Gravity dams serve the same purposes as arch and buttress dams; however, they differ in structure and their method of retaining water. This type of dam is solid and triangular in shape; therefore, it requires a large amount of concrete or other construction material. The immense weight of the concrete provides stabilization and allows the dam to maintain control of the water. As you can see, gravitational force holds the dam to the ground...hence the name.
I. Forces acting on gravity dams
The forces acting on gravity dams include the force of the water in the reservoir (acting horizontally and vertically), the weight of the concrete (acting in a downward direction), and the uplift force that results from the water pressure under the dam pushing in an upward direction (a result of the buoyant force of water) if there is not a drainage source. The overall force acts one-third of the way from the bottom of the dam, which is its center of gravity.
II. Pressure acting on the dam
Gravity dams must be able to withstand the pressure of the reservoir water. As a result of the triangular shape of the dam, the pressure of the water is also distributed in the shape of a triangle. The amount of pressure on the dam increases proportionally to the depth of the water; therefore, the pressure is at a maximum at the bottom of the reservoir. A portion of the weight of the dam is cancelled by the water pressure resulting from the uplift force, so if the dam is not properly constructed, it could be uplifted and overturned.
III. Energy Production
As far as energy production is concerned, gravity dams are also used in the generation of hydroelectric power by the conversion of the potential energy of the water into mechanical energy.