7.1. Functions of Ballast
Ballast is crushed stone, generally between sizes of 10 mm and 70 mm. It should be cubic in shape andnot flakey. Ballast should be hard enough not to crush but not so hard that it does not interlock. It must not dissolve or soften when wet.
(a) Ballast for toad distribution
Ballast is packed tight under sleepers adjacent to and under each rail. The deeper the ballast, the better the loading is distributed to the formation.
(b)Ballast to hold position
Ballast also needs to be packed tightly between sleepers and at sleeper ends to hold sleepers firmly in place. The vibration of passing traffic will gradually shake the ballast into a tight condition.
Tight ballast is necessary to prevent track buckles and to stop rail creep.
(c) Ballast for drainage
Ballast should be kept clean, particularly outside the sleeper ends, to ensure that water drains freely from the track. Any growth of vegetation in the ballast will lead to clogging of the drainage.
7.2. Ballast Cross Section
On track where no electric track circuit exists ballast is to be smoothed off level with the tops of sleepers and out to the edge of the ballast shoulder. On tracks where there are electric track circuits, ballast should be kept 25 mm below each rail so that electrical cause a gradual run-down of batteries, or a sudden signal failure. Such failures are particularly likely to occur after a medium rainfall following a dry period.
While this is a 'safe' failure, it is particularly inconvenient and disruptive to rail traffic. However, sufficient ballast in the cribs is essential for protection against track buckling, which is an 'unsafe' failure.
Ideally, ballast should be kept below rails by 25 mm, but up to and even above sleeper level from a point 150 mm outside the foot of the rail.
Ballast should slope away from the shoulder at a slope of 1 in 2. If it is profiled by a regulator at a steeper slope, it may fall away with traffic vibration, and reduce the shoulder width to an unsafe condition.
Ballast depth below underside of sleeper varies and is generally beyond the authority of the trackman to decide. The amount of lift will be directed by the Roadmaster.
Edit: Source is AN Track Maintenance Guide 1988.