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 Greymouth, 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 a