In this study, biotechnical characteristics of the root system of Quercus brantii and Crataegus aronia, were studied in Zagros forests, the west of Iran. For each species, a number of eight individual trees were selected and 73 root samples were tested for measuring tensile strength using Instron Universal Testing Machine. Results showed a negative relationship between the diameter and tensile strength. The minimum and maximum tensile strength values in the diameter range of 0.13-2.93 for Q. brantii were 15.01 and 116.08 MPa and in the diameter range of 0.32-2.97 cm for C. aronia were 12.98 and 83.10 MPa, respectively. The parameters for soil cohesion and angle of internal friction of soil particles for soil samples of each individual trees were measured using direct shear test apparatus after transporting samples to the laboratory. Calculation of soil reinforcement of the two species is based on a modified Mohr-Coulomb concept (Wu model) and on measured data of tensile strength and root area ratio (RAR). The maximum values of soil shear strength, considering the root cohesion effect of Q. brantii and C. aronia, have been estimated with 100.40 MPa and 98.28 MPa respectively. The results showed that the soil shear strength of Q. brantii is higher than C. aronia. The results confirm the effect of forest on slope stability and the findings of our study can be used in reinforced soils and lower the risk of landslides.
