New Zealand is lucky: the rock known as greywacke, an ideal source material for aggregates, is widespread in mountain belts. Other widely used rocks are argillite, basalt and andesite. They are quarried throughout the country by drilling and blasting, and then crushed, washed and screened.
The South Island’s braided rivers, which carry rocks down from the Southern Alps, have helped create a huge, accessible resource on the Canterbury Plains. Most of the weathered and softer rock has been washed away, leaving harder greywacke in the form of river gravels. These are crushed to produce angular surfaces and different sizes and shapes. Most are for bulk use, but some are not suitable for road-sealing chips as they have rounded surfaces.
Return to the stone age
Metal machinery used to crush rock has a high rate of wear and tear. In the late 1960s and early 1970s, quarryman Bryan Bartley and roading engineer Jim Macdonald invented a more durable rock crusher, based on a simple idea – the rocks themselves would break each other into smaller pieces, as they tumbled inside a drum. This reduced the need for metal components. It took 20–30 years to be recognised, but this Kiwi invention proved a worldwide success. Thousands of ‘Barmac’ rock crushers have been used on projects including the massive Three Gorges Dam on China’s Chang (Yangtze) River, and sand manufacture in the deserts of the Middle East.
Roading aggregate must comply with certain specifications. For example coarser, more angular rock chips are used to increase traction on corners, and on roads such as the Desert Road or in the Kawarau Gorge, which are likely to ice up in winter.
For road foundations clay is helpful to bind rock chips. In the layer on top of the foundation the chips are harder. The top layer of sealing chips must be resistant to chemical attack and rough, so that it does not become polished by tyres passing over it.
In Gisborne and other areas which lack more suitable rock types, limestone is commonly used as a road aggregate, giving the roads a lighter colour. As limestone is a softer rock these roads tend to wear faster.
Aggregates are used in concrete, whose other components are cement, sand and water. To ensure the concrete is strong enough, the rocks must also be non-reactive – they must not contain minerals that react with the other constituents of cement. In New Zealand, potentially reactive rocks occur mainly in the volcanic region of the central North Island. The andesite from Mt Taranaki is also too reactive for many uses.
Bulk fill, rip rap and pea metal
Almost any rock is suitable as fill for reclaiming land from the sea. Fist-sized stones become railway ballast, and large rocks, known as rip rap, are used to contain harbours and rivers. Pea metal, made of rock chips about the size of a pea, is used extensively for drainage.
Silica sands, which are brilliantly white, have in the past been mined at Pārengarenga Harbour for making bottles and other glass products in Auckland.
In the 2000s sand for concrete had to be dredged offshore to supply Auckland’s building industry. Since 1953 about 750,000 cubic metres of sand have been mined from the entrance to Mangawhai Harbour in Northland. This was a contentious issue in the early 2000s, as residents were concerned about coastal erosion.