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…An introduction to minerals
…An introduction to aggregates
…What are aggregates?
…Where do aggregates come from?
…How are aggregates formed?
…How are aggregates mined?
…How are aggregates produced?
…What are aggregates used for?
…How are aggregate quarries rehabilitated?
…WORKSHEET No 1: Aggregates
…WORKSHEET No 2: Investigating New Zealand's Mineral Resource in Detail
On a world scale New Zealand quarries are mostly small, but they tend to produce a wider range of products. The largest quarries in New Zealand have worked basalt deposited from geologically young volcanoes in the Auckland region. Greywacke – an older sedimentary rock - is now replacing basalt which is becoming depleted or built over. More than 600 open pit workings produce all the rock, gravel and sand used in New Zealand. It can be seen that the majority are small when it is considered the eight largest quarries in the Auckland region produce around 25% of the combined total output of the 600. Rocks for making concrete and building roads account for 75% of all the rock quarried in the country. The use of aggregate is closely related to population. Aggregates have a relatively low value for their bulk, so the cost of transporting these materials is high. For this reason most large quarries are located on resources close to main centres. The growth of towns and cities increases the demand for aggregates but restricts the supply as sources near cities run out or are built over. The cost of building and road making then rise as transport distances increase.
Most regions in the country have a range of resource types sufficient to supply their needs. Two exceptions are the Taranaki and Gisborne areas where high grade materials are unavailable.
Often a company will own a number of small quarries throughout a region and operate them as needed with portable plant. There have been few new quarries opened in the last 20 years.
Aggregates are the crushed rocks, gravel and sand which are produced by quarries throughout New Zealand. Most North Island aggregate is produced from quarries that extract sedimentary and volcanic rocks. In the South Island most aggregates are produced by working gravel deposited by rivers. River extraction is limited to the estimated renewable quantity of the river. Material smaller than 2mm is classified as sand. This is extracted from deposits near the coast and is dredged from rivers and the seabed. Aggregates account for about one third of the value of annual New Zealand mine output.
Where do aggregates come from?…
Aggregates are found in most areas of New Zealand. As with all minerals, aggregates can only be mined where they exist. This makes planning for their future use of critical importance.
How does rock for aggregates form?
Aggregates quarried in New Zealand are mainly from sedimentary (greywacke) and volcanic (basalt and andesite) rocks.
How are aggregate rocks mined?…
Rock is generally drilled and blasted with explosives then loaded onto trucks or conveyors and transported to a nearby processing plant for treatment. As the quarry usually has near neighbours the blasts are controlled by design and are often fired in a delayed sequences rather than all at once to reduce vibration.
At the process plant the rock is crushed, screened and washed to produce a range of products. Large quarries produce 20 or more different types of material. Two or three stages of crushing and screening may be used depending on the required size of the final material.
The type of rock and the way in which it is processed is selected according to its end use. The rock chip used to tarseal roads needs to have the correct size, shape, and surface texture. Rock used in concrete which must flow freely has to be free of low strength materials, and also not include any minerals which could react with the cement.
above left: A rock crushing plant in a quarry.
below right: Aggegates are mainly used for building roads from bottom to top.
What are aggregates used for?…
Aggregates are mainly used for building roads from the bottom to the top - from the foundation layer through to the asphalt or chip surfacings - making concrete, and in the manufacture of concrete and masonry products. A typical new house in New Zealand contains about 250 tonnes of aggregate. A large city building may contain up to 100,000 tonnes, used in concrete, foundation materials, roads, drainage systems and carparks.
Each year in New Zealand every person creates the demand for up to nine tonnes of aggregate.
The main uses of aggregate are:
- base course material used as foundation for sealed and unsealed roads
- repairing and surfacing roads
- maintaining railway lines
- manufacturing concrete products such as blocks, pipes and paving
- making bitumen paving for surfacing roads, airport runways and port facilities
- drainage and filtration
- making concrete (concrete is made mostly from aggregate)
Each use requires different material. Most large quarries produce several types. New Zealand's most productive quarries are operated by Winstone Aggregates Stevenson Group in Auckland. They produce 30 different types of aggregate, all made from the sedimentary rock greywacke.
How are aggregate quarries rehabilitated?…
Many quarries are located close to built up areas. Often cities and towns expand to surround the quarry which was probably originally located not far to the urban area to ensure lower transport costs. Expert help is used to help control effects such as noise, dust, vibration from blasting, and water quality. All stormwater runoff from working areas must be controlled by using settling ponds. Large quarries work to long term plans to maintain the quality of the aggregate they produce while at the same time ensuring that the operation of the site is environmentally acceptable and that the site will move towards a new productive use at closure.
New and proposed quarry operations are investigated in detail so that the design and proposed operating procedures allow the effects on the surrounding environment to be controlled. Operating conditions are developed to suit the particular site. Where practical, ongoing rehabilitation is planned as part of the operation. The final long term plan for the site is considered during the planning stage, although because of the relatively long duration of quarrying the planned end use of the land is usually left undefined.
Trees are planted during initial development to act as a visual screen. Sometimes an earthen noise bund may be built. Topsoil and overburden is removed and stockpiled. When the pit has reached its final depth the overburden may be used to progressively backfill, with rehabilitation being carried out at the same time. When rehabilitation is completed the area can begin a different productive life as a recreational area, water storage facility, wildlife refuge, or any number of other possibilities. Isaacs quarry in Christchurch is now a wildlife park and salmon farm.
above, below: Isaac's quarry in Christchurch is an example of a mining site which has begun a different productive life after mining has finished.
Ericsson Stadium in Auckland is the venue of many international sporting and entertainment events. It was the Mount Smart quarry. To view photographs of Mt Smart as it changed click here. Some quarries are backfilled with refuse and used as a landfill site by the town or city the quarry was started to service. To view a series of graphics which show a typical quarry rehabilitation click here.
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For more information about aggregates and quarrying visit the Aggregate & Quarry Association of New Zealand website www.aqa.org.nz
Click here for recent facts on aggregates.