Concrete piles explained to non-engineers

Sometimes my non-engineer friends ask me about how the buildings stand above the soil. I usually reply that they lay over concrete foundations which transmit the loads to the soil. Those foundations mainly work on a compression basis. The weight of the building is distributed all around the foundation area, which is usually larger than that of the building in order to increase its stability, and then it passes to the soil which supports them all.

In the case explained above we are assuming that the soil is a good enough one, one that will not collapse under these weights. But what would happen if we would like to build something on top of less consistent soils, such as different types of clay, gravel or even perhaps sand?

In this cases we would have to use piles. Piles! That word directly blows their minds. “What is a pile?” or “I’ve never heard of piles!” are some of the sentences I usually listen from them. So….what is exactly a pile and how does it work?

A pile is usually a slender concrete cylinder/square which is hammered/casted into the ground and whose top is linked to the foundation to help resisting the building loads and avoiding the collapse.

But…how does it exactly work?

The truth is that it collaborates in two ways. The first one is exactly the same principle as that of a “normal” foundation. Its bottom lays on top of a “good” soil and they simply transmit the loads deeper which is usually the same as saying “to a better soil”.

The second and more important one is through friction. They work really similar as a nail works when stuck into the wood. Do the boards fall apart even when the nail is upside down? They don’t, right? It’s because of the friction between the nail and the wood.

It happens the same here: as all the length of the pile is in contact with the soil when a hypothetical load tries to bury it deeper, a friction load appears between the concrete and the soil that prevents it from moving. This way, if we thrust several piles into the ground and we then place a foundation on top of them, the friction created between all these piles and the ground would prevent the building from failing even if the soil is not a good one.

I hope that this quick and easy explanation could help you all (engineers or not) to understand how piles work.