Highway Drainage System
- 1. HIGHWAY DRAINAGE SYSTEM V.GOKULAKRISHNAN III YEAR B.E-CIVIL ENGINEERING BIT,SATHY
- 2. INTRODUCTION One of the major reasons of deterioration of roads is rain water. During Rainfall, Part of water flows on ground surface and part of it percolates through soil mass until it reaches the ground water below water table. Due to percolation of water in highway pavement moisture content of soil increases which reduce the bearing capacity of the soil. Thus stability of highway is reduced. Drainage of the highway is the process of removing and controlling surplus water on the surface and sub soil water in sub surface with in the right way.
- 3. WHAT IS HIGHWAY DRAINAGE ? Highway drainage is an essential part of highway design and construction which remove the surplus water with in the highway limits and dispose it. Road way drainage is mainly due to surface runoff from adjacent area, precipitation of rain and moisture rising by capillarity from the ground water table. Removal and diversion of surface water from road way and adjoining land is known as surface drainage. Removal of excess sub soil water from the subgrade is termed as sub surface drainage.
- 4. IMPORTANCE OF HIGHWAY DRAINAGE To prevent subgrade failure: Excess moisture reducing the stability of pavement which leads to subgrade failure. To prevent reduction in strength of pavement material: The strength of pavement material like stabilized soil and WBM (water bound macadam) is reduced. To prevent frost action: In flexible pavement the formation of waves and corrugation takes place due to poor drainage. To prevent pavement edge: Excess water on pavement edge cause considerable damage.
- 5. Prevent slope failure: Excess moisture causes increase in weight and thus increases in stress and simultaneously reduction in strength of soil mass which result into failure of earth slopes and embankment foundations. Prevent erosion of soil: Due to surface water, erosion of soil from top of road and slope of embankment. Decrease volume changes: Volume of subgrade is changed especially in clayey soils due to variation in moisture content. This sometimes leads to pavement failure.
- 6. SURFACE DRAINAGE Under the surface drainage, water is to be collected in longitudinal drains and then disposed of at the nearest stream, valley or water course. Surface water is removed from surface of road by providing cross slope to the road surface. The road surface is also made impervious to prevent the water entering the subgrade. The recommended ranges of cross slopes for different types of pavement are as per Table-1.
- 7. Table-1 Recommended values of cross slopes for different types of road surface S.NO PAVEMENT SURFACE RECOMMENDED RANGE 1 Earthen road 1 in 20 to 1 in 25 2 Gravel road 1 in 25 to 1 in 30 3 Water bound macadam 1 in 35 to 1 in 48 4 Bituminous road 1 in 48 to 1 in 60 5 Cement concrete road 1 in 60 to 1 in 72
- 8. SURFACE DRAINAGE METHODS For rural highways on embankments, runoff from the roadway should be allowed to flow evenly over the side slopes and then spread over the adjacent terrain. This method, however, can sometimes adversely impact surrounding land, such as farms. In such instances the drainage should be collected, for example, in longitudinal ditches and then conveyed to a nearby watercourse. When a highway is located in a cut, runoff may be collected in shallow side ditches.
- 9. INLETS These are parts of a drainage system that receive runoff at grade and permit the water to flow downward into underground storm drains. Inlets should be capable of passing design floods without clogging with debris. The entrance to inlets should be protected with a grating set flush with the surface of gutters or medians, so as not to be a hazard to vehicles. There are several types of inlets. A drop inlet is a box-type structure that is located in pipe segments of a storm-water collection system and into which storm water enters from the top.
- 11. STORM SEWERS These are underground pipes that receive the runoff from a roadside inlet for conveyance and discharge into a body of water away from the road. Storm sewers are often sized for anticipated runoff and for pipe capacity determined from the Manning formula. In general, changes in sewer direction are made at inlets, catch basins, or manholes. The manholes should provide maintenance access to sewers at about every 500 ft.
- 13. OPEN CHANNELS The ditches may be trapezoidal or V-shaped. The trapezoidal ditch has greater capacity for a given depth. Most roadway cross sections, however, include some form of V shaped channel as part of their cross-sectional geometry. In most instances, it is not economical to vary the size of these channels. As a result, this type of channel generally has capacity to spare, since a normal depth must be maintained to drain the pavement subbase courses. When steep grades are present, the possibility of ditch erosion becomes a serious consideration. Erosion can be limited by lining the channel with sod, stone, bituminous or concrete paving, or by providing small check dams at intervals that depend on velocity, type of soil, and depth of flows.
- 15. CULVERTS A culvert is a closed structure for passage of runoff from one open channel to another. One example is a corrugated metal pipe under a roadway. For small culverts, stock sizes of metal pipe may be used. For larger flows, however, a concrete box or multiple pipes may be needed. If the culvert foundation is not susceptible to erosion, a bridge may be constructed over the waterway (bridge culvert). The section of a culvert passing under a highway should be capable of withstanding the loads induced by traffic passing over the culvert. Since corrugated metal pipes are flexible, they can carry gravity loads. Reinforced concrete culverts, however, have to support gravity loads without such assistance.
- 17. EFFECT OF ROAD GEOMETRY ON DRAINAGE Road surfacing materials are traditionally designed to be effectively impermeable, and only a small amount of rainwater should percolate into the pavement layers. It is important that any such water is able to drain through underlying pavement layers and away from the formation. Rainfall which does not permeate the pavement surface must be shed towards the edges of the pavement. Drainage is a basic consideration in the establishment of road geometry and vertical alignments should ensure that: a) outfall levels are achievable and b) subgrade drainage can discharge.
- 18. THE USE OF SPECIALIST DESIGN SOFTWARE While highway drainage schemes are relatively straightforward to model when compared to things like housing estates or large urban areas they do require computer modelling unless extremely small. This is done to calculate peak flow rates, storage volumes and flow control devices and are based on further calculations done using computers to calculate peak “green field” run-offs.
- 19. SUB-SURFACE DRAINAGE SYSTEM The change in moisture content of soil affects its bearing capacity of soil. The increase in moisture content of soil reduces its bearing capacity. Thus keeping this point in view, there should not occur any change in moisture content of the subgrade of the road. The change in moisture content of the subgrade is due to fluctuations in the ground water table, seepage flow, percolation of rain water and capillary water. Thus the aim of the sub soil drainage is to prevent changes in the moisture content of the subgrade.
- 20. CONCLUSION Based on above study we can say that Highway drainage is process of removing and controlling excess surface and sub soil water with in the right way. Highway drainage is an important part for design and construction of highway. In surface drainage, surface water is intercepted and diverted to a natural stream. If it is not done the surface water causing erosion. In sub surface water drainage, sub surface water is interrupted and disposed of to safe place.