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
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.
Catch water drain in hard strata
This will also reduce the load of road side gutters.
2. Read side Gutters.
This is most efficient cheaper method of draining of water in cuttings and other places. These gutters should be atleast 600 mm deeper than the bottom of road crust. The typical sections for gutters are shown in below image. The longitudinal gradient may usually be the same as for 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 mtr. 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.
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.
This may be provided across the road if the blockage of subsoil flow along the road is experienced and if no other method is successful. A French drain of dimentions 50 cm. x 60 cm. may be provided across the road below the crust loading moisture out of the banking. Sub Soil Drains should be filled with drainage media consisting of gravel, Pebbles, shingles, coarse sand etc. sub soil drain should have minimum bottom slope of 5% draining away from the read centre. This will ensure drainage of the subgrade and also to some extent of pavement. These drains should be provided at 5 m. to 10 m. centre to centre as shown in image below. The perforated pipe drains should also be provided in case of heavy soils (comprising of predeminently clayee soil like black cotton soil).
It should be noted that unless the side gutters are sufficiently deep this method will not prove much efficient. Sub soil drains are very effective in quickly draining the moisture from clayee soils. Life and serviciability of sub soil drains can be increased by providing geotextile around the drainage media in the trenches.
5. Treatment at valley Curves
At valley curves and at change of grade locations, water flowing below (seeping) needs an outlet. A subsoil drain of minimum dimension of 500 mm. x 600 mm. be introduced across the road as shown in below image. If the valley curve is very long, it is desirable to provide two or three such drains at such curve. The crust in usual fashion may be built up at such location after introducing the sub soil drains.
6. Shoulder Drainage.
Shoulder should be preferably in granular material. The camber of 5% should be provided for shoulders. Wherever shoulders are higher than the pavement edge the shoulder should be shaved off in a camber from pavement edge, outwards ensuring smooth flow of surface water.
Grass should not be allowed to grow on side shoulders and should be cut regularly. As a purely temporary measure water cuts may also be tried, but care should be taken that they are shallow and sufficient to drain off water immediately.
7. Shoulder Drainage (Sub Surface Type).
where the shoulder material is not sufficiently pervious (clayee, mixture of clay, sand/granular material) its permeability needs to be increased. This can be ensured by shoulder drains consisting of cross cuts of size 300 mm. x 600 mm. in the shoulder at 5 to 10 meter centre to centre filled with drainage media like gravel, shingle, coarse sand etc. The bottom of the shoulder drains should have a cross slope of minimum 5% draining away from the pavement. In case of pavement on grade it is desirable that the shoulder drains are 45 degrees to the centre line of the road in the direction of the slope.
In case of pavements in level portion should drains may be at right angles to the centre line.
8. Water Stagnation.
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 to excavating a suitable gutter upto 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. Boundry drain
In low embankment portion, it is desirable to provide deep parallel drains at extreme boundry leading to natural drainage. This will ensure keeping moisture away from the bank and demarcation of the road boundry. 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 over laying 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 cleared time to time other wise seal gets broken and water enters through gaps.
12. Curves in Chat 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 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 burried 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.
17. Camber and Superelevation.
To ensure efficient drainage it is desirable to provide minimum 3% camber and superelevation to the bituminous pavement. Steeper camber takes care of imperfections and depressions on the carriageway more efficiently than flatter camber.
18. Providing Drainage Media Below Crust.
To prevent entry of moisture into pavement from below and to quickly drain away the moisture that has entered the pavement from above drainage layer below the road base is very effective. This consists of a layer of gravel, pebbles, shingle, coarse sand etc. having thickness 300 mm. to 50 mms. laid for the entire width of the bank work. The drainage layer/sub-base should be laid at a slope not less than 5%. It should be laid for the full width of the bank work as shown in image below.
Drainage Media Below Crust
It is desirable to provide a membrane of geotextile separating the embankment (substrata) and the sub-base. This membrane will prevent entry of clay into the drainage media and percolating of drainage media into the substrata.
19. Other Improvised Methods.
Other improvised methods like sub surface drains at the edge of the base in place of side gutters in place of jungle gutters etc, as shown in below images.
STREET INLET
However, these measures require elaborate maintenance input. Initial cost and the cost of maintenance is very high in these options. However, in certain circumstances, their use is inevitable.
20. Raised Asphaltic Edges.
It is observed that due to flow of asphaltic materials under traffic leads fat asphalt gets pushed towards edges, resulting raised edges. Also due to wearing of the wearing surface which is more below the wheel line and almost nil at the extreme 300 mm. to 400 mm, of the edges on either side, negative camber conditions are forms at 1.5 mt. wide strips near edges. This leads to formation of tiny channels along the pavement leading to erosion of the pavement, This can be avoided by -
i) providing steeper camber.
ii) Using harder grade of bitumen and
iii) laying renewals for the 6 m, central width only and not for the full width of 7 m.
In case of curves also renewal should be provided only for 6,00 m. On outer side of the carriageway instead of usual practice of 7 m. This will take care of the negative camber.
21. Drainage at the junction of existing and widened pavement.
It is a common observation that whenever existing carriageway is widened by adding additional pavement on the outer sides of the carriageway (in the direction of camber or superelevation) in the next monsoon the original pavement shows signs of damages. Reason of this failure is due to excavation and laying of the new pavement on the leeward side of the camber the old established natural flow falls are interrupted causing built up of moisture in the old pavement leading to its damage. Newly laid pavement also suffers from this sogginess and causes cracking and heaving in the edges.
It can be avoided if at the time of laying of an additional pavement a drainage layer as shown in Fig.13, is laid below the new pavement. Care should also be taken not to knock off the existing offset between oversize metal/soling and the size metal. This provides a shear key between the two pavements.
22. selecting the particular type of treatment.
Initial cost and maintenance cost should also be taken into account while selecting the particular type of treatment.
i) In case of non urban roads built up gutters are costlier. Better alternative is to provide open excavated side drain. The latter alterative is cheaper and more drainage efficient as bigger cross sections can be provided which will reduce the maintenance input.
ii) In case oF restricted land widths and deep cutting built-up gutter is better alternative than the open excavated gutter or sub-surface edge drainages comprising of filter media and pipe outlets. Judicious selection is, therefore, necessary.
In addition to the provisions of surface drainage and subsurface drainage, efficient peripheral drainage is also essential for longer life and better serviceability of the pavement. Few points are noted below :
a) The waterways provided by the way of C.D. works ahe bridges should be inspected and cleared before monsoon. It should be entrusted that down stream is also clear for sufficient distance.
b) Trenches should not be allowed to be excavated in the body of the road for allowing telephone cables and electric lines or water for farming purpose.
c) Whenever pipe or cables are allowed to cross the road, they should be encased atleast 600 mm. below the road surface. They should be allowed only through the conduit of sufficient diameter. The excavated trench should be filled with only drainable materials like gravel, pebble, shingle sand, metal etc., no soil should be allowed in the refilling.
d) while allowing access roads from highways, the access road should always be lower than the main highway and should be provided with proper C.D. work to allow for the side drainage. Long lengths of hume pipe drains in approaches are rarely maintained properly so it is preferable to provide cattle trap type drains as shown in image below.
e) Coping on the headwalls of H.P. drains should not be higher than the edge of the shoulder. This obstructs the flow and causes sogginess.
f) Laying of pipes carrying water supply, telephone cables etc. should not be allowed on the bridge deck which may cause blocking of water spouts.
g) In case of spring flows or cronic leakages from water supply pipe line below pavement following method is effective.
1) Excavate and open the area around the trouble spot, give rectangular shape to the pit.
2) Depth of the trench should be such that it should accommodate the filter media and outlet pipe.
3) In case of pipe leaks depth below the water supply pipe should be sufficient to accommodate filter media and outlet pipe.
4) The trench should then be filled with filter media and outlet pipe should be installed to drain out the water.
5) It is desirable to use geotextile around the filter media to prevent choking of the filter media. Outlet should have suitable slope Leading to out fall above image .
Posts
0 Comments
If you have any doubts, suggestions , corrections etc. let me know