Friction Performance Of Tree Root-Soil Interface Under Cyclic Loading 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.The roots of plants in nature are affected by the mountain torrents and wind and snow loads,which is equivalent to exerting fatigue loads on the roots directly or indirectly.Betula platyphylla and Larix principis-rupprechtii are the main species for the forestation in northwestern Hebei province,China.Taking the Betula platyphylla and the Larix principis-rupprechtii as the research object,a systematic investigation was made to explore the effects of root diameter,embedment length of roots,soil moisture content,altitude,growth direction of root on the mechanical properties of root and soil interface under the monotonic loading.Cyclic loading test was also conducted for roots undamaged in monotonous loading,root diameter,root burial depth,soil moisture content,tree species and different cyclic loading amplitudes were considered as research variables to explore and compare the root-soil interface friction under monotonic and cyclic loading,the results are of great significance to the selection of tree species and the protection of ecological environment in northwest Hebei province.The results showed that:(1)In the process of pulling out the Betula platyphylla roots under monotonous load,there were two failure modes: the pull-out and breakage of roots.Besides,the Larix principis-rupprechtii root with smaller diameter damaged in skin slipping.After cyclic loading and unloading,the root had different failure modes under different cyclic loading amplitudes.(2)The maximum pull-out force of Betula platyphylla root increased,with the increase in the diameter and embedment length of root.With the increase of soil moisture content from 11.85% to17.85%,the friction between the root and the soil presented a trend of increased first and then decreased.The altitudes of the sample positions and the root growth directions also have influence on the root-soil interface friction.Redundancy analysis(RDA)results show 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.Root of Larix principis-rupprechtii with smaller diameter is prone to be broken,and the root with larger diameter has stronger soil-fixing ability than that of Betula platyphylla on the whole.(3)Under cyclic loading,the uncertainty of the roughness of root epidermis,the random distribution of soil particles around root and the variable bonding conditions between root and soil result in different friction characteristics of root-soil interface,which led to different shapes of pull-out force-slip curves(cyclic loading and unloading sections)of roots under different cyclic loading amplitudes.But the common characteristic of pull-out force-slip curves under repeated loads is that when cyclic loading and unloading were conducted,the loading and unloading paths did not coincide,which forming hysteretic curves,showing certain periodic characteristics,and the evolution laws of hysteretic curves were different under different cyclic loading amplitudes.(4)Cyclic loading and unloading caused the root pulled and retracted continuously,resulted in the accumulation of bond damage at the root-soil interface,led to the continuous attenuation of anti-slip ability of the root-soil interface.The value of slip and residual slip increased with the increase of repeated load cycles.Different bonding conditions have significant impacts on the bonding between root and soil under cyclic loading.Root diameter,root buried depth,and soil moisture content affect the distribution of the root-soil bonding force.Load cycling of constant amplitude produces only a gradual deterioration of bond between root and soil.