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Gravity Dam

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The Grand Coulee Dam
A gravity dam[1] is a massive sized dam fabricated from concrete and designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it.[2] This is why it is called a gravity dam. Gravity essentially holds the dam down to the ground, stopping water from toppling it over.[3]

Gravity dams are well suited for blocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is key that they are built on a solid foundation of bedrock.[4] In fact, an earth rockfill dam is a gravity dam.

The one advantage of a gravity dam is its rather simple design, with most dams being a straight vertical wall across a valley or gorge way. However, gravity dams can also be designed curved, as in the Hoover dam.

Gravity dams are very durable and still highly preferred over buttress dams and arch dams. The one drawback is that gravity dams require a large amount of material and construction to build are therefore relatively expensive.[5] 

[edit] History

Gravity dams were the first type of dam ever to be constructed, and were made from stone bricks or concrete bricks. One of the first gravity dams on record was built by the Egyptians around 2950 to 2750 BC. The dam, called Sadd el-Kafara and meaning "Dam of the Pagans," was 121.4 feet (37 m) tall and 348 feet (106.1 m) wide at the crest. The materials used to build the dam included a rubble masonry type wall, and 10,000 tons of gravel and stone filler. A covering of limestone was applied to the dam to protect it from erosion. Due to shoddy and hasty workmanship however, the dam failed to be watertight eventually resulting in its erosion after only a a few years.[6]

Another rudimentary form of gravity dam was supposedly fabricated around 3000 B.C. in the town of Jawa, located in Jordan. These gravity dams were part of a very elaborate water supply system. The town satisfied its water needs by damming runoff to form reservoirs. A 15-foot (4.5-m) high and 263-foot (80-m) long gravity type dam impounded one of these storage reservoirs.[7]

The Romans built their own gravity dams in the Iberian Peninsula region, North Africa, and in the Middle East. A dam located at Homs, Syria in 284 A.D. impounded one of the largest reservoirs built by the Romans. The dam was 6,562 feet (2,000 m) in length and held back about 118 million cubic yards (90 million m3) of water.[8]

Around 100 A.D., the Romans became the first civilization to use concrete and mortar in the construction of gravity dams. One example was the dam at Ponti di San Mauro, fabricated out a large slab of concrete as evidenced by the great block of concrete found amongst its remains.[9]

Later gravity dams used a trapezoidal cross section. This cross section eventually evolved into the common triangular cross section design of today's gravity dams. The first version of a modern gravity dam was built between 1765 and 1800 in Mexico.[10]

In 1850, J. Augustin Tortene de Sazilly hypothesized that the most advantageous cross section for a gravity dam was triangular with a vertical upstream face.

By 1872, the use of concrete in the construction of gravity dams made a comeback for the first time since the Roman era. This was a result of the widespread use of Portland cement.[11]

With the development of Portland cement came the development and construction of giant, super gravity dams such as the Hoover dam that was nearly 60 percent higher than, and two and half times the size of, any other dam in existence. The amount of water this colossal gravity impounded was also record breaking -- 46,498 million cubic yards (38,550 million m3) of water.[12]

Today, gravity dams are still being fabricated out of concrete, but with the incorporation of post-tensioned steel, a new development in their construction. However, the construction of gravity dams piqued in the 1960s and the building of these massive structures has slowly been tapering off due to the high labor and construction costs involved. 

[edit] References

  1. http://blog.thecivilengg.com/types-of-dams. thecivilengg.com
  2. PBS. Dam Basics. Building Big, 2008-09-30.
  3. About Dams. The British Dam Society, 2008-09-30.
  4. About Dams. The British Dam Society, 2008-09-30.
  5. Gravity Dam, 2008-09-30.
  6. Yang, Heloisa; Haynes, Matt; Winzenread, Stephen; Okada, Kevin. The History of Dams, 1999. (accessed: 2008-09-30)
  7. Key Developments in the History of Gravity Dams. Cracking Dams, 2008-09-30.
  8. Key Developments in the History of Gravity Dams. Cracking Dams, 2008-09-30.
  9. Yang, Heloisa; Haynes, Matt; Winzenread, Stephen; Okada, Kevin. The History of Dams, 1999. (accessed: 2008-09-30)
  10. Key Developments in the History of Gravity Dams. Cracking Dams, 2008-09-30.
  11. Key Developments in the History of Gravity Dams. Cracking Dams, 2008-09-30.
  12. Key Developments in the History of Gravity Dams. Cracking Dams, 2008-09-30.