Investigation of the Effects of Tree Roots on Shear Strength of the Karoon Banks

Downslope soil movement along riverbanks is a significant erosion process. Plant roots, particularly of woody vegetation, apparently stabilize soil on slopes because in most areas where the vegetation is removed, frequent bank failure occurs. Plant roots increase soil-shearing resistance both directly by mechanical reinforcement and indirectly through removal of pore water by transpiration. In this study, the effects of two plant species on the stability of the Karoon River has been investigated. To determine the In-situ shear strength of soil, a special device was designed and manufactured. This device is capable of measuring the shear strength of soil blocks as large as two cubic meters. In this study, twelve soil blocks, four blocks with roots of each tree and four blocks of root permitted were measured. Comparison of the soil shear strength with roots and root permitted soil shows that tree roots can significantly increase the shear strength of the soil. The amount of increase depends on the type of plant, the age of plant, the diameter of the roots and the percentage of roots in the block. In this study, the amount of increase varied between 20-66%. From the analysis of the data, two equations were developed to determine the increased shear strength.