Ballast is the coarse, open layer around the sleeper. Sub-ballast is a finer layer between ballast and the formation. It spreads load further and keeps formation fines from pumping up into the ballast.
Do not mix the two
A buyer who asks for ballast and receives a mixed 0/63 mm pile has not received ballast. The two products have different gradings and different jobs. Name the layer in the enquiry.
Four jobs of sub-ballast
Sub-ballast spreads stress, filters upward migration of formation fines, limits capillary water movement and provides a working platform during construction. Its grading is therefore finer and more continuous than the open ballast above. A separator geotextile may be added when the formation soil is especially mobile.
Design the interface
The boundary between layers must prevent intermixing without trapping water. Designers check particle compatibility, drainage outlets, shoulder width and compaction. Replacing one layer with the other changes stiffness and void structure, so it should never be done by nominal thickness alone.
