Graduate Thesis Or Dissertation
 

Consolidation, compression, and shear strength of four western Oregon forest soils

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  • Forest soils with low bulk densities are often considered less susceptible to compaction than soils with higher bulk densities. The objective of this study was to determine if soil strength controlled the compression of soils with low bulk density. Four soils were selected for this evaluation. Three of these were andic soils with low bulk density and the fourth soil was a more dense, cohesive soil. Undisturbed samples of saturated and partly saturated soil were compressed in a one-dimensional consolidation test apparatus. Measurements with separate samples were at one of 7 normal stresses between 0.033 and 1.96 MPa. Shear strength of saturated soil was measured in direct shear tests. Primary consolidation of saturated soil was completed in less than one minute at all normal stresses. Shear stress and bulk density increased continuously during shear strain. The compression index of the cohesive soil was significantly larger (p<0.05) than that of the andic soils. The shear strength of andic soils (average cohesion intercept of 0.016 MPa and friction angle of 33.3°) was significantly higher (p<0.05) than the cohesive soil (cohesion intercept of 0.028 MPa and friction angle of 28.9°). When saturated, the cohesive soil was more compressible than the andic soils because of lower soil strength. A nonlinear model of soil compression was developed that accurately predicted the compressed density of saturated and partly saturated soil as a function of normal stress, initial bulk density of undisturbed samples, and degree of saturation. As degree of saturation decreased, the compressibility of the cohesive soil decreased more rapidly than it did for the andic soils. As a result, bulk density of dry cohesive soil increased less than it did for dry andic soils. Differences in the compressibility of soils were attributed to texture and clay mineralogy. The differences in the compressibility of these soils were much smaller than were the differences in bulk density. Decreasing water content affected the compressibility of the cohesive soil more than it affected the andic soils. Because soil strength controls the compressibility of these forest soils regardless of bulk density, it will also determine the susceptibility of soils to compaction by machines.
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