What is subsurface Drainage?
Subsurface drainage is a process of removal of moisture (water) beyond the surface. The development of the science of soil mechanics has shown that the increase in the moisture content of the soil beyond certain leads to a decrease in the strength of the soil. It is, therefore, necessary to have an efficient sub-surface drainage system for keeping the road surface in good order.
It is natural that if the materials composing the road structure are placed on the wet, retentive or un-drained soil will get quickly disintegrated by traffic.
Causes of changes in moisture content in subgrade:
The moisture content of subgrade soil changes due to a number of causes and they can be enumerated as follows:
- movement of capillary water;
- percolation of water into the subgrade through cracks on the road surface;
- rise or fall in the level of the water table;
- seepage of water into the subgrade from higher ground adjacent to the road;
- transfer of moisture either from or to the soil in the verges as a result of difference in moisture content;
- transfer of moisture from or to lower soil layers by suction;
- transfer of water vapour through the soil; etc.
Interesting for You: Road (Highway) Drainage: Surface drainage system, types, methods & diagram
Situations requiring sub-surface drainage:
Following are the conditions or circumstances which require the provision of a suitable sub-surface drainage system:
- The road is at the foot of a hill and there are chances of the road being damaged by water coming from the top.
- The road is in cutting and there are chances of considerable seepage in the slopes.
- The road is passing through flat country and the water from the adjacent lands stagnates and makes the road bed soft and unstable.
- The soil below the road is subjected to the action of springs passing nearby
- The surface of the road has the normal underground water table which is sufficiently below the crust of the road but there is tendency of the moisture to rise to the surface of the road or subgrade through the capillary action.
Methods of subsurface drainage:
The main object of sub-surface drainage is to keep the highest water table level fairly w the level of subgrade so that the subgrade and pavement layers are not subjected to excessive moisture. The methods adopted for sub-surface drainage can be grouped into the following three categories:
- Method to control capillary rise
- Methods to control seepage flow
- Methods to lower water table
(I ) Methods to control capillary rise:
When there are chances for water to reach the level of subgrade through the capillary rise, it becomes necessary to arrest such action by providing suitable capillary cut-off between the subgrade and highest water table level. When the construction of road in embankment is in progress, the capillary cut-off may be provided in one of the following ways:
(a) Inserting an impermeable or a bituminous layer, as shown in fig. 2.
(b) Providing a layer of granular material of suitable thickness, as shown in fig. 3.
(ii ) Methods to control seepage flow:
If the ground is sloping and the seepage zone is at a depth of about 600 mm to 900 mm from the edge of road, the perforated drain pipe with filter material is provided as shown in fig. 4., to lower the seepage line. The trench for laying the drain pipe is kept above the sloping impervious layer.
If the road is partly in cutting and partly in embankment, as shown in fig. 5., the seepage flow can be arrested on the cutting side by perforated drain pipe and the Water can be disposed-off through cross drain pipes on the sloping side.
(iii) Methods to lower water tables
In order to bring down the level of water table for roads running in flat country with low embankments, the longitudinal sub-surface drains or pipe drains are placed below the surface of the ground in the permeable saturated stratum The pipe drains may be of vitrified clay with open joints butting against each other or they may be perforated at top. They may also be constructed of cement concrete or brick masonry with covering of perforated sheet.
The longitudinal pipe drains may be laid on both sides of the road, as shown in fig. 2. , or in the center of the road, as shown in fig. 3., depending upon the moisture conditions. These pipes are placed in the trench when placed on sides with proper slope and they discharge the water into the surface drain. The pipe drains are usually laid on both the sides of the road.
If the provision of two longitudinal pipe drains on the sides of the road does not prove adequate to lower the ground water table to the desired extent, the transverse drains are laid in sedition, as shown in fig. 8..
The transverse drains are laid with suitable slope and they discharge the water into the longitudinal drains. They are kept inclined at an angle of about 60° and staggered in plan. They are placed at a distance of 6 m to 20 m depending upon the moisture conditions.
The action of pipe drains will depend on the hydrostatic pressure, the water in the soil at the level of the drain is under pressure due to the hydrostatic head of the sub-soil water. This water under pressure then is forced into the drain through the open joints and thus the flow is started. The flow of water is continued till the head of outside water is less than the resistance offered by the soil to the flow of water
The pipe drains serve the following purposes:
- They intercept water before it reaches the road bed.
- It lower the level of the ground water and thus allow the ground to dry out.
- it will reduce the injurious action of frost by removing the moisture from the road surface.
- They remove water which is present in the road when the road thawing.
- They remove water which is prevented from overflowing when there is an impervious stratum underlying the road surface.
DESIGN OF FILTER FOR DRAINAGE TRENCHES
For a satisfactory filter design, the grain size distribution of the filter material should comply with the following two requirements:
- Permeability ratio:
The filter material should be such that it is sufficiently permeable and for this purpose, the permeability ratio should be greater than 5. The permeability ratio is defined as follows:
Permeability ratio = (15 per cent of the filter material) / (15 per cent size of the subgrade material)
- Piping ratio:
The silt and fine particles of the subgrade material should not be washed into the filter and for this purpose, the piping ratio should be less than 5. The piping ratio is defined as follows:
Piping ratio = (15 percent size of the filter material) / (85 percent size of the subgrade material).
The particle-size distribution curve for the subgrade soil is plotted and by applying the above two criteria, the grain size distribution curve of satisfactory filter material is obtained.
The top of the drainage trench in which the filter material has been placed must be properly sealed because the surface runoff generally contains some silty material and if it runs into the filter, it might clog the pours and reduce the effectiveness of the filter.
Road Construction in Waterlogged Areas
It sometimes so happens that the subgrade is subjected to extreme soaking conditions due to high level of ground water table as well as high capillary rise. Such an area is known as waterlogged area and the construction as well as maintenance of roads in such areas may prove to be very inconvenient and difficult. The methods which may be suggested in such cases are as follows:
- Control of capillary rise:
Suitable method to arrest the capillary rise may be adopted.
- Pavement thickness:
Depending upon the subgrade conditions, sufficient thickness of pavement should be provided.
- Raising the road level:
If the site conditions permit, the road level should be raised by constructing an embankment.
- Sand drains:
The provision of vertical sand drains at suitable spacing and horizontal sand blanket at the top ensure rapid drainage of water from the foundation soil.
- Sub-surface drainage system: The level of ground water table may be depressed by laying a suitable sub-surface drainage system.
If, in addition to waterlogging, the flooding of road surface is expected for a prolonged period, it becomes necessary to provide suitable cement concrete or bituminous concrete pavement.
I detrimental salts are also present in the waterlogged area, the problem of road construction becomes very serious and in such cases, extraordinary measures should be taken to resist the desirable effects of such salts on the performance of the road.
For surface Drainage system read here [ Surface drainage system ]
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