Development of a mathematical model for quantifying the effect of root-reinforcement on shear strength of soil

Abstract

The role of vegetation in the stability of slopes has gained increasing recognition in the last couple of decades. Performance of plants in stabilizing slopes is closely associated to the variation of the shear strength of root-reinforced soils. An attempt has been made in this study to investigate the root morphology; and to characterize the soil-root system. The main objective was to develop a mathematical model for predicting the additional shear strength of rooted soil. To determine the root architecture, at first three plants i.e. vetiver grass, tiger grass and wild cane were uprooted and their root morphology was closely monitored. It was found that vetiver has longer and denser network than the other two. Subsequently, vetiver was selected for detailed analysis. It has been found that, in sandy soil, vetiver root can grow up to 1 m within three months and the mean tensile strength of matured vetiver root is approximately 27 MPa. Extensive laboratory tests have been conducted to determine the additional shear strength of vetiver rooted soil. From laboratory test results, it has been found that due to inclusion of root, angle of internal friction (φ) increases for fine grained soil, however, for coarse and medium grained sand, φ decreases. Direct shear tests were also conducted on reconstituted samples by implanting roots at perpendicular root arrangement and an increase in shear strength up to 50% was observed. From unconfined compression test results, it has been found that axial stress of the rooted sample is 43% higher than that of the bare one. Tri-axial test results show 34% increase in shear strength of the rooted sample in comparison to that of the bare sample. Direct shear tests were also conducted on undisturbed samples. For both clayey and sandy samples, angle of internal friction of the rooted sample is higher but cohesion is lower than that of the bare soil. It has been found that vetiver root enhances the shear strength in most of the cases, but in some cases, inclusion of root decreases the shear strength of soil-root matrix. The effect of tensile force of the root acting at the base of slip plane which increases the stability of slope segment has not been evaluated by laboratory tests. In-situ shear strength tests were conducted in order to determine the shear strength parameters of vetiver rooted soil and additional shear strength for rooted soil was determined by comparing with bare sample. In this study, an approximately linear relationship between the additional shear strength provided by roots (Δs) and the tensile strength of roots per unit area of soils (tR) was obtained and based on the experimental observation, a mathematical model was developed to predict the additional shear strength of soil-root system. The model developed in this study is Δs=5.14tR which is comparable to other models. This simple, straightforward model will provide a convenient mean for stability analysis of vegetated slopes.