The protection of water ingress into the subgrade (foundation) of pavement is of utmost importance to the highway Engineer because water is an enemy of pavement especially those made with soils that have appreciable amounts of fines. If water is allowed to enter and soak the subgrade, it weakens the strength of the compacted soil under load and the pavement would soon fail. In this post I would show the three (3) ways water can enter and leave road subgrade and how to control them.
Three (3) notable ways water can subgrade of pavement include:
1. Seepage from high ground:
This is obtainable where a pavement is located between two high points. There would be hills on both sides of the road when the road is cut out between one or two adjacent high points. When rain falls, runoff would flow to the direction of the road and would infiltrate into the subgrade if the subgrade is not protected.
2. High water table
When the water table below the pavement is high, water can permeate the subgrade through capillary action. Capillary action is the phenomenon of upward movement of water. In that situation, the water table should be lowered and it should be ensured that the water table do not rise above 1200 mm (4 feet) below the pavement layer. Subdrains should be installed at this depth so that once the water table gets above 1200 mm the subdrains would drain off excess water.
3. Runoff water
When rain falls, every surface on the earth’s surface is soaked including pavement surface. Water percolate on the pavement surfaces. If the asphalt used to surface the pavement is porous and the pavement was not properly cambered (mild slopes falling on both sides of the pavement), the water would pass through the interstices of the pavement and soak the subgrade. On the other hand, if the surface is not porous and there is proper camber on the pavement but the verges (edge of the pavement) are exposed, the runoff water from the surface of the pavement would enter the pavement through the subgrade and soak it.
It is very important that ingress of water to subgrade of pavement is hindered by all possible but economical means to ensure the durability of the pavement. Figures 1 and 2 show camber and superelevation respectively on pavement surfaces. The prime importance of camber is to drain off excess water from percolating on pavement surfaces while the prime importance of superelevation to help moving vehicle resist centrifugal forces when it is negotiating a curve at speed. However, the slope of the superelevation also helps to drain off excess water.
The methods commonly used to achieve this end is the use of various types of drains. There two broad classes of drains common in this practice viz:
1. Surface drains: Surface drains are the type of drains that are usually open on the surface. They can come in rectangular, triangular or trapezoidal shape. This type of drain seems to be the commonest because it is cheap to construct. However, they have disadvantages that they may restrict the use of machines, take up some useful land and require a number of bridges and culverts for road crossings. They may also require frequent maintenance and de-silting. Surface drain can be effective to control water entering the subgrade if it is properly done for this purpose.
2. Subsurface drains: these types of drains are usually installed under the earth’s surface. The most popular is usually the pipe drains which consist of buried pipes through which soil water can enter. The pipes which could be made of clay, concrete or plastic convey water to the collector drains. Water enters the pipes through perforations distributed over the entire length of the pipe. Pipe drains may be more costly than surface drains but it requires limited maintenance and could be very effective. Pipe drains can be used to control seepage to subgrade of pavement for case one and two. However, for the third case, sand blanket/filter would be very effective. Figures 4 to 7 shows different ways pipe drain and sand blanket can be used to control seepage to subgrade pavement.
