Highway Drainage (Highway Engineering)
- 2. Introduction Significance of Drainage Requirements of Highway Drainage Surface Drainage Methods of Surface Drainage Shoulder Drainage Median Drainage Sub-Surface Drainage Methods Of Sub-Surface Drainage Road Construction in water logged area
- 3. Highway drainage is a process of removing and controlling excess surface and sub-soil water within the right of way. It includes interception and diversion of water from the road surface and sub grade.
- 4. Excess moisture in soil subgrade causes considerable lowering of its stability. Increase in moisture cause reduction in strength of many pavement materials like stabilized soil and water bound macadam. Sustained contact of water with bituminous pavements causes failure due to stripping of bitumen from aggregates like loosening or detachment of some of the bituminous pavement layers and formations of pot holes Excess water on shoulders and pavement edges causes considerable damage.
- 5. In clayey soil variation in moisture content causes considerable variation in volume of sub grade. High moisture content causes increases in weight and thus increase in stress and simultaneous reduction in strength of soil mass. Erosion of soil from top of unsurfaced roads and slopes of embankment, cut and hill side is also due to surface water. In cold regions presence of water in the subgrade and a continuous supply of water from the ground water can cause considerable damage to the pavement due to frost action.
- 6. The surface water from the carriageway and shoulder should effectively be drained off without allowing it to percolate to sub grade. Surface water from adjoining land should be prevented form entering the roadway. Side drain should have sufficient capacity and longitudinal slope to carry away all surface water collected. Flow of surface water across the road and shoulders and along slopes should not cause formation of cross ruts or erosion.
- 7. Seepage and other sources of under ground water should be drained off by the subsurface drainage system. Highest level of ground water table should be kept well below the level of subgrade, preferably by atleast 1.2m. In waterlogged areas special precautions should be taken, especially if detrimental salts are present or if flooring is likely to occur.
- 8. During rains, part of the rain water flows on surface and part of it percolate through the soil mass as gravitational water until it reaches the ground water below the water table. Removal and diversion of surface water from the road way and adjoining land is termed as surface drainage. Diversion and removal of excess of soil water from the subgrade is termed as Surface Drainage.
- 9. 1. By Longitudinal side drains. 2. Catch basins and Inlets in urban areas 3. Providing damp proof course 4. Providing proper camber 5. Providing sufficient slope to the sides 6. Keeping the level of carriage way at least 60 cm above the HFL.
- 10. A) Side drains for road in embankment -
- 11. It is necessary to provide side drain on one side or both sides, when road is constructed in embankment. Side drains should be at least 2.0 m away from bottom edge of an embankment. Depth of side drains is kept 1.0 to 1.5 m to prevent the entry of drain water into the embankment.
- 12. B) Side drains for road in cutting
- 13. A) Catch basins
- 14. Catch basin is a structure like chamber constructed on a sewer line. Water from pavement surface is collected in catch basin and discharged to the sewer line. The catch basin (catch pit) is provided with grating to prevent the entry of rubbish into the drainage system.
- 15. Inlets is a concrete box with grating either at the top or in the side.
- 16. For a quick drainage it is necessary to ensure that shoulder surface is properly sloped and free from irregularities and depressions. In impervious type, it is practiced to extend the subbase course with drainage across the shoulder upto side drain. Alternatively we can provide cont layer of 75- 100 mm thick laid under the shoulder at the bottom layer of sub base about 150 mm thick extended upto the edge. The paved shoulder should be at least 0.5% steeper then camber subjected to min of 3%
- 17. The unpaved shoulder should be further 0.5% steeper along the horizontal curve, Shoulder on inner side of the curve should have slope steeper than that of pavement. On the outer side, the shoulder should be made to drain away from the pavement, a low rate of super elevation are provided. On the other hand, where higher rate of super elevation, the outside shoulder are kept level or rounded.
- 19. Except for a median less than 5m wide, it is not desirable to drain the median area towards the pavement surface. 1.2-1.8 m wide median – Provided with kerbs and always paved. 1.8-5 m wide median – Usually turfed and crowned so that surface water could run towards the pavement maybe with or without kerbs.
- 20. Median wider than 5m are without any kerbs at edge If carriage way slopes towards the median, central drain maybe made to carry the runoff and slope should not be steeper than 6:1. When the median has only earthen surface, it should not be crowned because it can result in soil particles being carried to the pavement surface making it slippery.
- 21. Removal or Diversion of excess soil-water from sub-grade is termed as sub-surface drainage. The change in moisture of sub-grade are caused by the following Fluctuations in GroundWaterTable Seepage Flow PercolationOf RainWater Movement of CapillaryWater In sub-surface drainage it is practiced to keep the variation of moisture in sub-grade to a minimum.
- 22. When road is in cutting and water seeping from sides. When road is near the foot or hill and is likely to be damaged by water flowing down the hill. When road is passing through plain area and water is likely to accumulated on sides. Where water rises up to sub-grade by capillary action. Where sub-grade soil is affected by the drain passing near the road.
- 23. Lowering ofWater Table Controlling Seepage Flow Controlling Capillary Water
- 24. In order to that the sub-grade and pavement are not subjected to excessive moisture the water table should kept at least 1.0 to 1.2 m below the sub-grade. In places where water table is high to take the road formation on embankment of height not less than 1.0-1.2m is the best approach. ButWhen the formation level is at or below the general ground level it is necessary to lower the water table.
- 25. (a) If soil is relatively permeable, it may be possible to lowerWT by constructing longitudinal drainage trenches with drain pipe and filter sand and top of trenches is covered with clay seal The depth of trench depend on: (i) The required lowering of water table (ii) distance b/w the drainage trenches (iii) Types of soil
- 27. (b) If the soil is relatively less permeable, the lowering of groundWT may not be adequate. Hence in addition to the longitudinal drains traverse drains have to be installed at suitable intervals in effectively drain off the water.
- 28. When the general ground as well as impervious strata below are sloping, seepage flow is likely to exist. If seepage zone is at depth less than 0.6-0.9 m from sub-grade level, longitudinal pipe drain in trench filled with filter material and clay seal may be constructed to intercept the flow.
- 30. If the water reaching the sub-grade due to capillary rise is likely to be detrimental, it is possible to solve the problem by arresting the capillary rise. It can be done by following methods: (a) A layer of granular materials of suitable thickness is provided during the construction of embankment, between the sub-grade and the highest level of sub surface water table. (b) Alternate method is providing the capillary cut off is by inserting an impermeable or a bituminous layer in place of granular blanket.
- 32. The size and spacing of the sub-surface drainage system depend on the quantity of water to be drained off, the type of soil and type of drain. The filter material used in sub-surface drains should be designed to have sufficient permeability offering negligible resistance to the flow. And also resist the flow of the fine foundation soil resulting in soil piping. The Steps are as follows: Draw grain size distribution curve on log scale %passing vs. particles size for the foundation soil. Find the value of D15 size of foundation soil. *Note D15- particle size such that 15%of soil is finer than this size
- 33. The size of filter material should be 5 times more than the size of foundation soil D15 of filter/D15 of foundation > 5 To fulfill the condition to prevent piping. D15 of filter/D85 of foundation < 5 Hence plot a point represent the upper limit of D15 size of filter given by 5D85 of foundation. Find the size of the perforation in the drain pipe or the gap in the open jointed pipes and let this be = Dp Plot a point representing D85 size of filter given by the size 2Dp. The Shaded area thus obtained represents the region within which the grain size distribution curve of satisfactory filter material should lie.
- 35. In case of sub-grade subjected soil water to soaking cond., capillary rise, and in extreme cond. Even flooding for prolonged periods is termed as water logged areas. In such areas construction and maintenance of road is a problem. Methods adopted to Overcome Raising the road level by constructing embankment Providing a capillary cut off to arrest capillary water Providing Cement Concrete Block Depressing the sub surface water level by drainage system