Stevenson Resources has been moving vast quantities of overburden around its Drury Quarry to get at resources that were never expected to be quarried in the old days.
Drury Quarry is located at the base of the Hunua foothills in Auckland’s southern boundary and is one of the last ‘city’ quarries in the region.
The quarry contains a significant and substantial aggregate resource with a potential life in excess of 150 years.
Future proofing the access to that resource has been on-going for the Kiwi company, which has commercial interests in engineering, concrete, mining and agriculture.
The company was founded by William Stevenson in 1912 as a drain laying business. Between 1938 and 1939 the family-run business bought the Drury Quarry, 30 kilometres south of Auckland.
At the time the quarry was considered so far from Auckland that Stevensons had to provide housing for its workers. Over the decades, millions of cubic metres of aggregate have been extracted from the quarry to build Auckland’s roads and major civic projects, and the quarry is expected to contribute to the region’s growth for many decades to come.
Over the past decade Stevensons has invested huge resources in protecting its Drury Quarry from residential developments that are forever encroaching on the city’s retreating green fringe. The company owns about 2000 acres of farmland around its Drury resource to act as a buffer zone, and has planted a large greenbelt to mitigate any problems with the local community over dust and noise.
However, one of its latest projects, essential to future extraction, is within the quarry boundaries. This involves a 35-year, five-stage project to move overburden off the north-east side of the pit to get at the aggregate underneath.
This overburden was placed there in the quarry’s early days when no one, least of all the Stevensons, thought the quarry would be working back this far.
The company is halfway through the project, which involves preparing the site at the very top of the quarry’s north-eastern edge and preparing the fill site (sloping farmland and deep gullies raising up from the quarry’s eastern edge towards the dam on the top) where the material will be dumped.
While preparing the old overburden site (pictured) workers found an unexpected pile of huge basalt boulders, some of at least 60 tonnes that are natural basalt intrusions.
“We never knew they were there and they slowed us down a bit getting rid of them,” comments Stevenson Resources aggregate manager, Steve Ellis.
The first stage of the fill site on the eastern slopes involved earthworks and stone water races, and moving and ‘lifting’ a significant stream running down the eastern slopes.
While planting extends up the slope some 30 metres, rainwater coming off the fill is captured in watercourses and treated in a flock station before joining the diverted creek.
Ellis says around 250,000 metres has already been stripped and moved to the new fill site and another 600,000 to 700,000 metres will be moved in the next stage (some overburden will be used down on the Drury South project – a new 223 hectare industrial zone being developed by the company).
This will provide access to another 3.5 million tonnes of aggregate.
“Yes, we are still very much alive here with 40 years of resource in the current pit and another 100 odd years from the southern rock at the back.”