I’m currently studying for my FE Civil exam and going over Geotech material. When classifying soils using USCS, after you’ve determined the soil is a fine-grained soil, how can you know if the soil is organic or inorganic?
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There are a few parameters that you can use to judge whether a soil is organic or inorganic.
Organic soils have fairly lower bulk densities than inorganic soils (0.2 to 0.3 as compared to 1.2 to 1.5). Organic soils will feel significantly lighter.
Organic soils can retain way more water than inorganic soils (in proportions if their own densities).
The color of organic soils is usually darker than inorganic soils. But this is not a good parameter for comparison since the color is affected by other things as well apart from organic nature.
Cation exchange capacity (CEC) is higher in organic soils (4 times as that of inorganic soils). This means that organic soils will have a lower pH, higher exchangeable hydrogen, more calcium and magnesium ions, and higher exchangeable calcium content.