It was one of the most ambitious projects in the world, but how was the Hoover Dam constructed?
In the early-20th century, the lower region of the Colorado River was considered as a site for flood control and potential source of hydroelectric power for the growing demands of western US states. What would be known as the Hoover Dam was authorised by President Calvin Coolidge in 1928 and construction began by building conglomerate Six Companies in March 1931.
At the time the Hoover Dam was the largest in the world requiring over 3.3 million cubic metres (118 million cubic feet) of concrete to build, including the power station.
Before the dam could be built, foundations had to be laid and over 1.3 million cubic metres (48 million cubic feet) of loose rock and sediment were removed from the bottom of the Colorado River to reach stable bedrock. The foundation was reinforced with a grout curtain and the canyon walls were similarly stabilised by drilling holes up to 46 metres (150 feet) deep into them and filling cavities with more grout.
Chief engineer John Savage had decided it would be an arch-gravity dam, which combines the main features of two different types of dam: the arch part is a concave face that leans towards the water, deflecting some of the pressure onto the canyon walls, while the gravity part of the design is the enormous weight of the dam that thickens considerably from the top (13.7 metres/45 feet) to the bottom (201 metres/660 feet). This helps to resist the immense force that the Colorado River can generate with its 5.5 million-ton weight.
Diverting the Colorado River
Constructing the Hoover Dam would have been impossible with the Colorado River still flowing through Black Canyon. So, the builder endeavoured to divert the course of the river. Four tunnels with a combined length of over five kilometres (three miles) were dug into the canyon walls and around the dam site. The river was diverted into these tunnels by blocking its natural course with rubble as well as detonating a hole in a cofferdam – a temporary enclosure that stopped the river draining into the diversion tunnels. This was only performed for the two tunnels on the Arizona side of the canyon. The two drainage tunnels on the Nevada side were held in reserve for the higher waters in spring and summer.
Generating power
The Hoover Dam’s hydroelectric power plant is located on the downstream side at the bottom of the dam. Excavating the area for the station was finished in 1933, after the dam itself. Concrete began to be poured for the plant at the end of the same year and continued for the next two years, even overlapping with the filling of Lake Mead behind the dam in 1935. With war brewing in Europe and the vulnerability of the plant considered, 1.1 metres (3.6 feet) of concrete, rock and steel topped with tar formed a robust ceiling. Three Francis turbine generators were installed in 1937 when the power plant went on line, with 14 more added over the decades. Its average power generation in nearly 80 years has been around four terawatt-hours, helping to fulfil the huge power requirements of Las Vegas among other west coast communities.
Facts about Hoover Dam
Lake Mead – At maximum capacity, the lake on the upstream side of the dam can yield 219,700kg/m2 (45,000lb/ ft2) of force at its base.
Highway – For many years the top of the Hoover Dam served as a public highway between Nevada and Arizona until the Hoover Dam Bypass was opened in 2010.
Penstocks – The pipes that deliver water to the power plant are known as penstocks. The water in these pipes is under high pressure from the lake behind the dam.
Elevator – An elevator takes tourists down a 152m (500ft) shaft to the base of the dam to the power plant.
Power house – Originally rated at a 1.344GW capacity, the dam’s current installed power capacity is 2.078GW.