Dams are among the oldest structures built by humans for collective use. A dam is a barrier that is constructed across a river or stream so the water can be held back or impounded to supply water for drinking or irrigation, to control flooding, and to generate power. The main kinds of dams are earth fill, rock fill, concrete gravity, concrete arch, and arch gravity. The last three types are all made of concrete, reinforced concrete, or masonry. (The term masonry can mean concrete, bricks, or blocks of excavated rock.) Fill dams include all dams made of earth materials (soil and rock) that are compacted together. One type of fill dam called a tailings dam is constructed of fine waste that results from processing rock during mining; at mine sites, this soil-like waste is compacted to form an embankment that holds water for the mining and milling processes or to retain the tailings themselves in water. Of the main categories of dams listed above, all have been built since ancient times although many refinements were developed in the nineteenth and twentieth centuries with improved engineering technology. Dams that leak have failed to do their job, either because they simply can't hold water or because the water seeping through them eats materials away from the inside of the dam causing it to fail structurally. In modern times, most fill dams are also built with zones including a clay center or core, filter and drainage layers, coarser materials sandwiching the clay core, and rock on the upstream (water) face to prevent erosion. These zones can be seen clearly when a cross section is cut from the upstream to the downstream side of the dam. All fill dams depend on weight to remain stable. Fill embankments are usually less expensive to construct than concrete dams. Soil or rock are present at the site, and construction techniques, though complex, are also less costly than for concrete construction. For these reasons of available materials, low cost, and stability with mass, fill dams are often built across broad water courses. They also are more flexible than concrete structures and can deform without necessarily failing if foundation materials under the dam compress with the weight of the dam and the water. History
Quite naturally, early dam builders began by using plentiful materials like sand, timber and brush, and gravel. Their construction method consisted of carrying the materials by the basketful and loosely dumping the fill, so many of these dams may have survived only a few years. Scientists have not been able to pinpoint dates for the earliest dam construction, but they do know dams were needed where food was grown and in areas prone to flooding. Design of fill dams is based on experience; while failures are unfortunate and sometimes catastrophic, they are also the best teachers, and many engineering advances have been founded on careful study of earlier failures. The engineers of ancient India and Sri Lanka were the most successful pioneers of fill dam design and construction, and remains of earth dams can still be seen in both countries. In Sri Lanka, long embankments called tanks were built to store irrigation water. The Kalabalala Tank was 37 mi (60 km) long around its perimeter. The most famous earth fill dam recently constructed is the Aswan High Dam that was built across the Nile River in Egypt in 1970-1980. An earth fill dam was also the victim of a spectacular failure in June 1976 when the Teton Dam in Idaho eroded from within due to incorrect design of the zones inside the dam that allowed seepage, failure, and flooding of the valley downstream. Although earth dams tend to be short and broad, Nurek Dam in Tajikistan is 984 ft (300 m) high. Raw Materials
The materials used to construct fill dams include soil and rock. Soil is classified by particle size from the smallest, submicroscopic particles called clay; silt, which is also very fine; sand ranging from fine to coarse, where the fine grains are the...
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