Science Spin January 2007

Quarrying, a misunderstood industry?

By Paul Lyle

Quarry operators have made great efforts in recent years to minimize the effects of quarrying on the environment and to improve their image. Changes in environmental legislation have brought tighter controls on quarry working and the operator has to re-habilitate the site at the end of its working life. The days when worked-out quarries were abandoned to become unauthorised rubbish tips should, thankfully, be over.

This article aims to shed some light on the significance of the industry, in this case in the north of Ireland. I describe the desired properties of a good aggregate for roads and a few of the vast range of products produced by the quarrying industry.

Aggregates

Aggregates are rock fragments, which may be combined or aggregated to produce a mixture that can be used in construction. The aggregate may be bonded with cement or bitumen, as in concrete and macadam, or used un-bonded as filler. This article is largely about rocks that have been quarried, crushed and then screened to produce rock fragments of a desired size; it does not include sand and gravel.

Compared to many areas in Great Britain, particularly in southeast England, Northern Ireland has an abundance of good quality hard rock available for the construction of buildings and roads. I wonder how many people reading this article are aware that when they are travelling along the notorious circum-London M25 motorway, they are as likely as not to be on a road surface made from sandstone from a quarry in County Down, the aggregate for the wearing course, as the top surface of the road is known, having been shipped from the docks at Belfast directly to Kent?

There are few natural resources in Northern Ireland available in large enough quantities for export to Great Britain and continental Europe, but high quality roadstone is one of them. What is it about County Down sandstone that makes it special enough to be worth the trouble of shipping it to England, Holland and Germany?

Roads

To answer that question it is necessary to look at the changes that have taken place in transport and driving over the last 50 years. Also the parallel changes that have taken place in road construction methods. We now drive much faster, there is far more traffic and much greater loads are carried by lorries. This means correspondingly increased demands on the materials used to construct the roads.

The weight of the vehicles travelling on the wearing course is carried, not just by the top layer, but also by the various layers below the surface, down to bedrock. These lower layers of aggregate are required to resist pressures from the crushing and impact loads imparted by the weight of traffic, but there are additional properties required by the top surface. The wearing course consists of crushed rock fragments of varying sizes combined with some form of binder, commonly bitumen in this country.

The requirements of the aggregate particles in the wearing course are that they resist wear and tear, known as abrasion, and thus maintain the integrity of the road structure. They must resist as far as possible the tendency to be smoothed and polished by the frictional action of wheels moving at speed over the road surface.

Smoothing and polishing would lead to a surface that is prone to skidding. More skidding means more accidents and loss of life. More than any other properties, it is the ability to resist polishing that makes County Down sandstone or gritstone, as these rocks are termed in the quarrying industry, such a desirable commodity for modern-day roadmakers.

Many hard rocks of all categories, igneous, sedimentary and metamorphic, have the necessary resistance to abrasion and the inherent strength to resist crushing and sudden impact. The resistance to polishing is more complicated. Understanding it requires detailed examination of the rock, its mineral content and proportions, texture and grain size.

Polished Stone Value (PSV)

The ability of a rock to resist polishing is measured by a British Standard Test to determine the polished stone value.

The apparatus simulates the action of dust-laden tyres on samples of aggregate set in epoxy resin, mounted in standard moulds on a rotating "road wheel". A solid rubber tyre rides on the simulated road while abrasive powder and water are continuously fed on to the surface of the tyre. After a set time the degree of polishing of the sample is measured using a standard pendulum arc friction tester.

The deflection of the pointer on a calibrated scale gives the coefficient of friction as a percentage - the Polished Stone Value - and a higher value means greater resistance to polishing. Values of PSV greater than 60 are generally required for roads with high density or high speed traffic. The best gritstones from County Down can give PSV's of 65 or greater, while the values for basalts tend to be lower, usually around high 50s or low 60s, and limestones are commonly around 50 or lower.
Research has shown that those aggregates with high Polished Stone Values are those that contain mineral grains of contrasting hardness.

The County Down gritstones consist predominantly of hard grains, mostly quartz, contained in a softer matrix comprising mostly clay minerals. Photographs taken down a microscope (known as photomicrographs) of very thin slices of the rock can reveal details of the shape and textures of the grains and their relationship to each other.

As the gritstone aggregate is trafficked, the relatively soft matrix of clay minerals holding the hard grains allows these to be "plucked out" before they can reach a state of high polish. Detailed measurements of the forces involved between the passing tyre and the road surface has shown that a powerful torque or twisting motion is imparted to each aggregate particle on the road surface. So there is a tendency for the hard grains to be pulled out, rather like a tooth extraction.

The result is a constant replacement of the outer surface of each aggregate chip and so a continual renewal of the rough surface and therefore a continuing high level of skid resistance. Crucial is the proportion between the hard grains and the softer matrix. Too much soft clay mineral would weaken the strength of the rock and it would have a poor resistance to abrasion, while an absence of softer minerals quickly leads to a high degree of polishing. A road made from quartzite, for example, which is almost entirely made up of hard quartz grains, would be very durable but would soon be like an ice rink.

You may have noticed in recent years the appearance of a much lighter coloured road covering at the approaches to roundabouts or at other complex junctions. This is a high PSV material designed to reduce the incidence of skidding during braking. It is called calcined bauxite and has exceptionally high Polished Stone Values of more than 70. It is manufactured by heating bauxite, which is largely hydrated aluminium oxides formed by the tropical weathering of basic igneous rocks such as basalt.

The bauxite used on our roads is usually obtained from South America. Visitors to the Giant's Causeway will be familiar with bauxite in the red Interbasaltic laterites exposed along the cliff path. When heated to temperatures of more than 1500 degrees Celsius, the hydrated aluminium oxides are transformed to the very hard mineral corundum, which has very good polish-resisting properties when applied to a wearing course.

The process is expensive so the high-PSV layer is usually applied as a thin skim to the existing road surface. The example shown here is at a busy road junction controlled by traffic lights. The contrast in colour between the normal macadam wearing course and the pale-coloured calcined bauxite is clearly seen.

Stone products

As well as aggregate for roadstone, the quarrying industry in Northern Ireland produces a wide range of stone products that are essential to modern life. An important product is the cement block, which appears to be universally used in the building industry, either as a replacement for traditional bricks or in a complementary role. Kerbstones, concrete beams, paving slabs, concrete pipes of a wide range of diameters, all of these essential components of the construction industry are pouring out of

Northern Ireland's quarries in phenomenal numbers as the country upgrades its roads, water and sewage facilities and buildings, domestic, industrial and commercial. The value to the economy of the region is immense. For the year 2005, Northern Ireland quarries produced nearly 28 million tonnes of material, with a total selling value of about £110million (€160million) and employing just over 1,650 people across Northern Ireland.

The quarrying industry has a responsibility to ensure that its products are extracted with the minimum of disruption and cost to the environment. The public demands higher standards of infrastructure, leading to greater extraction of the aggregate resources and eventually to more and larger quarries. We must recognize the importance of the role played by the quarry industry in giving us a higher and safer standard of living.

To visiting a virtual quarry click here .

This website is brilliantly presented and teachers will find lesson plans linked to the curriculum as well as the listing of a few quarries that offer school visits.