Friction performance of root-soil interface of Betula platyphylla in Northwestern Hebei Province, China

Deeply penetrating roots can greatly contribute to the shear strength of soil, particularly anchoring the soil mantle to the underlying bedrock. Mechanical properties of tree roots generally include the tensile strength of roots and the root-soil interface friction. Friction between the roots and the soil is essential to the reinforcement of the slope soil. White birch (Betula platyphylla) is the main species for the forestation in northwestern Hebei Province, China. Taking the white birch as the research object, this study tested 127 roots in the soil using the pullout method. A systematic investigation was made to explore the effects of root diameter (ranging from 1 mm to 10 mm), embedment length of roots (50, 100, and 150 mm), moisture content of soil (11.85%, 13.85%, 15.85% and 17.85%), altitude (ranging from 1695 m to 1898 m), growth direction of roots (along the downhill, horizontal and uphill direction) on the mechanical properties of single root and soil interface under the load velocity of 10 mm/min. The results showed that there were two failure modes in the experimental process: the pull-out and breakage of roots. The pullout force and root slippage curve of roots were divided into three stages: ascending, steep descending, and stable pull-out stage. Two types of breakage were observed: broken in soil, and broken on the connecting surface of root and soil. The root with a relatively small diameter broken, when the friction between root and soil exceeded the tensile strength of root, where the tensile force of root cannot reach the maximum force. The load dropped sharply after the root broken. The residual load was borne by the remaining root buried in the soil, where the second half of the curve was similar to that of pull-out failure. In the root broken at the root-soil contact interface, the first half was similar to that broken in the soil. The root broken, while the load instantly dropped to zero, when the pull-out force reached the ultimate bearing capacity of the root. The maximum pull-out force of root increased, with the increase in the diameter and embedment length of root. When the diameter of root was relatively smaller, the maximum pull-out force was not much different in the embedment lengths of 50, 100 and 150 mm. The difference was greater in the maximum pull-out force between the root and the soil at the different embedment lengths, as the diameter of root was larger. The increase was smaller in the friction at the root-soil interface, as the embedment length increased. With the increase of soil moisture content from 11.85% to 17.85%, the friction between the root and the soil presented a trend of increasing first and then decreasing. The altitudes of the sample positions and the root growth directions also had influence on the root-soil interface friction. There was a power function between the maximum pull-out force and the diameter of root, where the determination coefficient were larger than 0.82. No relationship was found between the maximum displacement, corresponding to the maximum pull-out force and the root diameter. Redundancy Analysis (RDA) results showed that all influencing factors were positively correlated with the friction of root-soil interface. The diameter of root and altitude greatly contributed to the friction of root-soil interface. The other variables were ranked from large to small: the moisture content of soil, growth direction and depth of tree roots.