Aug 30, 2011

7 Wonders of the Industrial World - Part VII

Hoover Dam

A National Historic Landmark, Hoover Dam stands tall as an engineering marvel high above the Colorado River. Originally known as Boulder Dam, it’s a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the US states of Arizona and Nevada. It was constructed between 1931 and 1936 during the Great Depression and was dedicated on September 30, 1935, by President Franklin Roosevelt.

Its construction was the result of a massive effort involving thousands of workers. When it was completed, it was the largest dam of its kind in the world, standing at more than 725 feet above the Colorado River. With 17 generators producing 4 billion kilowatts of electricity a year, it also is one of the country's largest hydroelectric power facilities. Operation and maintenance of the facility are solely supported by revenue from power sales.

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In 1922, the Reclamation Service presented a report calling for the development of a dam on the Colorado River for flood control and electric power generation. Originally the dam was to be built in Boulder Canyon, but Black Canyon was found to be more suitable. Despite the change in site, the project was still called the Boulder Canyon Project.

President Coolidge signed the bill authorizing the dam on December 21, 1928. Officials decided on a massive concrete arch-gravity dam, which would be thick at the bottom and thin near the top, and would present a convex face towards the water above the dam. The curving arch of the dam would transmit the water's force into the abutments, in this case the rock walls of the canyon. The wedge-shaped dam would be 660 ft (200 m) thick at the bottom, narrowing to 45 ft (14 m) at the top, leaving room for a highway connecting Nevada and Arizona.

Soon after the dam was authorized, increasing numbers of unemployed converged on southern Nevada. Men came from around the country, many bringing families and hoping for employment. The site of Hoover Dam endures extremely hot weather, but the summer of 1931 was especially sweltering, with the daytime high averaging 119.9 °F (48.8 °C).

The first step was to divert the Colorado River. The riverbed had to be dredged clear of deep silt and sediment to expose a bedrock foundation for the building of the dam. The process of digging four diversion tunnels through canyon walls to divert river flow around the dam site was tedious. High scalers worked on the canyon walls, removing loose rock to make sure the walls were smooth so the dam’s concrete would adhere. After a year, the river was routed through the tunnels, and the main work began.

The first concrete was poured into the dam in June of 1933, 18 months ahead of schedule. A total of 3,250,000 cubic yards (2,480,000 m3) of concrete was used. In addition, 1,110,000 cubic yards (850,000 m3) were used in the power plant and other works. Two batch plants onsite were created to produce the concrete that was transported on railcars in large four and eight cubic yard buckets. An overhead cableway system lifted the buckets and lowered them to the forms. At peak production, one bucket was delivered about every 78 seconds. Overall, there is enough concrete in the dam to pave a two-lane highway from San Francisco to New York.

The dam was built in interlocking blocks, and concrete. More than 582 miles (937 km) of one-inch steel cooling pipes were placed within the concrete to offset the chemical heat generated by concrete setting. There was an ammonia refrigeration plant that cooled the water and was capable of creating a gigantic 1000 pound ice block every day.

Once the cement hardened, the cooling pipes were back-filled with concrete to create added strength. The massive water pressure of up to 45,000 pounds per square foot at the base of Hoover Dam, is held back by gravity. The arch-curved structure against the lake reservoir dissipates that pressure into the canyon walls equally on the Arizona and Nevada side.

The dam is named after America's 31st president, Herbert Hoover, who played a large role in bringing the nearby states into agreement about water allocations, settling a 25-year controversy. The dam has been called Boulder Canyon Dam as well as Boulder Dam, but Hoover Dam was reinstated as the official name by Congress in 1947.

Ninety-six men were killed in industrial accidents while building the dam. Several dozen others died from the heat or carbon monoxide poisoning while on the job, and hundreds of other people, wives and children of the workers, died from heat, polluted water or disease.

Hoover Dam was the largest federal project of its time. The Boulder Canyon Project Act appropriated $165 million for the Hoover Dam along with the downstream Imperial Dam and All-American Canal. Building the dam was hot, dirty and often dangerous work, but more than 20,000 men were happy to be working on the Hoover Dam, the biggest dam project in the world when it was completed.

Because of Hoover Dam, the Colorado River was controlled for the first time in history. Farmers received a dependable supply of water in Nevada, California and Arizona. And Los Angeles, San Diego and Phoenix and a dozen other towns and cities were given an inexpensive source of electricity, permitting population growth and industrial development.

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