# Shear Strength Analysis and Slope Stability Study of Straight Root Herbaceous Root Soil Composite
## 5 Conclusions  
This study investigated the shear performance of root soil composites of herbaceous plants represented by _Tagetes erecta_ and the soil stabilization and slope protection effects of their roots. Based on the results, the following key conclusions were determined:  
(1) Both RUS and USRS exhibited strain-hardening characteristics during the stress-strain shearing process. The maximum deviator stress during the failure of USRS reachedup to 1.29 times that of RUS. Under identical moisture content conditions, the soil-root composite shear performance increased with the confining pressure and root content. Moreover, the shear strength of the soil-root composite decreased with increasing moisture content. (2) The shear strength index of the soil-root composite was closely correlated with the moisture content and root content. At a constant moisture content, the cohesion and its added increment rose until they peaked and subsequently declined after an initial increase as the root content increases. Simultaneously, the internal friction angle gradually decreased with increasing moisture content and steadily increased with the root content. (3) The multiple linear regression analysis results of the shear resistance indicators and the influencing factors were consistent with the results of the significance and correlation analyses. The root content significantly influenced the cohesion of undisturbed soil with the following characteristics: root content (+) > root burial depth (\(-\)) > moisture content (\(-\)). The influence of the moisture content on the internal friction angle of undisturbed soil was significant, with the following characteristics: moisture content (\(-\)) > root burial depth (\(-\)) > root content (+). (4) The equivalent plastic strain and maximum displacement in the \(x\) and \(z\) directions were negatively correlated with the root content. The maximum equivalent plastic strain was 57.9% lower for the vegetated slope than for the bare slope at the slope angle of 50\({}^{\circ}\). The reduction ratio of the maximum displacement was similar in the \(x\) and \(z\) directions under different working conditions. (5) The root content and root depth were positively correlated with the safety coefficient and the safety coefficient slope under the same working conditions, while the slope angle was negatively correlated with the safety coefficient. The bare slope exhibited the lowest safety coefficient. The reinforcement effect by the plant root system resulted in a 3.2-11.2% increase in the safety coefficient and improved the stability of slopes with an angle exceeding 1.5%. The results of this study serve as a valuable reference in the selection of plant species for ecological slope protection in the Taihang Mountains region. They also provide strong support for ecological slope design, shallow landslide prevention, and soil erosion control. In addition, although herbaceous plant roots can improve the shear resistance of shallow slopes, the reinforcement range is limited. Future research will focus on developing a comprehensive slope-protection mechanism that combines vegetation planting with other slope-protection approaches. Conceptualization, B.W. ; methodology, B.W. ; software, B.W. ; validation, S.W. ; resources, S.W. ; data curation, B.W. ; writing--original draft preparation, B.W. ; writing--review and editing, B.W. and S.W. ; visualization, B.W. ; project administration, S.W. ; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the Hebei Province Construction Science and Technology Research Guidance Plan Project, China, grant number 2016-236. Not applicable. The analyzed datasets are available from the corresponding author on reasonable request. The data are not publicly available due to the reason below. The project sponsor believes that the research is progressiveness in the field of ecological environment, and the data should not be publicly available.