I was going over in my head how to explain something to non-geotechnical engineers when I realized I didn't understand it myself (embarrassing). Non-plastic silts are, imho, the worst construction material, weak, erodible, subject to collapse - assuming you won't use peat as a construction material. That got me to wondering what the physical explanation was for why some silts are plastic (i.e., why they hold onto water). Which brings me to my question, if silts are plastic, why aren't they classified as clays? Kaolinite, a clay, for example, can classify as either a low plasticity or high plasticity silt (generally plotting just below the A line). Terzaghi & Peck divide silts into inorganic (rock flour, non-plastic) and organic (plastic), but don't say why organic silts are plastic. Lambe and Whitman divide clays from silts at 0.002 mm (No. 200 sieve is 0.074 mm) which is unsatisfying as it tells you nothing about the material behavior. I have a feeling that an MH soil with a LL of 90 and a PI of 40 probably isn't dilatent (does anyone know?). Should we really classify low and high plasticity silts as another variety of clay and only classify non-plastic fines as silt?
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