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Expansive clay
Expansive clay or expansive soil is a clay or soil that is prone to large volume changes (swelling and shrinking) that are directly related to changes in water content.〔(Hobart king, "Expansive Soil and Expansive Clay: The hidden force behind basement and foundation problems". Geology.com. Accessed March 19, 2015. )〕 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, 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.〔(Science.gov clay related articles )〕
Mitigation of the effects of expansive clay on structures built in areas with expansive clays is a major challenge in geotechnical engineering. 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. A laboratory test to measure the expansion potential of soil is ASTM D 4829.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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