Granular Sub-Base (GSB)

The Granular Sub-Base (GSB) is the layer of well-graded natural or crushed aggregate placed directly on the prepared subgrade in a flexible pavement, performing three jobs: spreading the load from the upper layers to keep subgrade stress safe, acting as a drainage/filter layer that removes water entering the pavement, and providing a stable working platform for constructing the base course above. It is constructed and graded per MoRTH Specifications and used as a structural layer in the IRC 37 pavement composition.

Its effectiveness hinges on gradation, plasticity and compaction: the material must meet the specified grading envelope and a low plasticity index, be free-draining where it serves the drainage function, and be compacted (commonly to ≥ 98% of modified Proctor MDD) and verified by field density and CBR. A clogged, plastic or under-compacted GSB is a classic root cause of premature rutting and pothole failure because trapped water weakens the subgrade and destroys the load-spreading function — so it is as much a drainage element as a structural one.