Definitions

1. noun

A layer of material, such as stone or gravel, placed under a track or railway, to improve drainage and prevent erosion.

“The subballast was designed to support the weight of the heavy train.”

2. noun

A material or substance used to provide support or stability, especially in a situation where the ground is unstable or uneven.

“The subballast helped to prevent the building from sinking into the soft earth.”