Work on Glen Canyon Dam began in 1956. Four years were needed to prepare the site, including blasting two river bypass tunnels and spillway tunnels, erecting two huge cofferdams, constructing a highway bridge just downstream from the dam site, and deeply blasting and excavating the river bed and cliffs to assure a solid dam foundation. The highway bridge was a project element not employed at Hoover Dam, where its absence led to decades of increasingly troublesome highway congestion and dam operations conflicts on the crest of the dam. Three more years were needed to place the concrete for the dam.