A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by using only the weight of the material and its resistance against the foundation to oppose the horizontal pressure of water pushing against it. Gravity dams are designed so that each section of the dam is stable and independent of any other dam section. Characteristics Gravity dams generally require stiff rock foundations of high bearing strength (slightly weathered to fresh), although in rare cases, they have been built on soil foundations. The bearing strength of the foundation limits the allowable position of the resultant force, influencing 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 without damage, 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 destabilising force. Uplift pressures (buoyancy) can be reduced by internal and foundation drainage systems. During construction, the setting concrete produces an exothermic reaction. This heat expands the plastic concrete and can take up to several decades to cool. While cooling, the concrete is stiff and susceptible to cracking. It is the designer's task to ensure this does not occur. Design 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. There are three different tests that can be done to determine the foundation's support strength: the Westergaard, Eulerian, and Lagrangian approaches. Once the foundation is suitable to build