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Engineering: Hoover Dam Author(s): Henry Petroski Reviewed work(s): Source: American Scientist, Vol. 81, No. 6 (November-December 1993), pp. 517-521 Published by: Sigma Xi, The Scientific Research Society Stable URL: http://www.jstor.org/stable/29775051 . Accessed: 06/01/2013 08:28

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Hoover Dam

Henry Petroski

Everything seems to move slowly at Hoover Dam. Long lines of cars, buses and trucks in low gear wind along the two-lane road

down one steep side of Black Canyon and up the other. The lines stop frequently as carloads of tourists crane their necks at the grandeur of the

site, and drivers search for a parking space among the tiers of overlooks. For all the conges? tion, few motorists seem impatient or interested in searching maps for alternate routes. Everyone driving in this vicinity must know that U.S. High? way 95, which arcs along the crest of the dam, is the only road across the Colorado River between

Davis Dam, over 50 miles to the south, and Nava

jo Bridge, over 150 miles to the east. Pedestrians move about the top of the dam at a

snail's pace in the desert heat, crisscrossing the crest over which, by design, no water has ever flowed. Several thousand tourists may visit

Hoover Dam on a summer day, most of them

standing in long lines to board the large elevator that takes them, 20 at a time, 600 feet down into the inner workings of the dam. Many come from Las Vegas, 35 miles to the northwest. Even there, amid all the noise and neon, a visit to Hoover Dam is hawked as one of the area's must-do

things. The pitch may be engineering achieve? ment as awesome entertainment, but few who visit the works at Black Canyon seem disappoint? ed by the decided calmness and harmony of it all.

Just as the Mississippi River wreaked havoc in the Midwest last summer, so the Colorado River used to be alternately a blessing and a bane for the Southwest. Although the Colorado seemed a

potential source of water for irrigation in rich but arid regions such as Arizona and southern Cali? fornia, it did not provide a consistent supply of

water. It often flooded low-lying lands in the

spring and early summer and then slowed to a trickle during late summer and early fall. When? ever crops, cattle and Californians were not

awash, they went thirsty. Two promising regions with rich alluvial soil

were the Colorado Desert and the below-sea-lev? el Salton Sink. Near the turn of the century they were renamed Imperial Valley by land developers

who promised through their California Develop? ment Company (CDC) to supply enough water from the Colorado to make the otherwise arid land attractive. For a few years the scheme

worked and the region was booming, but soon the CDC's irrigation canal silted up and, in the face of lawsuits from landowners, a new way to divert the Colorado's water had to be found

quickly. A hastily engineered scheme?one that also blunted the impact of the newly formed Bu? reau of Reclamation's charge that CDC had mo?

nopolized the water supply?brought water up from Mexico. That worked fine at first, but in 1905 so much water came down the Colorado in

spring and fall floods that the river changed its course and flowed into the Salton Sink, which then became the inland Salton Sea. Lost crops, lost topsoil and a lost irrigation system amount?

ing to millions of dollars presented a disastrous

prospect for Imperial Valley. It was two years be? fore the Colorado was put back on course, but the root problems associated with both exploitation of and protection from the river remained.

The great amount of silt carried southward by the Colorado kept raising the river's channel, and so required constant maintenance of the protec? tive levees and other components of the irriga? tion system, much of which was located in Mexi? co. Problems with getting work crews back and forth across the border led eventually to support for a new canal located entirely on American soil. It was in such a climate that a young lawyer named Phil Swing, supported by southern-Cali? fornia water interests, was sent in 1917 to repre? sent them in Congress and to promote the idea of an "All-American Canal."

Swing's effectiveness led quickly to the intro? duction of legislation, but the proposal was de? feated largely because of the opposition of Arthur Powell Davis, a 40-year veteran of government service who knew about as much about the Col? orado River basin as anyone. Davis, nephew of the Colorado canyon's explorer, John Wesley Powell, had served as chief hydrographer when a Panama Canal route was under investigation and as an engineer with the U.S. Reclamation Service since its origins in 1902. The driving force behind the design and construction of many dams and ir?

rigation canals, he was director of the Reclama? tion Service when the All-American Canal bill

1993 November-December

Henry Petroski is the Aleksandar S. Vesic Professor and chair?

man of the Department of Civil and Environmental Engineering at Duke University, Durham, NC 27706.

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Figure 1. Hoover Dam photographed in 1990. (All photographs courtesy of the Bureau of Reclamation.)

came before the Congress. He argued successfully for that bill's defeat in 1919 so that a more com?

prehensive and long-range plan for the Colorado

might be explored first.

A Grand Plan Years earlier, as a supervising engineer in the Reclamation Service, Davis had thought about a

grand plan for the entire drainage system of the Colorado. According to Joseph Stevens, who sub? titled his telling of the story of Hoover Dam "an

American adventure," Davis's scheme was to be

"an undertaking to rival or even surpass in scale and importance the construction of the Panama Canal." Congressman Swing went further and added the Pyramids, the Great Wall of China and Solomon's temple to the list of feats of engineer? ing that were less complicated than what came to be known as the Boulder Dam Project. Congress agreed that the great problem of the Colorado basin should be studied by the Interior Depart? ment, and its secretary, Albert Fall, assigned the task to Davis's organization. The Fall-Davis Re?

port, issued in 1922, "contained an exhaustive hy drological and geological profile of the Colorado River and its canyons," but most attention was drawn to its recommendation that the govern? ment erect "at or near Boulder Canyon" a large dam, which could generate power to repay in time the construction expense.

Seven states?Arizona, California, Colorado, New Mexico, Nevada, Utah and Wyoming? would be affected by the larger plan, and they would first have to reach an agreement about their

respective claims to water. Conferences were held, with the federal government represented by the

Secretary of Commerce, Herbert Hoover, whom Phil Swing claimed to have had a part in suggest? ing as a "neutral" member of the Colorado River

Commission. It was Hoover who evidently broke an impasse over state-by-state allocations by proposing the establishment of Upper and Lower Colorado River Basins, and this led all but one of the states to agreement. According to Hoover, "a blunderbuss of a governor in Arizona, who knew

nothing of engineering, bellowed that it would 'rob Arizona of its birthright/" After an amend?

ment required ratification by only six of the seven affected states, the Colorado River Compact was

accomplished late in 1922. A Boulder Canyon Project Act was introduced in

1923 by Congressman Swing and California Sena? tor Hiram Johnson, and it became the focus of bitter debate inside and outside of Washington. The pub? lisher of The Los Angeles Times, Harry Chandler, was concerned about future irrigation for the almost one

million acres he owned just south of the Imperial Valley in Mexico. On the other hand, William Ran?

dolph Hearst of San Francisco, Chandler's Califor? nia newspaper rival, favored the bill. The saga of

518 American Scientist, Volume 81

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the Swing-Johnson legislation's debate through sev? eral sessions of Congress has been written about in detail, mostly from Congressman Swing's perspec? tive, by Beverley Moeller. The legislative struggle fi?

nally came to an end when President Calvin

Coolidge signed the bill into law in December 1928. Even before the first Swing-Johnson bill was in?

troduced, the Reclamation Service had begun de? tailed explorations of possible dam sites. When the Fell-Davis report was written, the choices had been narrowed down to five possible sites in Boulder

Canyon and two sites about 20 miles downstream in Black Canyon. Boulder Canyon's foundation

was already known to be granite, a preferred rock, whereas Black Canyon's was volcanic tuff (com? pacted ash), so Davis used the language "at or near Boulder Canyon" in the report. Further investiga? tion, however, revealed that the lower site at Black

Canyon was indeed the best of the lot. Among oth? er factors, there was less jointing and faulting, less silt and debris to be removed, easier prospects for

tunneling and a narrower gorge that equated to a need for less concrete. Furthermore, beds of sand and gravel for use in the concrete were located

nearby, the potential reservoir was larger and

nearby Las Vegas provided comparatively easy ac? cess to the canyon.

Designing the Dam In addition to a site for the dam, the details of the

design itself had to be specified. As with all engi? neering structures, judgment was employed to ar? rive at initial alternative geometries, which were then subjected to successively more-refined de?

grees of analysis until a final design emerged. About 30 geometries were investigated at the Denver office of the Bureau of Reclamation, as the Service had been renamed, and its engineers sub

jected the hypothesized dams to analyses of stresses, including those that would result from the cooling and contraction of the concrete as it cured. As was customary in the days before digi? tal computers, models (rubber and plaster, in this

case) were employed to guide and check theory and hand calculations. Initial specifications called for stresses no higher than 30 tons per square foot

anywhere in the dam. In the end this proved to be difficult to meet, and stresses up to 40 tons per square foot were allowed in the final design. This is equivalent to about 550 pounds per square inch, which is well below the compressive strength of even common concrete, thus provid? ing a considerable factor of safety against the pos? sibility that the dam would fail by being crushed under its own weight or under the pressure of water it had to resist.

Although similar in vertical cross section to a

gravity dam (one whose sheer weight prevents it from being tipped over or pushed downstream

by the water), Hoover Dam acts principally as an arch dam, transferring the pressure of the water behind it to the walls of the canyon, which act like abutments. The great height of the dam, about 725 feet above bedrock, and the consequent weight of the concrete, requires its transverse pro? file to spread like a gravity dam from 45 feet at the crest to 660 feet at the base. The structural in?

tegrity of the dam was a matter of some debate when the plans were first revealed by Elwood

Mead, then Commissioner of Reclamation, in a 1930 article in Civil Engineering. Mead outlined succinctly some "extraordinary

problems met in design" in a paragraph that showed a sensitivity to scale effects and design philosophy that were essential to producing a successful outcome:

In designing a dam more than 700 ft. in

height, stress factors become very important, which in the design of dams of nominal size are comparatively insignificant. Possible errors in basic design assumptions must be carefully studied and checked; the physical properties and volumetric changes of so great a mass of concrete must be carefully deteirnined; prima? ry stresses caused by the weight of the materi? als and the horizontal water pressure must be

accurately calculated, as well as secondary stresses due to all possible causes.

Mead did not elaborate on such technical mat?

ters, however, and soon an article by M. H. Gerry, Jr., a consulting engineer from San Francisco, ap

Figure 2. Boulder City, shown in 1934, housed the construction workers.

1993 November-December 519

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^^^^

Figure 3. Hoover Dam under construction in December, 1933, reveal?

ing the interlocking cells.

peared in Civil Engineering, challenging the safety and stability of the dam. Letters challenging the

challenger followed, and about a year after Com? missioner Mead's article, Harald M. Westergaard, a

stmctural-engmeering professor at the University of Illinois and consultant to the Bureau, published "Safety of Hoover Dam," in which he discounted

Gerry's misinterpretation of structural principles and declared that, "It is the business of the struc? tural engineer to imagine each undesirable thing that might happen to the structure and provide against that." Westergaard and the Bureau engi? neers had felt they had done just that before Mead transmitted to the Secretary of the Interior specifi? cations and drawings for the dam, power plant and appurtenant works. These were approved in late 1930, and construction bids were invited.

Building the Dam In his memorandum of December 15,1930, trans?

mitting dam specifications, Mead reminded the

Secretary of the Interior that the Depression had created very great "pressure for action on this

matter, as a means of finriishing employment and

encouraging a revival of business." The specifi? cations spelled out various conditions that were related to these economic concerns, including that

employment preference be given to ex-service? men and citizens and that, specifically, "no Mon?

golian labor shall be employed." Other non-tech? nical conditions required that Boulder City be

created 23 miles southeast of Las Vegas and close to the canyon as a construction camp site. Al?

though bid specifications stated that buildings erected in Boulder City were to have "a reason?

ably attractive appearance and no unpainted shanties or tar paper shacks will be permitted/' and even though there was much to be admired in the planning and construction of the town, 60

years later Stevens would relate many stories of shameful working conditions at Black Canyon.

Bids were due in Denver on March 4,1931, but few construction companies had the experience or resources, including the five-million-dollar bond, required to compete. The successful bid?

ding scheme was put together by a group named for the task, Six Companies, Inc. It comprised:

Utah Construction Co., Pacific Bridge Co., Kaiser

Paving Co., MacDonald-Kahn Construction Co., Morrison-Knudsen Co. and J. F. Shea Co. Each of the partner firms naturally had its own expertise, and Morrison-Knudsen included "America's foremost dam builder," Frank T. Crowe.

A 1905 civil-engineering graduate of the Uni?

versity of Maine, Crowe had gained cutting-edge experience in building high, concrete dams while he worked for the Bureau of Reclamation. After al? most 20 years in the field, he was offered and took a desk job as general superintendent of construc? tion for the Bureau, but he quit after a year to join the Morrison-Knudsen Company so that he could once again engage directly in dam building. It was Crowe who spearheaded the effort to come up with a bid figure for the Boulder Canyon Project, and he presented it to Six Companies representa? tives at a meeting early in February at the Engi? neers Club of San Francisco. When the bids were

opened in Denver the next month, Six Companies' low bid of just under $49 million was within five hundredths of one percent of the price tag esti? mated by engineers at the Bureau of Reclamation. The contract remained, until World War II, the

largest ever awarded by the government. In order to build the dam proper, the river had

to be diverted through tunnels driven through the canyon walls. An upstream diversion dam,

which had to be built between the annual floods, and a downstream coffer dam would keep the construction site dry. After the main dam was

completed, most of the diversion tunnels would be blocked off, but some parts would be incorpo? rated into the system of penstocks that would feed the turbines in the hydroelectric power plant. After about two years, the river bottom had been cleared to bedrock, and the first forms to receive concrete were erected. The pouring of concrete

began on June 6, 1933, and continued day and

night over the next two years. Three million cubic

yards of concrete, from two specially built mixing plants, were distributed among cube-like cells that interlock in the completed dam. Cooling pipes embedded five feet apart throughout the concrete carried away the heat of hydration; oth? erwise, the dam would still be cooling down and

520 American Scientist, Volume 81

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developing cracks as the concrete contracted. The

completed dam was turned over to the govern? ment on March 1, 1936, more than two years ahead of schedule, and energy began to be pro? duced by the power plant that fall.

The dam was dedicated on September 30,1935, by President Franklin D. Roosevelt. First to speak at the ceremony was his Secretary of the Interior,

Harold Ickes, who, after repeatedly referring to the structure as Boulder Dam, declared, "This great en?

gineering achievement should not carry the name of any living man but, on the contrary, should be

baptized with a designation as bold and character? istic and imagination stirring as the dam itself." He

was implying that the dam should not be named after Hoover, who was, of course, still alive. Ickes had in fact reopened a debate over the name of the dam that went back to an earlier dedication cere?

mony, one that acknowledged Congress's first ap? propriations for the entire Boulder Canyon Project with the driving of a spike of Nevada silver for the rail line that was to connect the construction site

with the Union Pacific Railroad in Las Vegas. At that ceremony, Ray Wilbur, the Secretary of the In? terior under President Hoover, who signed the bill, had asserted, to the surprise of many in attendance, "I have the honor to name this dam after a great engineer who really started this greatest project of all times, the Hoover Dam."

From the beginning, then, the name of the dam was a contentious and confusing issue. In 1939,

the American Society of Civil Engineers (ASCE) readopted Hoover Dam for use in society publi? cations, pointing to correspondence between Sec?

retary Ickes and Attorney General Homer Cum

mings. Cummings declared the name Hoover Dam to be official, because of its use in the appro? priations bill and government contracts for the dam, as opposed to the collective Boulder

Canyon Project, which included also the power plant and appurtenant works. In 1947, the Re?

publican 80th Congress, called "do-nothing" by President Harry Truman, passed legislation rein?

stating the name Hoover Dam. Whatever its name, more than 27 million people have visited the dam over the years, and there appears to be

general agreement with a plaque?placed near the center of the crest by the ASCE in 1955?de?

claring the dam to be one of the country's Seven Modern Civil Engineering Wonders.

Bibliography Bureau of Reclamation. 1930. Hoover Dam, Power Plant and Ap?

purtenant Works: Specifications, Schedule, and Drawings. Wash?

ington, D.C.: United States Department of the Interior.

Hoover, Herbert. 1952. Memoirs: The Cabinet and the Presi?

dency, 1920-1933. New York: Macmillan.

Mead, Elwood. 1930. Hoover Dam. Civil Engineering Octo? ber: 3-8.

Moeller, Beverley Bowen. 1971. Phil Swing and Boulder Dam.

Berkeley: University of California Press.

Stevens, Joseph E. 1988. Hoover Dam: An American Adven? ture. Norman: University of Oklahoma Press.

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