Have you noticed that potholes in roads only happen during the monsoon? Why not when the weather is dry?
Imagine the point of impact when a tyre goes over a roughness in the smooth asphalt surface and comes down with a thump on the area just beyond. The force disperses through the dry soil below in a conical form so that each horizontal layer of soil below is subject to a smaller and smaller force per unit are as you move downwards. Until you reach a stratum where the force exerted is too small to matter.
Now imagine the same soil in a wet state. The interstices between soil particles are filled with water, not air. Water is incompressible. With the impact of the tyre above, the force will be taken by both the soil particles and by the water. The water pressure below the tyre is thus immediately felt by the water particles below and all around the area of impact of the tyre. The water pushes equally in all directions, into the water adjacent to it and the water below it, but equally it exerts an upward force on the asphalt above. That is what causes the adjacent asphalt to break and you have the beginnings of a pothole.
How should a road be constructed? Assuming the soil is soft and can take only a little force without distortion you need to begin by covering it with a layer of large stones. These have to be rolled over with a heavy roller so the stones squash into the soil below. These large stone interlock with the soil that supports them. But between the individual stones above the level where the soil has reached are empty interstices through which water can flow. These large stones provide support for the next layer of somewhat smaller stones, too large to fall into the interstices of the larger stones below. That layer too is rolled over with a heavy roller. Then comes a layer of still smaller stones, finally sand and the topping of asphalt.
What we have built is what is called a water filter. As you move downwards it has a series of increasing larger empty interstices through which water can flow freely. Water entering from above, if it does so at all, emerges from the sides of the lowest layer as clean water, not carrying away any of the particles supporting the road because of the way the filter’s layers have been constructed. All you have to do is to make sure the water emerging from the sides below is carried off in a drain.
And that is the key to having a pothole-free road: drainage, drainage, drainage. Make sure the road is well drained. If the area is flooded and the drains are full there is probably nothing much you can do, except slow down the heavy traffic to minimise impact forces. And, of course, repair potholes as fast as possible, making sure to retain the parent water filter construction in the process of repair. But above all, make sure the road drains are working and water from the sub-surface of the road is removed as fast as possible.
And finally, let us not forget that repairing potholes is a flourishing business. Not everyone wants potholes gone.
The author is a civil engineer and urban planner, one of the three authors who suggested the idea of Navi Mumbai.
