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Transcript for The Story of Hoover Dam, segment 05 of 12
A second earth and rock dam was strewn across the river above the tunnel outlets downstream, keeping water from backing into the foundation area. Isolated and protected from the river by the two coffer dams, the site was pumped dry. Men and machines dug one hundred and thirty-five feet below the old river level to reach bedrock for the dam's foundation, excavating over two million cubic yards of rock, earth, and sand. As cleanup of the dam site exposed the ancient bed of the Colorado River, geologists read the history of what happened ages ago. Workmen cleaned and prepared bedrock surfaces to receive the first concrete, assuring utmost stability for Hoover Dam's foundation. Twelve miles upstream, drag lines excavated sand and gravel for the dam's concrete from an old stream bed deposit on the Arizona side. The train hauled this raw material to an aggregate plant across the river a few miles above the dam site. Here the sand and gravel passed through various processes of screening, grading, and washing until it emerged as unexcelled aggregate. Then it was stockpiled according to sizes to await its trip to the dam site.
This processed aggregate moved as called for in a steady flow over the railroad to two mixing plants, one in the canyon bottom and the other on the Nevada rim. There, sand and gravel were blended with cement into a uniform mix meeting rigid specifications for the four and one half million cubic yards of concrete to be placed in the dam structures.
From the mixing plants, concrete was dispatched to all points of construction. Nine anchored aerial cableways, spanning the canyon from rim to rim, lowered the concrete into the forms and handled other supplies and equipment as well.