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. Ballast also holds the track in place as the trains roll over it. A variety of materials have been used as track ballast, including crushed stone, washed gravel, bank run (unwashed) gravel, torpedo gravel (a mixture of coarse sand and small gravel), slag, chats, coal cinders, sand, and burnt clay. The term "ballast" comes from a nautical term for the stones used to stabilize a ship. Construction 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 thick, and high-speed railway lines may require ballast up to 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 thick can lead to vibrations that damage nearby structures. However, increasing the depth beyond confers 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 ingress 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 is especially important, because the ballast shoulder is the main restraint to lateral movement of the track. The ballast shoulder should be at least wide, and may be as wide as . The shape of the ballast is also important. Stones must be irregular, with sharp edges. That ensures they properly interlock with each other and the ties so as to fully secure them against movement. Spherical stones cannot do that. In order to let new ballast fully settle and interlock, speed limits are often reduced for a period of time on sections of track where fresh ballast has been laid. Maintenance If ballast