topics.