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Mathematical Evaluation of Factors Affecting Gully Stability¹

ABSTRACT

Because large amounts of sediment come from gully banks in the coarse loess soils of the Lower Missouri River Basin, a study was made of the factors influencing the stability of gully banks. The approach was to consider the action of forces within the soil mass forming the wall of the gully. A two-dimensional stability analysis was made using the Simplified Bishop Method of Slices.

If the angle of internal friction is 35°, calculated factors of safety indicate that vertical, saturated, or near-saturated gully walls in loessial soils will fail if a ground water table exists at the base of the wall and if the cohesion for the saturated soil system is zero at a hydrostatic pressure of zero. Although gully bank stability is sensitive to the angle of internal friction, the calculated stability is relatively insensitive to slope height and soil bulk density. Tension cracks that frequently open at some distance back from the gully face do not materially influence stability. The effect of infiltration rate into the soil on the stability of the gully wall depends upon the rate of water conductivity through the soil and upon the cohesion of the soil or the level of the ground water table. Increasing the infiltration rate decreases the stability.

¹ Contribution from the North Central Region, Agricultural Research Service, USDA, Columbia, Missouri, the Agronomy Dep. of the Missouri Agr. Exp. Sta., Journal Series no. 6343, and the Minnesota Agr. Exp. Sta., Scientific Journal Series no. 7934.

² Soil Scientist, USDA, Columbia, Missouri, and Soil Scientists, USDA, St. Paul, Minnesota, respectively.

Received for publication July 7, 1972. Accepted for publication August 31, 1972.