Raw Materials

The materials used to construct fill dams include soil and rock. Soil is classified by particle size from the smallest, submicroscopic particles called clay; silt, which is also very fine; sand ranging from fine to coarse, where the fine grains are the smallest soil particles our eyes can see; and gravel. Coarser fragments called cobbles and boulders are also used in dam construction but usually as protective outer layers.
Specific soil types and size ranges are needed to construct the zones within the dam, and explorations of the dam foundation area, the reservoir where the water will be stored, and surrounding areas are performed not only for design of the dam but to locate construction materials. The costs of fill construction rise dramatically with the distance materials are hauled. Samples of potential construction materials are tested in a soil laboratory for grain size, moisture content, dry density (weight), plasticity, and permeability. Clay is not only very fine in size but has chemical characteristics that cause it to stick together. The combination of fine size and plastic behavior also causes the clay to be less permeable to water. If clay is available near the site, the dam can be built with an impermeable core or central zone that prevents water from passing through the dam; otherwise, the dam must be designed so water can seep slowly and safely through a different combination of materials in its zones.
Water is also a raw material. The various soil types have compaction characteristics that can be determined in the laboratory and used during construction. Soil can be compacted to its best functional density by adding moisture and weight and impact, called compactive effort. Large vibrating rollers press thin layers of soil into place after an optimal amount of water has been added. The water and weight bond the soil particles together and force smaller particles into spaces between larger particles so voids are eliminated or made as small as possible to restrict seepage.
Increasingly, fill dams also include geotextiles and geomembranes. Geotextiles are nonwoven fabrics that are strong and puncture-resistant. They can be placed between lifts as the dam is raised to strength weak materials. They are also used as filter fabrics to wrap coarser drain rock and limit the migration of fine soil into the drainage material. Geomembranes are made of high-density polyethylene (HDPE) plastic and are impermeable. They can be used to line the upstream face of a fill dam or even to line the entire reservoir.


Read more: How fill dam is made - material, making, history, used, processing, dimensions, product, History, Raw Materials, Feasibility and Preliminary Design, Design, The Construction Process, Quality Control http://www.madehow.com/Volume-5/Fill-Dam.html#ixzz1MP7pWxn8