Effect of Grass, Legume, and Tree Roots on Soil Shearing Resistance

Plants enhance soil stability against downslope mass movement through the removal of soil water by transpiration and by the mechanical reinforcement of their roots. To assess the magnitude of this reinforcement, direct shear measurements were made on 0.25-m diam cylindrical soil columns packed both homogeneously and in layers which simulated water and/or root-impeding horizons. In all cases the matric potential was adjusted to zero before shearing. Twelve plant species were used including seven grasses: Phalaris tuberosa, Lolium rigidium, Dactylis glomerata, Bromus mollis, Sorgum bicolor sudanense, Triticum oestivum, Hordeum vulgare. Two legumes were used: Vicia dascarpa, Medicago sativa. and two trees were used: Pinus ponderosa and Quercus agrifolia. The ratio of the shear resistance at 25-mm displacement of the rooted and unrooted specimens was used as a measure of root reinforcement. Roots of several grasses planted in early fall and sheared the following spring gave about a threefold increase in shear resistance at the 0.3-m depth in homogeneous saturated clay loam. In the same material, roots of oak produced a similar increase only after 3 years' growth. One-year-old alfalfa produced a fourfold increase. At the 0.45-m depth at the interface between soil and a dense gravel-sand mixture simulating weathered rock, yellow pine gave a 1.5-fold increase after 16 months and a 2.5-fold increase after 52 months. Hardinggrass was almost equally effective after only 7 months. In almost all cases where roots increased soil shear resistance, the resistance continued to increase beyond 25-mm displacement so that the selection of 25-mm displacement was conservative, i.e., it may underestimate the root reinforcing effect. Factor of safety calculations for shallow planar slides using measured shear strengths show that plant roots can make large increases in slope stability.