Classification of Organic Soils

The presence of organics in soils is generally associated with high compressibility, significant secondary compression, often unsatisfactory strength characteristics, and low unit weight. As a result of the above, many state DOTs (Departments of Transportation) in the United States have strict limits on the maximum value of the organic content (2-7%) that can be present in soils to be used as subgrades and backfills. The loss on ignition test is the most widely used technique for...
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The presence of organics in soils is generally associated with high compressibility, significant secondary compression, often unsatisfactory strength characteristics, and low unit weight. As a result of the above, many state DOTs (Departments of Transportation) in the United States have strict limits on the maximum value of the organic content (2-7%) that can be present in soils to be used as subgrades and backfills. The loss on ignition test is the most widely used technique for measuring organic content. However, especially for low organic content soils, this method can lead to significantly overestimate the true organic content. As a result, certain soils may be incorrectly classified and erroneously considered unviable for certain applications; in other cases unnecessary costly treatments may be requested, even if not required. These are the issues motivating the research presented in this report, which addressed the classification of organic soils and the quantification of organic matter in soils. The research reviewed existing classification systems for organic soils, the effects of organic matter on the geotechnical properties of soils, and the methods for determination of organic content. In addition to the review of the existing literature, this research also included experimental work conducted on natural soils with varying organic content, as well as on laboratory prepared (“artificial”) organic soils. The experiments performed included loss on ignition tests, Atterberg limits, colorimetric tests, dry combustion tests, thermal analyses, and X-ray diffraction analyses. The work led to propose a system for classifying organic soils which is based on the percentage of organic matter present: soils with organic content <3% are termed mineral soils; if the organic content is >3% and < 15%, soils are classified as mineral soils with organics; when the organic content exceeds 15% but is <30%, the term organic soil is employed. Finally, soils with organic content higher than 30% are termed highly organic soils or peats. Given the potential errors associated with measuring organic content using the LOI method, this research proposes an approach based on the combined use of the LOI test, the liquid limit test conducted both before.