"Four and a half million cubic yards of concrete — enough to build a two-lane highway from San Francisco to New York — and 19 million pounds of reinforcing steel somehow had to be moved into the middle of the Nevada wilderness to construct both the dam and a 1.2-million-horsepower electric plant."
My understanding of the design for the dam was that the safety factor was TEN. The folks behind building it wanted to be able to build more and didn't want the first one to have any issues.