Subballast

Subballast is a layer of material placed beneath the ballast in railway track construction. Its primary purpose is to provide a stable, well-drained base for the ballast layer and the track structure as a whole. The subballast acts as a filter, preventing fine particles from the subgrade from migrating into the ballast, which can lead to poor drainage and instability. It also helps to distribute the load from the train wheels more evenly onto the subgrade below, reducing stress concentrations and preventing settlement.

The composition of subballast varies depending on the specific requirements of the track and the available local materials. Common materials include crushed stone, gravel, and slag. The particle size and grading of the subballast are important to ensure good drainage and load-bearing capacity. A well-designed subballast layer contributes significantly to the overall longevity and performance of the railway track, reducing maintenance requirements and ensuring a smoother ride for passengers. It is a critical component in preventing track deformation and ensuring safe operation. The thickness of the subballast layer is determined by factors such as the expected axle loads, the type of subgrade, and environmental conditions.