Volume 35, Issue 4 (July 2012)
Identification of Low-Organic-Content Soils: An Engineering Approach
From a geotechnical engineering perspective, the presence of organic matter in soils can often be a concern, because of its negative impact on many mechanical properties and its potential interference with soil stabilization reactions. For this reason, many regulating agencies have strict limits on the maximum allowable organic content in subgrade soils and backfills, requiring that it fall below a threshold value in the 2 %−7 % range. Methods currently used in practice for the identification of organic soils and for the quantification of organic matter have shortcomings when applied to soils with organic matter content less than ∼10 %−15 %. For such soils, the loss on ignition often overestimates the true organic content, and the criteria employed by the ASTM and the American Association of State Highway and Transportation Officials classification systems are generally insensitive to the presence of these amounts of organic matter. This paper presents the results of a study conducted to identify a practical approach for the identification of soils with organic content in the 3 %−15 % range. The study explored the relationship between true organic content, as determined through the dry combustion test, and the results of three tests: loss on ignition, Atterberg limits (with and without oven drying), and the colorimetric test. Tests were conducted on a number of natural soil samples, select clay minerals, and three types of laboratory-prepared soils. It was found that the combined use of these tests is effective in screening soils for the presence of percentages of organic matter in the 3 %−15 % range. Results of thermal gravimetric analyses, differential scanning calorimetry, and x-ray diffraction analyses performed on select tests provide an improved scientific understanding of the results.