One of the country's largest blasts on record went unnoticed last month when Winstone Aggregates detonated 20 tonnes of explosives to loosen rock at their Hunua Quarry south of Auckland.

Part of the local community for more than 50 years, Hunua Quarry operates within a framework of District and Regional Plans defined by the Resource Management Act as well as Winstone Aggregates' internal policies. Blasting is stringently regulated - four blast monitors record environmental effects (ground vibration and airblast) to ensure safety and environmental limits.

Quarry Manager Tim Hazell describes modern blasting technique as a precise and highly engineered activity. "These days, we use laser profiling to determine the optimum location for explosives. Noise and vibration levels are kept to a minimum by using a series of small blasts which actually reduce the side effects of the main blast".

By New Zealand standards, Hunua is considered a large "hard rock" quarry. Total site land holdings are in the order of 240ha and the processing plant commissioned in 2001 has an annual average output of about 2 million tonnes. This represents about 20 - 25% of Auckland's regional demand for aggregate.

Facts:

Shot designed, loaded and overseen by Orica Mining Services

144 holes (102mm diameter, depth of 19m) drilled by Drillco Specialists Limited

Electronic detonators initiated 20 tonnes of bulk explosives

Approximately 170,000 tonnes of greywacke rock blasted

This image: Tim Hazell, Hunua Quarry Manager