Proper and adequate drainage of water is critical to good pavement performance. Whilst the effect of water on soil (especially cohesive and silty soils) is well recognised, the effect on granular material and asphalt pavements should not be ignored.
There is a misconception that surface courses are impermeable i.e. do not let any water through. No asphalt material is completely impermeable, although mastic asphalt is, for all practical purposes; Hot Rolled Asphalt and proprietary 'Agricultural' asphalt material can be, though they may suffer from micro-cracking during installation. The permeability of Stone Mastic Asphalt, Dense Asphalt Concrete and Thin Surfacing depends upon the mix design and whether or not it has been fully compacted. Daines M (1995) (TRL Project Report PR78, Tests for voids and compaction in rolled asphalt surfacings) gives values in the range (<10-11 to 10-8 ) m/s for these materials. In accordance with PD 6691 Guidance on the use of BS EN 13108, Bituminous mixtures – Material specifications, these materials can have up to 7% air voids. Depending upon aggregate shape, anything with voids above about 5% will not be impermeable.