Abstract: In order to explore factors influencing the shrub plant roots and soil interface friction, we carried out the direct pull-out test on Indigofera Amblyantha Craib root to study its anti-pullout force under different soil dry densities. The results show that (1) the curves of anti-pullout force vs. displacement of the roots can be divided into three phases of steep rise, steep fall and gentle decline, increasing linearly to peak value at the start phase, and then presenting non-linear trend; (2) the maximum anti-pullout force of Indigofera Amblyantha Craib root increases obviously linearly with the increasing of root diameter under the same soil dry density, and the coefficient of correlation reaches 0.9; (3) the maximum anti-pullout force of Indigofera Amblyantha Craib root increases with the increasing of soil dry density with given root diameter, and the modes of pull-out failure of all roots are friction failure when the soil dry density varies from 1.35 g/cm³ to 1.45 g/cm³ and 1.55 g/cm³, while when the density is 1.65 g/cm³, root diameter of 0.586 mm is the critical point from friction failure to tensile failure, which means that the pull out failure mode is tensile failure when the root diameter is less than 0.586 mm, and friction failure when the root diameter is larger than 0.586 mm. Therefore, root diameter and unit weight of soil have influence on the anti-pullout force of root-soil interface.

Key words: principle of soil fixation, root-soil interface, pull-out test, anti-pullout force, soil density