Effects of organic matter on the physical and the physicochemical properties of an illitic soil

The main thrust of this study is to investigate the effects of organic matter on the physical and the physicochemical properties of illitic soils. For this purpose, organic matter (peat) was added to inorganic illitic clayey soil at eight levels (0%, 5%, 10%, 12.5%, 15%, 17.5%, 20%, and 30% by weight). The physicochemical properties of the resulting soils were determined using a Grain Size Analyzer (GSA) with specific surface area measurement, Scanning Electron Microscopy (SEM), and Infrared Spectroscopy (IR). The physical properties of the mixtures were determined by conducting a series of laboratory tests including Atterberg Limits, compaction, unconfined compressive strength, and swell characteristics tests. The results showed that at low organic contents (in general less than 15%) the soil particles tend to aggregate, whereas at higher organic contents the soil particles tend to disperse. Also, the IR tests showed that direct chemical interactions took place between the organic and the inorganic fractions of the mixtures. The tests on the physical properties showed that at low organic content (up to 10%) the plasticity index slightly increased then after, the plasticity index decreased with increasing organic content. Organic matter has shown to decrease the maximum dry density and increase the optimum water content, nevertheless, although organic matter decreased the soil's compactability, the feasibility of compaction of slightly organic content soils still exists. Moreover, it was shown that organic matter decreased the peak strength values and increased the water contents at these peak strengths. The final free swell for illitic soils increased with increasing levels of added organic matter. (C) 1999 Elsevier Science B.V. All rights reserved.