Railway ballast or track ballast refers to crushed stones placed under the railway track. It forms the trackbed for sleepers to lay on. Although some tracks are ballastless, the ballasted track remains a dominant infrastructure of the most railroad tracks. Why is that? Ballast in railway may seem inconspicuous, but its function plays a key role.
The function of ballast in railway track
Ballast is a common trackbed structure in the railway transport system, which is used to support track sleepers. A layer of crushed stones will be laid on the roadbed before the tracks are laid, which will then be compacted. Then sleepers and tracks will be laid.
- Distribute the pressure of the sleepers to the roadbed
Think about the gravity of the train passed to the subgrade from the rail, sleeper, and ballast. The strength of the subgrade is often not enough to bear so much pressure. Though the sleeper undertakes quite an amount of pressure, the ballast is needed to scatter pressure by increasing the functional area and reducing the pressure of per unit area.
- Preventing track creep
When the train is running, the wheels on the track generates not only vertical and lateral forces but also the rolling and longitudinal sliding of the wheels, making the rail undulate under the action of dynamic load. a longitudinal horizontal thrust is produced. This longitudinal horizontal thrust can cause the longitudinal displacement of the rail and sometimes drive the sleeper to move together. This movement is called track creep. The movement of the track can be reduced by making the ballast packed the sleepers.
- Absorb the impact of train
The train runs on the steel rail with a steel wheel. Because the wheels are not perfectly round, they will cause considerable impact when passing the rail joint. The instantaneous pressure on the track can be up to twice the static weight, and there is no rubber to absorb the impact. Such instantaneous impact, if acted directly on the subgrade, will cause subgrade settlement, as well as pumping (the phenomenon of repeated impact water flowing out of the surface, which can be simulated by stomping on the wet ground). If the reaction is directly on the track, it can cause fasteners, rails and even sleepers to break. Therefore, a relatively “soft” bit of ballast is needed to absorb the shock. (the “soft” here is relative to the rails and sleepers, the ballast is not at all “soft”).
Drainage is by no means a matter of gutters alone. Good ballast is essential to drainage. One may ask whether drainage is the primary or secondary function of ballast. Assuming that poor drainage will not affect the strength of the subgrade, then ballast need not have drainage function, but this is not the case. If you want to have good subgrade strength, a good job of drainage has to be done. So I would say that drainage is a function that ballast has to have.
- Easy to maintain
For example, with the operation of the train, the superelevation may change. At this point, if the line needs maintenance, it can be easily corrected by raising or reducing ballast. As a contrast, although the line parameters of the concrete solid ballast bed are less likely to change with the train operation, once they change, its maintenance is very difficult.
Why stones are used in the railway track?
Not any kind of stone is suitable for use as ballast. The ballast used in the railway mainly consists of crushed stone, crushed slag, and improved gravel, etc. Among them, the improved gravel is generally not used except for branch lines or station lines with minimal volume. The most widely used are broken rocks such as Igneous Rock and Hard Limestone. Granite, quartzite, basalt, and limestone are four kinds of rocks widely used.
The use of the right type of stones makes it easy to drain and adjust the track. Because stones distribute the weight of the train and track on the subgrade, it can reduce the vibration and noise caused by the passing of the train and increase the comfort of passengers.
At the same time, the stones can disperse the vibration and high heat generated when the train is passing by. It helps keep the gauge of the rail unchanged, avoid derailment accident, and the cracks between the stones can quickly drain the rain. Ballast has strong permeability and can prevent the growth of weeds on the track.