Subbase and capping layer
In terms of the overall structural strength of the pavement, the subbase is an
extremely important layer. If the design life traffic volume is less than 2msa, the
CBR of the subbase must exceed 20%. If it is greater than 2msa, the minimum
CBR of the subbase rises to 30%. Use of a capping layer (with a lower specification than the subbase material) will allow a thinner layer of the more expensive subbase to be used in the pavement