Improvement in Stability of a Tropical Hillslope via Mechanical Root Reinforcement

Vegetation can be a natural and cost-effective bioengineering solution to improve the stability of hillslopes. The mountainous hilly terrain of tropical islands including Guam is plagued with numerous rainfall-induced shallow slope failures. Even several houses are located along coastal hillslopes, and as such, there is a dire need for finding eco-friendly solutions to protect the slope failures and loss of soil into oceans. In this study, soil hydrological and geotechnical properties including the soil water characteristic tests were conducted on native soil collected from hillslopes in southern Guam. Also, Pandanus tectorius, a native tropical plant, was selected, and its roots were tested for their tensile strength. It was found that the tensile strength offered by roots decreased with increasing root diameter. The best-fit exponential and power-fit relationship between root diameter and tensile strength were established. Root morphological characteristics were documented, and root bundle theory was used to translate the tensile strength offered by roots into additional root cohesion, i.e., root enhanced mechanical shear strength by soil-root matrix. Numerical simulations show that vegetation was able to provide a decent amount of stability through an increase in factor of safety via root water uptake and mechanical root reinforcement.