Although they may not be considered as part of the drainage system, a pervious type of asphalt treated surface layer, known as porous asphalt, has become common in Europe in recent years. The main advantages attributed to porous asphalt layers are noise reduction, improvement of skid resistance in wet weather, and enhancement of runoff water quality. Asphalt treated drainage layers of this type are discussed further in Chapter 5, Section 5.7.
13.3.4.2 When Drainage is Unnecessary
Some authors suggest that subsurface drainage may not be necessary if:
• annual rainfall is not significant;
• the subgrade has a relatively high permeability value;
• the pavement is structurally adequate without drainage;
• lateral and vertical drainage in the pavement section exceeds infiltration; or
• heavy traffic level is negligible.
For example, Christopher (1998) found that drainage provides no additional benefit if average annual rainfall is less than 400 mm and permeability of the subgrade exceeds 3.5 x 10-5 m/s, however Dempsey (1988) and Forsyth et al. (1987) suggested different values, 3 x 10-6 m/s and 1.7 x 10-4 m/s, for this parameter.
