Role of roots in the shear strength of root-reinforced soils with high moisture content

This paper presents an experimental study on the effect of soil moisture content on the shear strength of root-reinforced soils by conducting in situ shear tests. The plant used in the shear tests was Prickly Sesban (Sesbania cannabina Merr.). Performance of plants in stabilizing slopes against shallow landslides in a rainfall event is closely relevant to the variation of the shear strength of root-reinforced soils with changes in the moisture content, which may take place due to seasonal effects (e.g. rainfall and temperature changes) as well as the influence of vegetation. The shear strength of soils may decrease noticeably with increasing soil moisture content. The role of roots in providing the shear strength to root-reinforced soils turns out to be important if soils are subjected to an increase in the moisture content. The experimental results show that root efficiency, defined as the ratio of the additional shear strength provided by roots over the shear strength of soils, may reach more than 1.0 if soils are at a high degree of saturation. In addition, an approximately linear relationship between the additional shear strength (ΔS) provided by roots and the tensile strength of roots per unit area of soils (tR) was obtained in this study, and the ratios of ΔS/tR are about 0.39 and 0.42 in terms of the peak and the residual shear strength, respectively. These results are considerably less than the theoretic value (=1.2), based on a simple force equilibrium model derived by Wu et al. [Wu, T.H., McKinell, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Can. Geotech. J. 16 (1), 19–33].