Track ballast forms the trackbed upon which railway sleepers or railroad ties
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. This
also serves to hold the track in place as the trains roll by. It is typically
made of crushed stone, although ballast has sometimes consisted of other, less
suitable materials. The term "ballast" comes from a nautical term for the
stones used to stabilize a ship. Construction
The thickness of a layer of track ballast depends on the size and spacing
of the ties, the amount of traffic expected on the line, and various other
factors. Track ballast should never be laid down less than 150 mm thick;
high-speed railway lines may require ballast up to 1⁄2 metre thick. An
insufficient depth of ballast overloads the underlying soil; in the worst cases,
this can cause the track to sink. If the ballast is less than 300 mm thick, this
can lead to vibrations, which can damage nearby structures.
Track ballast typically sits on a layer of sub-ballast; the latter is typically
made of small crushed stones. It gives a solid support for the top ballast, and
seals out water from the underlying ground. Sometimes, an elastic mat is
placed under the ballast layer as well; this can allow for significant
reductions in 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 being especially important, since this ballast shoulder
is, for the most part, the only thing restraining lateral movement of the
track. The ballast shoulder should be at least 150 mm wide under any
circumstances, and may be as large as 450 mm.
Maintenance If ballast is badly fouled, the clogging
will reduce its ability to drain properly; this, in turn, causes more
debris to be sucked up from the sub-ballast, causing more fouling.
Therefore, keeping the ballast clean is essential. Bioremediation can be used to
clean ballast. It is not always necessary to replace
the ballast if it is fouled, nor must all the ballast be removed if it is to
be cleaned. Removing and cleaning the ballast from the shoulder is often
sufficient, if shoulder ballast is removed to the correct depth. While this
job was historically done by manual labour>, this process is now, like many
other railway maintenance tasks, a mechanised one, with a chain of
specially-designed railroad cars handling the task. One wagon cuts the
ballast and passes it via a conveyor belt to a cleaning machine, then the
cleaning wagon washes the ballast, and deposits the dirt and ballast into other
wagons for disposal and re-use, respectively. Such machines can clean up
to two kilometres of ballast in an hour. Cleaning, however, can only be done a
certain number of times before the ballast is damaged to the point that it
cannot be re-used; furthermore, track ballast that is completely fouled can
not be corrected by shoulder cleaning. In such cases, it is necessary to
replace the ballast altogether. One method of "replacing" ballast, if
necessity demands, 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. The dump and jack method cannot of
course be used through tunnels, under overbridges, and where there are
platforms. Where the track is laid over a swamp, such as the Hexham swamp in
Australia, the ballast continuously sinks, and needs to be topped up to
maintain its line and level. After 150 years of topping up, there appears to be
10 m of sunken ballast under the tracks. Chat Moss in the United Kingdom is
similar. Regular inspection of the ballast
shoulder is important;