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A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by primarily using the weight of the material alone to resist 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.
Gravity dams are useful for a number of reasons. In winter and spring, gravity dams help control the flow of melting snow in a river, and in summer they store water to provide it year-round to the neighboring land. Without gravity dams, snow and ice would build up in winter and eventually melt, flowing downhill to a lake or river or the ocean, leaving a trail of destruction. By the time all the snow has melted and reached flowing water, the towns and cities in its path are left with flooded and destroyed properties. Then, in summer, these cities will run short on water. A gravity dam helps in capturing the melted snow and rain in a large area, containing the water for use through the entire year as it slowly drains over the dam or is guided away through streams, ditches, or pipes, allowing controlled flow without destruction.
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 de-stabilising 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.
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 or stone blocks, and are built into a triangular shape to provide the most support.
The most common classification of gravity dams is by the materials composing the structure:
- Concrete dams include
Gravity dams can be classified by plan (shape):
- Most gravity dams are straight (Grand Coulee Dam).
- Some masonry and concrete gravity dams have the dam axis curved (Shasta Dam, Cheesman Dam) 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 and ecosystems
Gravity dams are built to withstand some of the strongest earthquakes. Even though, the foundation of gravity dams are 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 in 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 down stream 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 50–150 years, they need to be maintained and regularly replaced.
Another problem with gravity dams deal with ecosystems. Because the flow and amount of water changes when a dam is built, it generally has an impact on the area of the dam and everything afterwards. If water that normally flows two weeks out of the year in an area is now flowing constantly, new life is going to start living and growing there. Similarly, if you cut off water to somewhere that has water flow year round, things are going to start dying. Many environmentalist have problems with dams because of their effects on the environment.
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- Design of Small Dams, Bureau of Reclamation, 1987
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