EVALUATION OF THE RELATIONSHIP BETWEEN COMPRESSIBILITY AND SHEAR-STRENGTH OF ANDISOLS

Andisols, and other soils with low bulk density, are often considered less susceptible to compaction than soils with higher bulk densities. This study was initiated to test the hypothesis that high soil strength reduces the compressibility of soils with low bulk density. The compressibility and shear strength of three Andisols were compared with that of a fine-textured soil of crystalline mineralogy with a higher bulk density. Soil compressibility was measured in one-dimensional consolidation tests and shear strength was measured in direct shear tests. Cores of undisturbed soil were collected from the 7- to 12-cm depth of four forested soils in western Oregon. Cores were saturated prior to testing. Compression indexes were 0.231 for the coarse-textured Andisol, 0.262 and 0.275 for two fine-textured Andisols, and 0.320 for the fine-textured soil of crystalline mineralogy, which was significantly higher than that of the Andisols. The angle of internal friction of the Andisols ranged from 33.0 to 33.7-degrees, but was 28.9-degrees for the fine-textured soil of crystalline mineralogy. A nonlinear model was successfully adapted to describe the relationship between bulk density and normal stress using data of these sample sets. These samples had different initial and compressed bulk densities. We concluded that Andisols are less compressible than other denser soils because of a high shear strength. The question of the susceptibility to compaction of soils with low bulk density must be reconsidered.