The soft spot foundation is another interesting technical solution that appeared in the marked some years ago.
Basically, it is similar to a standard shallow foundation with a certain amount of a foamy, soft material just in the middle below the anchor cage. I understand that different alternative materials can be used, being a standard solution Expanded Polystyrene (EPS, the type of material used to protect big electronic items during shipping).
The result is a change in the distribution of the forces that makes the foundation behave like a circular, ring-like structure.
As a consequence there is an increase of the lever arm of the stabilizing moment, and so ultimately a smaller foundation can be built.
The benefit are the savings in concrete and steel – some percentage point, that in big farms can mean a lot of money.
I know that it seems a bit strange – and possibly even counterintuitive – that just by putting some soft material in the middle of your foundation you will achieve savings in the quantities of material needed.
Me too I was surprised when I first discovered the solution. However, it looks like it really works.
The solution has been using in dozens of project worldwide. Additionally, I have seen that it has been certified by DNV-GL.
You have to consider that to work this foundation need to transmit an increased pressure to the soil below (because due to the soft spot that does not transmit pressure there is less area left).
Therefore it is usable only in situations where the bearing capacity is good – for instance in rocky areas.
I have seen this foundation for the first time on a project developed by CTE, a French engineering company specialized in wind energy.
It also look like they have some kind of copyright on the solution (or possibly only on the name).
However, lately I have noticed that it has been offered also by other engineering firms.