Editing Track ballast (section)

==Construction==
The appropriate thickness of a layer of track ballast depends on the size and spacing of the [[railway sleeper|ties]], the amount of traffic on the line, and various other factors.{{sfnp|Solomon|2001|p=18}} Track ballast should never be laid down less than {{convert|150|mm|in|0|abbr=in}} thick,{{sfnp|Bonnett|2005|p=60}} and high-speed railway lines may require ballast up to {{convert|0.5|m|in|0|abbr=off}} thick.<ref name="bell396">Bell 2004, p. 396.</ref> 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.{{sfnp|Hay|1982|p=399}}  Ballast less than {{convert|300|mm|in|0|abbr=in}} thick can lead to vibrations that damage nearby structures. However, increasing the depth beyond {{convert|300|mm|in|0|abbr=in}} confers no extra benefit in reducing vibration.<ref name="bachmann121">Bachmann 1997, p. 121.</ref>

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.{{sfnp|Solomon|2001|p=18}} Sometimes an elastic mat is placed between the sub-ballast and ballast, significantly reducing vibration.<ref name="bachmann121" />

It is essential for ballast to both cover the ties and form a substantial "shoulder"{{sfnp|Bonnett|2005|p=60}} to restrain lateral movement of the track.{{sfnp|Hay|1982|p=407}} This shoulder should be at least {{convert|150|mm|in|0|abbr=in}} wide, and may be as wide as {{convert|450|mm|in|0|abbr=in}}.{{refn |{{convert|150|mm|in|0|abbr=in}} is with {{convert|300|mm|in|0|abbr=in}} recommended for use in heavy traffic, or with [[Rail tracks#Continuous welded rail|continuous welded rail]] or concrete ties. A {{convert|450|mm|in|0|abbr=on}} shoulder significantly increases lateral stability and reduces required maintenance, though little or no resistance to buckling is gained above this size. See {{harvnb|Hay|1982|pp=407-408}}; {{harvnb|Kutz|2004||ps=, Section 24.4.2.}} }} Most railways use between {{convert|300|and|400|mm|in|0|abbr=in}}. 

[[File:Chelvey MMB 05 Bristol to Exeter Line.jpg|right|thumb|Ballast must be irregularly shaped to work properly.|alt=Irregularly cut brown stones with edges]]

Stones must be irregular, with sharp edges to ensure they properly interlock with each other and the ties to fully secure them against movement. Speed limits are often reduced for a period of time on sections of track where fresh ballast has been laid in order to allow it to properly settle.<ref name="Train Wreck">{{cite book|last=Bibel|first=George|title=Train Wreck: The Forensics of Rail Disasters|date=2012|publisher=[[Johns Hopkins University Press]]|location=Baltimore, MD|isbn=9781421405902|pages=287–88|url=https://books.google.com/books?id=HiU7KmXmx3oC&pg=PT287}}</ref>

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.<ref name="sol43">Solomon 2001, p. 43.</ref> 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.{{sfnp|Solomon|2001|p=41}} Alternatively, the ballast underneath the track can be removed with an undercutter, which does not require removing or lifting the track.<ref name="sol43" />

The dump and jack method cannot be used through tunnels, under bridges, or where there are platforms. Where the track is laid over a swamp the ballast is likely to sink continuously, and needs to be "topped up" to maintain its line and level. After 150 years of topping up at Hexham, Australia, there appears to be {{convert|10|m|ft|abbr=on}} of sunken ballast under the tracks.<ref>{{Cite web|url=https://www.academia.edu/8326131|title=Railway Materials Case Study|access-date=2016-08-04|last1=Nasir|first1=Enamul}}</ref> [[Chat Moss]] in the United Kingdom is similar.{{Citation needed|reason=reliable source needed for the last three sentences, particularly the {{convert|10|m|ft|abbr=on}} figure|date=March 2012}}

Regular inspection of the ballast shoulder is important.{{sfnp|Bonnett|2005|p=60}} 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.<ref name="hay408">Hay 1982, p. 408.</ref>{{sfnp|Kutz|2004|ps=, Section 24.4.2.}}

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 tamper|ballast tamping machine]]. A more recent, and probably better,<ref name="bell396" /> technique is to lift the rails and ties, and to force stones, smaller than the track ballast particles and all of the same size, into the gap. That has the advantage of not disturbing the well-compacted ballast on the trackbed, which tamping is likely to do.{{sfnp|Anderson|Key|1999}} The technique is called pneumatic ballast injection (PBI), or, less formally, "stoneblowing".{{sfnp|Ellis|2006|p=265|ps=, Pneumatic Ballast Injection}} However, it is not as effective as fresh ballast, because the smaller stones tend to move down between the larger pieces of ballast and degrade its bonds.<ref name="ifsc9">IFSC #37, ch. 9.</ref>