Abstract
Maximum stable heights of steep, friable loess slopes of western Iowa and western Tennessee were found to relate to the soil density and shear strength in accord with the sliding-wedge method of analysis. Shear strength was determined in situ with the borehole shear test, which allows a rapid field determination of the Mohr-Coulomb failure envelope. Tests at six sites in Iowa showed no consistent difference between horizontal versus vertical shear strength, except that horizontal strength was more variable. The frequently cited vertical cleavage of loess appears to be a result of tension in the surface layer in accord with Rankine theory and is not unique to loess. Vertical slab or cleavage failure should result in an average slope angle of about 77°, which coincides with an observed modal frequency in western Iowa loess. Failure of a 77° cut in shear should then result in an average slope of 51° dependent in part on the angle of internal friction, ; the 51° angle also coincides with an observed mode. A third observed modal frequency around 38° is attributed to the angle of repose of failed loess, and lower slope angles must relate to other mechanisms. The internal-friction angle, , of friable loess is fairly constant (20°-30°), and the cohesion, c, can be estimated by measuring the maximum heights of steep loess cuts.
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