Background Moisture content is the most frequently changing indicator of soil physical properties, which not only affects the apparent strength of the soil, but also affects the root-soil mechanical action. In the current shallow slope stability assessment researches, the impact of soil moisture content on the root reinforcement is rarely considered.

Methods In order to clarify the relationship between the root-soil interfacial characteristics and the soil moisture content, and to study the influence of soil moisture on the friction coefficient of root-soil complex, root shear strength and root failure mode, we carried out Symplocos setchuensis root tensile test, root-soil friction experiment based on nine soil moisture content gradients. Then we estimated the additional cohesion of root (c_r) under three models through BSTEM (bank stability and toe erosion model) to contrastively analyze root reinforcement. At last, we used GEO-Studio to evaluate slope stability and to analyze the change of slope stability under different soil moisture contents.

Results 1) The tensile strength and pull-out strength of S. setchuensis roots decreased with the increase of root diameter, and the two curves usually had an intersection point and its corresponding root diameter first increased and then decreased (0.12 - 2.75 mm). 2) The friction coefficient of root-soil interface decreased with the increase of soil moisture content. 3) The additional cohesion of roots (c_r) both increased with the increase of soil moisture content when considering the frictional characteristics of root-soil interface under different soil moisture content: c_r(f) < c_r(FBM) < c_r(Wu), the deviation of c_r(f) to c_r(FBM) and c_r(f) to c_r(Wu). 4) The slope factor of safety (Fs) decreased with the increase of soil moisture content, and it was more conservative when considering the characteristics of root-soil interface. Fs(Wu) and Fs(FBM) were about 1.53 and 1.02 times of Fs(f).

Conclusions Soil moisture content influenced the failure mode and pull-out strength of roots by affecting the frictional characteristics of root-soil interface, and thus influenced the soil reinforcement. This indicated that dynamic changes of frictional characteristics of root-soil interface should be considered in analyzing the stability of planted slope under dynamic soil moisture content. This research also provided a more conservative dynamic root reinforcement model compare with Wu and FBM model, which took the weakening mechanism of soil moisture content into account.