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Earthen dam

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Earthen dam



An embankment dam is a massive artificial water barrier. It is typically created by the emplacement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock. It has a semi-permanent waterproof natural covering for its surface, and a dense, waterproof core. This makes such a dam impervious to surface or seepage erosion.[1] The force of the impoundment creates a downward thrust upon the mass of the dam, greatly increasing the weight of the dam on its foundation. This added force effectively seals and makes waterproof the underlying foundation of the dam, at the interface between the dam and its stream bed.[2] Such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than by the use of a cementing substance.[3]

Types

Embankment dams come in two types: the earth-filled dam (also called an earthen dam or terrain dam) made of compacted earth, and the rock-filled dam. A cross-section of an embankment dam shows a shape like a bank, or hill. Most have a central section or core composed of an impermeable material to stop water from seeping through the dam. The core can be of clay, concrete or asphalt concrete. This dam type is a good choice for sites with wide valleys. Since they exert little pressure on their foundations, they can be built on hard rock or softer soils. For a rock-fill dam, rock-fill is blasted using explosives to break the rock. Additionally, the rock pieces may need to be crushed into smaller grades to get the right range of size for use in an embankment dam.[4]

Safety

The building of a dam and the filling of the reservoir behind it places a new weight on the floor and sides of a valley. The stress of the water increases linearly with its depth. Water also pushes against the upstream face of the dam, a nonrigid structure that under stress behaves semiplastically, and causes greater need for adjustment (flexibility) near the base of the dam than at shallower water levels. Thus the stress level of the dam must be calculated in advance of building to ensure that its break level threshold is not exceeded.[5]

Overtopping or overflow of an embankment dam beyond its spillways will cause its eventual failure. The erosion of the dam's material by an overtopping surface runoff will remove masses of material whose weight holds the dam in place against the hydraulic forces acting to move the dam. Even a small sustained overtopping surface flow can remove thousands of tons of overburden soil from the mass of the dam within hours. The removal of this mass, unbalances the forces that stabilize the dam against its impoundment. The mass of water still impounded behind the dam presses against the lightened mass of the embankment, (made lighter by surface erosion). As the mass of the dam erodes, the impoundment begins to move the entire structure. The embankment, having almost no elastic strength, begins to break into separate pieces, allowing the impounded water to flow between them, eroding and removing more material as it passes through. In the final stages of failure the remaining pieces of the embankment offer almost no resistance to the flow of the water as they continue to fracture into smaller and smaller sections of earth and/or rock. The overtopped earth embankment dam disintegrates into a thick mud soup of earth, rocks and water.

Therefore safety requirements for the spillway are high, and require it to be capable of containing a maximum flood stage. It is common for its specifications to be written such that it can contain a five hundred year flood.[6] Recently a number of embankment dam overtopping protection systems have been developed.[7] These techniques include the concrete overtopping protection systems, timber cribs, sheet-piles, riprap and gabions, reinforced earth, minimum energy loss weirs, embankment overflow stepped spillways and the precast concrete block protection systems.

The earthen dam at Lake Elsman was damaged in the 1989 Loma Prieta earthquake.[8] The performance of Austrian Dam during that earthquake has reinforced concerns about damage to the tops of earth dams by earthquakes.[8]

See also

Notes

External links

  • Embankment dams Table of contents
  • An introduction to embankment dams
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