The alluvial grounds made up of mudflats, reedbeds and a sandy and muddy foreshore posed a problem for the construction of the Pont de Normandie between the Seine-Maritime and Calvados departments. Deep foundations were necessary to overcome the layers of sand, peat, gravel, and clay and reach the limestone. Piles 164 ft (50 m) long were installed by drilling under the inverted Y pillars. Geometric monitoring permanently scans the supports of the pylons and the 26 piles of the viaducts, whose ground is regularly leveled.