This is an attempt to share information about roads. Lot of information is also available in the internet, so I have taken care not to repeat the same.
So, how is a road different from regular soil surface, and why so?
Let’s take regular surface; the playground. It is fine to play when there is no rain. We can even go cycling or ride a motorbike. However, when it rains, it becomes soft and slushy. Cycle wheel or feet go deeper into the soil.
In such cases, what do we do? We place a big stone and step over it. The stone gives a hard surface. Any surface that will not get soft with water will allow us to move over it easily. So, ideally, we must be having concrete slabs as roads everywhere!
However, the concrete is costly. There must be cheaper options too. By the way, in olden days there was no concrete – it was yet to be discovered. So what did they do?
Simple! They paved the soil with stones, pressed them together to give the same effect as a single large stone. It worked! However, there was a problem. There were gaps in between the huge stones. They filled it with small stones. It helped, but it did not solve the problem fully. The small stones, under pressure, got pushed inside easily.
Then they came up with the idea of putting layers of stones. Large stones were laid deeper to withstand the weight and pressure. Over this, they put a layer of smaller stones, which gave them uniform surface. The number of layers and the size of the stones depended on the weight that will move over the road.
Then there came another problem. How to keep the stone intact? The first binder was soil itself, at times mixed with lime. However, the binder used to get washed away during rains.
Then came ‘asphalt’ (bitumen) – also known as ‘tar’. Asphalt is solid or semisolid. It is heated, and mixed with stones, and then the stones are levelled using a roller.
The bitumen acted as excellent binder. It held the stones together, and it did not get affected by water. The technology developed, but the basic principle remains the same till date. The stones are known as ‘aggregates.’
The roads are built with a slight slope so that water can drain off to the edges. Then they are collected by the drains on the sides of the road.
What if the water does not drain? It leads to two things. Firstly, any water on the road, will damage the surface, when a heavy vehicle moves on it – as it will be pressed between the tyres and the road surface with huge force, thus expelling the small weak stones in the road surface. Once a small gap comes, water will keep on seeping in and removing the stones, one by one, leading to a crater, what we call as ‘pothole’!
Secondly, if the water is not drained by the drains, it will accumulate on the sides of the roads, start seeping in, and will make the soil at the bottom loose. If that becomes loose, then it will affect the stone layer which is over it. Thus, the stone layer will start sinking in. Thus, you will see cracks or undulation in the road. That is how the roads on the wet regions, where the water table is near the surface, get undulations (bumps) very easily!
Look at the cross section of a road, in the picture below:
The white pieces are the stones (aggregates). The black area is the asphalt (bitumen). You can see the sizes of the stones are bigger at the bottom and smaller at the top. The topmost layer is called as ‘wearing layer’ or ‘super pave’, which keeps wearing-off, and re-laid as and when required.
Now, you understand why it’s important that the engineer keeps an eye on the contractor when every layer is made and compacted! Next time, when you see a road construction, watch it closely!