Highway drainage

 "Highway Drainage may be defined as the process of interception and removal of water from over, under, and vicinity of the road surface."

• Importance of Highway Drainage : 

Water is an important material needed for the construction of embankments, pavement layers, and structures on a highway. 

But once the road is constructed, water is detrimental to the good performance of a road and must be drained off quickly. 

It is well said that cheap drainage is preferable to the costly pavement.

During rain or irrigation, the water infiltrates into the soil and is stored in its pores.

Highway Drainage is required to mitigate the effects due to water and moisture variations that are listed below as:

• Road surface becomes soft and loses its strength.

• Road subgrade may be softened and its bearing capacity is reduced.

• Variation in moisture content in expansive soil causes variation in the volume of subgrade

and thus failure of the road.

• Presence of moisture at freezing temperature may damage the road due to frost action.

• Erosion of side slopes, side drains, and formation of gullies may result if proper drainage

conditions are not maintained.

• Flexible pavement failure by the formation of waves and corrugations is due to poor

drainage.

• Formation of potholes.

• Failure of rigid pavement by mud pumping.

• Destruction of highways by water:

• Softening the road surface constructed of soil or sand-clay or gravel or water-bound

macadam.

• Erosion of side slopes forming gullies, erosion of side drains, etc.

• Softening the subgrade soil and decreasing its bearing power.

• Chances of landslides and slips.

• Ways of Protecting above effects:

• Interception and diversion of the surface water which would otherwise flow across the road

or along it and cause erosion.

• Interception and rapid removal of seepage of subsurface water.

• Proper soil treatment

• Change of the water course 

Why highway drainage system requires? 

• Surface water from the carriageway and shoulder should be effectively drained off without allowing it to percolate to the subgrade.

• Surface water from the adjoining land 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.

• 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 1.20m.

• Components of Highway Drainage System

a) Surface Drainage System

b) Subsurface Drainage System

 A). Surface Drainage System :

• A part of rainwater falling on the road surface and the adjoining area is lost by evaporation and percolation.
• The remaining water is known as surface water.

Removal and diversion of this surface water from the highway and adjoining land are known as surface drainage. 

 

• Surface drainage systems typically consist of an outlet channel (existing natural stream or constructed channel emptying into a natural stream), lateral ditches, and field ditches.
• Such systems are used primarily in flat areas having poor natural drainage to remove water that collects on the land surface when the rainfall rate exceeds the infiltration capacity.
• Additional improvements for surface drainage may include land smoothing or land grading to fill in shallow depressions and to assure a continuous slope in the field toward the field ditches.

Collection of Surface Water

The surface drainage may be divided into three categories:

a) Drainage in rural highway

• There is the provision of side drains in these areas which are generally open, unlined, and trapezoidal cut to suitable cross-section and longitudinal slopes. 
• Camber is applied to the pavement to drain the surface water and has to drain across the shoulders which are provided with more cross slope. Usually, drains are provided on one or both sides in embankments while drains are provided on both sides in case of roads with cutting.
• Open drains are dangerous in places where space is restricted in cutting and hence covered drains are used with layers of coarse sand gravel.

b) Drains in Urban Street

• In urban roads, underground longitudinal drains are provided due to the limitation of land width, the presence of footpaths, dividing islands,s, and other road facilities. 
• This is provided where there is a lesser number of natural water courses and in the presence of impervious surfaces. 
• Water is collected in the catch pits at suitable intervals and led through underground drainage pipes.

c) Drainage on hill roads

• On 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.
• Catch water drains at the upper hillside, sloping drains, and cross slopes are provided to drain out the water whereas side drains are provided only at the hillside.
• If hill roads are not properly drained, rockslides and slips may occur blocking the road during monsoon season. 
• The shape of the side drains is made in such a way that vehicles can park at that space during emergencies, crossing, or parking.

b) Subsurface Drainage System

• The stability and strength of the road surface depend upon the strength of the subgrade.
• With the increase in moisture content the strength of the subgrade decreases.

The variation in moisture content of the subgrade is caused by the free water and the groundwater.

Every effort is needed to reduce the moisture content to a minimum. From the usual drainage system, only gravitational water can be drained by the provision of subsoil drainage.

Drainage of infiltrated water

• During the rainy season and snow melting season, water will find its way to the subgrade soil through the permeable surface of the adjoining land, carriageway, shoulder, side slope, and cracks.
• Removal of such infiltrated water from the subgrade may be accomplished by the arrangements shown in the figures below. The control of subsurface water is classified under three headings:

1. Control of seepage flow

2. Lowering of the water table

3. Control of capillary rise 

Control of seepage flow 

• Seepage may occur from the higher ground in hilly topography or in road cuttings where a layer of permeable soil overlies an impermeable stratum which affects the strength characteristics of the subgrade.
• The best solution to this type of problem would be to intercept the seepage water on the uphill side of the road.
• If the seepage level reaches a depth less than 60-90 cm from the road subgrade, it should be intercepted to keep the seepage line at a safe depth below the road subgrade.

Lowering of the 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.

If the underground water table is more than 1.2m below the surface of the road, it does not require any subsurface drainage but when it is less than 1.2m the best measure would be to raise the road formation.

• The water table is lowered to the desired depth by providing sub-drains on either side of the road. It may be possible to lower the water table by merely constructing longitudinal drainage trenches with drain pipes and filter sand if the soil is relatively permeable.

• But if the soil is relatively less permeable, the water table lowered at the center of the pavement or between the two longitudinal drains may not be adequate. Thus, transverse drains may have to be provided in order to effectively drain off the water and lower the water table.

• The depth to which the drains should be laid depends upon the width of the roadway, the amount of water table to be lowered, the type of subgrade soil, and the lateral distance between the trenches.

• The pipe in the drainage system should be laid such that silting and scouring do not occur.

• For the maintenance of these systems, manholes and inspection chambers can be provided.

Control of capillary rise

In waterlogged sections, there will be the possibility of rising water to the subgrade level due to the phenomenon of capillary action which affects the strength of the subgrade. Thus, capillary cut-off measures need to be provided to free the subgrade from excessive moisture. If the subgrade soil is of permeable type, the lowering of the water table is economical but in the case of the retentive type of soil, drainage becomes very difficult and costly. In these cases, capillary cut-offs become more economical. There are two types of capillary cut-off:

1. Granular capillary cut-off:

• Provision of granular material of suitable thickness between the subgrade and the highest level of the subsurface water table during the construction of the embankment.
• The granular capillary cut-off layer should be sufficiently higher than the anticipated capillary rise within the granular layer so that the capillary water cannot rise above the cut-off layer.
• Suitable sand blankets and gravel blankets can be used for cut-off.

2. Impermeable capillary cut-off:

• Provision of the impermeable membrane such as prefabricated bituminized surfacing is used instead of a granular blanket.
• Bitumen stabilized soil, heavy-duty tar felt or heavy-duty polythene envelope can also be used.