Interaction of root pattern with tropical residual soil in bio-anchorage system for slope stabilization

The primary objectives of this study were to quantify how Eugenia Oleina (with H-type root architecture) improves slope stability in tropical residual soils and to assess the factors controlling this bio-anchorage effect. Laboratory tests measured soil shear strength (cohesion c′ and friction angle ϕ′) and root tensile properties, and a numerical slope analysis (finite element method) evaluated Factor of Safety (FOS) with and without vegetation under various rainfall scenarios. Results showed that planting E. Oleina increased shear strength: cohesion rose by 8-13 kPa and friction angle by 5-8° compared to unrooted soil, yielding higher FOS values. Root pull-out tests revealed that thinner E. Oleina roots exhibited higher tensile strength than thicker ones, consistent with a power-law trend. Specifically, mature E. Oleina roots (age > 10 yrs) attained tensile strengths 50 MPa, about 2,5 times higher than comparable M-type roots under dry conditions. However, in fully saturated soils, pull-out strength dropped by 33 %, illustrating moisture sensitivity. Finite element analysis confirmed the experimental findings: vegetation consistently improved slope stability (higher FOS) across rainfall conditions, although heavy rainfall (100 % saturation) reduced the FOS relative to moderate rain. These findings have practical implications: E. Oleina (an H-type species) is most effective on slopes up to 20° and 3,5 m high, especially in regions with intense rainfall, where its extensive horizontal roots can bind the soil surface. Guidelines are provided for implementing E. Oleina in bioengineering. The combined experimental and numerical results demonstrate E. Oleina’s value as a sustainable slope reinforcement strategy in tropical residual soils.