Effect of Organic Material on Soil Shear Strength

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Abstract
The term organic soil ordinarily refers to combinations of weathered rock products and partially decayed plant matter. The organic fraction tends to dominate soil physical properties for organic contents in excess of 20 percent by weight. To simulate these materials and to include precise control over organic content, fiber type and size, and mineral type, kaolinite and pulp fibers were used to prepare model organic soils. For comparison, kaolinite and all fiber samples were included. The effect of organic material on soil shear strength is shown in terms of consolidation and stress-strain behavior, undrained shear strength, and the shear strength parameters c' and φ'. Decomposition effects on vane shear strength are reported for selected soil samples and one consolidation pressure. The experimental work gives a relationship between void ratio and logarithm of pressure that is dependent on both organic content and pressure level. A small initial increase in organic (fiber) content significantly increased the compressive and vane shear strengths of the model organic soil. Higher organic contents changed the material behavior from one with a peak compressive strength at about 7 percent axial strain (kaolinite) to one with an almost straight line stress-strain curve for strains in excess of 20 percent (all fiber). Decomposition in the model organic soils led to a large decrease in vane shear strengths. An increase in organic (fiber) content increased the shear strength parameter φ' for both the CIU¯ and CID triaxial tests. A failure criterion, based on the maximum ratio of shear stress to the effective normal stress (maximum obliquity), gave φ'-values intermediate to the CIU¯ and CID values for high organic contents.

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