Shear Resistance Enhancement of 1.22‐Meter Diameter Soil Cross Sections by Pine and Alfalfa Roots

Abstract

AbstractPlant roots have been shown to increase soil shear resistance through direct mechanical reinforcement and thereby enhance soil stability on slopes. Because of their potential large diameter and length, the roots of trees may be especially significant in such soil reinforcement. To provide large rooting volumes simulating natural conditions and shear cross sections many times larger than potential tree root diameters, we constructed 12 1.22‐ by 1.22‐m cylindrical soil containers in which two artificial soil profiles were prepared. Replicates of each profile type were planted to alfalfa, yellow pine, or were kept clear of plants. A large pneumatic direct shear device sheared both root‐free and root‐permeated materials along a horizontal plane at the 0.6‐m depth at either constantly maintained shear stress levels (creep shear) or at constant shear displacement rates.Creep shear at the 0.6‐m depth showed that roots of 14‐month‐old alfalfa increased the shear resistance of homogeneous clay loam and a clay loam/gravel interface simulating a soil‐weathered rock boundary by 32 and 50%, respectively. Constant shear displacement rate experiments at the same depth on similar samples permeated by roots of yellow pine planted 54 months earlier showed shear resistance increasing steadily with displacement over the entire test displacement range. At 75 mm displacement, the shear resistance of the pine‐rooted soil was about two times that of the nonrooted in both profile types. The superiority of pine roots to alfalfa roots in increasing soil shear strength in these experiments is in accord with field observations that woody plants are more effective than herbaceous plants in stabilizing soil against slips and slides. Pine root‐size distributions determined 11 months after the last shear test were combined with root tensile strengths and Young's moduli previously measured and used in model simulations of root reinforcement with good results.