How to Use the Images  Inquiry QuestionReadings Photo 1 and 2 Photo 3 Photo 4 Photo 5 Activities Table ofContents Visual Evidence Photo 6: Laying the Concrete for the Dam (Bureau of Reclamation; Ben Glaha, photographer) Photo 7: The Dam Under Construction (Bureau of Reclamation; photographer unknown) Hoover Dam was not a solid block of concrete, but rather a jigsaw puzzle of individual blocks, called "lifts," measuring 25 to 60 feet to a side and no more than five feet in depth. Construction proceeded in a series of blocks because the volume of concrete being poured at one time had to be limited to prevent excessive heat and cracking. The liquid concrete was poured into a wooden framework called a "form" that held the concrete in place as it hardened. When a section had cooled and hardened sufficiently, the form was dismantled and moved up and the next five-foot high layer was poured. Eventually the blocks formed columns as tall as the dam was high, interlocking like a giant Lego set. Finally, highly liquid cement was poured into the spaces between the columns to bond them together into a single massive structure. Questions for Photos 6 and 7: 1. Study Photos 6 and 7 carefully. Why do you think it was necessary to follow this complicated procedure? 2. Why do you think the blocks were different sizes and shapes? Refer to Illustration 1, if necessary. 3. Buckets like the one shown in Photo 6 carried liquid concrete to the forms. How big do you think the bucket was, based on the size of the men standing near it? Can you find the pulley that carried it? The pulleys ran on a system of cables that spanned the canyon. The system allowed buckets to move up and down as well as from side to side. How many buckets and pulleys can you find in Photo 7? 4. After a bucket was lowered to the form, the bottom opened and the concrete poured out. Workers spread the concrete evenly and worked out air pockets. Examine the photos for evidence of these processes. What do you think the men are doing in Photo 6? Would you feel comfortable standing that close to a bucket of concrete that large? 5. How important do you think a fast and reliable system to deliver concrete was in building the dam? Think about what was involved in preparing concrete and getting it down into the canyon, and the problems that would occur if there were delays. A cable system was the "tired and true" method used on dams for decades. Can you think of other ways Six Companies could have gotten the concrete to where it needed to be without a cable system? If so, compare the strengths and weaknesses of each option. 6. How many workers can you find in the photos (the ones in Photo 7 are hard to see because they are so small)? What kind of protective clothing are they wearing? Based on everything you have learned so far, what do you think working conditions would be like doing this job? What dangers would these workers face? Click here for a larger version of Photo 6 and Photo 7.