Background  A large number of studies have shown that root system has a significant reinforcement effect on soil. However, due to the complicated plant species and roots architecture, the reinforcement of roots on soil is not easy to be quantified. Also, because the internal friction angle is not significantly affected by independent variables, there are few studies on the mechanism of roots affecting soil friction angle, and the conclusion is not clear yet. Therefore, in order to eliminate the synergistic interference of complex factors, the bionic root soil composite model was used to study the effects of xanthan gum gel concentration, the synergistic change of root amount and root diameter with the constant root amount ratio, root amount, and root diameter on soil reinforcement in this research.

Methods  PLA (polylactic acid) was used as raw material and 3D printing technology was used to produce simulated root fiber. Standard sand and xanthan gum gel was used to simulate soil and root exudates, respectively. Here, dry standard sand was chosen to eliminate the influence of cohesion. Through the test of indoor direct shear instrument and the test results analyzed by SPSS software, influence of the above variables on internal friction angle of soil was studied, and the reinforcement mechanism of soil was analyzed by ANOVA (analysis of variance).

Results  1) When the diameter and amount of the roots were constant, internal friction angle increased with xanthan gum concentration increasing and the fluctuation was stable when only the gel concentration of xanthan gum changed. The increase of xanthan gum gel concentration promoted the internal friction angle increasing, but this effect was not obvious after reaching a certain degree. 2) When RAR (root area ratio) was constant, the root content and xanthan gum gel concentration were given, only the root amount and root diameter changed. With the increase of root number, in other words, root diameter decreasing, the internal friction angle firstly increased and then decreased, and the overall fluctuation was stable. 3)When keeping the xanthan gum gel concentration and root diameter constant while only changing root amount, internal friction angle increased with root amount increasing. The internal friction angle increased with the increase of the amount of roots. 4)When keeping the xanthan gum gel concentration and root amount constant while only changing root diameter, internal friction angle increased with root amount increasing, but generally stable.

Conclusions  The ideal scenario is set by bionic root soil matrix, the test parameters are reasonably controlled, the influence of root related variables on the internal friction angle of soil is studied, and then the reinforcement mechanism of root system to soil is explored. The results provide a theoretical basis for further revealing the mechanism of root reinforcement of soil.