In drainage design, an undamaged asphalt surface is considered almost impervious. However, water can infiltrate into the structure through cracks and joints (see Chapter 5, Sections 5.3 and 5.4.2). Also, shoulders and slopes with higher permeability and high water tables can allow significant amounts of water into the structure. Whether the water arrives via cracks in the asphalt and is flowing through the pavement according to regime A or B, is threatening to arrive from the margins, or is simply close to the underside of the pavement due to a high phreatic surface, it is then necessary for longitudinal interceptor drains to be provided or other drains that will lower the water table or keep it in a low position. Many types of longitudinal drain are available as described in Section 13.3.9.
In many countries, especially the Mediterranean ones, where roads have open verges and slopes (especially where there are flat areas and hollows) it is customary to construct the verges/slopes with an impermeable surface cover using soils with a percentage of fines typically more than 25% of its weight (D25 < 80 ^m), with a minimum thickness of 20 cm to limit water seepage into the pavement structure.
