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Hoover Dam
Colorado River, Black Canyon, Nevada/Arizona border, USA
Hoover Dam
associated engineer
John Lucian Savage
US Bureau of Reclamation
Frank Crowe
date  May 1931 - 1st March 1936
era  Modern  |  category  Dam/Reservoir  |  reference  HQ403374
photo  courtesy US Bureau of Reclamation
The tallest dam in the world when completed. Also known as Boulder Dam, the Hoover Dam was constructed to control flow in the downstream Colorado River and harness its energy for hydro-electric power. A flagship project of the Depression era, the concrete gravity-arch dam was completed ahead of schedule, and later designated one of seven modern civil engineering wonders of the USA.
The Colorado River runs 2,330km from the Ricky Mountains to the Gulf of California, through seven states in western America and two states in northern Mexico. From about 1900, attempts were made to harness it for irrigation, water supply and energy generation, particularly in the desert regions of the USA.
On 17th June 1902, the US Reclamation Service was established to oversee the management of water resources in the western states. Engineers embarked on a series of surveys and reports into the control and use of the flood-prone Colorado, investigating 70 sites along the river and identifying the southern tip of Nevada as a suitable dam location.
In 1918, Arthur Powell Davis (1861-1933), then Director of the service, proposed building a rubble dam in Boulder Canyon, which was rejected in 1922 as the canyon was found to be on a seismic fault. The service recommended siting a dam for flood control and electric power generation in Black Canyon, 17km to the south west, in an area of hard and durable volcanic rock (andesite breccia).
In 1923, the service was renamed the Bureau of Reclamation. On 21st December 1928, the Boulder Canyon Project Act was signed by US President (John) Calvin Coolidge (1872-1933), and ratified on 6th March 1929. Everything about the project was at a huge scale not seen before in construction projects.
The bureau’s chief design engineer, John Lucian Savage (1879-1967), supervised the design of a 221m high reinforced concrete arched gravity dam, straddling the state line between Nevada and Arizona. The wedge-shaped structure was to be located in the narrowest part of the ravine, with a convex upstream face to transmit the pressure of the impounded water into the sides of the canyon, effectively jamming the dam into the gap.
On 4th March 1929, wealthy mining engineer Herbert Clark Hoover (1874-1964), formerly presiding officer of the Colorado River Commission, was inaugurated as America’s 31st President. He was a leading protagonist in getting the dam project underway, and it was named the Hoover Dam in his honour.
The Great Depression, which began with the Wall Street Crash of 29th October 1929 and continued throughout the 1930s, caused worldwide economic hardship. Thousands of workers and their families travelled to the dam site in the hope of employment, living in makeshift desert camps and later in the purpose-built settlement of Boulder City, south east of Las Vegas, Nevada.
In September 1930, construction began on a 53km railway link between Las Vegas, Boulder City and the dam site. Other infrastructure for the project included construction camps, rail lines along the canyon floor (after dewatering), 11km of access road from Boulder City and power supply lines stretching 357km from San Bernardino in California, plus on-site plants for aggregate crushing and screening, concrete batching, air compressors and steel fabrication.
Tender documents for the contract were released on 10th January 1931. Interested contractors had to provide a $2m bid bond with their submission. The winning contractor would also have to provide a $5m performance bond and complete the dam within seven years. At the time, it was almost impossible for a single contractor to raise the required bonds.
On 11th March 1931, the Bureau of Reclamation awarded the contract to Six Companies Incorporated, a Californian joint venture between Henry J. Kaiser Co of Oakland and Bechtel Corporation of San Francisco (with a 30% share of the company), MacDonald & Kahn of Los Angeles (20%), Utah Construction Company of Ogden (20%), Morrison-Knudsen of Boise, Idaho (10%), and Pacific Bridge Company (10%) and J.F. Shea (10%) both of Portland, Oregon. The bid of $48.9m was one of only three received, and some $5m less than the nearest other bid.
On 20th April 1931, Six Companies was given notice to proceed and construction began soon after. The company’s superintendent and chief engineer was Francis (Frank) Trenholm Crowe (1882-1946) of Morrison-Knudsen, acknowledged as America’s leading dam builder. He had worked at the Bureau of Reclamation, where he developed a system for delivering and pumping concrete in large dam pours. He refined the Hoover design with architect Gordon Bernie Kaufmann (1888-1949), who gave it and its power house a distinctive Art Deco look. Walker Rollo Young (1885-1982), the bureau’s construction engineer in charge, became resident engineer.
Before the dam could be constructed, the site had to be dry, which meant diverting the river during the low water season. Four diversion tunnels were driven through the canyon walls, two on either side of the river, using blasting and drilling. Progress was speeded up by the development of a motor-driven jumbo drill, equipped with thirty 65kg rock drills working simultaneously.
Each tunnel was excavated to a diameter of 17.1m, followed by rock grouting and lining with a 900mm layer of reinforced concrete — making finished bores of 15.2m. The concrete was produced in a batching plant on the river bed and delivered by electric trains. The combined length of all four tunnels was 4.9km. Internal lighting was provided by sunlight reflector dishes.
Embankment cofferdams were constructed upstream and downstream of the dam site, using the tunnel excavation arisings, to seal the site and force water to flow through the tunnels. In September 1932, construction of the upper cofferdam began and on 14th November 1932, the river was diverted. The upper cofferdam, approximately 180m downstream of the diversion tunnel inlets, was 137m long, 30m high, 229m wide at the base and contained 515,000 cu m of earth and rock. Its upstream face was protected by 150mm of concrete over a 900mm rock blanket, while the downstream side had a rock face.
The lower cofferdam was of compressed earth with rock facings. It was 107m long, 20m tall, 168m wide at the base and contained approximately 224,000 cu m of earth and rock. A rock barrier 16.5m high, 114m long and up to 61m thick was built at the downstream toe of the cofferdam to moderate the force of the water. All of the cofferdam, rock barrier and diversion tunnel works were completed before the spring floods of 1933.
The enclosed site was dewatered, and excavation down to bedrock commenced for the dam and power plant. In all, more than 350,000 cu ms of spoil was removed, with the bedrock foundation 33.5m to 42.4m below low water level.
The steep canyon sides were to act as the dam abutments, and to ensure they were sound and solid, a group of intrepid workers known as 'high scalers' abseiled down them, chipping away loose rocks with heavy jackhammers. They also planted explosives to remove unwanted rock outcrops. To protect them from falling debris, their cloth hats were dipped in tar and allowed to dry before use — the forerunners of modern hard hats.
Once the site was cleared, dam construction proper could commence. A line of 393 holes up to 45.7m deep were drilled into the base and sides of the canyon, and injected with cement grout. The resulting grout curtain was designed to stabilise the bedrock, prevent water seepage and limit uplift pressure on the dam wall. However, some of the grout holes penetrated geological faults and others struck hot springs. A number of holes were not grouted properly and 54 were abandoned.
On 4th March 1933, Franklin Delano Roosevelt (1882-1945) became US President. Hoover had lost office largely because voters blamed him for the Depression. The Black Canyon structure was swiftly renamed Boulder Dam.
On 6th June 1933, the first concrete was placed in the dam structure. An automated batching plant on the canyon rim produced 18.4 cu ms of high-strength low-water content concrete every three and a half minutes. It was delivered in hoppers by five 20 tonne cableways strung across the site.
Owing to the heat given off by chemical reactions that take place during concrete curing, exacerbated by desert temperatures of up to 51 deg C, the dam could not be constructed in one piece. Apart from the impossible logistics, such a large pour would have cracked and collapsed, and taken 125 years to cool down!
Instead, reinforced concrete was placed in a honeycomb of some 215 discrete columns — up to 18.3m square on the upstream face and 7.6m square on the downstream side. Concrete was poured in lifts of not more than 1.5m in 72 hours and adjacent columns were interlocked withby vertical keys to the upstream/downstream faces and horizontal keys to the canyon wall faces. The pours were cooled by more than 936km of 25mm diameter steel pipes filled with ice-cold water embedded in the concrete. Once the pours had cured, the joints and the empty cooling pipes were pressure grouted to stitch the columns together, forming a monolithic structure.
But the dam is not completely solid. A series of horizontal galleries for grouting and drainage run through the lower two-thirds of it, from one side of the canyon to the other. They are accessed from the crest via two elevators (lifts) in vertical shafts. Inclined inspection galleries follow the abutments on either side and a horizontal gallery near the surface runs below most of the roadway. The main gallery and elevators are accessed via the dam's four towers on the crest.
On 1st February 1935, the reservoir behind the dam began to fill. Called Lake Mead, it was to be the largest reservoir in the United States, capable of holding a volume equivalent to two years of the Colorado River's normal flow.
The river diversion tunnels now became part of the permanent works. The two inner tunnels were plugged with concrete about one-third their length below the canyon wall inlets, and were lined with 9.1m diameter steel pipes to convey water from the intake towers in the reservoir to the power plant at the base of the dam (downstream face) via additional 11.3m diameter tunnels.
The four reinforced concrete hollow intake towers in Lake Mead (beside the dam) control the water supply to the hydro-electric turbines in the power plant. They are linked in pairs to the upstream side of the dam crest, one pair founded on each side of the canyon. Each ribbed tower is 120.4m high and 25m in diameter at the base, stepping in to 19.3m diameter at the top, with an internal diameter of 9m.
Flow through the towers into the tunnels is controlled by cylindrical gates, 9.75m in diameter and 3.35m high. One gate is near the bottom and the other near the middle of each tower (eight gates in total), all protected by debris screens.
Altogether the tunnels contain 4.5km of 9.1m diameter pipe (max thickness 70mm) and 610m of 2.6m diameter pipe for reservoir outlets. The 9.1m diameter pipes are connected to the power plant by 16 steel penstocks, 4m in diameter, inside concrete-lined tunnels of 5.5m bore.
The quantity of pipework required for the penstock system exceeded 44,700 tonnes in gross weight — too heavy to be prefabricated and transported to site by rail or road. Instead, the pipes were made on site by bending and welding steel plates together.
Valve houses, about 55m above river level, control the 2.6m reservoir outlets with 2.3m jet flow gates, designed to bypass water around the dam under emergency or flood conditions. The 9.1m pipe outlets to the river below the dam can be closed by steel gates measuring 15.2m x 10.7m, if the penstocks need to be emptied for inspection, maintenance or repair. The inner tunnel plugs near the outlet ends of the penstocks have four 1.7m jet flow gates to allow the passage of sufficient water for downstream users’ needs.
Spillways to prevent the dam being overtopped are located 8.2m below its crest, one on each side. Each consists of an open channel 198m long, 46m wide and 52m deep, lined with 450-600mm of concrete. Water discharging into the top of the spillways flows through inclined shafts, 15.2m in diameter and 183m long, into the former outer diversion tunnels. The tunnels were plugged with concrete approximately halfway along, with the downstream sections used as 670m long spillway outlets. Their canyon inlets are closed with 15.2m square bulkhead gates set in steel frames, each weighing about 1,360 tonnes.
Discharge is controlled by four steel drum gates, 30.5m long and 4.9m high, on each spillway crest. The 2,270 tonne gates can be operated automatically or manually. The combined maximum spillway capacity is about 14,000 cu m/sec, or more than twice the flow over Niagara Falls. Water in the spillway tunnels can attain a maximum velocity of about 53m/sec, or 193kph (120mph).
The dam structure reached crest height in March 1935, with concreting completed on 29th May. By then the dam contained 2.5 million cu m, or 6.7 million tonnes, of concrete with 20,400 tonnes of steel reinforcement, making it the world’s largest concrete structure. On 30th September 1935, Roosevelt visited the site for a formal dedication ceremony, describing it as "an engineering victory of the first order".
The dam was finished by 1st March 1936, some two years before the seven stipulated in the original contract. The project cost a total of $165m. The dam is 201.2m wide at the base and measures 221.4m from bedrock to roadway, with the intake towers and crest towers on the parapet 12.2m higher. The curved crest is 373.1m long and the 13.7m wide two-lane roadway along it completed a direct road link between Las Vegas in Nevada and Phoenix in Arizona.
A total of 21,000 men worked on the project, peaking at a daily maximum of 5,218 in June 1934. In less than five years they had managed to create a structure of greater volume than Egypt’s largest pyramid. However, the project was not without loss &mash; 112 deaths were recorded between December 1922 and December 1935, of which 96 were 'industrial fatalities'. Despite vehicle exhausts producing carbon monoxide in the tunnels, no deaths were verified from carbon monoxide poisoning.
Sculptor Oskar J.W. Hansen (1891-1971) designed much of the artwork for the project, including the plaque commemorating those who died during construction, the monument of dedication on the Nevada side — a pair of 9m winged bronze figures with a 43m flagpole on a black diorite base — and the concrete bas-relief panels on the elevator towers. Artist Allen Tupper True (1881-1955) was responsible for the decoration of the dam’s internal walls and floors, and devised a colour coding system for the pipework and machinery.
Though controlling the Colorado River was the primary aim of the project, generating hydro-electricity allowed it to be self-financing. The power plant, constructed 1933-6, is located at the foot of the dam and housed in a U-shaped building with wings 198m long rising 91m above the river bed. Its roof is 1.1m thick, bombproofed using layers of concrete, rock and steel, and surfaced with sand and tar.
Eventually the power house contained 17 Francis turbine generators — nine in the Arizona wing and eight in the Nevada wing. The machinery and equipment was lowered into place from an electric aerial cableway spanning 366m across the canyon, capable of transporting loads of 150 tonnes. The cableway remains in place and is still used as necessary.
By late 1936, Lake Mead was full enough to enable energy generation. The power plant’s first three generators in the Nevada wing went into operation on 26th October, 14th November and 28th December 1936, providing electricity to cities in southern California, Nevada and Arizona. Two new generators started in 1937, on 22nd March (Nevada) and 16th August (Arizona).
In 1938, a two more Nevada generators wente live on 26th June and 31st August. In 1939, a pair of Arizona generators became operational on 19th June and 12th September. The installed capacity was then 704.8MW, making the Hoover Dam power plant the world’s largest hydro-electric facility, a distinction it held until 1949.
Meanwhile, by June 1937, problems with seepage around the grout curtain had become apparent and water was pouring from the Nevada side of the canyon above the power house. Alkaline water from hot springs seeped through the outlet tunnel, corroding the steel penstock. Abnormally high uplift pressure developed beneath the centre of the dam, peaking in September 1938.
Investigations in 1938-9 revealed that the grout curtain was too shallow to seal the fissures in the abutments, which were criss-crossed with faults not fully noted during the original site surveys. As a result, high pore water pressures had developed in the bedrock. Between 1938 and 1947, undercover remedial work to extend the grout curtain was carried out from the dam’s internal lower drainage gallery. New 91.4m deep holes were drilled and grouted under the pressure of the full reservoir head. Additional grouting was also undertaken beneath the Nevada spillway intake. The total cost of the grouting works was $1.84m.
More generators were brought online in the power plant and, in 1940, its annual hydro-electric power generation was 3TWh. Another Arizona generator went into service on 9th October 1941.
By 30th July 1941, Lake Mead was around 177m deep and had reached the top of the spillway gates, 372m above sea level. It covered 64,145 hectares and extended 177km up the Colorado River and 56km up the Virgin River.
On 6th August 1941, the spillways were tested for the first time. A week later, on 14th August, the drum gates of the Arizona spillway were raised for several hours to assess the effects of the test. All was satisfactory and spillway operation continued until 1st December 1942, when the reservoir had drained to 367.5m above sea level. However, on 28th October 1941, one of the drum gates dropped without warning, increasing flow through the Arizona spillway from 382 to 1,075 cu m/sec. The falling water eroded a cavity 35m long, 9m wide and up to 14m deep in the concrete lining of the spillway outlet tunnel, which was repaired and polished smooth to help minimise future abrasion. In 1947, flip buckets were added to both spillways to slow the flow.
On 12th April 1945, Harry S. Truman (1884-1972) succeeded to the US Presidency. In 1947, the American Congress passed legislation to reinstate the structure’s name as the Hoover Dam.
In 1955, a plaque inscribed, "A MODERN CIVIL ENGINEERING WONDER OF THE UNITED STATES. ONE OF SEVEN SELECTED BY THE AMERICAN SOCIETY OF CIVIL ENGINEERS. 1955" was erected on the dam’s crest.
On 1st December 1961, power installation at the Hoover Dam was completed when the final Nevada generating unit was switched on. The maximum installed generating capacity, including the two station service units, totalled 1,335MW.
On 24th July 1983, floods raised the water in Lake Mead to a record level, with water more than 600mm deep cascading over the raised gates of both spillways. Flooding caused widespread damage throughout the site, and the increased flow through the spillway tunnels again resulted in cavitation of the concrete. Repairs included the installation of spillway aerators.
The power plant’s original turbine generators were replaced between 1986 and 1993, increasing its total maximum capacity to 2,080MW. The plant now had 17 main generators, all Francis-type vertical hydraulic turbines — 15 x 132.7MW, one 74.6MW and one 64.1MW. It also has additional generators — 13 x 130MW, two 127MW, one 68.5MW and one 61.5MW. Two 2.4MW station service turbine units are driven by Pelton water wheels, and provide electricity for the site's own requirements.
The annual net energy generation for the Hoover Dam power plant has varied from a minimum of 2.65TWh in 1956 to a maximum of 10.35TWh in 1984. The average annual net generation, from 1947 to 2008, was about 4.2TWh.
By 2006, the combined population of Las Vegas and Phoenix had increased more than 100 fold, from 55,500 to around 5,700,000 people, and more than 16,000 vehicles crossed the dam every day. Traffic was limited to 24kph (15mph) but was often slower as visitors parked beside on the dam to take in the view.
In October 2010, the new Mike O'Callaghan-Pat Tillman Memorial Bridge bridge opened, 460m downstream of the dam. Situated 270m above the river, its concrete arch spans 324m and carries a four-lane highway. The road over the dam is no longer a through route. In daylight hours, pedestrians can walk the dam’s crest, and vehicles can cross from the Nevada side without stopping but must turn round and re-enter Nevada.
The rising population has also increased the demand for water. Lake Mead supplies 18 million people in three states and irrigates more than 400,000 hectares of land. Since 1999, the area has been experiencing drought conditions and by June 2014, the reservoir held only 40% of its maximum capacity. The water level dropped from 369m to 329.5m above sea level, exposing the dam outfall pipes during the summer months.
In July 2014, the power plant was de-rated from 2,074MW to 1,592MW, and in 2015, the annual net energy generation fell to 3.6TWh. With water levels still receding in Lake Mead, the dam now generates power only during periods of peak demand.
Since the present turbines operate at heads of between 128m and 180m, additional generators capable of working efficiently with less flow are required. Five new wide-head turbines, which operate under lower head waters should be operational by 2017. They can be used at reservoir levels down to 289.6m above sea level.
On 15th December 2014, a new 5km tunnel beneath the reservoir to safeguard drinking water supplies to Las Vegas was completed (commenced in 2011).
Architect: Gordon Bernie Kaufman
Resident engineer: Walker Rollo Young
Contractor: Six Companies Incorporated
Research: ECPK
bibliography
"Hoover Dam: Grout Curtain Failure and Lessons Learned in Site Characterization" by J. David Rogers, Dams Symposium, Association of Engineering Geologists Annual Meeting, Las Vegas, 22nd September 2005
http://xroads.virginia.edu
https://wrrc.arizona.edu
www.bcmha.org
www.bechtel.com
www.constructioncompany.com
www.hooverdamnotes.com
www.nce.co.uk
www.onlinenevada.org
www.usbr.gov
Location

Hoover Dam