A rather common subsurface drainage system used to remove the infiltrated/seepage water from the pavement structures is by providing a permeable layer. Permeable layers should be at least 10-15 cm thick and extend under the full width of the roadway. They can be used under both concrete (PCC) and asphalt (AC) pavement surfaces. Permeable bases are usually located just above the subgrade and are discussed in more detail in the second part of Section 13.3.6. Permeable unbound granular bases must be separated from high plasticity subgrade soils by mean of geotextiles or impervious materials.
The drainage layer should drain into a longitudinal drainage pipe. In order to encourage the lateral flow of water, a minimum cross-fall should be considered, of, at least, 2%. For curved lengths of road and those with a permeable central reserve, the pavement bed must have a cross-fall of between 2% to 4% inclination, starting 1.0 m away from the paved area (marked with E in Fig. 13.6).
Fig. 13.6 Typical pavement cross-falls. E = position 1 m inwards from the edge of the pavement. Similar cross-falls will exist at road edges
