Introduction to Architecture. Lesson 13: Hoover Dam

Transcription

1 Introduction to Architecture Lesson 13: Hoover Dam Back in 2010, my family and I flew to California and then drove to the Grand Canyon via the Hoover Dam. Named after President Herbert Hoover, the Hoover Dam spans the Colorado River and provides electricity to 20 million people in Arizona, Nevada, and California. Despite the fact that the country was more than a year into the Great Depression, with no end in sight, an idea was formulating in the minds of architects, businessmen, and government officials. The increasing population of the Southwest required a source of energy. So, in 1931, the Bureau of Reclamation chose to construct a dam in the Black Canyon area of the Colorado River for four reasons: 1. It was to help irrigate the Southwest.

2 2. It was to help control the Colorado River, which was often unpredictable, and reduce the flooding potential. 3. It could collect silt carried by the river. 4. It could generate a sizable amount of electricity. The Black Canyon has high walls, and the river is narrow, so a relatively small dam could be built here. By the time of its completion, it was 700 feet tall, more than 1,200 feet wide at its crest, and 660 feet thick at the base. It doesn t seem so small, but today it ranks as the 20 th tallest dam in the world and doesn t even make the top 10 list in terms of energy production. Until cities in Southern California agreed to purchase some of the electricity that would be created by the dam, construction was simply a dream. It didn t take long, however, before they were diverting the river so workers had a dry area in which to construct the dam. Four tunnels, each three quarters of a mile long and 56 feet in diameter were dug, and cofferdams were built by the end of (Cofferdams are temporary dams used to create a dry area in which to work.) Work then began on the walls of the canyon. Loose rock had to be cleared. This was the most dangerous job and was completed by men known as high scalers. They were tethered by a rope and used dynamite and jackhammers to clear the rock. ACTIVITIES: (*These activities are not optional for those interested in academic credit.) *1. Locate the Hoover Dam on your world map. You may have to use the Internet or an atlas to find the exact location. *2. The Hoover Dam, originally called the Boulder Dam, was completed in Mark this on your timeline. *3. Read more about high scalers. *4. One of these historic photos on the PBS website shows construction of one of the cofferdams (slide 16). Another slide (14) shows a steam shovel like the Bucyrus that was used to build the Panama Canal. By mid-1933, work on the actual dam was able to begin. Because of the scale of the project and the amount of concrete needed, two mixing plants were built at the site and a railway was built to transport that concrete. Do you remember the Panama Canal Expansion Project? They built mixing plants there, too, so that they wouldn t have to wait for concrete to be shipped or transported. Though it may seem like extra work and expense to build a mixing plant at the site, it saved an enormous amount of money and time. As the dam was being built, two major issues arose. First, as concrete is mixed, it creates a significant amount of heat. If allowed to cool too quickly, it tends to crack. The other issue was the size of the dam. There were to be 230 columns of concrete with dimensions of 25 x 60 feet. It would have taken 50 years for this amount of concrete to dry if left alone. Engineers devised a plan to accelerate the cooling process in such a way as to prevent cracking. Pipes were placed in the columns as they were poured, and ice cold water was pushed through them. Because of these pipes, the entire dam had completely cooled within 20 months of the last pouring in 1935.

3 ACTIVITIES: On display in the Hoover Dam Museum, this replica shows what the columns of concrete looked like during construction. *5. View some more historic photos. Be sure to look at Concrete Base for an actual photo of the columns. *6. This History.com video will provide an overview of construction. It is 4:09 long. Hoover Dam is an arch-gravity dam. It is a gravity dam in that it uses its own weight to resist the force of water against it. It was constructed with the arch shape for additional strength. Consider the design of an arched bridge or the arches in the Pantheon and the Coliseum. An arch provides stability. When the force of compression works against the top of the arch, the sides are pushed into the ground (against the abutments). In the case of the Hoover Dam, the abutments are in the solid canyon wall. It is believed, however, that the arch design was unnecessary because of the sheer mass of the concrete. When the Colorado River was dammed, Lake Mead Reservoir was created (the communities that existed were forced to evacuate). The water from this reservoir enters four 395-foot-tall intake towers, which can be closed. It then travels through steel pipes, penstocks, and on to either the powerhouses or one of two outlet buildings. These outlet buildings are to facilitate irrigation downstream. If the reservoir gets too high, two 500-foot-long spillways can be drained. The water level is a concern today, but it is not because it is getting too high.

4 Lake Mead Recreation Area (behind Hoover Dam) in May 2010 Intake Towers. The water used to lie just under the walkway that connects the towers.

5 Notice the water line. It has receded even more since The water flows through the pipes at a rate of 1.5 million gallons of water every second. When we took our tour of the dam, we stopped above one of the pipes. We could hear the rush and feel the rumble of the water as it passed through the pipe.

6 Our tour took us to one of the powerhouses. Between the two, there are a total of 17 generators, eight on the Nevada side of the river and nine on the Arizona side. Each of the generators can supply power to 100,000 homes. ACTIVITIES: *7. For an explanation of the science behind the hydroelectric power, watch this PBS Learning Media Video. You can stop at 2:27. The remainder of the video discusses environmental aspects of dam building, from the perspective of environmentalists. *8. Here is another explanation and an animation from the US Geological Survey (USGS).

7 Powerhouses

8 Penstocks for irrigation Powerhouses ACTIVITIES: *9. View this 5-minute History.com video about the construction. It includes clips from people who worked on it and an explanation of the name. *10. Take some time to view the historic photos and read the captions on this website. Hoover Dam was completed on September 30, It came in under budget at $49 million, which is $1 billion in today s money. Unfortunately, more than 100 people died during construction. Until October 19, 2010, there was only one way around the Hoover Dam across it. The winding roads and tourists made for an extremely long journey. In January 2005, construction began on the Mike O Callaghan Pat Tillman Memorial Bridge (Colorado River Bridge), the Hoover Dam Bypass. When we were there in May 2010, we were able to see some of the construction. I ve included a few pictures, since we just finished studying bridges.

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10 ACTIVITIES: *11. This site has a wonderful color diagram labeling the structures of the dam. If you are keeping a notebook, draw and label the structures.

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