What is Railway Ballast?
Railway ballast is an aggregate made up of crushed granite rocks. It is used to create a level and strong foundation for rail tracks and sleepers. It consists of 30-50mm angular stones and is sieved to remove fine particles that are not as durable.
Ballast is tightly packed below, between, and around the tracks. It is also known as rail ballast and track ballast. It should not be confused with regular ballast, which is used to create concrete.
Rail ballast can also be used to create strong sub-bases and for haul roads that transport heavy loads of construction materials.
Why Are Rail Tracks Laid on Ballast?
Railway tracks are laid on ballast to create a stable base capable of withstanding extreme weights. Using ballast helps keep the tracks in place and evenly distributes the strain placed on sleepers when trains pass. The deeper the layer of ballast, the better it is at reducing pressure.
In addition, ballast raises the height of the track and the gaps in its stones improve drainage, which prevents waterlogged lines and sodden sleepers. It also suppresses weeds and vegetation.
What Stones Are Used For Railway Ballast?
Granite stones that measure 30-50mm in size are typically used for railway ballast in the UK. Granite is a strong rock and is the ideal material. Limestone may also be used, but is not as durable and may require more maintenance than granite.
The ballast has sharp edges that interlock with one another to form a strong bond. If the stones were round, they would roll away when a train passed by, which would significantly compromise their ability to form a solid foundation.
Recycled Railway Ballast
Recycled railway ballast is generated when tracks are cleaned. Machines remove dirty, worn ballast and replace it with new stones that are sharper and effectively interlock. The old ballast is washed and crushed so it can be reused in the rail and construction industries.
Until 2011, the old ballast, which is known as spent ballast, was sent to landfills, rather than being recycled. Network Rail estimates that they are now recycling more than 1.2m tonnes of railway ballast each year.
Railway Ballast Specifications
When laying railway ballast, there must be a layer of at least 150mm below the railway sleepers. However, a depth of 200mm is often used for extra security. There must also be ballast either side of the track, which is known as a ballast shoulder. For straight tracks, the ballast shoulder should be no less than 380mm wide.
The European standard BS EN 13450 specifies the properties of aggregates that can be legally used as the upper layer of railway track. They must meet strict standards for strength, cleanliness, and the ability to resist freeze-thawing.