Drainage Improvement Measures

Measures to be followed for Drainage Improvement

Improper drainage causes damage to road structure, CD works in several ways and also hampers the functionality of roadways, so for long life and functionality proper drainage arrangement need to be selected and selection of proper arrangement depends upon several factors such as type of soil, topography and intensity of rainfall etc.

Providing effective drainage arrangement during original construction of the road or during improvement works and maintaining the same efficiently during service are very important for the life and serviceability of any pavement. It is experienced that even most Costly and sophisticated pavement gets damaged prematurely, if proper and efficient drainage is not provided. At the same time well drained pavement with cheaper specifications and less sophistication have given longer life and tolerable serviceability. So it is preferable to go for better and cheaper drainage effective methods than only "Sophistication".

Some failures in road structure due to improper drainage

Road width utilisation affected

Footpath functionality affected

Drainage system affecting functionality

Failure of C.D. work due to improper drainage

Failure of road structure

1. Catch Water Drains and Jungle Gutters

In case of roads in hilly areas, water from the hill slopes rushes towards the carriageway and finds access in road structure causing damages. As such the drainage of hill slopes should be carefully planned so that as far as possible the runoff is arrested at appropriate locations and diverted towards the natural drainage. This will also reduce the load of road side gutters.

Catch water drain in hard strata

Catch water drain in Clayey strata

2. Road Side Gutters

This is the most efficient and cheaper method of draining water in cuttings and other places. These gutters should be at least 600 mm deeper than the bottom of road crust. The longitudinal gradient may usually be the same as for the road, but if gutter grading is flatter than 1 in 100 then it shall be drained into nearest nallah/water course/valley etc. Minimum depth of gutter should be 1.00 m, and the side slopes of the gutter will depend on the strata met with. Bottom width should be as wide as possible but should not be less than 0.3 metre.

Typical sections for gutters

3. Impervious layer at sub-grade level (Rock cutting)

In case of rock cutting, undulations in parent rock should be filled in with concrete and then pavement should be built on it. This prevents entry of moisture in the crust from bottom and side. At the same time it prevents sogginess below the pavement due to ingress of water into the rock pockets. Following image shows typical arrangement in such case as per C.A.C. technique.

4. Sub Soil Drains

Proper drainage is essential for road longevity and safety.

Water stagnation along the road side either in borrow pits or rock pockets should be drained off. Where water gets ponded due to local depressions/sinking the same should be either raised to the general surrounding level or drained off by excavating a suitable gutter up to natural drainage.

9. Gaps in Parapet in Ghat sections

Parapets where provided should be discontinuous above the road level. And below the road level sufficient weep holes may be provided, so as to drain the sub-soil water.

10. Boundary drain

In low embankment portion, it is desirable to provide deep parallel drains at extreme boundary leading to natural drainage. This will ensure keeping moisture away from the bank and demarcation of the road boundary. This will also ensure that road land is not used for cultivation and consequent problem of ponding and seepage in the bank work.

11. Water Spouts

Water spouts on bridges and R.O.Bs should be cleaned properly and if necessary extra spouts may be provided. At the time of renewal or overlaying the wearing coat, care should be taken that width of the overlay/renewal coat on two lane bridges is kept as 7.00 metres, instead of usual 7.50 metres.

Expansion Joints also need to be cleared from time to time otherwise seal gets broken and water enters through gaps.

12. Curves in Ghat Section

Water flows towards the inner edge of the curves due to superelevation on curves. Therefore for curves with inner edge on hill side, unless the longitudinal drain is large and steep enough, the water normally spills on pavement top and stagnates on inner side of curves. At such location deep gutter with sufficient gradient shall be provided. Alternatively pipe culvert/CD work may be provided. It would also be generally desirable to black top the shoulders on inner side of such curves with rubble wall at edge defining the side gutter.

13. Interceptor Drain

In certain cases where the road gradient is more than the cross camber provided for the pavement, rain water has a tendency to flow along the pavement slope for considerable long distances. This flow on the pavement top causes erosion of the surface. It has also been observed that longer the flow more the damages. This length can be reduced by intercepting the flow by providing interceptor drain at suitable location as shown in below image.

Interceptor drains should discharge into the side drains.

14. Divided Carriageway - Sealing the Median verge

Simple method of preventing ingress of rain water into the pavement in case of divided carriageway is to seal the raised median with impervious layers. The impervious layers may consist of concrete, stone tiled paving or bituminous paving etc. Care should be taken that the paving should be sufficient strong to prevent dislocation causing ingress of water.

Top of paving should have sufficient camber (Minimum 5%) in the direction of the camber provided to the carriageway pavement.

15. Divided carriageway Introduction of Central Gutter

A central gutter of sufficient dimension should be provided in the median verge. Full surface of the verge should be paved as explained above. At suitable locations (say 30 to 40 m) the water should be carried out of embankment by providing buried pipes across one of the carriageway.

16. Divided Carriageway - Treatment on curves

On curved alignment superelevation is provided for both the carriageways in one direction only (towards inner side of the curve). Wherever raised median is provided the surface flow from the outer carriageway gets blocked due to raised median and inner side of the outer carriageway gets damaged. Drainage of such location can be achieved by providing inlets admitting flow of the outer carriageway and leading the flow to the outside of the bank work through pipe below carriageway.

Drainage Media Below Crust

Effective roadside and subsurface drainage is critical for pavement durability and road safety.