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Hoover Dam

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Aerial photo of the Hoover Dam
The Hoover Dam was constructed between 1931 and 1936 for flood control, irrigation and power generation. Named after U.S. president Herbert Hoover, the dam is one of the largest in the world, standing 726 feet (221 m) high and 1,244 feet (379 m) long. It stretches along the powerful Colorado River between Nevada and Arizona.[1]

It is able to irrigate 650,000 acres (263,045 ha) in Arizona and Southern California as well as 400,000 acres (161,874 ha) in Mexico.[2] While power generation fluctuates from year to year its maximum annual production was in excess of 10 billion kilowatts in 1984.[3]

Ninety-six workers were killed during construction accidents and many more died due to related illnesses.

Contents

[edit] Construction History

[edit] The Initial Stages: Development of a Plan

The Hoover Dam, finished in 1935, is a monumental engineering achievement displaying man's ability to control the natural world. For decades the Colorado River played havoc with agricultural land all over the West. Farms were destroyed by great floods and then suffered through months of droughts.

In 1902, President Theodore Roosevelt signed the Newlands Reclamation Act, authorizing construction of dams throughout the West to control sources of natural water for the irrigation of agricultural lands.[4]

Former Bureau of Reclamation employee Blaine Hamann described the intensity of the Colorado River: "The river was an enemy, and only in short periods of time could you look at it as a useful river. Most of the time it was something that would kill you or ruin your farm."[5]

After researching the entire Imperial Valley, where the river flowed, Reclamation director and chief engineer Arthur Davis authored a report entitled "Problems of the Imperial Valley and Vicinity." The report outlined a plan to develop a large dam on the river near Boulder Canyon and a related power plant that could help recoup the project's great cost.

The project passed through Congress and was signed by President Calvin Coolidge in 1928.

Downstream of the Hoover Dam

[edit] Projected Costs and Six Companies

Initial research on the Hoover Dam was prepared by the Bureau Reclamation Department, who were responsible for dam, power plant, and canal developments throughout the western states. Bureau president Arthur Davis, aided by civil engineer Frank Crowe, developed a cost estimate for the project. Construction was to be contracted out to private companies and overseen by the Bureau.

By the 1930s, the Great Depression had spread across the entire nation making the project far too expensive for any one company to afford. So Harry Morrison, president of Morrison-Knudson Co., brought together a consortium of companies to raise the money and skills needed to carry out the project. He also hired Frank Crowe, who desperately wanted to be involved in construction. Other members of the partnership included Utah Construction, JF Shea Co., Pacific Bridge Co., MacDonald and Kahn, and Henry Kaiser and Warren Bechtel. Together the company was called Six Companies.

With Crowe named construction superintendent, Six Companies won the contract on March 4, 1931 with a bid of $48.9 million.[6] Their bid was $24,000 more than the cost estimate, which Crowe developed, and $10 million lower than the next closest bid.

[edit] The workers arrive: Ragtown and Boulder City

The Great Depression caused unemployment to rise, causing a flood of people migrating to the construction site in hopes of getting work. Thousands of men brought their families and possessions to the Colorado River with just the hope of a paycheck. Some families made a home in Las Vegas, making the 30-mile (48-km) trek from the city to the site while others simply set-up tents along Black Canyon. This tent city became known as Ragtown.

Ragtown was nothing more than a series of tents and cardboard boxes fashioned into makeshift homes. Living conditions were extremely poor and temperatures were uncomfortable. While Las Vegas was 90 degrees, the floor of Black Canyon reached 120 degrees with little to no escape from the heat. Families came up with all sorts of ideas to help brave the heat: they soaked their roofs and floors with water to cool the wood. They also covered their windows with wet burlap sacks in hopes that wind blowing through the sacks would cool their homes. However, heat was not the only problem. The water provided by the Colorado River was filled with sediment, causing a number of related ailments.

One Las Vegas citizen, Murl Emery, a Colorado River ferryman, came to help the citizens of Ragtown. He and his family opened a store that sold food and other items from Las Vegas. Store credit was provided to many customers, and all but one debt was paid in full (that one customer died).

Six Companies had always intended to provide housing for 80 percent of its workers, but dam construction began ahead of schedule delaying the building of "Boulder City." They wanted to keep their workers away from the temptations of Las Vegas and so decided to establish housing just seven miles (11 km) from the dam site. The town would house 1,500 workers and include a general store, churches, a theatre, and even several schools. Individual houses were available for families and dormitories for single men. Six Companies also constructed the massive Anderson Mess Hall, which could feed as many as 1,300 people in one sitting.

[edit] Diversion Tunnels & Penstock System

Before construction of the dam could begin, workers had to drain the Colorado River. They did this by digging four diversion tunnels, two on either side of the canyon. In March 1931 Six Companies began digging with the use of "Jumbo Trucks." These trucks consisted of a multi level platform holding 20 to 30 men drilling the rock at various points simultaneously. The holes were dug for explosives. They used about a ton of dynamite for every 14 feet (4.3 m) of rock. Eventually these tunnels were 56 feet (17 m) in diameter and 4,000 feet (1,219 m) long.

Due to severe construction time constraints, work continued 24 hours a day, split into three shifts. Each shift competed against each other to see who could complete the most digging. The competition served the project well allowing for the tunnels to be completed nearly a year ahead of schedule.

The men who worked in the tunnels faced severely unhealthy conditions. Not only were the tunnels excruciatingly hot in the summer months (reaching 140 degrees Fahrenheit), resulting in numerous deaths from heat prostration, but workers were also subjected to excessive amounts of carbon monoxide from work trucks.

After the tunnels were excavated, they were lined with three feet (0.9 m) of concrete. The concrete was made from excavated rock and sand gathered from a streambed on the Arizona side of the river. The raw materials were brought to a facility established near the site to convert them to concrete.

Meanwhile, special Penstock pipes were being built at the Bobcock & Wilcox Co. factory specifically built for the project. Each piece of pipe was 8.5 to 30 feet (2.6 to 9.1 m) in diameter, 0.63 to 2.75 inches (1.6 to 7 cm)  in thickness and 11 feet (3.4 m) long. Two pieces of pipe together formed one section of pipe, weighing approximately 150 to 184 tons. Steel for the pipes was brought in from the East in large sheets. Each sheet was curved and formed one-third of a complete piece of pipe.[7]

Each section of pipe was then transported from the Bobcock & Wilcox Co. facility to the tunnels and put in place.

In fall of 1932 the Arizona side of the tunnel was breached and with the help of cofferdams they were able to divert the river to prepare for construction of the dam.

[edit] Preparing the Bedrock and Constructing the Monolith

Before any concrete could be laid down for the dam, more groundwork had to be done. The cliffs of the canyon had to be cleared of all loose rocks and the floor had to be excavated of both rocks and sediment until solid bedrock below was reached.

"High scalers" cleared debris from the cliff walls by hanging on ropes and drilling holes for more explosives. The danger of the job was accompanied with higher pay. In all, approximately 1.5 million cubic yards (1.1 million m3) of material was excavated from the canyon walls and floor.[8]

The first load of concrete was laid in June 1933. Concrete was transported along high lines in eight-cubic yard (6.1-m3) bottom-dump buckets. The Hoover Dam was formed from 230 separate pieces of concrete, each five feet (1.5 m) thick, but varying in width. Together the pieces created a 726-foot (221-m) high, 1,244-foot (379-m) long monolithic wall.

If the dam had been poured as a single piece of concrete, it would have taken more than 100 years to cool.[9] So each section was compacted and cooled separately. The cooling process was achieved by imbedding steel piping and circulating ice water through the interlocking blocks of concrete. The pipes were eventually back filled with concrete to increase the walls strength.

The last piece of concrete was poured in May 1935.

[edit] The Power Plant

Arthur Davis' original plan for the Hoover Dam included a power plant because he believed it could help recuperate much of the construction costs. The plant sits at the foot of the dam with sections stretching out on both sides of the river. Each section is 650 feet (198 m) long, with a combined 10 acres (4 ha) of floor space.

There are 17 main turbines combining 2,998,000 horsepower. Of these 17 turbines, 15 are 178,000 horsepower, one 100,000 horsepower, and one is an 86,000 horsepower Francis-type vertical hydraulic turbine. The average annual net generation for the power plant for 1947 through 2005 was about 4.4 billion kilowatt-hours.[10]

Water is brought to the plant through four intake valve towers, two on each side of the river. Wicket gates on the units control the amount of water in and out of the valves. The water then flows down 500 feet (152 m) of Penstock piping to spin the turbine wheels and then the water is eventually discharged back into the river.

The hydroelectric energy produced is sold to the surrounding states. Income created by the plant recuperated the majority of operation, maintenance and construction costs by 1987.[11]

[edit] Lake Mead, the Reservoir

Lake Mead is the largest man-made lake and reservoir in the United States. It is a by-product of the Hoover Dam construction. Named after Elwood Mead, commissioner of the U.S. Bureau of Reclamation from 1924 to 1936, the lake extends 110 miles (177 km) behind the dam and contains 31 million acre feet (3.8 million ha m) of water.[12]

[edit] Mike O'Callaghan – Pat Tillman Memorial Bridge

The Mike O'Callaghan – Pat Tillman Memorial Bridge (aka Hoover Dam bypass) opened on October 20, 2010.

[edit] Equipment Used

[edit] Unique Facts

  • At its base, the Hoover Dam is as thick as two football fields measured end-to-end (660 feet) (201 m).
  • Up to 20,000 vehicles drive across the top of the dam each day.
  • The dam uses enough concrete to build a two-lane road from Seattle, Washington to Miami, Florida (4.5 million cubic yards) (3.4 million m3).
  • Lake Mead is the largest reservoir in the United States. It contains enough water to flood the entire state of New York with one foot (0.3 m) of water (26 million acre feet) (3.2 million ha m).
  • Each of the 17 generators can supply electricity to 100,000 homes.
  • 96 men were killed while constructing the Hoover Dam between 1931 and 1936.
  • More than 8.5 million pounds (3.9 million kg) of dynamite were used to blast the foundation for the dam and the eight miles (13 km) of diversion tunnels.

[edit] References

  1. Answers.com Hoover Dam, 2008-09-24.
  2. Answers.com Hoover Dam, 2008-09-24.
  3. US Bureau of Reclamation. FAQ: Power, 2008-09-24.
  4. National Governors Association. Timeline, 2008-09-24.
  5. PBS. Hoover Dam: Timeline, 2008-09-24.
  6. GeneralContractor.com Historic Construction Projects: Hoover Dam, 2008-09-24.
  7. US Bureau of Reclamation The Story of Hoover Dam. Google Video, 2008-09-24.
  8. Answers.com Hoover Dam, 2008-09-24.
  9. Arizona-Leisure.com Construction History of Hoover Dam, 2008-09-24.
  10. US Bureau of Reclamation. FAQ: Power, 2008-09-24.
  11. US Bureau of Reclamation. FAQ: Power, 2008-09-24.
  12. Answers.com Hoover Dam, 2008-09-24.