Road Drainage Systems

In roadwork, the main drainage requirements fall into two categories.

1. Sub-soil Drainage 

There must be a provision of sub-soil drainage to cope with water in the ground and to ensure that the water table can be kept low enough not to allow saturation of the road structure and its surrounding soil. Excess water of this nature causes the structure and ground to become plastic and incapable of withstanding traffic loads. Subsoil drainage also helps to prevent frost damage to the road structure by keeping it drained. roads, but it may also become necessary to lay a new drainage system for an existing road system, particularly where there have been other works, such as large-scale building projects, adjacent to existing roads.

In new works, great care must be taken to keep the water table below the road formation level. This, in lowlying wet areas, may require the provision of a system of land drains, collector drains and a long outfall pipe or open ditch before the ground can be drained sufficiently
for the structural work of the road building to be started.

 

2. Drainage of the Carriageway
The surface of the road must be kept clear of standing water in order not to increase the dangers to road users. To accomplish this, roads should be cambered when straight, laid to crossfalls on bends, and adequately provided with gullies or grips to dispose of rain water which falls on the carriageway. In many cases, the drainage of this surface water and the sub-soil water is connected to the same systems of piped drains or open ditches through which the water is removed to a safe distance from the carriageway itself.

Springs may be encountered in a position close to or under the proposed road structure. These must be piped clear of the roadworks. With most new works in rural areas, the construction alternates between cut and fill, with the design, where possible, using the surplus from the cutting to form the embankments. Before the work of cutting and filling begins, it is good practice to lay French drains (see Figures below)

along the limits of the roadworks, on completion of earthworks, similar drains
should immediately be laid in the verges and central reserve, in order to keep the formation of the new road
clear of surface water. All these drains are laid to falls and interconnected and linked to a suitable natural watercourse by an outfall drain.