It took until the 20th century for engineers to build anything bigger than the Great Pyramid at Giza. E&T looks at one of the largest public works projects ever undertaken in the US.
It couldn't have come at a better time. America was caught in the grip of the worst depression it had ever seen, and the announcement of the largest public works project was seen as a catalyst for economic recovery. The President, Herbert Hoover, saw the 'Boulder Dam' as an opportunity to create employment, restore national pride and rescue a failing political career.
Construction began in 1931, and was completed five years later. It was built as a source of hydroelectric power, to provide irrigation for agriculture and to prevent flooding, which was a constant threat to the River Colorado delta. It features 17 turbines, of which one is rated at 86,000 horsepower, one at 100,000 horsepower and the remaining 15 at 178,000 horsepower each.
On the down side, despite generating huge amounts of electricity and protecting the Colorado Delta, the dam has had a catastrophic impact on the downstream ecosystem with occasions when freshwater barely reaches the sea, causing a 'reverse estuary'. Four species of fish – once plentiful – have responded to the change by becoming critically endangered.
The brief was to produce a dam on the River Colorado 'at or near Boulder Canyon' for the purposes of flood control and the generation of hydro-electricity. The design selected was a concrete arch-gravity design, presenting a convex face towards the upstream water. It was to measure 200m in thickness at the base narrowing to 14m at the crest – wide enough to allow a road to run across connecting Nevada and Arizona. The successful bidder was a joint venture consortium called Six Companies, Inc.
Budget and costs
Six Companies put in a bid of $48,890,955 – five million dollars less than the next lowest bid and within $24,000 of the undisclosed government estimate for the project. In today's money that's about $750m. Despite careful financial control the project was dogged by labour disputes over wages and conditions.
More than 15,000 engineers, surveyors and labourers worked on the project in all, with the workforce peaking in July 1934 at 5,251. For most of the five-year construction phase, the average number of site workers was 3,000. Official statistics reveal there were 112 deaths associated with the construction, including JG Tierney, the original surveyor, who died looking for a site for the dam in 1922. His son Patrick was the last to die, 13 years later. Controversy surrounds the true number of fatalities which could be as many as 1,000. Workers succumbing to carbon monoxide poisoning in the dam's tunnels were classified as having pneumonia, and so were not part of the official death toll.
At the time construction projects using concrete were untested and so the design principle was that of ultra conservatism. Should the dam fail the 35km2 reservoir that it impounded – Lake Mead – contained enough water to wash away 200 human settlements downstream. To allow for exceptional weather events the design for the 379m long dam also included two drum-gate controlled spillways capable of diverting an 11,000m3/s overflow. The spillways have only been used twice: once in testing in 1941 and again in response to a flood in 1983. Both events caused considerable cavitation damage to the interior of the tunnels.
The Hoover Dam incorporates over 2.3 million cubic metres of concrete and weighs more than 6.5 million tonnes. The problem with concrete is that as it cures it generates large amounts of heat – the body of material involved would take more than century to cool with the resulting stresses causing the dam to crumble and fail. To get around this the dam was cast in 15m2 x 1.5m high blocks containing 25mm steel pipes to allow river water to cool the block from within.
The Hoover Dam – named after the 31st US President Herbert Hoover – wasn't officially named so until 1947. Until that point it had been widely known as the Boulder or Boulder Canyon Dam – a reference to the proposed original site of the construction – despite the dam finally being built in Black Canyon. Although there is a tradition of dams being named after presidents, Hoover's successor who dedicated the dam in 1937 Franklin D Roosevelt – made sure that the 'Great Engineer' who presided over the Great Depression didn't get a mention.
Delivery and legacy
The dam was formally delivered to the Federal Government on 1 March 1936, two years ahead of schedule. Today its annual power output is in the region of four billion KWh, supplying the states of California, Nevada and Arizona. The dam is now also a tourist attraction, receiving one million visitors per year. Security concerns after the 2001 terrorist attacks mean that much of the interior of the dam is now out-of-bounds, while the highway crossing it has been closed and replaced by the Hoover Dam Bypass.