Pull-out characteristics of herbaceous roots of alfalfa on the loess in different growth stages and their impacts on slope stability

Plant roots can provide additional cohesion to enhance soil strength, effectively prevent soil erosion, and improve slope stability. In this study, loess samples with herbaceous roots of alfalfa were obtained through artificial planting in order to measure and compare the shear strength and root pull-out force of loess with alfalfa roots in different growth periods, to analyze the enhancement effect of alfalfa roots in different growth periods on the strength of loess, to improve Wu and Waldron's model (WWM) based on the pull-out characteristics of alfalfa roots in different growth periods, to determine that the correction coefficient of WWM model is a time-related function, and to analyze the influences of alfalfa in different growth periods on loess slope stability under rainfall conditions using the infinite slope model. The results show that the cohesion of loess containing alfalfa roots is larger than that of plain loess, and the cohesion of loess containing alfalfa roots initially increases and then decreases as the alfalfa growth period progresses. The growth period of alfalfa has a significant effect on its root pull-out strength. As the alfalfa growth period progresses, the cohesion of the root-soil complex increases based on the improved WWM model, and it is significantly higher than the value obtained from direct shear tests. In the initial stage of alfalfa growth, the correction coefficient increases continuously, reaches its peak in one year, and then decreases. In addition, the stability coefficient of a loess slope within the alfalfa root distribution depth is always greater than that outside of the alfalfa root distribution depth, and the stability coefficient of a loess slope within alfalfa root distribution depth increases as the growth period progresses. The results of this study provide a reference for slope stability and soil and water conservation in loess areas.