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How The Hoover Dam Was Conceived?
The beginnings of the Hoover Dam go back to 1922, when the Fall-Davis report presented by the Reclamation Service made the Colorado River a federal concern because of its basin covering several states before it ultimately entered Mexico. The cycles of drought and floods in the southwest of the United States also spelt out the need for a dam, and it was felt that its potential for hydroelectric power could make the dam self financing. Metropolitan centers could also benefit with water and electricity from the lake so created.
The bill for the project was approved in 1927, for a venture that many considered too ambitious, difficult because of the landscape and the fear that existing technology could not adequately support the technical requirements of such a huge dam. The height of over 700 feet and the requirement to close a gorge length of over 1200 feet was considered daunting. What was conceived was a dam that impounded the water by gravity of its weight and the spring action of the arch that it formed in the gorge which transmitted the hydraulic head of the water to the rocky sides of the canyon because of the arch action.
Building the Hoover dam and the people who died building it are a testimony to the engineers and people of those Depression-affected times. And a salute to the politicians and government of that time which created a lake with a surface area of 247 square miles and a depth of almost 600 feet. The dam created a generation capacity for hydro electrical power of 4.2 billion KWH annually from its 17 main turbines.
Image Source: Wikimedia
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The Building of the Hoover Dam
The site for the dam was originally at Boulder Canyon and the dam was at that time called Boulder Dam, but subsequent assessment by engineers moved the site to the Black Canyon where the dam was finally located. The name of Hoover Dam was finally given to the dam by the Truman administration. The location of the Black Canyon was so remote that the first job that the engineers had to undertake was the laying of roads and railways. Then they had to arrange to create stockpiles of the materials required for the construction.
As a first step to starting work on the foundation of the dam, the river itself had to be diverted.
For this, four concrete-lined diversion tunnels, two on each side of the river, were made through the canyon walls. The tunnels were 56 feet in diameter and ran to a total length of 16,000 feet. This work was started in 1931 and ran at different levels. The concrete lining of the tunnels reduced their effective diameter to 50 feet. Rubble was dumped into the river and its course blocked so that water went through the diversion tunnels in late 1932.
Coffer dams were also built at two places upstream and downstream of the dam location, so that the location of the actual dam could be protected from the river water. Once the water was diverted and the coffer dams were built, the base area of the dam was dewatered and the foundation was excavated to remove all earth and all fractured rock to reach a sound bedrock depth.
The sides of the canyon were also to bear the weight of the water because of the arch action and therefore also had to be excavated to virgin rock. This was done by teams of “high scalers" who removed the rock with pneumatic hammers and dynamite while remaining suspended at their work places with ropes. The danger from constantly falling rock and other material led to the use of hard hats by all workers.
To further strengthen the base and rock sides, holes, some as deep as 150 feet, were drilled into the rock and filled with cement grout. This grout curtain was meant to fill up any cavities and reduce uplift pressure on the dam. Leaks were noticed after the completion of the dam and were traced to improper grouting of some of the holes. New holes were drilled and the grout curtain strengthened- a job that took over nine years.
Concrete pouring for the dam started in June 1933 and was cast in blocks of 50 feet by 50 feet which were 5 feet high. Pipes were laid in the concrete through which iced water was circulated to keep down the temperature. Once the curing process was over these pipes were filled with grout. Joints between the blocks were similarly grouted. A total of 3,250,000 cubic yards was poured to complete the dam, which took place on May 29th 1935. Work on the powerhouses and other spillway structures were simultaneous.
Image source: Wikimedia:
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Fatalities during the Building of the Hoover Dam
The first fatalities connected with the dam were two surveyors who drowned while conducting surveys six years before the dam was even authorized. The official tally is said to be 96 and includes people whose deaths were related to industrial deaths or accidents caused during the work. Various causes led to these deaths, including drowning, blasting, and death from falling rocks, falls from heights, truck accidents, and injuries caused by heavy equipment. Despite recurring rumors to the contrary, none of the persons who died working on the dam are actually buried inside it.