A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by using only the weight of the material and its resistance against the foundation to oppose the horizontal pressure of water pushing against it. Gravity dams are designed so that each section of the dam is stable and independent of any other dam section.

Characteristics

Gravity dams generally require stiff rock foundations of high bearing strength (slightly weathered to fresh), although in rare cases, they have been built on soil foundations. The bearing strength of the foundation limits the allowable position of the resultant force, influencing the overall stability. Also, the stiff nature of the gravity dam structure is unforgiving to differential foundation settlement, which can induce cracking of the dam structure. Gravity dams provide some advantages over embankment dams, the main advantage being that they can tolerate minor over-topping flows without damage, as the concrete is resistant to scouring. Large over-topping flows are still a problem, as they can scour the foundations if not accounted for in the design. A disadvantage of gravity dams is that, due to their large footprint, they are susceptible to uplift pressures which act as a destabilising force. Uplift pressures (buoyancy) can be reduced by internal and foundation drainage systems. During construction, the setting concrete produces an exothermic reaction. This heat expands the plastic concrete and can take up to several decades to cool. While cooling, the concrete is stiff and susceptible to cracking. It is the designer's task to ensure this does not occur.

Design

Gravity dams are built by first cutting away a large part of the land in one section of a river, allowing water to fill the space and be stored. Once the land has been cut away, the soil has to be tested to make sure it can support the weight of the dam and the water. It is important to make sure the soil will not erode over time, which would allow the water to cut a way around or under the dam. Sometimes the soil is sufficient to achieve these goals; however, other times it requires conditioning by adding support rocks which will bolster the weight of the dam and water. There are three different tests that can be done to determine the foundation's support strength: the Westergaard, Eulerian, and Lagrangian approaches. Once the foundation is suitable to build on, construction of the dam can begin. Usually gravity dams are built out of a strong material such as

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

or stone blocks, and are built into a triangular shape to provide the most support.

Classifications

The most common classification of gravity dams is by the materials composing the structure: * Concrete dams include **

mass concrete Mass concrete is defined by American Concrete Institute Committee 207 as "any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of cement and attendant volume change ...

dams, made of: *** conventional concrete:

Dworshak Dam Dworshak Dam is a concrete gravity dam in the western United States, on the North Fork of the Clearwater River in north central Idaho. In Clearwater County, the dam is located approximately northwest of Orofino and impounds the Dwors ...

Grand Coulee Dam Grand Coulee Dam is a concrete gravity dam on the Columbia River in the U.S. state of Washington, built to produce hydroelectric power and provide irrigation water. Constructed between 1933 and 1942, Grand Coulee originally had two powerho ...

*** Roller-Compacted Concrete (RCC):

Willow Creek Dam (Oregon) Willow Creek Dam is a dam in Morrow County of the U.S. state of Oregon, located just east of Heppner's city limits. It was the first major dam in the United States constructed of roller-compacted concrete. The dam's drainage basin is of ari ...

Upper Stillwater Dam Upper may refer to: * Shoe upper or ''vamp'', the part of a shoe on the top of the foot * Stimulant, drugs which induce temporary improvements in either mental or physical function or both * ''Upper'', the original film title for the 2013 found f ...

masonry Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term ''masonry'' can also refer to the units themselves. The common materials of masonry construction are bricks, building ...

Pathfinder Dam Pathfinder Dam is a masonry dam, located on the North Platte River, approximately southwest of Casper, Wyoming. It was originally constructed from 1905 to 1909 as part of the North Platte Project and has been modified several times since then. ...

Cheesman Dam Cheesman Dam is a masonry curved gravity dam on the South Platte River located in Colorado. It was the tallest of its type in the world when completed in 1905. The primary purpose of the dam is water supply and it was named for Colorado business ...

** hollow gravity dams, made of reinforced concrete: Braddock Dam Composite dams are a combination of concrete and embankment dams. Construction materials of composite dams are the same used for concrete and embankment dams. Gravity dams can be classified by plan (shape): * Most gravity dams are straight (

). * Some masonry and concrete gravity dams have the dam axis curved (

Shasta Dam Shasta Dam (called Kennett Dam before its construction) is a concrete arch-gravity dam across the Sacramento River in Northern California in the United States. At high, it is the eighth-tallest dam in the United States. Located at the north ...

) to add stability through arch action. Gravity dams can be classified with respect to their structural height: * Low, up to 100 feet. * Medium high, between 100 and 300 feet. * High, over 300 feet.

Earthquakes

Gravity dams are built to withstand some of the strongest

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

s. Even though the foundation of a gravity dam is built to support the weight of the dam and all the water, it is quite flexible in that it absorbs a large amount of energy and sends it into the earth's crust. It needs to be able to absorb the energy from an earthquake because, if the dam were to break, it would send a mass amount of water rushing downstream and destroying everything in its way. Earthquakes are the biggest danger to gravity dams and that is why, every year and after every major earthquake, they must be tested for cracks, durability, and strength. Although gravity dams are expected to last anywhere from 50–150 years, they need to be maintained and regularly replaced.

References

Bibliography

* * {{Authority control Dams by type