Editing Track ballast

{{Short description|Trackbed upon which railway ties are laid}}
{{other uses|Ballast (disambiguation)}}
{{Use dmy dates|date=January 2021}}
[[File:Rails.and.ballast.bb.jpg|thumb|right|Good quality track ballast is made of [[crushed stone]]. The sharp edges help the particles interlock with each other.]]
[[File:Close-up of railway track.jpg|thumb|Track ballast supports railway sleepers, which carry railway track.]]

'''Track ballast''' is the material which forms the [[trackbed]] upon which [[railroad tie]]s (UK: sleepers) are laid. It is packed between, below, and around the ties.{{sfnp|Solomon|2001|p=18}}  It is used to bear the compression load of the railroad ties, rails, and [[rolling stock]]; to facilitate [[drainage]]; and keep down [[vegetation]] that can compromise the integrity of the combined track structure.{{sfnp|Solomon|2001|p=18}} Ballast also physically holds the track in place as the trains roll over it. Not all types of railway tracks use ballast.<ref> [https://www.railjournal.com/in_depth/tubular-track-tames-the-desert Tubular Modular Track]</ref>

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]], [[Chat (mining)|chats]], [[Ember|coal cinders]], [[sand]],<ref>{{Cite web|last=Kellogg|first=H. W.|title=Selection and Maintenance of Ballast|year=1946|url=https://www.arema.org/files/roadmasters/1946/1946-Selection-and-Maintenance-of-Ballast.pdf|website=American Railway Engineering and Maintenance-of-Way Association|access-date=27 March 2021}}</ref> and burnt [[clay]].<ref>{{Cite book|last1=Beyer|first1=S. W.|last2=Williams|first2=I. A.|title=The Geology of Clays|year=1904|url=http://ir.uiowa.edu/cgi/viewcontent.cgi?article=1078&context=igsar|archive-url=https://web.archive.org/web/20100813010824/http://ir.uiowa.edu/cgi/viewcontent.cgi?article=1078&context=igsar|url-status=dead|archive-date=13 August 2010|publisher= |location= |pages=534–537}}</ref> The term "[[ballast]]" comes from a nautical term for the stones used to stabilize a ship.{{sfnp|Solomon|2001|p=18}}

==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>

== Quantities ==

The quantity of ballast used tends to vary with gauge, with the wider gauges tending to have wider formations, although one report states that for a given load and speed, narrowing the gauge only slightly reduces the quantity of earthwork and ballast needed. The depth of ballast also tends to vary with the density of rail traffic, as faster and heavier traffic requires greater stability. The quantity of ballast also tends to increase over the years as more and more ballast is piled onto an existing roadbed. Some figures from an 1897 report listing requirements for light railways (usually narrower than standard gauge) are:

* first class line – {{convert|60|lb/yd|kg/m|1|abbr=on}} [[Rail profile|rail]] – {{convert|1700|cuyd/mi|m3/km|abbr=on|lk=on}}.
* second class line – {{convert|41.5|lb/yd|kg/m|1|abbr=on}} rail – {{convert|1135|cuyd/mi|m3/km|abbr=on}}.
* third class line – {{convert|30|lb/yd|kg/m|1|abbr=on}} rail – {{convert|600|cuyd/mi|m3/km|abbr=on}}.<ref>{{cite news |url=http://nla.gov.au/nla.news-article3658726 |title=Light Railways. |newspaper=[[The Brisbane Courier]] |date=29 September 1897 |access-date=21 May 2011 |page=5 |publisher=National Library of Australia}}</ref>

==See also==
{{portal|Trains}}
*[[Ballastless track]]
*[[Ballast tamper]]
*[[Gandy dancer]]
*[[Maintenance of way]]
*[[Rail tracks#Maintenance|Track maintenance]]

==Footnotes==
{{reflist}}

==References==
{{Refbegin|2}}
* {{cite conference | last1 = Anderson | first1 = W. F. |last2=Key |first2=A. J.  | book-title = Twelfth European Conference on Soil Mechanics and Geotechnical Engineering (Proceedings) | title = Two layer ballast beds as railway track foundations | year = 1999 | publisher = AA Balkema | isbn= 90-5809-047-7 }}
* {{cite book | title = Vibration Problems in Structures: Practical Guidelines | author = Bachmann, Hugo| isbn = 3-7643-5148-9 | publisher = Birkhäuser | year = 1997|display-authors=etal}}
* {{cite book | title = Engineering Geology and Construction | last = Bell | first = F.G. | year = 2004 | publisher = Spon Press | isbn = 0-415-25939-8}}
* {{Bonnett Practical Rail Engineering}}
* {{cite book |title=Ellis' British Railway Engineering Encyclopaedia |year=2006 | last = Ellis | first = Iain | publisher = Lulu.com | isbn =  1-84728-643-7 }}{{better source needed|date=May 2022|reason=This is a self published work, non-self-published is preferable}}
* {{cite book | title = Railroad Engineering | last = Hay | first = William Walter | year = 1982 | publisher = John Wiley and Sons | isbn = 0-471-36400-2 }}
*{{cite book |last1=Indraratna |first1=Buddhima |title=Advanced rail geotechnology--ballasted track |date=2011 |publisher=CRC Press/Balkema |location=Leiden, The Netherlands |isbn=978-0-203-81577-9 }}
* {{cite book | title = Urban Railways and the Civil Engineer | year = 1988 | author =Institution of Civil Engineers | publisher = Thomas Telford | isbn = 0-7277-1337-X }}
* International Federation for Structural Concrete (fédération internationale du béton) bulletin #37.
* {{cite book | title = Handbook of Transportation Engineering | last = Kutz | first = Myer | year = 2004 | publisher = McGraw-Hill | isbn = 0-07-139122-3 }}
* {{cite book | title = Track Geotechnology and Substructure Management | last = Selig | first = Ernest Theodore |author2=Waters, John M.  | year = 1994 | publisher = Thomas Telford | isbn = 0-7277-2013-9}}
* {{cite book | title = Railway Maintenance Equipment: The Men and Machines that Keep the Railroads Running | last = Solomon | first = Brian | publisher = MBI Publishing Company | year = 2001 | isbn = 0-7603-0975-2 }}
{{Refend}}

== Further reading ==
*'''''New South Wales: Ballast 1850-1987''''' Longworth, Jim [[Australian Railway History]], December, 2004 pp443–462

==External links==
{{Commons category|Ballast}}
*[http://www.ontrackplant.com/photos/tags/ballast-cleaner Photos of ballast cleaners in the UK]
*[http://www.ontrackplant.com/photos/tags/ballast-regulator Photos of ballast regulators in the UK]

{{Rail tracks}}

{{Authority control}}

[[Category:Aggregate (composite)]]
[[Category:Permanent way]]
[[Category:Rail infrastructure]]