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Strength and Stability of Dams

Consider a concrete gravity dam with a vertical upstream face, a common design for this type of dam. The pressure of the water stored in the reservoir acts equally in all directions (hydrostatic pressure); because the dam's upstream face is vertical the water pressure on the dam wall will act in the horizontal direction and will tend to push the dam wall downstream. The weight of the dam wall acts vertically downwards. In order for a dam to be stable and therefore safe the design engineers and engineering geologists must make sure that;

  • the foundation rock mass on which the dam is built must have adequate strength (bearing capacity) to support the dam wall, especially in the saturated state;
  • the dam must be able to resist the tendency of gravity dams to overturn about the downstream toe at the foundation;
  • the dam must be able to resist the tendency to slide downstream under the pressure from the water in the reservoir.
  • uplift pressures (water pressure acting vertically upwards in horizontal cracks within either the dam wall or the foundation rock) are taken into account in assessing the stability of the dam because these pressures tend to destabilise the dam. In early gravity dams uplift pressures were not allowed for during the design.

The strength of a dam depends on the materials used to built it. A gravity dam is proportioned so that its own weight resists the forces acting on it.

Gravity dams built by the Ancient Romans had a base width to height ratio of about 3 whereas, in modern concrete gravity dams, this ratio is significantly less than 1 eg Warragamba Dam has a base width to height ratio of 0.8. To achieve the same level of stability an embankment dam, built from broken rock and clay, must be very much wider at the base than a dam of the same height built out of concrete eg Windamere Dam, an earth and rock fill dam, has a base width to height ratio of 3.7.

An arch dam is much thinner than a gravity dam: it contains less concrete and is therefore cheaper to build but the weight of the dam wall is not enough, on its own, to resist the forces acting on the dam. Concrete arch dams rely on the strength of the rock masses forming the bottom and sides of the valley in which they are built to help resist the forces acting on them. It is very important that a good, high strength foundation be found for any proposed arch dam.

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