A gravity dam is a dam built of concrete or stone masonry and designed to retain water primarily by using the weight of the material only to withstand the horizontal pressure of the water pushing against it. Gravity dams are designed so that each section of the dam is stable, independent of any other prey section.
Gravity dams generally require high rolling resistance (slightly weathered to cool) hard rock foundations; Although they have been built on ground foundations in rare cases. The supporting strength of the foundation limits the permissible position of the resultant which influences the overall stability. Also, the rigid nature of the structure of the dam of gravity is implacable to the differential establishment of the foundation; It can induce cracking of the dam structure.
Gravity dams provide some advantages over embankment dams. The main advantage is that they can tolerate minors on the topping flows, since the concrete is resistant to washing. Large drainage flows remain a problem, as they can traverse the foundations if they are not accounted for in the design. A disadvantage of gravity dams is that because of their large footprint, they are susceptible to elevation pressures that act as a destabilising force. Lifting pressures (buoyancy) can be reduced by internal and bottom drainage systems, which reduces pressures.
During the construction, the concreting produces an exothermic reaction. This heat expands plastic concrete and can take up to several decades to cool. During cooling, the concrete is in a rigid state and is susceptible to cracking. It is the designer's job to ensure this does not happen.
The most common classification of gravity dams is by the materials that make up the structure:
• Concrete dams include
• damp concrete dams, made of:
• Conventional concrete: Dworshak dam, Grand Coulee dam
• Roll Compacted Concrete (RCC): Willow Creek Dam (Oregon), Stillwater Upper Dam
• Masonry: Pathfinder Dam, Cheesman Dam
• hollow concrete prey, reinforced concrete: Braddock dam
Compound 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 (form):
• Most gravity dams are straight (Grand Coulee Dam).
• Some concrete and concrete dams have the curved dam axis (Shasta Dam, Cheesman Dam) to increase stability through arc action.
Gravity dams can be classified with respect to their structural height:
• Low, up to 100 feet.
• High medium, between 100 and 300 feet.
• High, more than 300 feet.