Effect of Willow Tree Root on Slope Stability

Tree root, as a reinforcing agent, may improve the stability of soil slopes by two mechanisms: a decrease in the developed shear stress via distribution of applied loads within a larger area, an increase in the mobilized shear resistance via its tensile strength. The latter, in addition to mechanical properties of root, is a function of diameter, number, orientation and deflection angle of the roots with respect to the potential slip surface. Coppin & Richard analytical model was used to asses to what extend willow tree roots can improve the stability factor of a planar slip surface. The required data were provided by a series of in-situ tests in Taleghan region using a large scale field shear stress test apparatus. Tests were run in a natural deposit with and without willow root to measure cohesion and internal friction angle of soil under different root density. At the end of the tests, the diameter and angle at which the roots were deviated and cut were measured in the sheared zone. Experimental results were used in the model for different water levels, revealing that for a 45 degree planar slip surface and a root density of 5 to 26 percent the safety factor rises between 0.5 to 36 percent.