Railway sleepers often sit on a bed of small stones that act as ballast. This ballast material stretches well beyond the width of the sleepers and to quite a depth. But why does it have to be stones? Could anything else be used either physically or economically? (Continued)
鈥� An earlier correspondent discusses ballast without referring to the nature of the rock used, which is a critical parameter. Ideally, the rock should be from a tough and stable lithology.
Samples of the potential material for railway ballast are examined in thin sections by a mineralogist using a petrographic microscope. The aim is to look for evidence of minerals that are likely to break down when exposed to factors such as air, water and load pressure.
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鈥淚deally, rock used for railway ballast should come from a tough and stable lithography鈥�
In some states in Australia, much of the ancient rock available locally is weathered at the surface, and more ideal rock may have to be transported over vast distances. So to save on costs, the ballast used is often less than ideal.
Mining sites are useful because mine waste brought to the surface has not been weathered in the same way. However, this kind of material can cause other problems. In one example, ballast taken from a nickel mine caused degradation of the metal railway sleepers because it contained a lot of pyrrhotite, . The pyrrhotite oxidised and produced sulphuric acid, which rapidly corroded the metal.
Concrete beds are not foolproof either. The common 鈥渃oncrete cancer鈥�, caused when the cement paste reacts with the rock aggregate, is often a result of coming into contact with unsuitable ballast rock.
Roger Townend, Townend Mineralogy Laboratory, Malaga, Western Australia
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This article appeared in print under the headline 鈥淎 bed of rocks鈥�