Water Well Journal
Issue link: http://read.dmtmag.com/i/592194
Many years of fighting the alternating characteristics of the Colorado River were instrumental in convincing local power brokers that some means of river control had to be developed. After struggling with the Colorado River's unpredictable nature for years, several attempts at forming a solution to the problem were tried. Many of these attempts included bypass channels, canals, or other types of diversion structures to send river water to irrigable acreage. The majority of these solutions were man-made structures such as rock- or earthen-filled dams and similar types of abutments placed in the 1890s and 1900-1910 within the canyon region. All these attempts failed in one way or another. Almost all the failures were due to either the erosive force of the high currents imposed by the river or damage due to silt erosion and deposition. In addition to the problems resulting from flooding and silt deposition, many of the methods attempted to control the river were privately funded and built. This created a situation where the equitable means of dividing the use of water, particularly in Mexico, were often arbitrary and unfair. Soon, given the size and importance of the resource to the country, and the previously tried and failed attempts at con- trolling the river, the United States government decided inter- vention on its part was needed. The Ultimate Solution The Colorado River Commission was formed in 1922 to address the issues with the Colorado River. It consisted of rep- resentatives of all the states impacted as well as the federal government. The final outcome of this group was the Boulder Canyon Act, which provided authorization and funding to "construct, operate, and maintain a dam and incidental works on the main stream of the Colorado River at Black Canyon or Boulder Canyon adequate to create a storage reservoir with a capacity of not less than 26,000,000 acre-feet of water." In addition to construction of the dam itself, funding was provided to equip the structure with electric generating facili- ties to recover the cost of the dam and a canal to transport the water to designated locations. The total funding amounted to $165,000,000. The conditions of the Boulder Canyon Act, including the distribution of water and electricity, were finally satisfied and on June 25, 1929, President Herbert Hoover signed a procla- mation that construction of the dam could begin. The Dam The name assigned to the proposed dam was originally Boulder Dam in recognition of its intended location—even though that is not where the dam ended up! There were sub- stantial surveys and geological investigations conducted with seven selected potential sites. Five sites were at Boulder Canyon and two sites at Black Canyon. Following extensive engineering studies and rock drilling at all the sites, the two sites at Black Canyon were selected as the final choices. There were several important reasons why the Boulder Canyon sites were deemed unfavorable— accessibility, rock stability, strength, depth, and the volume of concrete required to build the dam in Boulder Canyon. Various types of dams were evaluated for the Boulder Dam. Among these were an earth- and rock-filled dam, a rock-filled dam with a concrete face, a concrete-arch dam, a combination multiple- arch and gravity dam, a conventional gravity dam, and a concrete dam in the form of an arch. The final type of dam selected was a gravity- arch dam. This style pro- vided the greatest benefit from the standpoint of safety as the dam would be able to take advantage of the best attributes of both types. As an engineer, I am impressed and somewhat in awe of the precautions that were taken in the design of this dam and the careful attention to detail that was undertaken by the engineering staff. There are typically two potentially dangerous aspects of a dam: sliding along its base and overturning due to the water pressure exerted on its upstream face. The design engineers recognized these potential failure modes and constructed certain features to combat them. For one, the dam is locked into the rock faces on both sides through the action of the arch. This ensures that as the water level rises, the force of the water is transferred into the rock walls. To overcome the risk of sliding, the base of the dam was built at 660 feet, almost the height of the dam itself! This provides the mass needed to resist sliding along the base. In addition to the base length of the dam, an extensive cur- tain drain was installed under the base to preclude seeping of water under the dam that could result in both of the conditions cited above. Following the completion of the dam, it was actually dis- covered the underdrain system had been installed improperly. It was too shallow and did not penetrate the rock base suffi- ciently to gather the water seeping under the dam in some spots, while the grout used was inadequate to seal off the seeping in other locations. These conditions could have potentially resulted in the fail- ure of the dam, but the dam engineers recognized the situation and implemented revisions to correct it. Interestingly, the repair of the underdrain system required more than a decade, twice as long as the original construction itself! The dam was built by Six Companies Inc., a consortium of six large construction firms from the west coast and was placed into service by President Franklin Roosevelt in Sep- tember of 1935, two years ahead of schedule. ENGINEERING continues on page 62 WWJ November 2015 61 Twitter @WaterWellJournl View from the Hoover Dam power- house up the canyon with the power lines.