The first and most significant delay to the project occurred during the initial design stages. An unusual geological formation was discovered. An ancient glacier had moved across the site pushing up an impermeable wall of iron oxide. This created two ground water tables each at different levels with a clear line of discontinuity across the proposed location of the basement. “There’s a height difference of around 50cm between the two water tables and to prevent significant inflows we had to keep the vertical water-tight layer intact when excavating the basement,” explains Eddy. “This unexpected complication had not been factored into the budget or the tight construction schedule – hence our decision to split the tender.”
A further challenge was to make the basement totally watertight. As much of the Netherlands is below sea level, Dutch Engineers are accustomed to factoring flooding into their basement designs. However, in this case, since the basement would become an exhibition space, there could be no risk of any water ingress. The innovative solution adopted to meet this requirement involves the use of concrete mixed with bacteria-filled capsules. These capsules break open if a crack forms releasing the bacteria into the crack. The bacteria react with water to create a limestone slurry that seals the crack making it water-tight. This is one of the first times that this product has been used in the Netherlands. The solution enables the Contractor to provide a robust guarantee for the integrity and water resistance of the basement walls.
Additionally, due to the high-water table the basement is continuously sitting in water, even during construction. Therefore, concrete cast underwater was used to form the floor of the basement pit. Tension anchors connected into this underwater concrete resist the uplift forces keeping the concrete floor in place. The concrete is reinforced with steel fibres, a relatively new technique in the Netherlands, that allows a thinner basement pit slab to be adopted and gives a reduced risk of deflection under the existing buildings. Casting concrete underwater is a common basement construction technique in the Netherlands but presents significant challenges with works carried out underwater with divers and consideration of the changing forced through the construction process.