Everything about Track Ballast totally explained
Track ballast forms the trackbed upon which
railroad ties (US) or railway sleepers (UK) are laid. It is packed between, below, and around the ties. 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 marine shipping term for the stones used to weigh down a ship. Early railway engineers didn't understand the importance of quality track ballast; they'd use cheap and easily-available materials such as ashes, chalk, clay, earth, and even
cinders from locomotive fireboxes. It was soon clear that good-quality ballast made of
rock was necessary if there was to be a good foundation and adequate
drainage. 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. in other areas, the ballast can be a mix of light and dark colors called "Salt and Pepper".
Construction
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. An insufficient depth of ballast overloads the underlying
soil; in the worst cases, this can cause the track to sink. If the ballast is less than 300 mm (12 inches) thick, this can lead to vibrations, which can damage nearby structures (though increasing the depth beyond this has no measurable effect).
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. The ballast shoulder should be at least 150 mm (6 inches) wide under any circumstances, and may be as large as 450 mm (18 inches).
Maintenance
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. While this job was historically done by manual labour, with a chain of specially-designed
railroad cars handling the task. 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 re-use or disposal, respectively.
Cleaning, however, can only be done a certain number of times before the ballast is damaged to the point that it can't be re-used; furthermore, track ballast that's completely fouled can not be corrected by shoulder cleaning. In such cases, it's necessary to replace the ballast altogether. One method of "replacing" ballast, if necessity demands, is to simply dump fresh ballast on the track, jack the whole track on top of it, and then tamp it down;
If the trackbed becomes uneven, it's 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. A more recent, and probably better, This technique is called pneumatic ballast injection (PBI; or, less formally, "stoneblowing"). However, this technique isn't as effective with fresh ballast, as the smaller stones tend to move down between the larger pieces of ballast.
Further Information
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