Track ballast forms the trackbed upon which railway sleepers (UK) or railroad ties (US) are laid. It is packed between, below, and around the ties.It is used to bear the load from the railroad ties, to facilitate drainage of water, and also to keep down vegetation that might interfere with the track structure. This also serves to hold the track in place as the trains roll by. It is typically made of crushed stone, although ballast has sometimes consisted of other, less suitable materials. The term "ballast" comes from a nautical term for the stones used to stabilize a ship.
Good quality track ballast is made of crushed stone. The sharp edges help the particles interlock with each otheer.
Properties Of Ballast
1) a good ballast should be strong, hard-wearing, stable, drainable, easy to clean, workable, resistant to deformation, easily available, and reasonably cheap to Purchase.
2)Good quality track ballast is made of crushed natural rock with particles between 28 mm and 50 mm in diameter.
3)A high proportion of particles finer than this will reduce its drainage properties, and a high proportion of larger particles result in the load on the ties being distributed improperly.
4) Angular stones are preferable to naturally rounded ones, as angular stones interlock with each other, inhibiting track movement.
5)Soft materials such as limestone are not particularly suitable, as they tend to degrade under load when wet, causing deterioration of the line; granite, although expensive, is one of the best materials in this regard.
CONSTRUCTION AS FOLLOWS
1)The thickness of a layer of track ballast depends on the size and spacing of the ties, the amount of traffic expected on the line, and various other factors.
2)Track ballast should never be laid down less than 150 mm (6 inches) thick; high-speed railway lines may require ballast up to half a metre (20 inches) thick.
3)An insufficient depth of ballast overloads the underlying soil; in the worst cases, this can cause the track to sink.
4) If the ballast is less than 300 mm (12 inches) thick, this can lead to vibrations, which can damage nearby structures.
5)Track ballast typically sits on a layer of sub-ballast; the latter is typically made of small crushed stones. It gives a solid support for the top ballast, and seals out water from the underlying ground.
Sometimes, an elastic mat is placed under the ballast layer as well; this can allow for significant reductions in vibration.
6)The ballast shoulder should be at least 150 mm (6 inches) wide under any circumstances, and may be as large as 450 mm.
MAINTAINANCE AS FOLLOWS
1)If ballast is badly fouled, the clogging will reduce its ability to drain properly; this, in turn, causes more debris to be sucked up from the sub-ballast, causing more fouling.Therefore, keeping the ballast clean is essential.
2)It is not always necessary to replace the ballast if it is fouled, nor must all the ballast be removed if it is to be cleaned. Removing and cleaning the ballast from the shoulder is often sufficient, if shoulder ballast is removed to the correct depth.
3)While this job was historically done by manual labour, this process is now, like many other railway maintenance tasks, a mechanised one, with a chain of specially-designed railroad cars handling the task.
4)One wagon cuts the ballast and passes it via a conveyor belt to a cleaning machine, then the cleaning wagon washes the ballast, and deposits the dirt and ballast into other wagons for disposal and Reuse,Respectively.Such machines can clean up to two kilometres of ballast in an hour.
5) If the trackbed becomes uneven, it is necessary to pack ballast underneath sunk ties to level the track out again. This is, in the mechanized age, usually done by a ballast tamping machine.