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 (RCC): Willow Creek Dam (Oregon), Upper Stillwater Dam masonry: Pathfinder Dam, Cheesman Dam 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 (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 Gravity dams are built to withstand some of the strongest earthquakes. 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 Dams by type