Evaluation of the effects of Tamarix roots on Kashafroud river banks soil cohesion and shear strength

Vegetation has a fundamental role in controlling surface landslides by its mechanical and hydrological effects. However, due to the complexity of vegetation-soil interactions, quantification of the effects of roots on soil remains as a challenge. This study tries to evaluate and quantify the effects of Tamarix roots in increasing the soil shear strength of Kashafroud banks in Khorasane_Razavi Province, Iran. After field observations, a number of 18 samples of soil containing Tamarix hispida roots and also three samples of soil without any root with the sizes of 10×10 cm were collected from the research area and transported to the laboratory. Direct shear test was carried out for the samples and by drawing Mohr-Coulomb failure envelope, the shear strength of soil reinforced by the roots was obtained and compared to those of the soil samples without any roots. Results showed that roots increase the soil cohesion and shear strength by 216.6 and 41.64 percent, respectively. Results also indicated that by increasing the vegetation indices of RAR and RDR, soil cohesion and as a result, shear strength is increased. Of course, this shear strength increase will support natural stability of river banks against erosion and destruction.