Mechanical properties of roots of Vetiveria zizanioides as protection slope plants under tensile and pullout conditions

Root anchoring in the soil can provide significant resistance for plants to erosion and potential soil shear failure on a slope. Typical failures of plant roots can be found under the pullout and tensile conditions. The pullout angle varies greatly for the different roots, where the root penetrates various directions in the soil. However, the influence of pullout angle on the characteristic of mechanical properties and failure types of plant roots under tensile and pullout conditions can still far from being fully understood so far. Among them, Vetiveria zizanioides has been widely used for ecological slope protection, and soil and water conservation, due to its excellent adaptability, strong resistance, and well-developed root system. Taking the Vetiveria zizanioides as the study material, a series of tensile and pullout tests were conducted to determine the mechanical properties and failure types of roots. The roots were also planted in a plant groove of 100 cm in length, 40 cm in width, and 50 cm in depth. A single root tensile test was firstly performed on the roots with different diameters (0.20-1.40 mm). A pullout test was used to simulate the root in different directions in the soil, particularly with the different pullout angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°). Some parameters were recorded, including the tensile stress-strain curves, the maximum tensile force, and tensile strength for the tensile test, while the pullout force, the pullout displacement curves, the maximum pullout force, and pullout strength for the pullout test. The failure types were identified, according to the resulting relations of the maximum tensile force, tensile strength, the maximum pullout force, and pullout strength to the root diameter. The results indicated that the maximum tensile and pullout force increased, with the increase of root diameter, whereas, the tensile and pullout strength decreased, indicating the power function of the relationships (R2≥0.75, P<0.001). The maximum pullout force increased in the range of 0.48-2.71, 0.59-4.16, 0.79-4.08, 0.95-4.47, 1.86-5.39, 2.20-6.39, and 2.95-7.46 N for the pullout angle of 0°, 15°, 30°, 45°, 60°, 75°, and 90°, respectively. The failure type of the root depended mainly on the root diameter and pullout angle. The thinner root was a more likely failure with the pullout failure, while the number of tensile failure roots and the critical root diameter increased with the pullout angle for the tensile failure type. The numbers of tensile failure roots were 0, 1, 2, 3, 3, 4, and 5 for the pullout angle of 0°, 15°, 30°, 45°, 60°, 75°, and 90°, respectively, and the corresponding of critical root diameter for the tensile failure type was observed at 0, 0.32, 0.43, 0.43, 0.49, 0.52 and 0.46 mm, respectively. Specifically, the root, suffered the tensile failure, when the root maximum tensile force was smaller than the maximum pullout force, due to the snap during the pullout test, and vice versa, the root suffered the pullout failure, when the root maximum tensile force was higher than the maximum pullout force. Therefore, the influence of root diameter and pullout angle on the failure type of the root was attributed to the increase in the maximum tensile force. As such, the root diameter and pullout angle posed significant effects on the root pulling resistance and root failure type. The finding can provide a strong reference for the mechanism of root reinforce soil, particularly for slope stability, erosion, surface landslides, and geological hazards prevention. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.