Road Layers Explained: GSB, WMM, DBM & BC

A flexible pavement consists of multiple layers, each serving a specific structural purpose. From bottom to top: Sub-grade → GSB → WMM → DBM → BC.

GSB is the first structural layer above the sub-grade. Material: Well-graded granular material (stone, gravel, or sand). Compaction: 98% of MDD by Proctor test. Thickness: typically 150–200mm.

WMM provides the base course. Material: Crushed aggregates with controlled grading. Compaction: 100% of MDD. Thickness: typically 150–250mm. It provides the stable base for bituminous layers.

DBM is the binder course. It provides structural support and acts as a bond between WMM and the surface course. Bitumen content: 4–5%. Compaction by tandem roller.

BC is the wearing course that traffic contacts directly. Fine, well-graded mix with higher bitumen content (5–6%). Provides smooth, waterproof surface.