Water enters the pavement layers through cracks, joints and open spaces in the pavement surface. The water reaches the soil sub-grade on which the pavement rests and softens it. The sub-grade loses its supporting power and the pavement settles. It is thus important to drain away the water that reaches the sub-grade. For this purpose, a drainage layer is provided at the bottom of the pavement and is extended to the full width of the embankment so that the water is discharged on the embankment slope (Figure 8). The drainage layer is often a part of the pavement structure.
Figure : Pavement Drainage