The Microstructure of Loess and Its Relationship to Engineering Properties.
Technical rept. May 68-Feb 69,
AIR FORCE WEAPONS LAB KIRTLAND AFB NM
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The microstructure of a loessial soil in an attempt to correlate structural or fabric anisotropies with variations in shear strength is examined. Oriented thin sections of undisturbed loess samples were prepared and measurements were made directly on photomicrographs to determine the degree of preferred orientation of silt grains. The overall effect of microstructure and grain orientation on strength properties was evaluated by direct shear and triaxial compression tests. Significant grain orientation was found in both horizontal and vertical planes of undisturbed loess samples. Preferred grain orientation and imbrication conformed to postulated paleowind directions. Silt-sized particles were found to be oriented parallel with the depositing wind current and imbricated in conformity with established criteria of sedimentation. The orientation of silt grains in the material influenced the directional shear strength considerably. A significant reduction of shear strength was found where specimens were stressed parallel to the plane of preferred grain orientation and imbrication. Microstructural anisotropies in Vicksburg loess also reflected definite variation in shear strength as determined by triaxial compression tests. Author
- Soil Mechanics