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Base equivalence

In Type 3 and 4 flexible roads, the thickness of granular bases may be reduced provided that the thickness of the bituminous binder course is increased proportionately in accordance with the following requirements:
  1. Each 10 mm increase in bituminous binder course thickness is equivalent to a 35 mm decrease in thickness of Type 1 Granular sub-base at base and/or sub-base levels and vice versa.
  2. This equivalence rule may be applied to include the total replacement of all granular materials at both sub-base and base levels, subject to the following restrictions:
    1. Binder course and surface course thickness in Type 3 and 4 roads are minimum values and must not be reduced by application of the 10:35 equivalence of bituminous/granular materials; and
    2. Where part of a granular base or sub-base is to be replaced by additional binder course material, the remaining total thickness of granular material at base and/or sub-base level must not be less than 150 mm.