Cohesion is the shear strength or the force that binds together like particles in the structure of a soil. This force exists without any compressive stress. Tests are often done on land sites to determine the soil's cohesiveness before building occurs.
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The types of soil that have cohesive properties have a high clay content. These soils usually have fine grains, do not crumble and are plastic when moist. Cohesive soils are difficult to break apart when dry. Examples of cohesive soils are claylike silt, sandy silt, silty clay and organic clay.
When added to soils, including those with sandy content, water plays a vital role in soil cohesion because of its surface tension. That property provides a weak bond among the soil grains to cause cohesion. Minerals such as salt and caliche can add to a soil's cohesive properties. Long-term pressure can also make soil cohesive.
Although water plays a big role in cohesion, too much of it actually causes soil to lose that property. Soil loses its plasticity and becomes nearly fluid in its mechanics.
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- Library 4 Farming: Soil Part 1 - Principles
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