Gh. Habibi Bibalani - Islamic Azad University, Shabestar Branch, Shabestar, East Azarbayjan, Islamic Republic of Iran.
B. Majnounian - Faculty of Natural Resources, University of Tehran, Karaj, Islamic Republic of Iran.

Slope instability due to landslides is particularly common in Iran. It is possible to fight against these hazards thanks to the roots of trees, which provide an important contribu-tion towards the stability of hillslopes. However, our knowledge of the effectiveness of roots for slope stability needs improvement. Therefore this study was carried out on the effect of tree roots on slope stability, in particular for the following species: tea (Thea sinensis L.) and citrus (Citrus spp.), which are of economic interest; and Lilaki (Gleditshia caspica Dsf.) and Angili (Parrotia persica D. C.), without economic interest. The study area was located at Roudsar Township in Gilan Province of Iran. A large part of the area had slopes of steep gradients on which natural vegetation was present. Other parts of the same area have been cleared and planted with tea and citrus crops. Soil samples with and without roots of the mentioned species were taken, on an area covering approximately ۷۰ ha, for testing in the laboratory. Soil shear tests were carried out on these samples and the Factor of Safety (FS) was calculated. Results showed that the FS was increased in soils with tree roots present. The global slope FS was then determined using Bishop’s method. We calculated FS in order to protect slopes where the gradient exceeds ۲۵%. In this case study, minimum FS corresponds to e.g. Parrotia sp. vegetation with ۴۰-۶۰% crown cover, a soil internal friction angle of ۱۵° and slope angle of ۲۱°. When the soil internal friction angle equals ۱۵° and the slope angle is >۳۱°, slope stability cannot be increased by any mentioned vegetation species. The most important results show that the main contribu-tory factors which affect slopes’ stability are: angle of slope, angle of internal friction, and percentage of vegetation cover.

Factor of safety, Root reinforcement, Roots, Roudsar, Slope stability