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5.1.1. General
Clay can have a number of meanings in engineering geology. The term can relate to mineralogy, implying an assemblage of clay minerals; to size, implying an assemblage of clay sized particles (in British engineering terminology < 0.002 mm); or to behaviour, implying a material which contains a sufficiently high clay size/mineral content to influence the material properties.
In traditional litho-stratigraphic nomenclature the term ‘clay’ is used loosely to refer to fine-grained deposits. These may comprise various constituents, including clay minerals, which have been derived from the weathering of the less stable components of the original rock, fine particles of quartz, feldspar and mica, and diagenetic minerals which have developed during the deposition, burial and uplift history of the deposit.
Broadly, the important clay mineral groupings are kaolinite, smectite and illite (Chapter 2) and one or more of these groupings tend to dominate a particular deposit. This is because the character of the clay mineral found in any particular soil depends on factors such as parent material, climate, topography, vegetation, and the length of time over which these factors have operated. In other words particular deposits form or are modified under specific environmental influences. Perhaps the most dominant of these are climate and time.
5.1.2. Climate
In tropical regions the relatively rapid chemical weathering associated with high annual temperature and high annual rainfall contributes to the formation of residual clay soils whose mineralogy is controlled by local environmental influences such
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