I have been on a tour of a chamber like that recently, and the blue material in the spikes has roughly the rigidity of cardboard. The material is reinforced by metal wires, however. Even though the wires are not very thick (you could bend them with your hands easily), I would imagine you would hurt yourself quite badly if you fell onto them.

In fact, the whole chamber is shaped like a cube, and the measurements must be made in the middle of the cube. In the chamber I visited, the "floor" was made of a wire mesh suspended at mid-height inside the cube, so there it was impossible to fall.