Experimental–Numerical Assessment of the Geomechanical Potential of Chrysopogon zizanioides (L.) Roberty for Root Reinforcement of Filtered Mine Tailings Under Controlled Conditions

Abstract

Mine tailings are highly disturbed technogenic materials whose low mechanical stability may limit mine closure and long-term land rehabilitation. This study evaluates the geomechanical potential of Chrysopogon zizanioides (L.) Roberty, commonly known as vetiver grass, to improve the shear-strength response of filtered mine tailings under controlled laboratory and numerical modelling conditions. The study does not constitute field-scale validation of phytostabilization; rather, it examines the contribution of vetiver roots to apparent cohesion and shallow slope stability. A combined experimental–numerical framework was implemented, including laboratory characterization of unreinforced and root-reinforced tailings, derivation of Mohr–Coulomb shear-strength parameters, and limit-equilibrium slope-stability analysis under predefined root-growth and root-orientation scenarios. The results indicate that vetiver roots increased apparent cohesion by up to 34.6%, whereas changes in friction angle remained below 10%, suggesting that the dominant reinforcement mechanism is pseudo-cohesive rather than frictional. The calculated factors of safety varied according to slope geometry, assumed root length, root orientation, and simplified water-condition scenarios. However, the findings remain limited to controlled experimental and numerical conditions. Field-scale validation, long-term root monitoring, moisture variability, nutrient availability, phytotoxicity, contaminant immobilization, and life-cycle performance should be assessed before practical implementation. This study provides preliminary geomechanical evidence of vetiver-induced root reinforcement in filtered mine tailings.

1. Introduction

Mining generates large volumes of tailings, which can create long-term geotechnical and environmental liabilities when stored in tailings deposits. Physical instability, erosion, and shallow failures can compromise closure works, vegetation establishment, and long-term land rehabilitation [1].

In recent years, the transition to sustainable resource management has highlighted the need to rethink the concept of mining tailings, considering them not only as waste but also as potential resources within the framework of the circular economy [2,3]. The principles of the circular economy advocate minimizing waste generation, improving resource efficiency, and promoting the recovery and reuse of materials [4]. In the mining sector, this approach has become increasingly relevant as a strategy to reduce environmental risks while simultaneously generating added value from waste materials [5,6].

Physical instability poses a critical barrier to the rehabilitation of mining tailings deposits, as surface failures, erosion, and poor surface integrity can impede vegetation establishment and limit long-term land recovery. In this context, vegetation-based reinforcement can be analyzed from a biogeotechnical perspective, particularly through the contribution of roots to the apparent cohesion and stability of surface slopes. In this study, phytostabilization is used only as the broader conceptual framework. The demonstrated evidence is restricted to the geomechanical contribution of vetiver roots under controlled laboratory and numerical conditions [7].

Despite its recognized environmental benefits, the application of phytostabilization techniques to mining tailings has been studied primarily from a remediation perspective, with little emphasis on its mechanical contribution to slope stability. In particular, the role of plant root systems in improving shear strength parameters and the overall stability of tailings deposits has not yet been sufficiently quantified [8].

Previous studies on vegetation in mining-affected substrates have focused primarily on environmental remediation, contaminant tolerance, or erosion control [9]. In contrast, the quantitative mechanical contribution of root systems to the shear strength parameters of filtered mine tailings is still not sufficiently documented, especially through studies that combine laboratory tests with slope stability models. Although vegetation-based remediation of mine-affected substrates has been widely discussed, the quantitative contribution of root systems to the shear-strength parameters of filtered tailings remains insufficiently documented, particularly when laboratory testing is coupled with slope-stability analysis [10,11,12].

Addressing this gap is crucial to promoting phytostabilization as a viable strategy within sustainable mining and waste valorization systems. Recent studies have highlighted the importance of integrating ecological rehabilitation with engineering approaches to achieve long-term stability and sustainability [13,14].

Vetiver is relevant to bioengineering because its dense, fibrous, and predominantly vertical root system can mobilize tensile resistance, interlocking, and anchorage within near-surface soil or tailings layers [15,16,17].

Therefore, this study aims to evaluate the geomechanical potential of vetiver roots to improve the shear-strength response and shallow slope stability of filtered mine tailings under controlled laboratory and numerical modelling conditions. The specific objectives are: (i) to compare the shear strength parameters of tailings without reinforcement and with roots, (ii) to quantify the apparent cohesion contribution associated with vetiver roots, and (iii) to evaluate the influence of predefined root length and orientation scenarios on calculated slope stability [18].

2. Materials and Methods

2.1. Methodological Framework

The methodological framework consisted of four sequential stages (Figure 1). First, filtered mine tailings were physically and mechanically characterized under unreinforced tailings. Second, vetiver plants were established in controlled-growth columns, and intact root and tailings specimens were prepared for laboratory testing. Third, shear strength parameters for unreinforced tailings and root-reinforced tailings were obtained from laboratory tests and interpreted using the Mohr-Coulomb framework. Fourth, the resulting parameters were incorporated into limit equilibrium slope stability analyses to compare unreinforced tailings and root-reinforced tailings conditions under defined slope geometries (FT), root orientation and growth length (PROG) configurations, and predefined water condition assumptions. This framework allows for a direct link between experimental evidence and the numerical stability response.

2.2. Regional Context and Characteristics of the Area

The tailings material was collected near the canton of Yantzaza, Zamora Chinchipe province, southern Ecuador, located in a Jurassic basin with managed lateral faults. The tailings developed before being covered by sediments, and are hosted in volcanic-andesitic rocks and sediments of the Santiago Formation, where the LSMR industries are located. The area has a mountainous terrain with diverse slopes and an irregular configuration. The altitude ranges from 1400 to 2020 m above sea level, and the mining area lies between 1420 and 1630 m above sea level, bordering the Machinaza River. The area features steep slopes between 24% and 50%, with local inclines where the structure crosses eroded deposits [19,20]. Figure 2 presents an overview of the regional and local environment.

2.3. Determination of the Characteristics of the Tailings

The behavior of the filtered tailings was evaluated within a dry soil mechanics framework, using representative material sampling, and was classified as a low-plasticity clayey silt (CL) according to the Unified Soil Classification System (USCS) (ASTM D2487-17e1) [21]. Laboratory tests were performed according to ASTM standards, including moisture content analysis (ASTM D2216) [22], density and absorption analysis (ASTM C128-25) [23], particle size analysis by sieving (ASTM E112-13r21) [24], Atterberg limits (ASTM D4318-17e01) [25], direct shear tests (ASTM D6528-24) [26], and triaxial compression tests (ASTM D7181-20) [27]. The experimentally derived values were subsequently used as input parameters for the slope-stability analyses summarized in

Table 1. Geotechnical properties of filtered mine tailings under bare conditions.

Parameters	Units	Documented Data *	Experimental Data
Moisture content	%	12	9.69
Unit weight	kN/m3	16–16.50	13.62
Saturated weight	kN/m3	20	14.06
Absorption percentage	%	-	3.24
Liquid limit	%	-	26.32
Plastic limit	%	-	18.24
Plasticity index	%	No plastic	8.08
USCS Classification	-	ML–CL	CL
Sand (0.075 mm < d < 2.0 mm)	%	52.8	32
Silt (0.002 mm < d < 0.075 mm)	%	31.7	51
Clay (d < 0.002 mm)	%	15.5	17
Cohesion	kPa	0–30	6
Angle of friction	°	25–35	30
Young’s modulus	kPa	1300	–
Poisson ratio	υ	0.25–0.30	–
Porosity	η	0.20–0.70	–

* Documented values were used only as reference ranges; experimental values were used as model inputs [28,29,30,31].

.

2.4. Characteristic Parameters of Tailings with Vetiver Grass

The biological and biomechanical characteristics of vetiver were compiled to define the root reinforcement parameters used in the experimental and numerical framework. These parameters included the number of roots, root diameter, tensile strength, root-soil contact area, and the assumed root growth pattern. The collected values were not used to demonstrate phytostabilization at the field scale, but rather to support the interpretation of the mechanical interaction between vetiver roots and filtered tailings (

Table 2. Biomechanical properties of plants.

Parameters	Units	Documented Data *
Number of roots in a plant	u	50
Root modulus of elasticity (EA)	MPa	2150
Average tensile strength	kN	188.50
Average area of a root	mm2	0.1–0.3
Surface area occupied in the soil by roots	m2/m2	0.3–0.5
Crop growth rate	m2	0.6
Maximum tensile strength	N	40
Growth spacing	m	1
		0.8
		0.6
Awaiting its growth	°	25
		35
		45
Root cohesion	kPa	–
Angle of friction	°	–
Sample alterations **	%	30 to 20

* The same research characteristics were adopted within the FEM program and parameters eight and nine with slope angles of 25°, 35° and 45° [32,33]. ** This refers to the approximate loss or alteration of root and leaf material during the experimental process. It does not represent biological degradation under field conditions.

). Using the compiled and experimental data from the filtered tailings, direct shear tests (ASTM D6528-24) [26] and triaxial compression tests (ASTM D7181-20) [27] were performed on specimens with the root system present, in order to compare the mechanical response between the unreinforced tailings and root-reinforced tailings conditions.

The novelty of this study lies in the controlled assessment of vetiver-induced root reinforcement in filtered mine tailings, with emphasis on the contribution of roots to apparent cohesion and slope-stability response, which is valuable within the framework of the International Vetiver Network [16].

2.5. Experimental Growth Column and Intact Preparation of Root Tailings Specimens

Two polyethylene columns were prepared by cutting the upper section of each container. Each column was 33 cm high and 18 cm in internal diameter. The columns were filled with filtered mine tailings to a height of 20 cm, and drainage holes were made at the base to prevent water accumulation during controlled irrigation (Figure 3). Drainage wells were integrated into the base of each container, and irrigation was performed using a percentage of the experimentally determined capacity (approximately 1.6% of the volumetric water content) of the tailings’ volumetric moisture content.

The plant material used in this study consisted of commercially cultivated Chrysopogon zizanioides (L.) Roberty (Poaceae), commonly known as vetiver grass. Plant material was obtained from the Municipal Nursery of Loja, Ecuador, on 6 October 2025. The nursery is located at 3°57′25.01″ S, 79°13′03.27″ W and maintains commercially propagated stock under municipal management, including controlled fertilization, propagation, maintenance, and public distribution.

Vetiver was established from cuttings placed directly into the filtered tailings substrate. Chrysopogon zizanioides (L.) Roberty is a non-woody perennial grass of the Poaceae family, native to South and Southeast Asia, recognized for its dense, fibrous root system. It is widely used in tropical and subtropical regions for erosion control, surface stabilization, and rehabilitation of degraded soils.

The material was not collected from wild populations and does not correspond to rare, threatened, or endangered taxa. Chrysopogon zizanioides is not included in the Appendices of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) [34,35]. The acquisition and use of plant material complied with applicable institutional and national regulations and were conducted in accordance with the principles established by the Convention on Biological Diversity (CBD) regarding the conservation and sustainable use of biological resources [36].

No genetic modification, molecular characterization, DNA sequencing, or access to protected genetic resources was undertaken as part of this research. The plants were used exclusively in controlled laboratory column experiments designed to evaluate root reinforcement effects and their contribution to the geomechanical stabilization of filtered mine tailings (Supplementary Materials Figures S1–S4).

Because the study did not involve taxonomic, floristic, phylogenetic, conservation, or biodiversity assessments, no herbarium voucher specimen was required for the research objectives. However, photographic records of the plant material, collection site, and cultivation source were documented and archived by the authors to facilitate future verification of taxonomic identity.

In bioengineering applications, this root architecture may improve the mechanical response of the substrate through anchorage, interlocking, and tensile stress transfer. However, establishment in mine tailings may be constrained by nutrient deficiency, salinity, extreme pH, metal toxicity (Pb, As, Cd, Cr, Cu, and Zn), water stress, and site-specific climatic conditions. In this study, these ecological limitations were not fully quantified; therefore, biological observations are interpreted as preliminary indicators of establishment, rather than as evidence of field-scale phytostabilization [16,37,38,39,40,41,42,43,44,45].

Two polyethylene columns were prepared for controlled vetiver establishment. Each column was 33 cm high and 18 cm in internal diameter and was filled with filtered mine tailings to a height of 20 cm. Drainage holes were made at the base of each column to prevent water accumulation during controlled irrigation. Vetiver cuttings were placed directly in the tailings substrate. The experiment lasted 30 days. No external fertilizer was added; however, the actual nutrient content of the tailings was not measured. Therefore, the study does not claim plant growth under complete nutrient absence. At the end of the experiment, the observed root length was approximately 37 cm. Root development may have been influenced by the initial cutting length, controlled irrigation, drainage, clayey-silt tailings texture, aeration, local temperature, and short-term plant adaptation. Initial wilting was observed during early establishment, followed by partial foliar regeneration from the second week. These observations indicate preliminary establishment in an unfertilized tailings substrate but do not constitute a complete assessment of nutrient limitation or phytotoxicity. Figure 4 shows the development of the vetiver grass after 30 days, along with the profiling scheme of the test pellets (Supplementary Materials Figures S1–S4).

2.6. Determination of the Geotechnical Reliability Index

Safety Factor in Bare State Versus Vegetation Cover

Dry stability analyses were performed to isolate the mechanical contribution of the root reinforcement. The pore water pressure adjacent to the proposed sliding sections for the zero scenario was set to zero (no infiltration, evapotranspiration, or suction). Additional scenarios with water conditions were included only as simplified comparative assumptions and should not be interpreted as transient seepage, rainfall infiltration, evapotranspiration, or unsaturated flow simulations. The mechanical characteristics of the sections and the shear strength parameters (c′, φ′) were assigned from the laboratory study of the material structure. Surcharge and seismic actions were not evaluated. The analysis focused on surface translational failures; consequently, the sliding surface record was limited to strata from 0 to 5 m.

The Morgenstern–Price limit-equilibrium method was applied because it satisfies both force and moment equilibrium for general slip surfaces. This method is suitable for evaluating two-dimensional surface stability scenarios in tailings slopes, using shear strength parameters derived from laboratory tests. Within this range, a complete 2D balance analysis generates reliable safety factors suitable for application in project-scale assessments [46]. The factor of safety (FS) is determined using the Morgenstern-Price limit equilibrium method, which analyzes the equilibrium of forces and moments in individual blocks divided on the surface with a constant force slope between sections λ. For section I, the existing shear strength along the base is

$$R_i={c_i}^{\prime }{b}_i+{N_i}^{\prime }·\tan \left({{\varphi }_i}^{\prime }\right)$$

(1)

and the safety factor is considered as:

$$FS={\displaystyle \frac{\sum i{R}_i}{\sum S_i{\left(FS,\lambda \right)}^{\prime }}}$$

(2)

Equations (1) and (2). ${c_i}^{\prime }=$ effective cohesion; ${{\varphi }_i}^{\prime }=$ effective friction angle; $b_i=$ section base length; ${N_i}^{\prime }=$ effective normal force; $S_i{\left(FS,\lambda \right)}^{\prime }=$ mobilized base shear stress; $\lambda =$ constant force inclination between sections.

Where $\sum Si{\left(FS,\lambda \right)}^{\prime }$ is the mobilized shear demand developed from the joint solution of the global force and moment equilibrium with constant slope of the force between sections λ. In the recent study under dry conditions, the infiltration into the pores along the slip zone is equal to zero, therefore $N{i}^{\prime }=Ni$.

The slope typology (ST) was established with a 3H:1V, 2H:1V, and 1H:1V geometry, reflecting configurations adopted in mining tailings. Reinforcement at the root of the VG acts as a variable that increases effective cohesion and redistributes shear loads under certain conditions. However, it is necessary to understand the mechanical contribution of the root system and its performance in critical situations, which is demonstrated through ongoing quantitative studies in mining deposits in Andean tropical climates.

Figure 5 presents the ten numerical scenarios used for each typology to evaluate the influence of the plant’s root configuration, orientation, and growth (PROG): bare FT (starting point) using proposed conditions for depth and direction ${c^{\prime }}_{FT}\left(z,\theta \right)$, and for plants using experimental ${{\varphi }^{\prime }}_{FT}\left(z,\theta \right)$ and achieved ${{\varphi }^{\prime }}_{FT+GV}\left(z,\theta \right)$ values ${c^{\prime }}_{FT+GV}\left(z,\theta \right)$ from direct shear and triaxial compression tests in the root-tailings matrix. The experimentally observed root lengths were limited to approximately 32–37 cm after 30 days and, in the numerical analysis, including 1 m and 5 m, were treated as parametric scenarios based on the potential root development reported in the literature, and not as lengths achieved during the laboratory experiment; simplified water scenarios were not coupled seepage or unsaturated-flow simulations.

Root anchorage increases effective cohesion to apparent cohesion and influences modifications of the friction angle, as observed in the filtered tailings; both configurations are considered in terms of resistance $R_i={c_i}^{\prime }{b}_i+{N_i}^{\prime }·\tan \left({{\varphi }_i}^{\prime }\right)$. For each scenario, the critical slip surface is generated from the minimum factor of safety (FS) within the 0 to 5 m margins; the results were classified and related to the quantification of the plant’s mechanical contribution. The scenarios were simulated in both dry and undersaturated conditions. Prototype and laboratory development was carried out in partially wet states, when the pore water pressure present on the proposed slip surface was moderate; this allowed for the analysis of the mechanical effects introduced by the roots (tensile reinforcement and interfacial entanglement) with the hypothesis of integrated hydromechanical consequences in the modeling data (water retention in the tailings, hydraulic conductivity, rainfall forcing/evapotranspiration).

2.7. Sensitivity Analysis

A local sensitivity analysis (one factor at a time) was performed to assess the influence of uncertainty in shear strength parameters on the calculated factor of safety and to determine its effectiveness. Cohesion (c′) and friction angle (φ′) were varied independently within predefined ranges of ±5% and ±10%, respectively, while the other parameters were held constant for both direct and triaxial shear (UU) [47,48]. For each section depth, and for both bare and vegetated approaches, the characteristics were modified independently and modeled using FEM to quantify the resulting variation in the factor of safety using the Morgenstern–Price method [25,46,49].

The objective was not to provide a probabilistic reliability analysis, but rather to examine the robustness of slope stability response to plausible variations in laboratory-obtained strength parameters. Non-physical, numerically determined outputs were noted and reported separately; valid simulation results were not excluded solely because they differed from the baseline.

3. Results

3.1. Geotechnical Evaluation of Mining Tailings (Filtration)

The tailings, with a representative sample, were classified under the Unified Soil Classification System (USCS) as low-plasticity clayey silt (CL). Laboratory tests performed according to ASTM standards (ASTM D2216, D4318, D6528-24, D7181-20) established the geotechnical basis presented in Table 1. The material, exhibiting a tendency towards cohesion, as CL, showed a higher liquid limit (26.32%) and plasticity index (8.08%). These results reflect a trend similar to other studies, and their classification as soil influences the failure mechanisms due to the development of vegetation reinforcement on the surface of the section.

The results of the direct shear and triaxial compression tests for the bare tailings are summarized in

Table 3. Root configuration numeric parameters.

Parameters	Units	Values	Conditions
Number of roots in a plant	u	50	Use in the model
Root modulus of elasticity (EA)	MPa	2150 *	Adopted
Average tensile strength	kN	188.50	Adopted
Average area of a root	mm2	0.2	Measured
Surface area occupied in the soil by roots	m2/m2	0.4	Measured
Crop growth rate	m2	0.6	Adopted
Maximum tensile strength	N	40	Adopted
Root cohesion	kPa	8–10	Use in the model
Angle of friction	°	31–32.62	Use in the model

* Considered a value adopted from research.

, which shows the cohesion (c′) and the internal friction angle (φ′). The results showed low variability among replicate tests, with cohesion values close to 6 kPa and friction angles close to 30°. These values are consistent with the conditions expected for the USCS classification category, validating the reliability of the data set obtained in the experiment.

3.2. Impact of Vetiver on Mechanical Strength Properties

Vetiver roots are typically found in greater density in the upper mantle and their concentration decreases with depth; they then act as anchors in the modeling process. Research has shown that the plant covers a diameter of 0.4 m, estimated to consist of 50 roots, each 8 mm long. Given that each root has a tensile strength of 75 MPa, the total tensile strength would be 188.5 kN. Similarly, with a modulus of elasticity ranging from 43 MPa to 2150 MPa, a tensile strength of 40 N is recommended. Direct shear and triaxial compression tests confirmed that vetiver improves cohesion (c′), as the angle of internal friction (φ′) in the filtered tailings (

Table 4. Tailings have shear strength properties under environmental conditions.

Floor	Resistance Parameters	No. 1	No. 2	No. 3	No. 4	No. 5	No. 6	No. 7	No. 8
CL	Cohesion c/kPa	6.02	6.05	6.00	6.03	6.02	6.07	6.01	6.03
	Friction angle φ/°	30.12	30.09	30.02	30.06	30.04	30.10	30.00	30.08

) showed a +20% increase in cohesion and a +5% increase in φ′. Laboratory results showed that apparent cohesion increased from approximately 6 kPa to approximately 8 kPa, whereas friction-angle changes were comparatively smaller.

3.3. Alteration of Vegetation on Slope Stability

The stability mechanism of mine slopes varies between bare material and vetiver reinforcement. In these areas, shear strength, reduced cohesion, and an increased friction angle relative to the surface are observed, and these characteristics change progressively with root development. The orientation of both perpendicular and random root fibers is comparable, and the planting location depends on the slope’s inclination; the steeper the slope, the lower the horizontal distance between plants, and vice versa. In the bare tailings state, the safety factors (SF) were modified by the slope structure and the saturation of the tailings system (WT) (

Table 5. Shear strength properties of tailings with plant anchors.

Floor	Resistance Parameters	No. 1	No. 2	No. 3	No. 4	No. 5	No. 6	No. 7	No. 8
CL	Cohesion c′/kPa	8.05	8.02	8.05	8.01	8.07	8.06	8.09	8.04
	Friction angle φ′/°	32.62	32.36	31.78	32.27	32.17	30.98	32.36	31.47

). For example, the SF in the 3H:1V ratio is 1.516, decreasing to 1.031 for 1H:1V, near the critical thresholds.

The numerical scenarios suggest that higher factors of safety (SF) are obtained when the assumed root-reinforcement zone is longer and more favorably oriented relative to the potential failure Surface. By integrating vetiver roots, the material demonstrated an increase in FS values in all three embankment types (

Table 6. Safety factors of filtered tailings in the bare external state.

Slope Geometry	Without WT	With WT
3H:1V	1.516	1.048
2H:1V	1.038	1.045
1H:1V	1.031	1.048

). For example, this increase suggests a direct correlation between root length and mechanical reinforcement efficiency, consistent with the root anchorage theory. The FS value ranged from 1.269 to 1.586, with root growth of 1 m to 5 m and varying angularity at −10% (25%) (

Table 7. Comparison of safety factors for bare and vetiver-reinforced filtered tailings slopes under defined scenarios of root growth, root orientation and water condition.

Slope Geometry	PROG		Absence of Groundwater Level (WT)	Groundwater Level Status (WT) *
	Length (m)	Direction (°)
3H:1V	0.3	90	1.035	-
	1.0	90	-	1.064
	5.0	80	1.295	-
2H:1V	5.0	90	1.030	-
	1.0	100	-	1.048
	5.0	80	1.036	-
1H:1V	5.0	80	1.030	-
	1.0	90	-	1.269
	5.0	80	-	1.586

* The highest values were considered based on PROG conditions and slope types.

). An FS value of 1.295 was particularly noteworthy at a depth of 5 m with a root orientation of 80°, compared to the absence of WT and the slopes of the three categories, which had FS values ranging from 1.03 to 1.036. These improvements were especially pronounced in the mining waste, based on the assumption that the deep, fibrous root system of vetiver develops and generates greater anchorage and shear strength.

3.4. Effects of Shear Strength on Plant Growth Characteristics and Orientation

In triaxial shear and compression tests, appreciable improvements in cohesion (c′) and internal friction angle (φ′) were confirmed with the presence of vetiver roots. Cohesion increased by up to 34.61%, whereas the maximum friction-angle increase was 8.41% (

Table 8. Changes in shear strength parameters induced by vetiver root reinforcement in filtered mine tailings.

Evidence	c′ Bare (kPa)	c′ Vetiver (kPa)	∆c′ Vetiver (%)	SE c′ Bare (kPa)	SE c′ Bare (kPa)	φ′ Naked (°)	Φ′ Vetiver (°)	∆φ′ Vetiver	SE φ′ Bare (°)	SE φ′ Bare (°)
1	6.02	8.05	33.72	0.007	0.009	30.12	32.62	8.30	0.014	0.173
2	6.05	8.02	32.56			30.09	32.36	7.54
3	6.00	8.05	34.17			30.02	31.78	5.86
4	6.03	8.01	32.84			30.06	32.27	7.35
5	6.02	8.07	34.05			30.04	32.17	7.09
6	6.07	8.06	32.78			30.10	30.98	2.92
7	6.01	8.09	34.61			30.00	32.36	7.87
8	6.03	8.04	33.33			30.08	31.47	4.62

). This response suggests that root-induced interlocking and tensile stress transfer mainly contributed to apparent cohesion.

The increase in cohesion and effective friction angles is consistent with the transmission of tensile forces (root strength reinforcement, shear resistance due to the stake effect), the tensile strength, and the mechanical blockage observed in the root-tailings interaction under the drying state. The percentage increase in shear strength properties (c′ and φ′) due to the anchorage of vegetation in the filtered tailings is illustrated in Figure 6, which highlights the recurring improvement in cohesion and friction angle.

3.5. Influence of Root Length and Orientation on Slope Stability

The numerical scenarios were designed to evaluate how assumed root length and orientation may influence shallow slope stability. Experimentally observed root lengths were limited to approximately 32–37 cm after 30 days. In contrast, the 1 m and 5 m root lengths used in the numerical analysis were treated as parametric scenarios based on potential mature root development reported in the literature, not as lengths achieved during the laboratory experiment. The simplified water-condition scenarios were used only for comparative sensitivity purposes and should not be interpreted as coupled simulations of rainfall infiltration, evapotranspiration, matric suction, or unsaturated flow.

3.6. Statistical Impetus and Confidence (Credibility of the Statistical Study)

The deterministic sensitivity analysis showed that unreinforced embankments are highly susceptible to variations in their characteristics. For example, cohesion and friction angle varied between ±5 and 10%, generating a deviation of up to 10.3% in their FS (Supplementary Materials Tables S1–S4). With the incorporation of vetiver grass anchors, the FS results changed and fell below 1.00, as stipulated by the Ministry of Urban Development and Housing of Ecuador (MINUVI) [50,51]. Therefore, the structure or material conditions become adverse in certain cases, making it essential to integrate external components for stabilization.

The majority of the statistical analysis supported this research. The FS values remained stable in most cases across different slope architectures and with varying levels of vetiver vegetation integration, while the coefficients of variation (CV%) were below 9%, thus supporting the validity of the findings. The standard deviation between the mechanical parameters of the tailings was significant (p < 0.01 kPa) in most analyses, ensuring consistency. Confidence intervals increased under reinforced conditions; for example, in the case of a 3H:1V slope with a 5 m growth rate and an 80° angle relative to the horizontal, FS changed from 1.082 to 1.19, compared to other more common ranges in modified materials. In summary, these results confirm a basis for integrating vetiver grass as anchors to reinforce mechanical and statistical variability, based on a circular economy approach with environmental integration.

3.7. Comparative Evaluation of the Orientation and Anchorage of Vetiver Grass

With the global comparative analysis (Supplementary Materials Tables S5 and S6) shows that vetiver grass gradually increases the FS in the mining metallurgical residue with the six PROG configurations, with greater growth development and orientation of 5 and 260°.

To visualize the improved changes caused by the vetiver anchors, Figure 7 summarizes the percentage increase in the Factor of Safety (FS) at distances of 0.3 m, 1 m, and 5 m, as well as the directions of 260°, 270°, and 280°. These figures demonstrate stable efficiency in stabilizing tailings slopes with low inherent shear strength, where the deep root structure of vetiver grass generates a high incidence of reinforcement.

4. Discussion

Using a parameterized limit-equilibrium framework, this study quantified changes in the factor of safety of filtered-tailings slopes under unreinforced and root-reinforced conditions. The results provide preliminary evidence that vetiver roots can improve the mechanical response of filtered mine tailings under controlled conditions, primarily by increasing apparent cohesion. Therefore, the analysis focuses on the geomechanical implications of root reinforcement, the limitations of the experimental-numerical framework, and the conditions that must be considered before applying phytostabilization at a field scale [47,48,49].

4.1. Influence of the Root System on the Mechanical Properties of Tailings

The results obtained show that the incorporation of vetiver grass induces a significant modification in the shear strength of the filtered tailings, primarily due to an increase in effective cohesion (c′), while the angle of internal friction (φ′) shows only marginal variations. This pattern is consistent with the theory of reinforced soils, where fibrous elements—in this case, the roots—introduce an additional resistant component of a tensile nature, which manifests as apparent cohesion.

The approximate 20–35% increase in c′ confirms that the reinforcement mechanism is controlled by the transfer of tensile stresses from the soil matrix to the root system, as well as by the anchoring and interlocking effects. In contrast, the limited variation in φ′ (<10%) suggests that the granular structure of the tailings does not undergo substantial changes, which is expected in fine-grained CL-type materials where friction is governed by inter-joint contacts and not by fibrous inclusions.

From a constitutive perspective, these results support modeling the root-tailings system using a modified Mohr-Coulomb failure envelope, in which the root contribution can be interpreted as an additional cohesive term dependent on root density, stiffness, and tensile strength. Consequently, the action of a biological agent in the pseudo-cohesive reinforcement mechanism is efficient, especially under low saturation conditions, where the soil-root interaction is not weakened by pore pressure.

4.2. Effect of Slope Stability with Biological Configurations

The comparison between bare and reinforced tailings shows that the incorporation of vetiver generates consistent increases in the factor of safety (FS), although of moderate magnitude (≈1.5–3.5%). However, these increases become significant when analyzed in the context of slopes close to the limit condition (FS ≈ 1.0), especially in more critical configurations such as 1H:1V. On bare slopes, the reduction in FS with increasing slope confirms the material’s high susceptibility to surface failures, especially under saturated conditions. The introduction of the root system modifies this behavior by redistributing shear stresses along the potential slip surface, acting as a distributed reinforcement system. It is important to note that the improvement in FS is not uniform across slope types. In flatter slopes such as 3H:1V, the effect of the reinforcement is less critical because the system already exhibits relatively stable conditions. In contrast, in steeper slopes such as 1H:1V and 2H:1V), the contribution of roots is more significant, as it directly helps to counteract the increased shear forces mobilized. Therefore, root reinforcement using vetiver should be interpreted as a complementary biogeotechnical measure that can improve surface stability under favorable conditions, but it should not be considered an independent stabilization solution for highly unstable slopes or for cases governed by high pore water pressures.

4.3. Relationship Between Root Growth, PROG and Shear Strength

The analysis of growth and orientation of articulated roots (PROG) shows that the efficiency of plant reinforcement depends largely on the geometry of the root system relative to the failure surface. In particular, it is observed that increasing root length (up to 5 m) produces substantial improvements in c′ and φ′, resulting in greater shear strength of the system.

This behavior can be explained by the concept of interception of slip surfaces, where roots act as reinforcing elements that mobilize additional resistance when soil deformation is demanded. The greater the root length, the greater the probability of intersecting with the critical surface, thus increasing the anchoring capacity.

Regarding orientation, the results indicate that angles close to 260–280° maximize mechanical contribution. This suggests that configurations with predominantly vertical components favor shear strength, allowing roots to work under tension in the face of relative displacements between soil blocks.

Similarly, the variability observed in φ′ may be associated with a mechanical entanglement effect induced by the presence of roots, although this mechanism is secondary to cohesive input. Overall, these results confirm that root reinforcement is not isotropic, but rather depends on the root-soil system architecture, which should be considered in advanced numerical modeling.

4.4. Evaluation of Geotechnical Performance According to Slope Type (ST) and PROG

The performance of the stabilization system clearly depends on the interaction between slope type (ST) and root configuration (PROG). The best results are achieved in experimental scenarios with longer root lengths (≈5 m) and optimal orientations (≈260°), where the system reaches stability conditions classified as “SF values”. In geotechnical terms, this indicates that the effectiveness of vegetation reinforcement is controlled by the geometric compatibility between the root system and the slope failure kinematics. On steeper slopes, reducing the distance between plants improves the continuity of the reinforcement, generating an effect similar to a three-dimensional containment mesh. On the other hand, in the pilot growth stages 0.3 m–1 m (experimental), scenario-dependent performance is limited because the roots have not yet developed their full resistance capacity. This highlights the importance of the time factor in stabilization, as the system evolves progressively until it reaches its maximum mechanical potential. These findings suggest that the design of vetiver-based solutions should consider not only the plant species, but also parameters such as planting density, expected root orientation, and establishment time, integrating agronomic and geotechnical criteria into a single design strategy.

4.5. Limitations

This study has several limitations that constrain the interpretation of the results. First, although the geotechnical tests and slope-stability analyses were designed to isolate the mechanical contribution of roots, vetiver establishment required controlled irrigation, which introduced partially saturated conditions in the growth columns. Second, the numerical representation of the root system was simplified; root area ratio, root density, spatial distribution, effective diameter, and anisotropic reinforcement were not directly quantified. Third, the experiment was conducted without external fertilizer addition, but nutrient content was not measured. Fourth, no geochemical or phytotoxicity characterization was performed; therefore, contaminant immobilization, metal tolerance, nutrient limitation, and plant toxicity response remain outside the demonstrated scope of this work. Finally, long-term root death, dormancy, or decay may progressively reduce tensile resistance, reinforcement continuity, and root–tailings interaction. The mechanical contribution of vetiver roots should therefore not be assumed to be permanent without long-term monitoring of plant survival, root renewal, below-ground biomass, and root degradation.

4.6. Reliability and Applicability

These results indicate model robustness within the tested parameter ranges; however, full probabilistic reliability assessment would require a larger dataset. However, its practical implementation requires field-scale validation and a comprehensive assessment of hydraulic, mechanical, biological, and geochemical factors. Moisture variability, root development over time, contaminant bioavailability, nutritional status, and maintenance requirements must be considered before this approach can be evaluated as a reliable phytostabilization strategy. This positions the study of mechanical reinforcement with vetiver grass not as an isolated solution, but as one component within a more complex stabilization system for mining slopes.

Coefficients of variation below 9% and statistical significance (p < 0.01) support the reliability of the results, although it is necessary to consider that the experimental model was developed under controlled conditions and in a dry state (u = 0). Under real conditions, factors such as matric suction, infiltration, and root degradation can modify the system’s response.

4.7. Sustainable Engineering Treatments as a Sustainable Solution

The results obtained under controlled laboratory conditions and numerical simulations suggest that vetiver roots can improve the mechanical behavior of filtered mine tailings, primarily by increasing apparent cohesion. This improvement is attributed to the formation of a root-tailings matrix capable of mobilizing additional tensile strength and interlocking resistance under surface failure conditions.

The experimentation with vetiver grass in mining tailings should be interpreted not only from a geotechnical perspective, but also within a broader sustainability framework. In this context, the present study could contributes to the understanding of nature-based solutions (NbS) as multifunctional strategies capable of simultaneously addressing environmental, technical, and resource efficiency challenges [52].

From a circular economy perspective, the in situ stabilization of mining tailings represents a shift from conventional linear waste management approaches toward regenerative systems in which waste materials are reintegrated into functional soil layers. Unlike traditional engineering solutions that rely on external materials (e.g., cement, geosynthetics, or structural reinforcements), phytostabilization minimizes the need for additional resources, thereby reducing material consumption and associated environmental burdens [13,18].

In quantitative terms, cohesion increased from approximately 6 kPa in unreinforced tailings to values close to 8 kPa in root-reinforced specimens. Likewise, the numerical scenarios showed varying safety factors depending on the slope geometry and assumed water conditions, with values close to the limit equilibrium in critical configurations and higher values under favorable root reinforcement scenarios.

Vegetation can provide additional ecosystem services, such as erosion control, dust reduction, and surface protection. However, contaminant immobilization was not assessed in this study and should not be claimed without geochemical and plant tissue analyses [53].

However, it is important to recognize that the sustainable performance of the soil-root matrix depends heavily on the context. Factors such as climatic conditions, water availability, long-term root development, and maintenance requirements can influence both its effectiveness and its environmental impact. In particular, the controlled irrigation required during the early stages of growth can introduce trade-offs related to water consumption, especially in semi-arid environments [13].

Overall, experimental and numerical evidence supports the potential use of vetiver root reinforcement as part of a broader tailings rehabilitation strategy. However, further evidence is needed regarding geochemistry, phytotoxicity, long-term plant survival, and field performance.

4.8. Future Implications and Research Needs

Future research should integrate detailed root-system quantification, geochemical characterization, phytotoxicity indicators, and field-scale monitoring. Priority should be given to root area ratio, root density, root diameter distribution, differentiation between fine and coarse roots, long-term root degradation, survival rate, chlorosis, biomass production, root elongation, and metal accumulation in plant tissues. In addition, life-cycle assessment, cost–benefit analysis, and socio-technical evaluation should be incorporated to determine whether vetiver-induced root reinforcement can evolve from a preliminary geomechanical response into a reliable phytostabilization strategy for mine-tailings rehabilitation.

5. Conclusions

This study evaluated the geomechanical potential of vetiver roots to reinforce filtered mine tailings under controlled laboratory and numerical modelling conditions. Root-reinforced specimens exhibited higher apparent cohesion than unreinforced tailings, increasing from approximately 6 kPa to values close to 8 kPa, with a maximum increase of 34.6%. Changes in friction angle remained below 10%, indicating that the dominant contribution of the root system was pseudo-cohesive reinforcement rather than a fundamental modification of the tailings frictional structure. Slope-stability analyses showed that the calculated factor of safety depends on slope geometry, assumed root length, root orientation, and simplified water-condition scenarios. However, the 1 m and 5 m root-length scenarios should be interpreted as parametric projections rather than root growth achieved during the 30-day test. The study does not demonstrate complete phytostabilization because nutrient content, phytotoxicity, contaminant immobilization, and field-scale ecological performance were not assessed. Future research should incorporate geochemical characterization, plant-response indicators, long-term root monitoring, field validation, and life-cycle or cost–benefit assessment before practical implementation.