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Hoover Dam

In 1955 the American Society of Civil Engineers named the Hoover Dam as one of the USA's seven civil engineering wonders.

Forty-five years later the citation is still valid. At the time of construction it was the highest dam in the world - 221m from bedrock to crest - and the reservoir it created could contain two years' normal flow of the Colorado River.

Irrigation of land in Southern California with the waters of the Colorado began in 1901.

But control of recurring drought/flood cycles and heavy deposits of sediment proved to be beyond the capacity of a system of levees. So the people of the South West turned to the government for help.

Following intensive investigation, a bill for the construction of a dam at Black Canyon on the boundary of Nevada and Arizona was passed in December 1928. The sum voted for the construction of the dam and irrigation canal system was $165M.

The contract for the dam was awarded by the Bureau of Reclamation to the Six Companies Inc consortium, but before construction could start a vast amount of preliminary work had to be undertaken to provide power, transport links and accommodation.

Temperatures at the top of the canyon could be as high as 51degreesC so a location some 11km away was chosen to house people involved in the project. No labour was available locally, so workers were recruited from across the USA.

A railway line was laid and a road constructed. Power was brought 357km across the desert from California. To accommodate the workforce a complete town was constructed: Boulder City, Nevada. In addition to workshops, screening plant and the like, a steel fabrication facility was set up for the 4.5km of pipe for the penstock system.

The site of the dam lay in a canyon just 88m wide at the narrowest point. The first task was to divert the river and this was done by driving four diversion channels through the solid walls of the canyon and constructing coffer dams.

More than 350,000m3 of muck was excavated from the area of the dam to reach solid rock. While this operation was taking place, 'high scalers' suspended hundreds of feet above the river removed loose rock from the canyon walls.

Concreting began in June 1933 and the dam crest was reached in March 1935. Altogether the concrete arch structure required 2.45Mm3 of concrete.

The method of construction was unusual in that the dam was constructed in blocks; 18m square at the upstream face and 7.6m square on the downstream face.

The blocks contained pipes through which cooling water was run in order to dissipate the heat from the concrete. Once cured, joints between the blocks were pressure grouted to form a monolithic structure.

Seven years had been allowed for construction. In fact impounding commenced in February 1935 and the dam was inaugurated by President Franklin D Roosevelt in September that year, less than five years after work began.

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