Sub-ballast

Track ballast forms the trackbed upon which railroad ties (sleepers) 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, for example burnt clay. The term "" comes from a nautical term for the stones used to stabilize a ship.

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 (5.9 inches) thick; and high-speed railway lines may require ballast up to ¹⁄₂ metre (19.7 inches) thick. An insufficient depth of ballast causes overloading of the underlying soil, and in unfavourable conditions overloading the soil causes the track to sink, usually unevenly. Ballast less than 300 mm (11.8 inches) thick can lead to vibrations that damage nearby structures. However, increasing the depth beyond 300 mm (11.8 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 (5.9 inches) wide, and may be as wide as 450 mm (17.7 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.

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. Bioremediation can be used to clean ballast.

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Wikipedia