15 July 2008

Dam tour

Ok this is the Hoover Dam, legendary not only for its great size, but also for its unprecedented construction. The great Colorado river, rivaled by only the Mississippi as "America's River", flows for 1,700 miles and drains 244,000 square miles of the American Southwest. It is the river that created the Grand Canyon, Glen Canyon, Imperial Valley, and many other magnificant landmarks, as well as frustrating those who tried to explore it for nearly 400 years. The powerfull river carries enough silt to cover 80,000 acres up to one foot, or more than all the dirt removed in building the Panama Canal. Its greatest problem, like many other rivers, is its tendency to flood. In 1905, the river overflowed a manmade canal in California's Imperial Valley, the resulting flood raised the water level of the Salton Sea by 73 feet by the time the canal was repaired in 1907. By the 1930's, over $7,000,000 had been spent protecting the Imperial Valley from floods, with a per year cost that had risen to over $1,000,000. The time had come for something new to be done. The first obstacle arose from the river flowing through several states who had conflicting interests in it. In 1921,Secretary of Commerce Herbert Hoover created the Colorado River Commision, including representatives from Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. On November 24, 1922, the commision finally came to some agreements about what to do with the river. Now it was time for the Bureau of Reclamation to step in. It made sense that a dam should be made in the canyon region of the river for ease of construction, but exactly where had to consider geologic, economic,and many other practical factors. This involved river explorations by water, land, air, core samples, and surveys conducted on the cliffsides of the canyons themselves. After proposals had been made to Congress and various bids made by contractors for the project, on April 20, 1931, a construction contract was signed by Secretary of the Interior Ray Lyman Wilbur, to Six Companies Incorporated for $48,890,995. Six Companies was created in February 1931 for the purpose of building the dam. It was actually made up of seven companies, W. A. Bechtel of San Francisco, Henry J. Kaiser of Oakland, Utah Construction Company of Ogden, Macdonald and Kahn Company of Los Angeles, Morrison-Knudsen Company of Boise, J. F. Shea Company of Portland, and the Pacific Bridge Company of Portland. In addition to Six Companies, the Boulder City Company was also formed to create the town of Boulder City for the dam workers, and the Hoover Dam Transportation Company was created to move men and materials around the area. These companies created an overall payroll of almost $100,000 monthly, with a schedule of three shifts a day, seven days a week, including holidays. Before anything could even be done on the dam, several things had to be created including Boulder City to house the workers, concrete mixing sites, rock quarries, power substations, roads, railroads, and more. It was estimated that $2,000,000 had been spent before any actual dam work had begun.
This diagram shows the amount of construction that had to go into the massive project. Before anything on the dam could be done, first diversion tunnels had to be dug to allow the river to flow around the site. The Outer tunnel on each side would later become part of the overflow path, while the inner tunnel on each side would become part of the path that takes water to the power generating station. Once the tunnels were built, large cofferdams had to be built above and below the site to prevent the river from entering, then the site had to be dried out and dug down to the bedrock. Then besides the dam, there needed to be built the power station, intake towers, spillways, and all the associated pipeways. In all, more concrete would be used on the project than all other dams built by the Bureau of Rec;amation combined.

Tunnels for the dam were dug using giant drill carriages, called Jumbos. These were trucks with massive platforms built onto them where 24 to 30 workers could drill at once. The diversion tunnels alone totalled 15,909 feet of 56 foot wide tunneling, and with the jumbo, crews averaged 16 feet every four hours. Three massive air compressor plants were constructed to operate the air operated drills in the tunnels, and a massive railroad network was created to remove the "muck", or drilled out rock. The tunnels were also coated with a three foot ring of concrete to minimize damage. This concrete, later to be used in the dam itself, was made of aggregates gathered from local side canyons, and organized by automated by machines, sorting the rocks by size. The various sizes were sent to the mixing plants where a specifically controlled process combined the exact amount of each size, with sand and water.
Following the completion of the diversion tunnels, the cofferdams needed to be completed to free the dam site of water. These were made of rock landfill from the diversion tunnels, covered with concrete under the same strict control as the tunnels and dam itself. The upper dam alone was 98 feet high, 480 feet long, and 750 feet thick. Finally, after the site was drained and silt removed, construction of the dam itself could begin. The most difficult challenge at this point was cooling the concrete. As concrete dries, it heats up around forty degrees due to the chemical reaction, and the subsequent cooling can cause large cracks to develop. This was counteracted here in two ways. The dam was built as a series of columns whose construction was staggered, offering more surface area to allow cooling. More improtantly, a series of small pipes were run throughout the concrete as it was poured, and these pipes were connected to an ammonia refrigeration plant, designed to remove excess heat and minimize the shrinkage. The concrete was poured using eight-cubic yard buckets carried by massive aerial tramways, as seen above. A crew of workers in rubber boots smooth the concrete in each block, as well as work it around pipes and instrumenst placed to measure temperature and pressure. The penstock headers, used to carry water to the power plant, were made of steel piping placed in tunnels in the rock. The manufacture of these giant pipes was such an undertaking that it was contracted to the Babcock and Wilcox Company of Barberton, Ohio for $11,000,000. They produced 14,500 feet of piping in diameters of up to 30 feet. The sections of piping, weighing around 170 tons each, required the construction of a special trailer with individually steerable wheels to transport them to the site. The hydro-electric power plant dwarfs in comparison to the massive dam, but it is actually 19 stories high with an area of four acres. When it was completed, it supplied all the power to the 8,500,000 people then living around the Colorado River Basin.
Inside one of the power plant "wings". The total power producing capacity of the plant is 1,860 Mega watts, or put another way, each cubic foot of water through the turbines produces 746 watts. Each wing also has a smaller turbine used to power local loads.
One of the controling stations for the power plant.
An Indian symbol for water or power

So, when all was said and done, the dam was completed in 1935, and the first power generated in 1936. Total cost was $49,000,000 and took 4.36 million cubic yards of concrete. 112 deaths were attributed to the construction of the dam. It has around 9 million visitors each year, and the reservoir it created, Lake Mead, is the fifth busiest US National Park unit. The lake is 157,900 acres, reaching as far as 110 miles from the dam. It was named for Elwood Mead, the director of the Bureau of Reclamation at the time the dam was built.

Some of the tools used in planning and constructing the dam.

One of the towers taking power from the power plant.
Monument to the construction of the dam
For your comic relief, his is the mens restroom, on the dam.
The giant intake towers, where the water flows into the pipes to the power plant.
Looking down the backside of the dam at the waters of Lake Mead.
Looking back at one of the concrete spillways, used only twice, in 1983 and 1999, when record precipitation caused the reservoir to reach the level of the spillway. The change in the rock color from white to red shows the highest level of the water.
The US 93 bridge being constructed in front of the dam. The road curently goes across the dam, but is being moved to the bridge due to the road's tight mountain turns and security concerns for the dam itself. The bridge is projected to be comleted in 2010.

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