page_title: O'Shaughnessy Dam (California)
section_title: Construction
hierarchical_section_title: O'Shaughnessy Dam (California) / Construction
caption_attribution_description: English: California's O'Shaughnessy Dam under construction in August 1922
context_page_description: O'Shaughnessy Dam is a 430-foot high concrete arch-gravity dam in Tuolumne County, California, in the United States. It impounds the Tuolumne River, forming the Hetch Hetchy Reservoir at the lower end of Hetch Hetchy Valley in Yosemite National Park, about 160 miles east of San Francisco. The dam and reservoir are the source for the Hetch Hetchy Aqueduct, which provides water for over two million people in San Francisco and other municipalities of the west Bay Area. The dam is named for engineer Michael O'Shaughnessy, who oversaw its construction.
  Although San Francisco had sought Tuolumne River water as early as the 1890s, this project did not move forward until the disastrous earthquake and fire of 1906, which underscored the insufficiency of the existing water supply. The Hetch Hetchy Valley – then compared to Yosemite Valley for its scenic beauty – was chosen for its water quality and hydroelectric potential, but the location within the national park generated controversy. An act of Congress was required to circumvent federal protection of the Tuolumne River, with the reasoning that public land should be developed for the public benefit.
context_section_description: Work on the Hetch Hetchy project began in early 1914 shortly after the passage of the Raker Act. The city hired John R. Freeman, who had previously worked on the water supply systems of Boston and New York City, to plan the complex dam and aqueduct system. Civil engineer Michael O'Shaughnessy would oversee the construction and design details of the Hetch Hetchy project. The dam in Hetch Hetchy Valley would subsequently be named in his honor. Before construction of O'Shaughnessy Dam could commence, the city completed a 70-foot (21 m) high dam at Lake Eleanor to provide water for the Early Intake Powerhouse, which was necessary to provide electricity for the construction site of the larger dam.
  Initial construction of the dam cost $6,121,000 ($91.9 million in 2019 dollars), and was largely financed by revenue bonds issued by the city of San Francisco. To transport workers and materials, the city hired Frederick Rolandi, a San Francisco engineer who had previous experience designing railways, to oversee construction the Hetch Hetchy Railroad. The railway would be a 68-mile (109 km) standard gauge line that followed the narrow canyon of the Tuolumne River to the remote dam site. Built from 1915 to 1918 by a workforce of roughly 900, the railway allowed for supplies to be shipped directly from San Francisco along the Southern Pacific and Sierra lines which connected to the Hetch Hetchy line. The railroad principally used geared Shay locomotives to negotiate its dangerous winding curves and steep 4 percent grades.
  Actual groundbreaking on O'Shaughnessy Dam was on August 1, 1919, when Utah Construction Company of San Francisco began preparing the dam site for construction. Workers began clearing the trees in Hetch Hetchy Valley to prepare it for receiving waters of the future reservoir. A 20-foot (6.1 m) diameter tunnel, later expanded to 23-by-25-foot (7.0 m × 7.6 m), was dug around the south side of the O'Shaughnessy Dam site, and a timber crib cofferdam diverted the waters of the Tuolumne River into the tunnel during construction. The riverbed on the site of the future dam was excavated over 100 ft (30 m) before hitting the granite bedrock. A retaining wall was poured on the upstream side to prevent water seepage into the foundation hole, and the granite was scoured and artificially roughened to prepare for receiving concrete.
  The concrete for the dam was processed in a plant located shortly upstream from the construction site, with sand and rock excavated from abundant alluvial deposits in the Hetch Hetchy valley. This was mixed with cement shipped in on the Hetch Hetchy Railroad and local boulders ranging from 1 ft (0.30 m) to several yards (metres) in diameter to produce a cyclopean construction material for the dam. Beginning in September 1921, the concrete was hoisted up a 375-foot (114 m) tower on the south side of the gorge, from which it could flow down movable chutes by gravity to the construction site. A total of 398,516 cu yd (304,687 m³) of concrete was poured to form a dam standing 226 feet (69 m) above the riverbed and 344 feet (105 m) above foundations. The last concrete was placed in February 1922 and the dam was completed in May 1923. At the time, it was the second tallest dam in the United States, after Idaho's Arrowrock Dam. On May 24, 1923, the reservoir filled for the first time. A peak labor force of five hundred worked on the project, which claimed the lives of 67 men and one woman.
  The first hydropower was delivered in 1925 with the completion of the Moccasin Powerhouse, fed by Hetch Hetchy water through the Canyon and Mountain Tunnels. However, the first water deliveries did not reach San Francisco until 1934, eleven years after the completion of O'Shaughnessy Dam and twenty years after groundbreaking of the Hetch Hetchy project. O'Shaughnessy Dam had been designed with adequate foundations and a unique stepped face in order to make possible a future increase in the dam height. This was done in anticipation of rapid growth in the demand for water and hydroelectricity. Indeed, between 1935 and 1938, t
caption: Construction work on the dam in August 1922
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