Graduate Thesis Or Dissertation

Characteristics of compacted soils from Eastern Oregon

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  • Soil samples and soil clods were collected from ten potato fields in Eastern Oregon. An index of grain size distribution was sought that would correlate well with compacted soil bulk density. The Hazen coefficient of uniformity was best related to bulk density. The limitations of various indices and their relationship to bulk density are discussed. Soil samples were equilibrated to three metric suctions and compacted by a series of static pressures. The semilog graphs of bulk density versus pressure were linear over the range of applied stress and moisture content of the experiment. For the same soils but with different moisture content, the slopes of the graphs were parallel to each other. This indicates that the slope of the graph, the compression index, is independent of the moisture content. The data obtained in this study were used to check Larson's equation for predicting soil compaction. His predicted values of compression index were higher than those measured for Oregon Aridisols, but when the measured values were used, Larson's equation was successful in predicting soil compaction. The minimum static stresses required to produce the clod bulk densities found in the field were estimated in the laboratory. The limitations of this calculation are discussed. A multiple regression of the dependent variables, compression indices and bulk densities on the independent variables of grain size, organic matter content and Hazen coefficient of uniformity was carried out. Fine silt content together with total sand accounted for 88% and 90% of the variation in compression index and bulk density respectively. Probe geometry of a soil penetrometer has an effect on measured soil strength. The 2 mm diameter, 10° (included semiangie = 5°) pointed probe was found more suitable than a blunt one in comparing strength of different soils. The different behaviour of soils equilibrated to similar matric suction and compacted at the same static pressure was explained in terms of soil-probe interaction, frictional forces and cohesive and adhesive properties of the system. Soil strength increased with increasing compacting pressure applied to the soil. The increase in soil strength was less for a sandy soil than a fine textured soil. The increase in soil strength was mainly due to increased bulk density of the soil. For a soil subjected to the same compacting pressure, the soil strength increased with increasing matric suctions. The effect of soil water content and grain size distribution on soil strength was difficult to separate.
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