PETER OWENS looks at Southern Aggregates’ hard rock quarry at Greenhills near Bluff, which produces new Ballast Track Specification 140.
In the 19th century a rapidly constructed and comprehensive rail network was essential for the development of the young colony.
Our rail links are still very important and KiwiRail operates services on 3898 kilometres of track, of which around 500 kilometres is electrified.
Track ballast is vital to the continuous and safe operation of this national rail network and KiwiRail is very particular about both the ballast and its suppliers.
The southernmost section of our rail links Dunedin and Bluff and Southern Aggregates of Invercargill is one of the main suppliers of rail ballast to the exact specifications set by KiwiRail.
Southern Aggregates’ hard rock quarry at Greenhills near Bluff produces the ballast, made to the new Ballast Track Specification 140, amongst other materials such as ornamental boulders and concrete sand.
At the quarry track ballast is stored in certified stockpiles and is handled in conditions that prevent material contamination, segregation and degradation.
Quarry manager, Kerry Sands, says there isn’t a ‘continuous’ demand for the specified ballast, but that it is always there whenever the customer needs it.
It is a world standard that the thickness of a layer of track ballast depends on the size and spacing of the ties, the amount of traffic on the line, and various other factors, but is never laid down less than 150mm thick. However, beyond a depth of 300mm adds no extra benefit in, say, reducing vibration.
Track ballast typically rests on a layer of small crushed stones – the sub-ballast – which provides support and reduces ground water seepage. Sometimes an elastic mat is placed on the layer of sub-ballast and beneath the ballast to reduce vibration.
The ballast is also piled as high as the ties and a substantial ‘shoulder’ placed at their ends that is a minimum of 150mm wide and up to 450mm wide. The shape of the ballast is also important. Stones must be irregularly cut, with sharp edges, so that they properly interlock and grip the ties in order to fully secure them against movement. Spherical stones cannot do this. In order to let new ballast settle and interlock, speed limits are lowered on sections of track for a period of time.