It is expected that a bituminous sub-base would only be selected where the base is also bituminous.
When placing bituminous material directly onto backfill it is important to ensure that the exposed surface of the backfill has been compacted. This operation is essential to minimise the risk of a build-up of pore water pressure causing the subgrade to become spongy. It is also imperative that construction is phased such that excavated areas are covered on the same day with the first layer of bituminous material to prevent the ingress of water. Care should be taken in compacting this first layer. If pore water pressure builds up in the backfill at this stage, then rolling should cease and the material left overnight, or longer if necessary, before placing of any further layers.
The condition of sub-base material can be assessed using field identification test No. 3 in A1.