Peter Schunck compared the structure of the building with a ship, floating on uncertain ground. The foundation is basically a concrete trough, that can 'give a little' if any mining shafts were to be dug under it. This 'ship' would then never be able to capsize. During construction, the moat of the old Roman settlement Coriovallum was uncovered, which was the reason to lay the foundation deeper than originally planned. Inside the old walls the subsoil was firm, but outside them it was very bad. Building such a structure on such ground in a mining area was deemed too risky and the ground was cleared until a single layer of equal overall weakness was encountered, a 12 m thick sand layer. This sand is generally considered a bad foundation soil, but in a mining area it is a good choice because it flows and 'gives' evenly in case of a cave-in in a mine below it. Still, Peutz decided that the soil pressure should not exceed 1 kgf/cm2 (100 kPa). So for the foundation he used a reinforced concrete slab extending under the whole building. This 50 cm thick beamless foundation slab is mushroom-shaped.