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 sub-ballast, small crushed stones which provide a solid support for the top ballast and reduce ingress of water from the underlying ground. Sometimes an elastic mat is placed between the sub-ballast and ballast, significantly reducing vibration. Ballast can only be cleaned so often before it is damaged beyond re-use. Ballast that is completely fouled can not be corrected by shoulder cleaning. One method of "replacing" ballast is to simply dump fresh ballast on the track, jack the whole track on top of it, and then tamp it down. Alternatively, the ballast underneath the track can be removed with an undercutter, which does not require removing or lifting the track. Sinkage at
Chat Moss in the United Kingdom caused challenges during construction, leading engineers to place a "floating" layer of lower density materials like peat underneath the ballast. Regular inspection of the ballast shoulder is important. The shoulder acquires some amount of stability over time, being compacted by traffic, but maintenance tasks such as replacing ties, tamping, and ballast cleaning can upset that stability. After performing those tasks, it is necessary either for trains to run at reduced speed on the repaired sections, or to employ machinery to compact the shoulder again. If the trackbed becomes uneven, it is necessary to pack ballast underneath sunken ties to level the track again, which is usually done by a
ballast tamping machine. A more recent, and probably better, == Quantities ==