How Dams Are Built

The methods of building dams can be envisioned by following the construction of Hoover Dam, built between 1930 and 1935. The engineers constructed a concrete arch-gravity dam at an approved cost of $174,000,000. It is as tall as a 60-story skyscraper. Its crest is 45 feet (14 meters) thick and its base, 660 feet (201 meters). It stores the entire flow of the Colorado River for two years. Much preliminary work had to be done. The engineers made geologic and topographic surveys to select the site. They made maps of 70 locations, bored holes to test the rock for a sound foundation, and studied the river's speed, high water level, and silting. Once the location was chosen, designers made their plans. They then made models to test their design. Where once had been burning desert, engineers built Boulder City to house about 5,000 workers. Construction gangs built railroads and highways for transporting great quantities of equipment and materials. Workmen strung cables across the canyon from pairs of towers, which travel on tracks along opposite sides of the site. Each of the five cableways could carry 25 tons (22,680 kilograms). Two of them had spans of nearly half a mile. Construction crews also built a great gravel screening plant and two huge concrete mixing plants.

On each side of the river two tunnels, each 50 feet (15 meters) in diameter, were drilled and blasted from the rock of the canyon walls. These tunnels were used to divert the river around the site. When construction was completed the tunnels served as spillway outlets and penstocks for the power plant. Next, cofferdams of earth and rock were built upstream and downstream from the dam site to block the river. "High scalers" stripped tons of loose and projecting rock from canyon walls. The overburden, or loose rock and muck, was dug out to expose the bedrock. Grout, a thin mortar of cement and water, was next forced into the foundation to fill seams and holes. Forms were made. These would be used for building the dam in enormous blocks. Concrete was poured into the forms from eight-cubic-yard buckets traveling on the cableways. As each block of concrete dried, grout was pumped between the blocks, making the dam into one solid piece. Allowing so gigantic a structure to cool naturally would have taken a century because of the heat given off by the setting cement. In addition the concrete would have shrunk and cracked. Cold water circulating through 528 miles (850 kilometers) of one-inch (25-mm) pipes embedded in the concrete carried off the heat. Refrigerating pipes were also used to freeze landslides of wet earth at Grand Coulee Dam. Another problem was the freezing of control gates in winter. A huge electrical heating apparatus was installed in the spillway and gates.

0 0

Post a comment