The process of removing and controlling excess surface and sub surface water within the right of way is called highway drainage. In includes collection and diversion of surface and sub surface water. Highway drainage system is required in highways due to the significant reduction on bearing capacity of soil foundation due to the presence of moisture content in it. The water standing during monsoon and rainy season on the carriageway is danger to high speed traffic. Presence of water on the bituminous surface reduces the life of roads. Excess water in shoulder and pavement causes considerable damage to the Roads from its edge. If there is clayey soil is present within the surface or on the surrounding of earthen road, considerable changes in volume of materials will happen. Unmanaged water could damage open embankment and filling areas. This leads to failure of pavement structure before its designed period.
Due to unmanaged excess water on the highway pavement, following defects will occur:
- Presence of moisture and water on the road surface of earthen roads, gravel roads damage the wearing course.
- Erosion of side slopes forming gullies, erosion of side drains, etc. takes place due to presence of surface runoff.
- Softening the subgrade soil and decreasing its bearing capacity will happen.
- Chances of landslides and mass movement etc. could be happen due to presence of water.
To overcome these considerable losses and damages due to the action of excessive water on the road and its right of way, following requirements are mandatory for us:
- Surface water from the carriageway and shoulder should be easily drained off without entering to the subgrade.
- Surface water from the adjoining land and slopes should be prevented from entering the roadway.
- The side drain should have sufficient capacity and longitudinal slope to carry away all the surface water collected during rainy periods.
- A Seepage and other sources of underground water should be drained off by the sub- surface drainage system.
- Highest level of ground water table should be kept well below the level of subgrade, preferably by at least 1.20 m.
- The water from the natural channels and gullies must be managed properly.
Types of highway drainage system
Highway drainage system consists of construction of drainage structures at proper locations. The highway drainage systems are of following types
A. Surface Drainage system
In highway engineering, surface drainage system is most important. A highway pavement without proper drainage facilities will not last for long time. Rainwater falling on the road surface and adjoining area, is lost by evaporation and infiltration. The remaining water is known as surface water. Removal and diversion of this surface water from highway and earth surface is known as surface drainage. The water or rainfall on road surface of earth should be collected by side drains which carries the drain water to stream or river. So, while construction process of road, the designer should design required space for providing proper drainage facilities as well as the pavement should also be constructed with minimum camber. Drainage system may differ according to type and location of Highway.
In general, the following conditions are considered:
- Drainage in rural Roads: There is the provision of side drains in these areas which are generally open, unlined and trapezoidal and are cut to suitable cross section and longitudinal slopes. Camber is applied to the pavement to drain the surface water. Usually, drains are provided on one or both sides in embankments while drains are provided on both sides in case of roads with box cutting. Where space is restricted in cutting Open drains are dangerous and hence covered drains are used with layers of coarse sand gravel.
- Drains in Urban Roads: In urban roads, underground longitudinal drains are provided due to the limitation of road width, foot path, dividing road and other road facilities. It is provided where there is lesser number of natural water courses. Water is collected in the catch pits at required intervals and connected through underground drainage pipes.
- Drainage in hill roads: In hill roads, there are complex drainage problems. Water flowing down the hill has to be efficiently intercepted and disposed of downhill side by constructing suitable cross drainage works. Side drains are provided only at the hill side where Catch water drains at the upper hill side, sloping drains and cross slopes are provided to drain out the water. If hill roads are not properly drained, landslide may occur blocking the road during monsoon season. During emergency, the shape of the side drains is made in such a way that vehicles can park at that space.
B. Sub surface Drainage System
The underground drainage system, which collects and escapes the excess moisture developed within the pavement layers safely is called sub surface drainage. The strength of subgrade depends upon the stability and strength of the road surface. Increase in moisture content, decrease the strength of the subgrade. There are various methods of sub surface drainage system:
- Drainage of infiltrated water
- Control of seepage flow
- Lowering of water table
- Control of capillary rise
A. Drainage of infiltrated water: During snow melting season and rainy season, water will find its own way to the subgrade soil through the permeable surface of the land, carriageway, shoulder, side slope and cracks. Removal of such infiltrated water from the subgrade may be by controlling seepage flow, controlling penetration of surface runoff in to the subgrade etc.
B. Control of seepage flow: Seepage may occur from the higher ground in hilly topography or in road cuttings where a layer of permeable soil. The best solution to this type of problem would be to intercept the seepage water on the uphill side of the road.
C. Lowering of water table: The water table may rise and may come up to the pavement layers in low-lying areas during rainy seasons which becomes very harmful to the pavement and the subgrade especially when the subgrade is made of fine-grained soils. Therefore, it becomes necessary to lower the water table safely below the pavement layer. If the underground water table is more than 1.2 m below the surface of the road, it does not require any subsurface drainage but when it is less than 1.2 m the best measure would be to raise the road formation.
D. Control of capillary rise: In water logged sections, there will be possibility of rising of water to the subgrade level due to the phenomenon of capillary action which affects the strength of the subgrade. Thus, capillary cut off measures needs to be provided to free the subgrade from the excessive moisture. If the subgrade soil is of permeable type, the lowering of water table is economical but in case of retentive type of soil, drainage becomes very difficult and costly. In these cases, capillary cut offs become more economical.
C. Cross Drainage system
The Roads shall facilitate to drain off the surface runoff, seepage and capillary rise from the road pavement structures. The water from natural streams, channels or gullies are also responsible losses and damage of the road structures. So, for proper water management in highway, and its associated structures, cross drainage structures are to be provided. The Cross-drainage structures are those structures which are provided whenever streams have to cross the roadway facility.
In Roads, Cross Drainage structures are of following types:
- Culvert
- Bridge
- Causeway
- Aqueduct
- Inverted Siphon
- Super passes
1.Culvert:
A closed conduit placed under the embankment to carry water across the roadway is termed as culverts. In Nepal Road standard-2070, culverts are the bridging structures of linear waterway span less than about 6m. In fact, more than 75% of the cross-drainage structures are culverts. Collection and transport of water across the road so as to not cause damage to the road bank or the stream bed by scouring. To provide sufficient waterway to prevent heading up of water above the road surface. Various types of culverts are pipe culverts, box culverts, slab culverts, and arch culverts.
A. Pipe culvert
B. Box culvert
C. Slab culvert
D. Arch culvert
2. Bridge:
A bridge is a structure constructed over water course to carry traffic over it. In Nepal Road Standard-2070, bridges are the structures having linear waterway span more than about 6m. In practical, Bridges are classified according to the materials of construction, structural basis, bridge span and load of carriageway etc.
3. Causeway:
They are constructed in the alternatives of culverts on less important roads where the maximum flow of depth does not exceed 1.5 m. construction of causeway over the culverts is more economical and saves cost on cross drainage structures. During the flood, the water flows over the road and traffic on both sides is stopped but as soon as the flood recedes, the traffic flow is restarted. It remains dry for most of the time.
4. Aqueduct:
Aqueduct is a cross drainage structure constructed to channel water from the rivers or stream to the distribution points. But modern engineering aqueduct is a cross drainage system constitute of pipes, ditches, canals, tunnels, and supporting structures constructed/laid to convey water from a source to the main distribution points. Aqueducts play and important role in maintaining the strategic flow between drainage water with canal water.
5. Inverted Siphon:
An inverted siphon is a hydraulic structure used to carry a conduit, such as a pipeline or a channel, beneath an obstacle like a valley or a depression, where it is not feasible or practical to construct an overhead bridge or culvert. It essentially operates as a water-filled tube that runs downhill and then back up again, transporting water or other fluids from a higher elevation to a lower one, and then back up to the original level. Inverted siphons are commonly employed in various engineering projects, including sewer systems, aqueducts, and irrigation networks, to facilitate the smooth passage of fluids across challenging terrain or obstacles.
6. Super pass:
A super passage structure is designed to carry drainage above a canal, particularly when the canal bed level lies below the drainage bed level. In this structure, a drainage trough is constructed at road level, enabling the flow of drainage water from upstream to downstream. The piers supporting the structure are positioned beneath this drainage trough, allowing the canal water to flow through them. Notably, the full supply level of the canal remains below the drainage trough in this arrangement. The flow of water in the canal occurs under the influence of gravity and atmospheric pressure. Essentially, the super passage structure represents a reversal of the traditional aqueduct design.
Importance of Highway Drainage
In general, highway drainage has following importance:
- To prevent the road surface becoming soft and losing its strength.
- To keep bearing capacity of road in its design value up to design period.
- To prevent the variation in moisture content in expansive soil which causes variation in the volume of subgrade and thus failure of road.
- To mitigate the effects due to presence of moisture at freezing temperature.
- To prevent erosion of side slopes, side drains and formation of gullies due to poor drainage conditions.
- To prevent formation of waves and corrugations due to poor drainage.
- To prevent formation of pot holes on road pavement.
- To prevent failure of rigid pavement by mud pumping.
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