earth dams
Introduction
Loose material dams are "Dam (hydraulic)" dams that are formed by rocks or loose uncemented earth.[2] To achieve the impermeability of the dam, impermeable screens of clay, asphalt or some synthetic material are built. They are preferably used when the site where the dam rests does not resist the loads that a gravity or arch dam could apply to it. They are usually used to take advantage of the materials available on the site.
Materials
The criterion used to choose a type of loose materials over a concrete one lies either in the poor quality of the natural foundation of the land (low bearing capacity) or in the fact that it is more profitable to collect and treat (crushing and classifying) the local material to configure the dam, than to manufacture the concrete with similar intentions. In any case, the particular case must be subjected to an in-depth analysis that includes both the characterization of the geological and geotechnical properties of the environment, as well as other factors, among which the following stand out: quality of indigenous materials, possibility of installing a stone crushing plant, transportation distances, environmental sensitivity, etc.
Loose material dams can be built from almost any material with rudimentary construction equipment. Earthen dams have been successfully constructed using gravel, sand, silt, rock dust and clay. If a large amount of permeable material such as sand and gravel is available and clay material has to be imported, the curtain would have a small heart or core of impermeable clay and the local material would constitute the bulk of the curtain.
Concrete has been used as a waterproof core, but it does not offer the flexibility of clay materials. If there is no permeable material, the curtain can be constructed of clay materials with bottom drains of imported sand and gravel below the downstream baseline to collect seepage and reduce pore pressures.
Stability
The slopes of an earth curtain are rarely greater than 2 horizontal to 1 vertical and are usually around 3 to 1. The usual criterion is the stability of the slopes against strike-slip failure.
Stability under the action of seismic forces is especially critical. For soils in which pore pressure changes form as a result of constant stress deformations induced by an earthquake, the determination of appropriate values for strain acceleration is very difficult. For some soil types, displacements do not occur over a wide range of accelerations.