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, independent of any other dam section.
Gravity dams generally require stiff rock foundations of high bearing strength (slightly weathered to fresh); although they have been built on soil foundations in rare cases. The bearing strength of the foundation limits the allowable position of the resultant which influences 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 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 which reduces the pressures.
During construction, the setting concrete produces a exothermic reaction. This heat expands the plastic concrete and can take up to several decades to cool. When cooling, the concrete is in a stiff state and is susceptible to cracking. It is the designer's task to ensure this does not occur.
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.
- Kollgaardand, E.B.; Chadwick, W.L. (1988). Development of Dam Engineering in the United States. US Committee of the International Commission on Large Dams.
- Dams of the United States - Pictorial display of Landmark Dams. Denver, Colorado: US Society on Dams. 2013.
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