Effects of Types and Amount of Organic Matter on the Engineering Properties of Soil

Organic soils exhibit highly variable geotechnical behavior due to differences in the type, content, and decomposition state of organic matter, which are often inadequately distinguished in engineering practice. This study investigates the influence of organic matter origin on soil behavior through a comparative analysis of sapric organic soils and sewage sludge soil tested under identical one-dimensional consolidation conditions. Laboratory tests included physical characterization, standard consolidation tests under vertical stresses of 200 kPa and 800 kPa, post-consolidation organic matter measurements, and microstructural analyses using SEM–EDS and XRD. The results show that increasing sapric organic matter content from 23% to 35% led to a substantial increase in liquid limit (from 82% to 117%), a reduction in maximum dry density to 0.857 g/cm3, and a marked increase in compressibility. The highest compression index (Cc = 2.78) and secondary compression coefficient (Cα = 0.129) were recorded for the soil containing 35% sapric organic matter under 800 kPa, indicating pronounced long-term deformation. In contrast, sewage sludge soil, despite having a comparable organic content (26%), exhibited a higher liquid limit 138% but lower compressibility (Cc = 1.62, Cα = 0.069) .Microstructural observations revealed that sapric soils are characterized by a loose, fibrous, and highly porous fabric dominated by carbon-rich organic constituents (up to 50.2% C), whereas sewage sludge soil exhibits a finer, denser matrix with higher oxygen content (44.6% O) and greater mineral contribution. Overall, the findings demonstrate that organic matter type and decomposition state exert a stronger control on compressibility and creep behavior than organic content alone, emphasizing the necessity of incorporating organic matter origin into geotechnical assessment and design of organic and sludge soils.