The weight of the gravity dam resists the horizontal force of the water pushing against it. Some early ones were built of stone, but-today they are made of concrete. The pressure of the water tends to tip the dam over, lifting the bottom of the dam, if it is not heavy enough.
Concrete can withstand great pressure, but it is not so strong if it is being pulled or stretched in any way. Dams are built in a way that minimizes any tension (pulling force) by ensuring that the pressure of the water and the weight of the dam combine to make a force that falls on the middle third of its base.
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.
Picture Credit : Google