Gravity dam
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. 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, although in rare cases, they have been built on soil.Stability of the dam primarily arises from the range of normal force angles viably generated by the foundation. Also, the stiff nature of a gravity dam structure endures differential foundation settlement poorly, as it can crack the dam structure.
The main advantage of gravity dams over embankments is the scour-resistance of concrete, which protects against damage from minor over-topping flows. Unexpected large over-topping flows are still a problem, as they can scour dam foundations. A disadvantage of gravity dams is that their large concrete structures are susceptible to destabilising uplift pressures relative to the surrounding soil. Uplift pressures can be reduced by internal and foundation drainage systems.
During construction, the exothermic curing of concrete can generate large amounts of heat. The poorly-conductive concrete then traps this heat in the dam structure for decades, expanding the plastic concrete and leaving it susceptible to cracking while cooling. 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. Three different tests 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.Classifications
The most common classification of gravity dams is by the materials composing the structure:- Concrete dams include
- * mass concrete dams, made of:
- ** conventional concrete: Dworshak Dam, Grand Coulee Dam
- ** Roller-Compacted Concrete : Willow Creek Dam, Upper Stillwater Dam
- * masonry: Aswan Low Dam, Pathfinder Dam, Cheesman Dam
- * hollow gravity dams, made of reinforced concrete: Braddock Dam
- Most gravity dams are straight.
- Some masonry and concrete gravity dams have the dam axis curved to add stability through arch action.
- Low, up to 100 feet.
- Medium high, between 100 and 300 feet.
- High, over 300 feet.
Earthquakes