A concrete arch-gravity type, in which the water load is carried by both gravity action and horizontal arch action. 726.4 feet tall weight equals 6, 600, 000 tons. Three and one-quarter million cubic yards of concrete. The dam was built in blocks or vertical columns varying in size from about 60 feet square at the upstream face of the dam to about 25 feet square at the downstream face. Adjacent columns were locked together by a system of vertical keys on the radial joints and horizontal keys on the circumferential joints (think giant Lego set). Concrete placement in any one block was limited to 5 feet in 72 hours. After the concrete was cooled, a cement and water mixture called grout was forced into the spaces created between the columns by the contraction of the cooled concrete to form a monolithic (one piece) structure.
