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The Shear Resistance of Root-Permeated Homogeneous and Stratified Soil¹

ABSTRACT

Mechanical reinforcement which stabilizes soil on slopes has been attributed to plant roots. To measure such reinforcement, direct shear tests were made on 25-cm diameter root-permeated soil columns. Roots of alfalfa (Medicago sativa), barley (Hordeum vulgare), and yellow pine (Pinus ponderosa), each increased the shear resistance of homogeneous and compacted layers of silty clay loam at 30-cm depth. One-year-old alfalfa had a much greater reinforcing effect than pine trees 16 months after transplanting or barley at its maximum growth. Barley had a greater effect in the clay loam than pine, but its effectiveness decreased as depth increased from 15 to 30 to 45 cm. Alfalfa roots were more effective than either pine or barley roots in increasing the resistance to shearing between a dense gravel-sand layer (simulating weathered rock) and the overlying soil, increasing shearing resistance to as much as 5 times that of fallow soil. A model is presented of soil reinforced by nonrigid roots. Calculations are given of slope safety factor increases from root reinforcement.

¹ Contribution from the Dep. of Soils and Plant Nutrition, Univ. of California, Berkeley, CA 94720. Research supported by the California Agric. Exp. Stn. and Natl. Sci. Found. Grant GK-43365.

² Associate Professor, Univ. of California, Berkeley.

Received for publication June 21, 1976. Accepted for publication April 18, 1977.