Good quality track ballast is made of crushed stone. The sharp edges help the particles interlock with each other.Track ballast supports railway sleepers, which carry railway track.
Track ballast is the material which forms the trackbed upon which railroad ties (UK: sleepers) are laid. It is packed between, below, and around the ties.[1] 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.[1] Ballast also physically holds the track in place as the trains roll over it. Not all types of railway tracks use ballast.[2]
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,[3] and burnt clay.[4] The term "ballast" comes from a nautical term for the stones used to stabilize a ship.[1]
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.[1] Track ballast should never be laid down less than 150mm (6 inches) thick,[5] and high-speed railway lines may require ballast up to 0.5 metres (20 inches) thick.[6] 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.[7] Ballast less than 300mm (12 inches) thick can lead to vibrations that damage nearby structures. However, increasing the depth beyond 300mm (12 inches) confers no extra benefit in reducing vibration.[8]
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.[1] Sometimes an elastic mat is placed between the sub-ballast and ballast, significantly reducing vibration.[8]
It is essential for ballast to both cover the ties and form a substantial "shoulder"[5] to restrain lateral movement of the track.[9] This shoulder should be at least 150mm (6 inches) wide, and may be as wide as 450mm (18 inches).[10] Most railways use between 300 and 400mm (12 and 16 inches).
Ballast must be irregularly shaped to work properly.
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.[11]
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.[12] 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.[13] Alternatively, the ballast underneath the track can be removed with an undercutter, which does not require removing or lifting the track.[12]
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 10m (33ft) of sunken ballast under the tracks.[14]Chat Moss in the United Kingdom is similar.[citation needed]
Regular inspection of the ballast shoulder is important.[5] 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.[15][16]
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,[6] 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.[17] The technique is called pneumatic ballast injection (PBI), or, less formally, "stoneblowing".[18] 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.[19]
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 – 60lb/yd (29.8kg/m)rail – 1,700cuyd/mi (810m3/km).
second class line – 41.5lb/yd (20.6kg/m) rail – 1,135cuyd/mi (539m3/km).
third class line – 30lb/yd (14.9kg/m) rail – 600cuyd/mi (290m3/km).[20]
↑ 150mm (6 inches) is with 300mm (12 inches) recommended for use in heavy traffic, or with continuous welded rail or concrete ties. A 450mm (18in) shoulder significantly increases lateral stability and reduces required maintenance, though little or no resistance to buckling is gained above this size. See Hay 1982, pp.407–408; Kutz 2004, Section 24.4.2.
In rail transport, track gauge is the distance between the two rails of a railway track. All vehicles on a rail network must have wheelsets that are compatible with the track gauge. Since many different track gauges exist worldwide, gauge differences often present a barrier to wider operation on railway networks.
A railway track or railroad track, also known as a train track or permanent way, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties and ballast, plus the underlying subgrade. It enables trains to move by providing a dependable surface for their wheels to roll upon. Early tracks were constructed with wooden or cast iron rails, and wooden or stone sleepers; since the 1870s, rails have almost universally been made from steel.
HO or H0 is a rail transport modelling scale using a 1:87 scale. It is the most popular scale of model railway in the world. The rails are spaced 16.5 millimetres (0.650 in) apart for modelling 1,435 mm standard gauge tracks and trains in HO.
The railway track or permanent way is the elements of railway lines: generally the pairs of rails typically laid on the sleepers or ties embedded in ballast, intended to carry the ordinary trains of a railway. It is described as a permanent way because, in the earlier days of railway construction, contractors often laid a temporary track to transport spoil and materials about the site; when this work was substantially completed, the temporary track was taken up and the permanent way installed.
A railroad tie, crosstie, railway tie or railway sleeper is a rectangular support for the rails in railroad tracks. Usually laid perpendicular to the rails, ties transfer loads to the track ballast and subgrade, hold the rails upright and keep them spaced to the correct gauge.
A light railway is a railway built at lower costs and to lower standards than typical "heavy rail": it uses lighter-weight track, and may have more steep gradients and tight curves to reduce civil engineering costs. These lighter standards allow lower costs of operation, at the price of lower vehicle capacity.
Maintenance of way refers to the maintenance, construction, and improvement of rail infrastructure, including tracks, ballast, grade, and lineside infrastructure such as signals and signs.
The track bed or trackbed is the groundwork onto which a railway track is laid. Trackbeds of disused railways are sometimes used for recreational paths or new light rail links.
High-speed railway track construction is the process by which Lignes à Grandes Vitesses, the land on which TGV trains are to run, is prepared for their use, involving carving the track bed and laying the track. This construction technique is used both for the French TGV network and other TGV-based networks outside of France.
A tamping machine or ballast tamper, informally simply a tamper, is a self-propelled, rail-mounted machine used to pack the track ballast under railway tracks to make the tracks and roadbed more durable and level. Prior to the introduction of mechanical tampers, this task was done by manual labour with the help of beaters. As well as being faster, more accurate, more efficient and less labour-intensive, tamping machines are essential for the use of concrete sleepers since they are too heavy to be lifted by hand.
A work train is one or more rail cars intended for internal non-revenue use by the railroad's operator. Work trains serve functions such as track maintenance, maintenance of way, revenue collection, system cleanup and waste removal, heavy duty hauling, and crew member transport.
The rail profile is the cross sectional shape of a railway rail, perpendicular to its length.
A ballast cleaner is a machine that specialises in cleaning the railway track ballast of impurities.
A concrete sleeper or concrete tie is a type of railway sleeper or railroad tie made out of steel reinforced concrete.
Barlow rail was a rolled rail section used on early railways. It has wide flaring feet and was designed to be laid direct on the ballast, without requiring sleepers. It was widely adopted on lightly trafficked railways, but was ultimately unsuccessful because of maintenance difficulties.
A rail fastening system is a means of fixing rails to railroad ties or sleepers. The terms rail anchors, tie plates, chairs and track fasteners are used to refer to parts or all of a rail fastening system. The components of a rail fastening system may also be known collectively as other track material, or OTM for short. Various types of fastening have been used over the years.
Baulk road is the name given to a type of railway track or 'rail road' that is formed using rails carried on continuous timber bearings, as opposed to the more familiar 'cross-sleeper' track that uses closely spaced sleepers or ties to give intermittent support to stronger rails.
Ladder track is a type of railway track in which the track is laid on longitudinal supports with transverse connectors holding the two rails at the correct gauge distance. Modern ladder track can be considered a development of baulk road, which supported rails on longitudinal wooden sleepers. Synonyms include longitudinal beam track.
A stoneblower is a railway track maintenance machine that automatically lifts and packs the sleepers with small grade ballast, which is blown under the sleepers to level the track. An alternative to the use of a ballast tamper, the totally self-contained machine levels track without the use of a large gang of workmen.
A ballastless track or slab track is a type of railway track infrastructure in which the traditional elastic combination of sleepers and ballast is replaced by a rigid construction of concrete or asphalt.
References
Anderson, W. F.; Key, A. J. (1999). "Two layer ballast beds as railway track foundations". Twelfth European Conference on Soil Mechanics and Geotechnical Engineering (Proceedings). AA Balkema. ISBN90-5809-047-7.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
