Expansive clay is a
clay soil that is prone to large volume changes (swelling and shrinking) that are directly related to changes in
water content. Soils with a high content of expansive minerals can form deep cracks in drier seasons or years; such soils are called
vertisols. Soils with
smectite clay minerals
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay mineral ...
, including
montmorillonite and
bentonite, have the most dramatic
shrink-swell capacity.
The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. All clays consist of mineral sheets packaged into layers, and can be classified as either 1:1 or 2:1. These ratios refer to the proportion of tetrahedral sheets to octahedral sheets. Octahedral sheets are sandwiched between two tetrahedral sheets in 2:1 clays, while 1:1 clays have sheets in matched pairs. Expansive clays have an expanding crystal lattice in a 2:1 ratio; however, there are 2:1 non-expansive clays.
Mitigation of the effects of expansive clay on structures built in areas with expansive clays is a major challenge in
geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It als ...
. Some areas mitigate foundation cracking by watering around the foundation with a soaker hose during dry conditions. This process can be automated by a timer, or using a soil moisture sensor controller. Even though irrigation is expensive, the cost is small compared to repairing a cracked foundation. Admixtures can be added to expansive clays to reduce the shrink-swell properties, as well.
[Biswas, Rajdip, and Nemani Kriscna]
“Effect of fly ash on strength and swelling aspect of an expansive soil.“
Department of Civil Engineering National Institute of Technology. Visited on November 19th, 2015.
One laboratory test to measure the expansion potential of soil is ASTM D 4829.
See also
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Argillipedoturbation Argillipedoturbation, sometimes referred to as self-mulching, is a process of soil mixing caused by the shrinking and swelling of the smectite clays contained in the soil. It is an effect specific to soils of the vertisolic variety, and is triggered ...
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Dispersion (soil)
Dispersion is a process that occurs in soils that are particularly vulnerable to erosion by water. In soil layers where clays are saturated with sodium ions ("sodic soils"), soil can break down very easily into fine particles and wash away. This ca ...
References
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Types of soil
Soil mechanics
Soil physics
Sediments