Artificial drainage of poorly draining soil is achieved by means of a network of small underground conduits known as drainage pipes, which are introduced into the permeable or poorly permeable soil, and collect and promote the evacuation of excess water without the need for special shaping of the overlying soil surface.
In order to properly investigate the design of a drainage system, it is necessary to identify groundwater infiltration. An in-depth study of the surface and underground hydrology of the territory must frequently be carried out, also through the statistical processing of rainfall, hydrometric and phreatometric data, together with a geopaedological investigation aimed at ascertaining the physical-chemical characteristics of the soils, mainly the permeability coefficient and the depth of the first impermeable layer.
There are no rules to follow for draining the soil, but it must be analysed on a case-by-case basis. For example, the body of a motorway (or a high-traffic road) generally consists of an impermeable carriageway, side verges and a central dividing wall, which is almost always permeable. Rainwater, filtered through the draining soil, will eventually cause damage to the stability of the road if it is absorbed by the road’s support structure. In these cases, drainage systems should be positioned longitudinally, along the edges of carriageways and in the centre of the traffic island.
In roads built halfway up the hillside, i.e. part earthwork and part relief, the technique of intercepting drainage is used by placing the drains upstream of the road in order to avoid infiltration between the impermeable zone and the fill.