Root tensile strength of 3 typical plant species and their contribution to soil shear strength; a case study: Sasumua Backslope, Nyandarua District, Kenya

Abstract

The effect of root reinforcement depends on the morphological characteristics of the root system, the tensile strength of individual roots, the soil-root cohesive strength, and the distribution of the root system in the soil. This research looks into Sasumua Backslope in Kenya, where a great deal of erosion is responsible for large soil losses. Shallow mass movements are evident throughout the slope. Root reinforcement effect of nine typical species was assessed, using Wu et al. (1979) model. The plant species comprised of shrubs (Atriplex halimus), grasses (Pennisetum clandistenum and Themeda triandra), and tree ferns (Asparagus species). Physical counting of roots per depth class was conducted to obtain root area ratio (RAR) values of the individual species. For each species, single root specimens were sampled and tested for tensile tests in the laboratory using the Hounsfield Tensometer apparatus. Maximum RAR values were located within 0.1 m for all the species, with maximum rooting depth of 0.7 m for fern tree. Shrubs species showed high RAR values between 0.1 – 0.3 m depth. Tensile force increases with diameter. Generally shrubs break at high tensile force (160 N maximum), followed by tree ferns (maximum 90 N) and lastly grass (maximum 75 N). Root tensile strength decreases with increasing root diameter, and follows a power law equation of the form . Generally, tensile strength can be well predicted by root diameter. The maximum root tensile strength values recorded was 39 N/mm² for grass. The results presented in this paper contribute to expanding the knowledge on root resistance behavior and on root density distribution within the soil. The studied location has allowed the implementation of soil–root reinforcement models initially used by Wu et al. (1979) and the evaluation of the vegetation contribution to soil stability.