In order to stop water getting into a pavement foundation and to avoid the consequent reduction of its support capability, the construction of ditches or trenches is a common procedure. They are usually filled with highly permeable material, wrapped in geotextile and with a perforated tube or porous material near the bottom. Alternatively a geo-composite based drainage system, known as a “fin” drain, may be used. Fin drains are, typically, only a few centimetres thick. Trenches or ditches are, usually, excavated by digging plant while narrow trenches for geo-synthetic fin drains may be dug or may be saw-cut.
These types of systems, named longitudinal drains, are placed in parallel with the road’s centre line, usually at the edge of the pavement structure, and will lie under the surface water channels or gutters whenever these are permeable.
Lateral drains, according to their function, can be divided in two main groups, interceptor drains and water table lowering drains.
Water table lowering drains allow the lowering of the water level in the pavement structure platform, when close to the pavement’s under-side. They are normally placed at depths varying from 1.2 to 2 m below the pavement surface so as to reach the water table (or to reach the height to which the water table may sometimes rise). Thus they keep the water table low and help to prevent the water from being pulled up to the pavement layers by capillarity.
Interceptor drains are drains of the same type as the previous ones, although they usually go down less far into the ground. They aim to ensure the internal drainage of the pavement and to intercept percolating water.
