Analysis of the Root Mechanical Reinforcement Effect of Typical Vegetation for Slope Protection in Open-Pit Mine Waste Dumps

In order to study the reinforcement effect of plant roots on soil in open-pit mine waste dump slopes, typical slope protection plants such as Alfalfa (Medicago sativa), Cogonggrass (Imperata cylindrica), and Goosegrass (Eleusine indica) were selected. The tensile mechanical properties of their roots and the shear performance of the root-soil composite were investigated. Additionally, the WWM model was used to calculate the shear strength of the soil samples, and model parameters were adjusted to quantify the additional cohesion provided by each plant's roots at different soil depths. The results showed that the Root Area Ratio (RAR) of all three herbaceous plants decreased with increasing soil depth. Among them, Alfalfa had the highest RAR, which was 3.33 and 2.68 times that of Cogonggrass and Goosegrass, respectively. The Root Length Density (RLD) of Alfalfa initially increased and then decreased with soil depth, while the RLD of Cogonggrass and Goosegrass decreased consistently with increasing soil depth. The RLD values of the three herbaceous plants at different soil depths ranged from 0.07 to 0.4 cm/cm³ for Alfalfa, 0.06 to 0.67 cm/cm³ for Cogonggrass, and 0.13 to 1.3 cm/cm³ for Goosegrass. The tensile properties of the roots of all three herbaceous plants exhibited a power-law relationship with root diameter. The tensile force increased with increasing root diameter, while the tensile strength decreased as the diameter increased. Roots can alter the mechanical characteristics of the shear behavior in the root-soil composite, increasing the shear peak strength, extending the peak shear strain, and enhancing the shear strength of the soil. When the water content is 10.5%, the cohesion and internal friction angle of the rootless soil and root-containing soil reach their maximum values, but decrease as the water content increases. Soil depth and plant roots have minimal impact on the internal friction angle. The shear strength of the root-soil composite was greater than that of rootless soil, with the increment in additional cohesion being highest for Goosegrass, followed by Cogonggrass, and Alfalfa. The correction factor k of the WWM model was optimized, and a more accurate method for quantifying the soil reinforcement effect of plant roots was established. The results of this study provide a scientific basis for the ecological restoration of slopes in open-pit mine waste dumps, guiding the selection and configuration of suitable protective plants to enhance the stability of these slopes.