What Is Ballast on a Railroad Track?
Railroad ballast serves as a bed for railroad tracks and provides track stability, drainage, and support of significant loads carried by railcars. In addition, it deters the growth of vegetation and allows for track maintenance to be performed more easily. Ballast is produced from natural deposits of granite, trap rock, quartzite, dolomite or limestone.
Broken stone is one of the best materials for railway ballast to be used on the railway tracks. Almost all the important railway tracks are provided with broken stone. The stone to be used as railway ballast should be hard, tough nonporous and should not decompose when exposed to air and light. Igneous rocks like quartzite and granite forms the excellent ballast materials. When these are not available then lime stone and sand stone can also be used as good ballast material.
The appropriate 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. Track ballast should never be laid down less than 150 mm (6 inches) thick; and high-speed railway lines may require ballast up to 1⁄2 metre (20 inches) thick.
An insufficient depth of ballast causes overloading of the underlying soil, and in unfavorable conditions overloading the soil causes the track to sink, usually unevenly. Ballast less than 300 mm (12 inches) thick can lead to vibrations that damage nearby structures. However, increasing the depth beyond 300 mm (12 inches) adds no extra benefit in reducing vibration.
In turn, track ballast typically rests on a layer of small crushed stones: the sub-ballast. The sub-ballast layer gives a solid support for the top ballast, and reduces the seepage of water from the underlying ground. Sometimes an elastic mat is placed on the layer of sub-ballast and beneath the ballast, thereby significantly reducing vibration.
It is essential for ballast to be piled as high as the ties, and for a substantial “shoulder” to be placed at their ends; the latter being especially important, since this ballast shoulder is the main restraint of lateral movement of the track. The ballast shoulder always should be at least 150 mm (6 inches) wide, and may be as wide as 450 mm (18 inches).
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 the stones fully settle and interlock, speed limits are often lowered on sections of track for a period of time after new ballast has been laid.