A road is only as strong as the layers below its surface. A simple guide to sub-base and base materials such as GSB and graded aggregates.
The smooth surface of a road sits on layers of compacted stone material. These hidden layers spread the load of vehicles and keep the road from sinking or cracking.
Typical road layers
- Subgrade: the prepared natural ground.
- Sub-base: often granular sub-base (GSB), a graded mix of stone and fines that drains water and adds strength.
- Base: graded crushed aggregates that carry most of the load.
- Surface: bituminous or concrete wearing course.
What makes good sub-base material
- Correct grading as per the specification in use.
- Hard, durable stone without excess clay.
- Good drainage so water does not stay in the layer.
Compaction
Even the best material fails if it is not compacted properly. Material is spread in layers, watered to the right moisture and rolled until the required density is reached.
Road projects usually follow specific standards; always check the grading and testing requirements in your contract.
