Granular Sub Base is critical components of road construction, providing support and stability to the base course & pavement layers. This layer helps distribute the load of traffic and prevent premature failure of the road surface.
The Granular Sub Base (GSB) is the layer of material placed on the compacted subgrade and beneath the base course. It provides additional support and helps in drainage. Common materials used for Granular Sub Base include crushed stone, gravel or recycled concrete. The Granular Sub Base layer is typically thicker than the base layer.
Before dumping and placing subbase material surface of subgrade should be prepared and any unsuitable material should removed.
Spread the sub-base material evenly over the prepared sub-grade using equipment like bulldozers, graders or excavators. The layer thickness should conform to design specifications. Granular Sub Base includes all naturally occurring materials, meeting the requirements mentioned in below Table:
Soil aggregates subbase available in nature consisting of pebbles, shingle and sand may be used. It may be a mixture of natural and crushed stone. The pebble size should not exceed 60 mm (2½”).
Begin compaction immediately after spreading the material. Use heavy rollers (either pneumatic-tired rollers or vibratory rollers) to achieve initial compaction. This process helps in settling the material and removing air voids.
Control the moisture content of the Granular Sub Base material. It should be moist enough to facilitate compaction but not too wet, which can cause instability. Moisture content is typically controlled to a specified range based on material type and conditions.
Depending on the thickness of the sub-base layer, it may be compacted in several layers. Each layer is typically compacted to a specified density using the compaction equipment.
Ensure that each pass of the compactor overlaps the previous pass by at least half the width of the roller to achieve uniform compaction.
The number of passes required for compaction depends on factors such as material type, thickness and moisture content. Generally, minimum 6 passes are required to achieve the desired density.
Compaction tests are performed on each 15cm layer to ensure that the Granular Sub Base layers meet the required specifications. Min Compaction (Field Density) 98% of MDD determined as per ASTM -D 1557
