Specification of aggregates

The aggregates used in the sub-base below pervious surfaces must consider the presence of water on the materials and the need to transmit water. CIRIA guidance includes advice on:

Recommended gradings and criteria to achieve a compromise between stiffness and water storage and transmission capacity.
Recommended criteria for strength and durability of the aggregate particles when saturated and subjected to wetting and drying.
Recommended criteria for materials to ensure they do not crush or degrade either during construction or in service.

These requirements are met by specifying appropriate values for ten per cent fines values, Los Angeles Abrasion Tests values and also by specifying the types of rock that can be used for aggregate.

It is important when specifying granular materials to ensure that they can be easily, economically and sustainably produced. The use of commonly available coarse crushed rock aggregates will ensure that pervious surfaces can be constructed economically.