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Colorado River, Between Arizona and Nevada
United States of America
The Hoover Dam, standing 726 feet over the bedrock of the Colorado River, between the states of Nevada and Arizona, just outside Las Vegas, Nevada, is a mute testimony to the spirit of a nation working together at a time of duress. The Hoover Dam was constructed during the Great Depression, when thousands of unemployed men traveled with their families to the Black Canyon to earn highly competitive wages ranging from $4 to $5.60 per day.
The dam is also proof that three and a quarter million cubic yards of concrete can tame the raging Colorado. It has been named a National Historic Landmark and was named one of America's Seven Modern Civil Engineering Wonders by the American Society of Civil Engineers.
Before work began on the dam, the river had to be diverted away from the construction site. This was accomplished by building four tunnels, each 56 feet in diameter, through the rock walls of Black Canyon. Total combined length of the tunnels which were started in 1931, was nearly 16,000 feet.
Two years later, construction of the dam itself began. The first concrete was placed into the dam on June 6, 1933. In August, the two inner diversion tunnels were plugged, leaving the river to flow through the outer tunnels. In 1935, 1000 ton steel grates were lowered over each of the tunnels to begin channeling the Colorado back towards the dam.
The construction of the Hoover Dam pushed the science of concrete to new frontiers. For example, before each section of the dam was placed, 1" diameter pipes were placed on top of the previously poured concrete section. River water was first circulated through the pipes to help remove some of the heat curing concrete radiates, then refrigerated water, chilled by a plant capable of producing a 1000-ton iceberg every 24 hours, was circulated through the pipes to finish cooling the concrete to an acceptable temperature. After the concrete was cooled, the pipes were filled with grout, resulting in a monolithic structure. Bureau of Reclamation engineers estimated that if the dam were made of a singly-poured block of concrete with no cooling, it would have taken 125 years for the concrete to cool to air temperatures, completely ruining its structural integrity.
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