How to Use
Reading 2: A Short Course on Dam Building
In 1938, a New York publisher put out an illustrated “cartoon guide” for tourists visiting Hoover Dam, then known as Boulder Dam. The guide included the instructions excerpted below:
15 Minute Course in Engineering
LESSON NO. 1
LESSON NO. 2
And you will need to build some roads and erect a power line. At Boulder Dam they had to get the electricity from Los Angeles to build the dam which now sends electricity back to Los Angeles.
LESSON NO. 3
LESSON NO. 4
When you’ve got ‘em ready, dump a bunch of rock and dirt in the river, just below the upper end of the tunnels to block off the river. Make this “cofferdam” about a hundred feet high, two blocks thick at the base and 70 feet thick on top.
At the other end of the tunnels build another cofferdam to keep the water from backing up, pump the puddles out from between, and there you have it—a dry spot in the river, with the stream running right around it, through the tunnels. Simple?
LESSON NO. 5
LESSON NO. 6
LESSON NO. 7
Concrete has a nasty habit of cracking. The lime in cement causes it to get hot when it is mixed with water. As it dries and cools, it shrinks and that’s what causes the cracks. Obviously you must not allow this to happen when you are building a dam. If cracks were to appear in your dam it might cause severe criticism from people living below the dam—or from their heirs.
It would take 150 years for all that concrete in the dam to cool under normal conditions. To hurry it up, string two or three miles of water pipe around through each five-foot layer of concrete as you pour it. From your ice plant, run ice cold brine through the pipes. This will cause the mass to cool and shrink quickly. You can then pump a very thin cement “soup” into cracks and spaces that have opened up. Your dam will be sealed tighter than a drum—and wedged between the canyon walls.
LESSON NO. 8
LESSON NO. 9
This completes our engineering course.
Questions for Reading 2
1. This guide contains many large numbers. Can you think of ways to translate those numbers into comparisons with more familiar things, so that it would be easier to get a sense of how big the project was?
2. The diversion tunnels were among the longest in the world when they were built. Six Companies thought that building them was the most critical part of the project. Under their contract with Reclamation, they would have to pay $3,000 for every day they exceeded the deadline for finishing this part of the work. Why do you think both Reclamation and Six Companies thought the diversion tunnels were so important? Do you agree? Explain your answer.
3. Why was it important to speed up the cooling process? How long would it take for the concrete in the dam to cool by itself? (The pipes are still there, buried in the concrete.)
4. The spillways at Hoover Dam are only used when the water in the reservoir is so high that it would otherwise overflow the top of the dam. Why do you think the engineers thought they had to keep this from happening?
5. In Lesson 1, the guide suggests, possibly not quite seriously, that you could take any canyon on the Colorado you wanted—"no one will miss it, maybe." There are many canyons on the river, some almost as dramatic as the Grand Canyon, located just upstream from Hoover Dam's reservoir. What might people "miss" about these canyons if they were filled with water? Do you think anybody today would suggest that no one might notice if you flooded one of them? Why or why not?