Landslides are a recurrent geohazard in Andean urban environments, where weak soils, intense seasonal rainfall, and unplanned urban expansion combine to increase slope vulnerability. In such settings, sustainable hillside management requires stabilization strategies that are both technically effective and environmentally compatible. This study evaluates the effect of root reinforcement by vetiver grass (Chrysopogon zizanioides) on slope stability in two representative soils from Loja, Ecuador: sandy silt (SM) and sandy clay (SC). A reduced-scale physical model with 30 days of root development was established, and consolidated–drained direct shear tests (ASTM D3080/D3080M-23) were performed to determine the shear strength parameters under bare and vetiver-reinforced conditions. These parameters were then incorporated into numerical slope stability analyses using Slide and PLAXIS 2D, considering three slope angles (30°, 45°, and 50°), six root-positioning configurations, and hydraulic conditions with and without a water table. Vetiver increased effective cohesion by 22.7% in sandy silt and 19.0% in sandy clay, while the internal friction angle increased by 21.8% and 12.2%, respectively. Across all modeled scenarios, vetiver produced a consistent improvement in the factor of safety. The most critical case, corresponding to sandy silt at 45° with a water table, increased from FS = 0.841 in the control condition to FS = 1.309 under the full-coverage configuration. Parametric sensitivity analysis yielded coefficients of variation between 4.97% and 7.03%, indicating a stable model response under controlled parameter perturbations. These findings support vetiver as an experimentally grounded and environmentally sustainable Nature-based Solution for slope stabilization and provide relevant evidence for sustainable management of hazard-prone urban hillsides in vulnerable Andean settings.

Plant roots can modify soil mechanical behaviour through anchorage, interlocking, and stress-transfer mechanisms; however, the magnitude of this effect is not uniform and depends on the soil matrix in which the root system develops. This study evaluates the biomechanical reinforcement induced by Chrysopogon zizanioides (L.) Roberty roots in seven contrasting Andean soil matrices classified as CL-ML, MH, CH, OH, OL, CL, and ML. Bare-soil and Vetiver-rooted specimens were compared across three depth intervals: 0.00–0.50 m, 0.50–1.50 m, and 1.50–2.00 m. Shear-strength parameters were determined through direct shear tests under controlled normal stresses of 50, 100, and 200 kPa, and the relative amplifications of apparent cohesion (Δc) and internal friction angle (Δφ) were calculated. ANOVA, Tukey HSD post hoc tests, effect-size analysis, descriptive depth-dependent regression analyses, and shear-resistance sensitivity analysis were then applied to identify soil-dependent reinforcement patterns. The results showed that the mechanical contribution of Vetiver roots was primarily controlled by soil type. The highest relative cohesion amplification was obtained in OL soil, where Δc reached 100.0%, whereas the strongest friction-angle response was observed in CH soil, with Δφ reaching 20.1%. MH soil exhibited an intermediate and technically balanced response, combining moderate cohesion gains with relevant frictional improvement. One-way ANOVA screening indicated that soil type was the main differentiating factor controlling the Vetiver-induced response, while depth had a less dominant effect. Tukey HSD comparisons confirmed that OL exhibited a distinct cohesion-amplification pattern relative to the other soil matrices. These findings indicate that C. zizanioides root reinforcement should not be interpreted as a uniform plant effect but as a soil-matrix-dependent biomechanical response governed by root–soil interaction. The initial mechanical condition of the soil, rather than root content alone, determines whether Vetiver reinforcement is expressed primarily as a cohesion gain, a friction-angle gain, or a combined response. Because the statistical evidence is screening-level owing to the absence of replication within soil–depth cells, these patterns should support soil-specific interpretation rather than be applied as universal amplification factors.

Vegetation-based slope stabilization is commonly evaluated from the final factor of safety, which can obscure whether an apparently favorable response reflects intrinsic slope resistance or the incremental effect of the vegetation treatment. This study tests whether the factor hierarchy associated with absolute stability differs from that associated with vegetation-induced stabilization and whether this distinction is preserved across limit equilibrium and finite element formulations. Seven fine-grained Andean soil classes, four vegetation systems, three slope geometries, and baseline and surcharge conditions were evaluated using laboratory-derived equivalent Mohr–Coulomb parameters. Each vegetated scenario was paired with an otherwise identical bare-soil reference, and the balanced numerical design was decomposed into factor and interaction contributions to variation across the investigated design space. Within the validated limit equilibrium domain, soil and geometry represented 72.1% and 26.6% of absolute stability variation, whereas species and soil × species represented 11.8% and 10.5% of relative vegetation-induced variation after normalization. Across 210 paired cases, agreement between methods was strong for absolute stability (r = 0.88; concordance coefficient = 0.83), but markedly weaker for vegetation-induced benefit. The results demonstrate a transferable distinction between the factors associated with system stability and those associated with intervention effectiveness. For engineering and restoration practice, plant selection should therefore follow site characterization and treatment-level numerical assessment, rather than a universal species ranking.

Expansive clay slopes are vulnerable to progressive strength loss induced by repeated wetting and drying, a mechanism that drives shallow failure in active moisture zones. Reproducing this degradation experimentally is time-consuming and resource-intensive. This study evaluates whether Fully Softened Strength (FSS) can serve as a practical substitute for five wet–dry cycles in expansive clay slope stability assessment. Direct shear tests were conducted on wet–dry-cycled and reconstituted FSS specimens across fourteen experimental water contents. Strength parameters were incorporated into homogeneous and heterogeneous limit equilibrium slope models, considering degraded layer thicknesses of 1–5 m and suspended water table conditions. Equivalence was assessed using root mean square error (RMSE), prediction bias, and physical representativeness. Five wet–dry cycles produced a dominant cohesion reduction of 70.4% with minor changes in friction angle, reaching a quasi-stationary degraded state. FSS reproduced an equivalent system response through mechanical compensation between cohesion and friction—not through equality of strength parameters—under shallow failure conditions. The best statistical fit was obtained at w = 43.5% (RMSE = 0.314); however, w = 42.0%, coinciding with the liquid limit, provided a physically more robust interpretation with near-zero bias. Equivalence was found to be valid only for normal stresses ≤ 50 kPa, representative of shallow failure depths of 1–4 m.