A city's heritage

Dunedin is proof that a big city can live comfortably with a quarry at its heart, reports HUGH DE LACY.

When the Edinburgh of the South finally gets its brand-new $200 million sports stadium in 2011, Tony Hunter will be able to stroll across the road from his office at the Logan Point quarry to see Rugby World Cup games.

And if they’d had such an event and such a stadium back in 1880, Hunter’s great-great-grandfather, James Palmer, could have done the same, because that’s how long Logan Point has been in operation.

Even today Hunter, who manages the quarry, never bothers to drive to inner-city business appointments; it’s just as easy to walk.

Operating within the stringent environmental standards of ISO1401, Logan Point has made the citizens of Dunedin both familiar with and relaxed about having a quarry within two kilometres of their central business district – and no doubt their Scots ancestors would have approved of the huge cost savings that have accrued to the city from having it so handy.

Logan Point features all the usual prohibitives to a quarry in a built-up area: the phonolite dome rock has to be drilled and blasted, generating the inevitable dust and vibration, and, as well as crushers, loaders and trucks the site also hosts both concrete and asphalt plants. Even more startling to those who believe quarries have no place in town is the fact that this one has been operating for so long it’s now 60 metres below sea-level, and floods three or four times a year.

Not that the floods are any problem in themselves: the rock is impermeable, and the water – which is routinely sampled and tested, and can take up to a week to pump out – is used to control dust, and to clean sealing chip and concrete aggregates before being discharged by way of a water treatment plant.

For the blasting, a hydraulic rig drills 89mm holes with a 2.8 metre burden and spacing on a 10 metre face, with vibrations controlled and fracture improved by way of electronic detonators. A 30-tonne excavator fitted with a Terminator rock-breaker, assisted by a 20-tonne wheel loader, dump the rock onto a couple of 20-tonne trucks filling a 50-tonne hopper with an apron feeder to an Allis 1208 jaw crusher with a capacity of up to 200 tonnes per hour.

Rock crushed to under 30mm is taken off to clean the bulk of clay fines, while the rest goes onto a 1000-tonne surge pile sitting atop a variable speed feeder supplying an Allis H4000 hydrocone crusher which reduces it to 55mm roadbase. Anything oversized circulates through a Barmac 9600 Duopactor, which puts it through a 16mm sieve for sealing chips, concrete aggregates and asphalt dust.

An unusual feature of Logan Point, arising at least in part from its urban location, is that it operates none of its own distribution trucks: all outside deliveries are done by independent contractors.

At 250,000 tonnes a year Logan Point is the biggest aggregate supplier in the Dunedin district, but it’s not the only quarry within the city’s environs. It is actually just one of the facilities operated by Blackhead Quarries which, with a full-time staff of 45, is probably the biggest quarrying company south of the Waitaki River.

The company is named after the Blackhead quarry which sits on a promontory of black basalt just five kilometres south of Dunedin’s St Clair beach, producing top-end products like M4 roadbase, as well as sealing chip and concrete aggregates. Blackhead was first operated by the Grey family in the 1940s until being sold in 1986 to a joint venture comprising James Palmer’s old company, Palmer and Sons, and local contracting company Fulton Hogan.

Today the Blackhead quarry produces about 120,000 tonnes of product a year, and features an on-site laboratory that provides external quality assurance auditing to ISO 9001-200 standard.

Blackhead rock is drilled and blasted on a pattern of 2.7 metre by 3.3 metre with 10 metre bench heights. A 30-tonne excavator, supported by a 20-tonne wheel loader, loads a 10 cubic metre truck to feed a Nordberg 40 by 30 single-toggle jaw crusher, and thence to a 6000-tonne surge pile. The rock is crushed to 65mm by an Allis H4000 cone crusher screened for roadbases, with the oversize going into a Number One Kumbee to produce railway ballast.

Overrun goes to another surge pile feeding a Barmac 9600 Duopacter producing sealing chips, concrete aggregate and sand. It’s a highly automated operation monitored by video from the 13 metre computerised weighbridge.

The joint venture proved so successful that in 1986 it was restructured as a single company, Blackhead Quarries, into which the partners’ rock drilling services were also folded five years later. Working both for the company and outside interests anywhere south of the Waitaki, this drill-and-blast unit employs two Tamrock Ranger 700 drill-rigs. At 150 tonnes a year, it is one of the biggest explosives users in the country.

The parent company was further augmented in 2004 by the addition of Logan Point, Balclutha Quarries, a sand plant at Walton Park in Dunedin and a mobile crushing service.

The Balclutha operation takes up to 30,000 tonnes of gravel a year out of the Clutha River from Manuka Island, where the quarry is sited, and from Clydevale. This is used for the production of sands and aggregates but because of the hydro-electric dams upriver, the resource will soon run out.

However, five kilometres upstream from the quarry the company has a 50 hectare block of land containing a 100 years’ supply of high-quality greywacke rock which is being won by drilling and blasting 10,000 tonnes at a time. This is put through a primary crusher-feeder at the quarry, and then through a 42 by 30 jaw crusher, with the lower grades screened out for base-course. A Nordberg GP 300 cone crusher travelling over another screen takes out roadbases, with the rest passing through a Twister crusher and through a washing and screening plant to produce high-value polished stone value sealing ships.

The Walton Park sand operation is based at Fairfield, just south of Dunedin, and produces about 60,000 tonnes a year of high-grade golden sand for concrete aggregates, specialised sand for resin flooring sold nationwide, and beach sand for the moulding of steel at Dunedin’s three main iron foundries.

Rounding out Blackhead Quarries’ inventory of operations is its two mobile crushers, which operate at up to 15 sites from Omarama to Owaka, and from Middlemarch to Millers Flat. The first of these is a Nordberg C100 jaw crusher with a Kumbee secondary crusher. It’s driven by a 500kva generator and is carted round on seven articulated trailers.

The second mobile plant is a Nordberg LT 1213S impact crusher which, in 2005-06 was the first of its kind in the country. It can produce twice as much as the older plant and also features an advanced quality-control capacity.

Blackhead Quarries is likely to be a significant provider of materials to Dunedin’s new sports stadium, which will be ready for the Rugby World Cup and will be the first such facility in the country with a roof. That latter feature may prove handy, given that a badly-directed boot from an errant rugby player in the downtown stadium could see the ball end up 60 metres below sea-level over the road at the bottom of the Logan Point quarry.