07-ROAD DRAINAGE

An effective drainage system is required to minimise access of water into the pavement layers and prevent damages to pavement resulting from the saturation of the subgrade or fill immediately below. The road drainage should be effective in disposing the surface precipitation and controlling the underground seepage. For the design of drains, the IRC publications SP: 42 & SP: 50 can be referred. Damages caused to the road by inadequate drainage are:

Reduction in bearing capacity of the subgrade soil due to the presence of water, leading to complete failure of the pavement.
Reduction in the design strength of pavement layers such as stabilized soil, water bound macadam etc., due to the ingress of water.
Potholes, rutting, waviness and corrugation of the bituminous pavement layers.
Erosion of shoulder and pavement edges due to the flow of surface water.
Loss of soil from the subgrade and fill layers below the pavement due to seepage of water.
Erosion of slopes of fill/cut or hillside due to surface water.

Suitability of different types of lining in side drains

Type of lining	Brief description	Suitable for	Unsuitable for
Turfing	Turfing is useful in humid area for preventing erosion.	Triangular drains having 4:1 to 3:1 slopes.	Rectangular, Trapezoidal drains since maintenance is difficult.
Stone/brick	Stone/brick masonry can be laid dry or bedded in concrete with joints filled with cement mortar.	When drains are required to carry a large amount of debris or if the velocity of flow is high.	In known unstable area.
Concreting	-	As above	As above.
Stone slab lining	-	For triangular section drains and in other section in combination with masonry/ concreting and in areas where flat stone is easily available.
Boulder pitching	-	For areas with stable slopes	In unstable loose strata.
Bituminous treatment	It is primarily quick sealing of the sides and base impregnated with bitumen cutback or emulsion 10 cm to 15 cm depth.	Used in conjunction with boulder pitching in catch water drains for prevention of seepage flow

Table: Gradation of material for drainage layer

IS sieve designation	% by weight passing the IS sieve
75 mm	100
53 mm	-
26.5 mm	55-75
9.5 mm	-
4.75 mm	10-30
2.36 mm	-
425 mic	-
75 mic	<5

RELATED CODES :

1. IRC-SP: 20 Rural Roads Manual - published by Indian Road Congress, New Delhi 2002.

2. Manual of specifications and standards for two lining of highways through PPP – published by the Planning Commission, Government of India.

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