Joao, thank you for your interest. I have just seen your blog. It looks great!

After the sheet piling were installed, we were able to continue.
What follows is a big hole and then pouring a thin layer of concrete.
This is called "Concrete cleanliness layer" in german. Thats at least what google translates :P.
It serves as a even layer for the bitumen seal and thermal insulation under the "real" foundation.