Ballast and formation

Ballast is the layer of crushed hard stone under and around the sleepers. It spreads the concentrated load from the sleepers to the formation below, resists the sleepers moving along or across the track, drains rainwater away quickly and allows the track to be lifted, lined and packed back into geometry by tamping machines.

Good ballast is angular, hard, tough and durable stone of a uniform size range, so that particles lock together and voids drain. Rounded gravel rolls under load; soft stone grinds to dust that clogs the voids, a condition called fouling, which holds water, weakens support and must be cleaned by ballast-cleaning machines. The depth of ballast cushion below the sleeper bottom is specified by route and axle load.

Below the ballast lies the formation: the top of the embankment or cutting, often with a blanket layer of graded granular material over the subgrade. Its job is to carry the reduced stress without excessive settlement and to keep the subgrade from pumping up into the ballast as mud. Weak or poorly drained formation, especially on expansive clays, is a common cause of persistent track faults; heavier axle loads on freight corridors have led to thicker blankets and stronger formation design.

Load spreads through ballast roughly like a pyramid, so the pressure on the formation falls as the ballast cushion increases. Simple spread calculations give a first estimate; detailed analysis uses layered elastic models.