National Geographic : 1986 Oct
Updating an old technology, project engineers constructed 130 huge sand-and-gravel mats to serve as foundations for the barrier's gargantuan piers. Able to absorb the changing water pressure in the soil of the seafloor, these "filter mattresses" prevent the seabed from being washed away. Without them the enormous forces battering the barrier during storms would cause the piers to shift and settle, destroying the gate mechanisms. For generations the Dutch have been laying mattresses as foundations for the earthen dikes that hold back the sea all along the nation's coastline. In a mid-1960s photograph workers construct a mat of willow and brushwood (far right) to be floated out to the position of a desired dike and sunk with stones. Often these mats were several acres in size. After being stabilized with tons of stones, they were loaded with sand and clay pumped or dug from the nearby seabed by huge dredges, until gradually a new dike emerged. The modern mats for the surge barrier, which were assembled at an on-site factory, consist of sand, fine gravel, and coarse gravel in three wire-reinforced layers (left). Separating the layers are rugged synthetic fabrics that may eventually rot away, leaving a "natural" foundation under each pier. Steel holding pins, driven through all the layers, help stabilize the fillings. Lines and section numbers painted on each mattress (below left) will act as guides for repairs, if necessary, after placement.