Quenching the fires that have been burning in the old Strongman mine is a hot job. HUGH DE LACY meets the firefighters.
For the past 18 years fires in the old Strongman underground mines near Runanga on the West Coast have been defying attempts to extinguish them, but tunnelling and ground stabilisation specialist Geotech thinks it’s at last making headway.
Strongman, which for much of last century was the country’s largest underground coalmine, was opened in the notorious Brunner seam in 1939, and closed after it exploded in 1967, killing 19 men.
By then the Brunner seam had already killed 74 men – 65 in the 1896 Brunner disaster and nine more in the Dobson mine in 1926 – and would take the lives of 29 more in the Pike River mine explosions of 2010.
All those mines were underground, but since the early 2000s Strongman has been gradually converted to an open pit operation, with Geotech contracted by owner Solid Energy to operate it under the state-owned collier’s technical and statutory guidance.
The pit is one of five currently operational areas at Strongman, high in the Papamoa Range north-east of Greymouth, with about 12,000 tonnes of high-grade coking coal being mined annually and processed through the Rocky Creek washery at nearby Rapahoe, for export to the global steelmaking industry.
The slump in the global coal price has seen Solid Energy expand Geotech’s brief from coal production and site rehabilitation to include firefighting, all within the original mining contract’s requirement to shift 100,000 cubic metres of over-burden a month.
For years Solid Energy had tried a range of methods to stifle the underground fires, which started by spontaneous combustion, using techniques like injected grout barriers to starve it of air, and removing the coal ahead of the advancing fires to deny it fuel.
These techniques are still being used where appropriate, but Geotech is targeting the fires more directly by digging tunnels to reach them, then saturating the hot areas with thousands of tonnes of water pumped up from the flooded shafts at the bottom of the old underground workings.
The site is itself a challenge, being 600 metres above sea level, and the recipient of no less than six metres – yes, metres – of rain a year.
The fires are sustained by oxygen seeping in through the fractured rock, and they vent into the open air through cracks and fissures near the surface, reaching temperatures of 1000°C in some places.
Geotech’s targets are the fires in the Strongman Two underground mine, which was opened – and has since been closed – not far above the original portal about 20 years after the 1967 disaster.
Employing 28 staff and a powerful phalanx of earthmoving equipment, Geotech’s strategy is to drive a series of pits down to the fires raging anything up to 70 metres below ground, operating within specified areas about 200m by 200m.
Riding with the operator on the digger is a heat-gun which, when pointed at the hot rock tells its temperature, while gas monitors are standard equipment on all machines.
“We don’t put any rock on the truck that’s over 70° – if it’s too hot we don’t work it until it’s cooled down,” Dwayne Solly, Geotech’s project manager for the Strongman job, told Q&M.
Water is pumped from the lower levels of the mine 170m below the still-existing portals and well below the fires, and poured onto the hot rock at the rate of 250t an hour.
“There’s plenty of water at the bottom of the mine – we’ll never run out,” says Solly, whose past experience has mostly been in the quarrying sector, and includes 15,000 hours on excavators and 6000 hours on bulldozers.
As the fires are cooled, the area around them is mined, giving a return on the operation but, more importantly, denying the fires further fuel.
Geotech’s first target fires were on what is known as the Nine-Mile ridge after the river and valley of the same name in which Strongman sits.
That required the removal of 800,000m3 of overburden and took six months.
The current efforts are aimed at the X-Y ridge where 1.7 million cubic metres is being shifted over a 15-month period.
The line-up of equipment is impressive, headed by a 65t Hitachi excavator backed up by a 75t Komatsu.
Komatsu 155 and Caterpillar D11N bulldozers push the stuff around, and spoil and harvested coal alike are trucked away by a couple of old 50t Terex trucks, three Komatsu 605 rigid dumpers and three Komatsu HM400 trucks.
The operation has substantial drilling capacity in the form of a Sandvik D50KS rig supported by various down-hole hammer rigs.
As well as drilling and earthmoving, Geotech does all the blasting, with blast management flexibility and efficiency ensured by the on-site use of a technically-advanced emulsion plant, one of only two believed to be in the country.
Geotech was founded in 2000 by Ant Black, a specialist in surface and underground mining technology with more than two decades’ experience in Australia and New Guinea as well as New Zealand.
Black has been working on the West Coast since the late 1980s, and Geotech was his response to growing demands for technological expertise from the hydro-electric as well as the export coal sectors.
The new company landed a contract on the Manapouri power scheme, doing rock stabilisation by shotcrete and bolting, and it was also giving advice on ventilation and gas management in the great tunnel cut through the rock to the underground generators.
The booming export coking coal trade drew the company to the West Coast, working mostly for Solid Energy, but also doing shaft recovery work for Oceana Gold and cliff stabilisation for the Department of Conservation.
In the latter role they rescued ‘Rocky’, a 15-tonne boulder that had crashed into a Morgans Valley house in Christchurch during the March 2011 earthquake, and found a new home for it on the Mount Hutt ski area near Methven.
Geotech was also called in to deal with another rock so notorious it had earned a nickname for itself, Scary Lucy, at Solid Energy’s Stockton mine Ridgeline Project.
Scary Lucy was a 60 tonne sandstone behemoth that held up progress on the Ridgeline for two months at a cost of $70,000.
Geotech stabilised the ground around the rock, which was eventually dealt with by being detonated inside a safety net.
At Strongman, coal-winning and firefighting are going hand in hand with post-mining rehabilitation and remediation, further broadening the demands the old mine is making on its latest miners.