Ballast is the layer of broken stone provided on the formation on which the sleepers rest. The stones are cubical in shape and pass through 50 mm square mesh. The functions of the ballast are:
(a) To distribute the load of traffic to the earthen formation to a safe value.
(b) To act as an elastic bed to absorb the impact of the wheels.
(c) To hold the sleepers firmly in the lateral and longitudinal directions.
(d) To act as a good drainage medium.
(e) To provide a convenient medium to pack the track to the desired levels.
(f) To prevent capillary rise of water which may cause decay/corrosion of sleepers?
(g) To check the growth of vegetation under the track.
The thickness of ballast for a high density BG track is 300 mm, and it is 250 mm for a low density BG track. Figure 4 gives the standard ballast section.
Figure: Standard BG Track Ballast Section