Effects of Vitex negundo root properties on soil resistance caused by pull-out forces at different positions around the stem

•Roots develop the highest soil resistance in the pull-out direction.•Root length plays a decisive role in soil resistance in pull-out direction.•Root diameter is significant in soil resistance against pull-out direction.•Around the plant stem, branching points have little effect on pull-out strength.

Plants roots are beneficial for slope stabilization as they can reinforce the soil shear strength. Though the effect of root distribution characteristics has been studied a lot, the spatial interaction mechanisms of soil-root blocks on pull-out tests around the tree stem are still poorly understood. This paper aims to analyze the effect of root spatial distribution and properties on the behavior of soil resistance around the tree stem by field pull-out tests on a slope covered by Vitex negundo L. var. heterophylla. The relationships between the pull-out strengths and root properties at different positions were compared; and the branch root mechanics were used to explore the correlation between root spatial distribution and soil resistance behavior. The results show that roots evenly distribute in a 0.7 m radius around the main stem. In the position on both sides of the pull-out direction, coarse roots are mainly located near the main stem, while fine roots are farther. Roots develop the highest soil resistance of 2207 N with respect to the pull out direction and 508 N against the pull-out direction. When a higher number of roots with diameter < 5 mm and longer length are located in the front of the tree stem and a higher number of roots with diameter > 5 mm behind the tree stem, the soil resistance are much greater. Branching points have little impact on pull-out strength. Besides, friction at the root-soil interface in either along or against the pull-out direction is greater than bilateral direction of the tree stem. In addition, the results of this paper can present the response of roots around the tree stem subjected to pull-out force, which can be considered in selecting and collocating of plant species in eco-engineering. The methods in this paper can be taken as a reference in analyzing the effect of root properties on soil resistance.