In order to make a contaminated site which has been sealed off by sheet piling or sheeting walls so suitable for building purposes that foundation piles can be placed therein, casing (10) are placed in the ground to a depth at which the bottom edge of the casings (10) is embedded in a watertight (clay) layer (6), foundation piles (2) are placed by way of the empty or emptied casing (10) to such a depth that the bottom part of the piles is embedded in a load-bearing soil layer (8), and a plastic or elastic material which seals off the space around the foundation piles in a liquid-tight manner is placed around the foundation piles, at least in the section where the casings pass from a non-watertight layer into said watertight layer.
