Reliability-Based Slope Stability Analysis Along the 
Bonga-Felegeselam Road, Ethiopia

Ermias Shitu; Aklilu Shitu Gebremariam; Sufiyan Ahmad; Mohd Sheob

doi:doi:10.11648/j.ajce.20261402.13

Research Article |

| Peer-Reviewed

Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia

Ermias Shitu^*

, Aklilu Shitu Gebremariam

, Sufiyan Ahmad

, Mohd Sheob

Published in American Journal of Civil Engineering (Volume 14, Issue 2)

Received: 26 February 2026 Accepted: 9 March 2026 Published: 27 March 2026

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Abstract

Slope instability is a common problem in mountainous road corridors, particularly in regions characterized by steep terrain, weak soil formations, and intense seasonal rainfall. The Bonga-Felegeselam road in southwestern Ethiopia has experienced several slope failures during construction due to these unfavorable geological and hydrological conditions. This study investigates the causes of instability and evaluates appropriate stabilization measures for two critical cut slopes located between Stations 16+900 to 16+980 (left-hand side, LHS) and Stations 35+200 to 35+300 (both LHS and RHS) of LOT I: Bonga Asphalt Road Project. Detailed site characterization was conducted through field observations, geometric configuration, soil classification, and laboratory testing to determine the relevant geotechnical properties. Slope stability was assessed using both deterministic and probabilistic approaches based on the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The analyses considered different groundwater conditions to evaluate the influence of rainfall-induced saturation on slope stability. The results indicate that the slopes become highly unstable under fully saturated conditions, with factors of safety ranging from 0.657 to 0.916 and failure probabilities between 41.8% and 95.3%. Sensitivity analysis further showed that slope stability is more sensitive to variations in the friction angle than cohesion. To mitigate the instability, a combination of masonry retaining walls, surface drainage, and subsurface drainage systems was proposed. Post-remediation analyses demonstrated a significant improvement in slope stability, increasing the factor of safety to values between 1.57 and 1.90. The findings highlight the importance of integrating deterministic and probabilistic approaches to develop reliable stabilization strategies for rainfall-prone mountainous regions.

Published in	American Journal of Civil Engineering (Volume 14, Issue 2)
DOI	10.11648/j.ajce.20261402.13
Page(s)	67-81
Creative Commons	This is an Open Access article, distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution and reproduction in any medium or format, provided the original work is properly cited.
Copyright	Copyright © The Author(s), 2026. Published by Science Publishing Group

Keywords

Slope Stability Analysis, Limit Equilibrium Method, Finite Element Method, Cut Slope Stability Analysis, Deterministic Approach, Probabilistic Approach

1. Introduction

Experience is the best teacher but often not the kindest. Failures provide invaluable lessons and insights for preventing future issues, especially in geotechnical engineering where slope instability poses serious risks

[1]

. Slope instability is one of the most common problems in road construction projects in hilly areas where large earthworks involving excavations of 3-4 million cubic meters are done to fulfill road standards

[2, 3]

. These problems often arise due to insufficient site investigation and inadequate slope design, leading to failures during or after excavation, resulting in significant project cost overruns and delays

[4, 5]

. Southwest Ethiopia is home to the Bonga-Felegeselam Road Project, exemplifies these challenges. Ethiopian Roads Authority (ERA) identified the area as landslide-prone due to its steep topography, high-intensity rainfall, and weak geological formations

[6, 7]

. Several studies have highlighted that such conditions; including geomorphological variability, old landslide scars, and shallow groundwater tables, contribute significantly to slope instability in Ethiopian highlands

[8-10]

. Furthermore, seasonal rainfall from March to October with high intensity accelerates the saturation of soil masses, leading to progressive failure

[6, 9]

Slope failures in Ethiopia have been extensively studied in various regions including the Blue Nile Basin, Dessie area, and Rift margins, all of which exhibit similar failure mechanisms dominated by rainfall infiltration and weak clayey soils

[7, 8, 11, 12]

. Researchers have used limit equilibrium and finite element methods to assess slope stability, showing that failure often occurs under fully saturated, drained conditions

[1, 9, 10, 13]

. As road construction expands across mountainous and escarpment terrains, such failures become increasingly common, posing severe risks to public safety and infrastructure reliability

[4, 5, 11]

. Therefore, this study focuses on analyzing the causes and mechanisms of slope failures at two critical cut-slope sections along the Bonga-Felegeselam Road Project. By integrating deterministic and probabilistic slope stability analyses using both LEM and FEM approaches

[1-3, 9]

, the study aims to identify primary failure causes and propose appropriate remedial measures. The numerical results from these analyses are verified with field-observed failures to ensure practical applicability

[10, 14]

2. Materials and Methods

2.1. Location of the Study Area

The project area is situated is in southwest Ethiopia and may be reached by the main asphalt route that runs from Addis Ababa to Bonga via Wolkite and Jimma. The road splits off in the direction of Felegeselam town after Bonga. Two failed cut slopes located near Bonga town were selected for this investigation. The terrain is characterized by steep topography, high rainfall, and deeply weathered residual soils, which contribute to slope instability

[4-6]

. The geological and geomorphological conditions of this area are similar to other landslide-prone highlands of Ethiopia

[7, 8]

2.2. Materials

The slope stability analysis used parameters derived from field and laboratory investigations. Subsurface profiling was carried out based on geophysical survey results and borehole drilling. Laboratory and in-situ tests were conducted to determine geotechnical parameters such as unit weight, cohesion, angle of internal friction, and shear strength

[1, 9, 10]

. These parameters were used for both deterministic and probabilistic analyses. The factor of safety (FOS) was computed under effective stress conditions by considering pore pressure and varying groundwater levels. For full saturation, a pore fluid unit weight of 9.81 kN/m³ was applied to model excess pore pressure, consistent with previous works

[11]

. This method aligns with established practices for analyzing slope behavior under high rainfall and saturation conditions

[12, 13]

. Field observation indicated circular failure surfaces typical of cohesive soil slopes, modeled using the Mohr-Coulomb failure criterion

[1, 9, 14]

. Representative failure geometry and soil conditions were recorded for further analysis (see Figures 1a, 1b for site conditions such as tension cracks and seepage zones).

Download: Download full-size image

Figure 1. Field observation at failed slope (a) Tension crack observed, (b) Seepage water emerging from the slope during the site investigation.

2.3. Methods

Data for the analysis were obtained from two main sources: primary field data and secondary data from road construction agencies. Primary data included direct field observations of scarps, cracks, seepage, and toe erosion, while interviews with local residents helped reconstruct the slope failure history

[5, 6]

. The field observations confirmed that failures occurred after heavy rainfall events and not due to seismic activity, consistent with findings from similar Ethiopian studies

[7, 8]

Data processing included several steps:

1) Selection of representative failure locations.

2) Development of slope geometry using survey data and AutoCAD templates.

3) Interpretation of borehole and triaxial test data.

4) Generation of finite element meshes using PLAXIS software

[3, 9]

These analyses followed well-established geotechnical methods that integrate LEM and FEM modeling

[1, 2, 10]

. The LEM employed the Morgenstern-Price method, while FEM simulations used the Mohr-Coulomb constitutive model to represent soil behavior

[3, 9]

. Uncertainty in soil parameters was addressed using the probabilistic approach described by

[1]

and

[15]

. The input parameters for Station 16+980 used in the LEM and FEM analyses are presented in Tables 1 and 2, respectively. Similarly, the input parameters for Station 35+270 used in the LEM and FEM analyses are provided in Tables 3 and 4, respectively.

2.4. Selection of Potential Failed Slope Location

Several failure-prone areas were identified along the project corridor, but two critical sites were chosen based on the availability of geological, geotechnical, and topographical data. Both locations Stations 16+980 and 35+270 exhibited prominent tension cracks, seepage zones, and toe erosion features (see Figure 2a and 2b for geometric representations). The geometry-based modeling provided realistic input for numerical analysis and improved interpretation of slope stability results

[9, 10, 13]

Download: Download full-size image

Figure 2. Slope geometry adopted for the stability analysis: (a) model of the cut slope at Station 16+980 and (b) model of the cut slope at Station 35+270.

2.5. Soil Properties

At Station 16+980, borehole data and laboratory testing revealed two main layers:

Layer 1 (0-6 m): Light brown, moist, high-plasticity CL with sand that is firm to extremely stiff.

Layer 2 (6-16 m): Light grey to pale yellow, low-plasticity CL with sand, firm to hard.

At Station 35+270, three layers were identified:

Layer 1 (0-7.5 m): Extremely stiff to hard, CL with pebbles, dark grey with orange specks.

Layer 2 (7.5-13.5 m): Firm to stiff, reddish-brown CL.

Layer 3 (13.5-22 m): Hard, light brown, dry, gravelly CL.

Laboratory results (Tables 1-4) summarize the geotechnical parameters used for the analyses. The soils exhibit low permeability and moderate to high plasticity, with reduced shear strength under saturation, consistent with prior findings by

[20]

and

[10]

. These soil properties make slopes vulnerable to failure during prolonged rainfall events

[5, 6, 12]

Table 1. Input parameters of station 16+980 for limit equilibrium analysis obtained from laboratory test.

Geotechnical properties	subsurface profile of the cut slope at Km 16+980
unsaturated unit weight (γun)	layer 1	layer 2
unsaturated unit weight (γun)	17.3	19.02
saturated unit weight (γsat)	18.12	19.02
cohesion (KN/m2)	20.45	31.66
groundwater table	2.5m	below

Table 2. Input parameters of station 16+980 for finite element analysis obtained from laboratory test.

Geotechnical properties	subsurface profile of the cut slope at Km 16+980
unsaturated unit weight (γun)	layer 1	layer 2
unsaturated unit weight (γun)	17.3	19.02
saturated unit weight (γsat)	18.12	19.02
cohesion c' (KN/m2)	17.01	28.77
angle of friction ∅’ [°]	4.2	6
young's modulus E'[KPa]	4526.33	14190.11
poisonous ratio ν’ [-]	0.35	0.35
dilatancy angle ψ [°]	0	0
ground water table (GWT)	2.5m	below

Table 3. Input parameters of station 35+270 for limit equilibrium analysis obtained from laboratory test.

Geotechnical properties	cut slope subsurface profile at Km 16+980
bulk unit weight (ɣbulk) [KN/m3]	layer 1 (0-7.5m)	layer 2 (7.5-13.5m)	layer 3 (13.5-22m)
bulk unit weight (ɣbulk) [KN/m3]	17.54	18.33	18.75
cohesion c' (KN/m2)	14.1	34.2	49.3
angle of friction ∅’ [°]	3.8	5.1	7.3
ground water table (GWT)	nill

Table 4. Input parameters of station 35+270 for finite element analysis.

Geotechnical properties	cut slope subsurface profile at Km 16+980
bulk unit weight (ɣbulk) [KN/m3]	layer 1 (0-7.5m)	layer 2 (7.5-13.5m)	layer 3 (13.5-22m)
bulk unit weight (ɣbulk) [KN/m3]	17.54	18.33	18.75
cohesion c' (KN/m2)	14.1	34.2	49.3
angle of friction ∅’ [°]	3.8	5.1	7.3
young's modulus E'[KPa]	5543.48	20702.27	38489.38
poisonous ratio ν’ [-]	0.35	0.35	0.35
dilatancy angle ψ [°]	0	0	0
ground water table (GWT)	nill

3. Analysis of the Failed Cut Slopes

3.1. Failure of the Cut Portion at Km 16+980 (LHS)

Field inspection confirmed that the slope failure occurred in a circular mode, extending approximately 12 m downslope from the crown, with a detachment depth between 1-2.5 m

[1, 2]

. The affected slope section is shown in Figure 3a. Sensitivity analysis was performed to define appropriate model boundaries, and as presented in Figure 3b, the factor of safety (FOS) stabilized as the domain width increased, consistent with the findings of

[1]

. The Limit Equilibrium Method (LEM) (Table 6) employed the Morgenstern-Price method, while the Finite Element Method (FEM) (Table 7) used the Mohr-Coulomb constitutive model

[3, 9]

. Variability in soil parameters was incorporated using the N-sigma (Nσ) rule, following the approach recommended by

[1]

and

[15]

. As shown in Figure 4a and 4b, the FOS decreased notably under fully saturated, drained conditions. For LEM, FOS values ranged from 0.916 to 1.054, while FEM results ranged from 0.762 to 1.021. These results confirm that saturation and elevated pore-water pressure are the principal destabilizing factors for the slope

[9, 10, 13]

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Figure 3. (a) View of the landslide affecting the roadside cut at Station 16+980 (LHS), and (b) sensitivity analysis conducted to determine the appropriate model geometry for the stability analysis.

Uncertainties in soil properties were evaluated using the Nσ rule, which considers the highest and lowest conceivable parameter values based on engineering judgment. The standard deviation and coefficient of variation (COV) were computed using Equations (1) and (2) and compared with the recommended ranges given by

[1]

in Table 5.

σ = \frac{HCV - LCV}{4}

(1)

COV = \frac{σ}{average value}

(2)

Considering that the slope material consists predominantly of clayey soil, and allowing for uncertainties arising from sampling, transportation, and laboratory testing, COV values of 30% for cohesion and 10% for friction angle were finally adopted for the probabilistic analysis

[19]

Table 5. Coefficients of Variation for Geotechnical Properties and in Situ Tests (Duncan, J. M., 2014).

Property or in Situ Test	COV (%)	References
Unit weight (𝛾)	3-7	Harr (1987), Kulhawy (1992)
Buoyant unit weight (𝛾b)	0-10	Lacasse and Nadim (1997), Duncan (2000)
Effective stress friction angle (𝜙′)	2-13	Harr (1987), Kulhawy (1992), Duncan (2000)
Undrained shear strength (su) and c′	13-40	Kulhawy (1992), Harr (1987), Lacasse and Nadim (1997)
Undrained strength ratio (su∕𝜎v′)	5-15	Lacasse and Nadim (1997), Duncan (2000)
Standard penetration test blow count (N)	15-45	Harr (1987), Kulhawy (1992)
Electric cone penetration test (qc)	5-15	Kulhawy (1992)
Mechanical cone penetration test (qc)	15-37	Harr (1987), Kulhawy (1992)
Dilatometer test tip resistance (qD)	5-15	Kulhawy (1992)
Vane shear test undrained strength (sv)	10-20	Kulhawy (1992)

Table 6. The outcome of station 16+980's LEM analysis.

Method of analysis	Conditions considered	Factor of safety		Probability of failure
Method of analysis	Conditions considered	At normal GWT	Full saturation	At normal GWT	Full saturation
LEM	undrained	1.054	0.964	41.80%	60.40%
LEM	drained	1.043	0.916	47.60%	66.90%

Download: Download full-size image

Figure 4. Results of slope stability analyses for Station 16+980: (a) LEM (b) FEM.

Table 7. The outcome of station 16+980's FEM.

Method of analysis	Conditions considered	Factor of safety
Method of analysis	Conditions considered	At normal GWT	Full saturation
Finite element analysis	undrained	1.026	0.785
Finite element analysis	drained	1.021	0.762

3.2. Failure of the Cut Section at Km 35+270

Both sides of the road were impacted by the failure at this station. Failure geometry extended about 15 m from the crest, and sensitivity analysis was conducted to determine the appropriate mesh and boundary (Figure 5). The LEM results (Table 8) showed FOS values between 0.678 and 0.965, with failure probabilities ranging from 60% to 95%. The FEM results (Table 9) confirmed a critical FOS of 0.657 under full saturation, as shown in Figure 6. These results correspond closely with

[9]

and

[10]

, who showed that fully saturated conditions produce minimum safety factors. Similar rainfall-induced failures have been reported in Ethiopian road projects

[5-8, 11]

Table 8. The limit equilibrium analysis result of station 35+270.

Method of analysis	Conditions considered	Factor of safety				Probability of failure
		At normal GWT		Full saturation		At normal GWT		Full saturation
		RHS	LHS	RHS	LHS	RHS	LHS	RHS	LHS
LEM	drained	0.79	0.965	0.678	0.856	87.80%	60.50%	95.30%	80.94%

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Figure 5. Analysis of station 35+270 using finite elements.

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Figure 6. RHS and LHS analysis of station 35+270 using LEM while taking full saturation conditions into account.

Table 9. The outcome of station 35+270's finite element analysis.

Method of analysis	Conditions considered	Factor of safety
Method of analysis	Conditions considered	At normal GWT	Full saturation
Finite element analysis	drained	0.813	0.657

4. Result and Discussion

The slope stability analyses for both investigated stations confirmed that failures occurred primarily under fully saturated conditions. The calculated factors of safety (FOS) from the finite element analysis ranged between 0.657 (for Station 35+270 under full saturation) and 1.021 (for Station 16+980 under normal groundwater conditions), indicating overall instability of the existing slopes. This pattern is consistent with findings by

[1]

and

[2]

, who emphasized the high sensitivity of cohesive slopes to increases in pore-water pressure and moisture content. Intense rainfall and human activities, such as irrigation, trigger slope instability

[22, 23]

. By consequence, To evaluate how soil strength parameters influence stability, a sensitivity analysis was performed by varying cohesion (c) and friction angle (φ) independently using the General Limit Equilibrium (GLE) method

[26, 27]

. Each parameter was increased by 25%, 50%, 75%, and 100%, while keeping other conditions constant. The results are summarized in Table 10.

Table 10. Sensitivity analysis of strength factors to determine how they affect FOS.

No.	Cut section (Km)	FS Design stability	Factor of safety
			Cohesion Increment by				The angle of friction increment
			25%	50%	75%	100%	25%	50%	75%	100%
1	16+980	0.762	1.136	1.1351	1.566	1.781	0.997	1.234	1.554	1.722
2	35+270 (LHS)	0.856	1.054	1.261	1.462	1.704	0.865	1.112	1.384	1.584

As shown in Table 10, the factor of safety exceeded 1.5 (stable condition) only when cohesion or friction angle increased by 75 - 100%. The slope at Km 16+980 achieved stability at both 75% and 100% increments, while at Km 35+270, stability was reached only at 100% increases. The right-hand side (RHS) of Station 35+270 remained marginally stable even under the most favorable conditions. Overall, variations in the friction angle improved stability more effectively than cohesion. This observation agrees with the classification of

[4, 21]

and

[5]

, where FOS < 1.0 indicates failure, 1.0-1.5 represents questionable stability, and FOS > 1.5 represents a safe condition

[25]

FEM provided more realistic stress-strain and pore-pressure distributions than the limit equilibrium approach, as also reported by

[9]

and

[28]

. Recent studies have further demonstrated FEM's capability to capture complex geological effects on seismic behavior in challenging terrains

[31]

. The probabilistic failure probability (Pf) ranged between 41.8% and 95.3%, reflecting significant variability in soil strength due to natural heterogeneity and sampling uncertainty

[13, 17]

. These probabilities are consistent with reliability studies by

[17]

and

[12]

. Both deterministic and probabilistic analyses demonstrated that slope failures in the study area are primarily controlled by high rainfall infiltration, low shear strength, and fluctuating groundwater levels. These findings underscore the necessity of proper drainage control and the installation of retaining structures to ensure long-term slope stability along Ethiopian road corridors.

There are two choices to improve the stability of any slopes: increasing resistance or decreasing driving forces. In the remediation of the landslide, the countermeasures may be either Control or Restraint work as shown in Figure 7.

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Figure 7. Slope failure countermeasures.

Remedial measure at 16+980

The main causes of failure were the steep cut angle, presence of springs and shallow groundwater, and weak fine-grained soils

[5, 6, 11]

. To mitigate these issues, a masonry retaining wall combined with surface and subsurface drainage was constructed. In mountainous road corridors, reducing the self‑weight of retaining and slope‑supporting structures through lightweight concrete made with volcanic scoria

[16]

can help lower driving forces on potential failure surfaces and thus contribute to improved slope stability. The area lies in a low-seismic-hazard zone (PGA = 0.03 g)

[24, 29]

. After remediation, both LEM and FEM analyses (Tables 11 and 12) indicated FOS values between 1.625 and 1.896, with negligible failure probability. As shown in Figure 8, FEM results revealed a substantial reduction in displacement, with a maximum of 60.32 × 10⁻³ m. These findings are consistent with

[10]

and

[28]

, who confirmed the effectiveness of retaining walls in stabilizing saturated slopes.

Table 11. The station 16+980 LEM result following corrective action.

Method of analysis	Conditions considered	Factor of safety		Probability of failure
Method of analysis	Conditions considered	At normal GWT	Full saturation	At normal GWT	Full saturation
LEM	undrained	1.896	1.687	0.000%	0.000%
LEM	drained	1.789	1.649	0.000%	0.000%

Table 12. The outcome of station 16+980's FEM following corrective action.

Method of analysis	Conditions considered	Factor of safety
Method of analysis	Conditions considered	At normal GWT	Full saturation
Finite element analysis	undrained	1.769	1.697
Finite element analysis	drained	1.703	1.625

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Figure 8. Comparison of slope stability analyses at Station 16+980 after remedial measures using (a) the Limit Equilibrium Method and (b) the Finite Element Method.

Remedial measure at 35+270

The slope at this location failed due to steep cutting, intense rainfall infiltration, and low-strength clayey soils. Stabilization involved constructing a masonry retaining wall, improving surface drainage, and adding toe protection. Post-remediation LEM results (Table 13) showed improved FOS values between 1.585 and 1.928, while FEM results (Table 14) ranged from 1.576 to 1.639. The FEM deformation map (Figure 9) displayed a maximum displacement of 23.69 × 10⁻³ m, confirming slope stability recovery. These combined countermeasures align with the stabilization approaches recommended by

[11]

and

[18]

Table 13. The station 35+270 LEM result following corrective action.

Method of analysis	Conditions considered	Factor of safety				Probability of failure
		At normal GWT		Full saturation		At normal GWT		Full saturation
		RHS	LHS	RHS	LHS	RHS	LHS	RHS	LHS
Limit equilibrium m analysis	drained	1.928	1.677	1.83	1.585	0.00%	0.00%	0.00%	0.50%

Table 14. The outcome of station 35+270's FEM study following corrective action.

Method of analysis	Conditions considered	Factor of safety
Method of analysis	Conditions considered	At normal GWT	Full saturation
Finite element analysis	drained	1.639	1.576

Download: Download full-size image

Figure 9. Stability analysis results for Station 35+270 after implementation of remedial measures: (a) LEM and (b) FEM.

5. Conclusion

This study explored the causes of slope failures and developed practical solutions for two critical cut slopes located at Km 16+980 and Km 35+270 along the Bonga-Felegeselam road. Using both deterministic and probabilistic analyses through the Limit Equilibrium and Finite Element Methods, the investigation clearly showed that steep slope geometry, shallow groundwater levels, and rainfall-induced saturation were the main factors behind the observed failures. These conditions reduced the soil’s shear strength and triggered instability during periods of intense rainfall. To restore stability, a combination of control and restraint measures such as masonry retaining walls, surface and subsurface drainage systems, and toe protection were designed and implemented. After these remedial works, the factor of safety (FOS) increased significantly, from below 1.0 to above 1.6, confirming the effectiveness of the interventions

[30]

. The finite element results also revealed a marked reduction in displacement, indicating the success of the stabilization measures.

Overall, the findings emphasize that effective drainage management combined with well-designed retaining structures can greatly enhance slope stability in Ethiopia’s mountainous and rainfall-prone regions. Moreover, integrating deterministic and probabilistic methods provides a more comprehensive understanding of slope behavior, improving both design confidence and long-term safety for future infrastructure projects.

6. Recommendation for Future Research

While this study provided valuable insights into the causes and control of slope instability, further research is needed to deepen our understanding and improve design reliability. The following recommendations are suggested:

1) Back-Analysis for Better Accuracy: Future studies should perform detailed back-analyses of actual slope failures to fine-tune laboratory-derived soil parameters and improve the accuracy of numerical models.

2) Stronger Probabilistic Assessment: Collecting more field and laboratory data will help refine the variability of soil parameters such as cohesion and friction angle, allowing for more dependable reliability-based designs.

3) Use of 3D Modelling: Employing three-dimensional finite element analysis will provide a more realistic picture of slope behavior, especially in complex terrains where 2D models have limitations.

4) Long-Term Field Monitoring: Installing piezometers and inclinometers on remediated slopes will make it possible to monitor changes in groundwater levels and slope movement over time, offering valuable feedback on the long-term effectiveness of the implemented measures.

5) Incorporating Remote Sensing and GIS Tools: Integrating satellite imagery and GIS-based analysis can help identify vulnerable zones early, enabling proactive mitigation planning for road networks across similar terrains.

These steps will not only strengthen the scientific understanding of slope behavior but also contribute to safer, more resilient, and cost-effective road infrastructure across Ethiopia’s highlands.

Abbreviations

CL	Lean Clay (Unified Soil Classification System)
COV	Coefficient of Variation
ERA	Ethiopian Roads Authority
FEM	Finite Element Method
FOS	Factor of Safety
GLE	General Limit Equilibrium
GWT	Ground Water Table
LEM	Limit Equilibrium Method
LHS	Left-Hand Side
$N σ$	N-sigma (Statistical variability rule)
$P_{f}$	Probability of Failure
PGA	Peak Ground Acceleration
RHS	Right-Hand Side
RI	Reliability Index
RSBF	Reduced Set of Base Functions
SBFEM	Scaled Boundary Finite Element Method
UIRM	Unit Impulse Response Matrix

Author Contributions

Ermias Shitu: Investigation, Data curation, Methodology, Software, Formal analysis, Writing - original draft

Aklilu Shitu Gebremariam: Investigation, Data curation, Methodology, Software, Formal Analysis, Writing - original draft

Sufiyan Ahmad: Investigation, Data curation, Methodology, Software, Formal Analysis

Mohd Sheob: Software, Formal Analysis, Writing - review & editing

Data Availability Statement

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conﬂicts of interest.

Appendix: Notations and Symbols

1. Greek Letters

Table 15. Symbols used in the paper.

Symbol	Description	Unit
$Γ$	Unit weight	kN/m³
$γ_{un}$	Unsaturated unit weight	kN/m³
$γ_{sat}$	Saturated unit weight	kN/m³
$γ_{bulk}$	Bulk unit weight	kN/m³
$ϕ^{'}$	Effective angle of internal friction	degrees (°)
$c^{'}$	Effective cohesion	kN/m²
$Ψ$	Dilatancy angle	degrees (°)
$ν^{'}$	Poisson's ratio	dimensionless [-]
$Σ$	Standard deviation	varies
$σ^{'}$	Effective stress	kN/m²
$σ_{v}^{'}$	Effective vertical stress	kN/m²
$Τ$	Shear stress	kN/m²
μ	Mean value	varies

2. Subscripts and Superscripts

Table 16. Expression of some parameters used in the paper.

Notation	Meaning
' (prime)	Effective Stress Parameter $(e . g ., c^{'}, ϕ^{'})$
un	Unsaturated Condition
sat	Saturated Condition
bulk	Bulk Property
v	Vertical (as in $σ_{v}^{'}$ )
w	Water (as in Unit Weight of Water)

3. Dimensionless Numbers & Field Tests

Table 17. Abbreviations used.

Symbol	Description
$COV$	Coefficient of Variation $(COV = σ / μ)$
$N$	Standard penetration test (SPT) blow count
$q_{c}$	Cone penetration test (CPT) resistance
$q_{D}$	Dilatometer test tip resistance
$s_{u}$	Undrained shear strength

References

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[2]	Cheng, Y. M., & Lau, C. K. (2014). Slope stability analysis and stabilization: New methods and insight. CRC Press. https://doi.org/10.4324/9780203927953
[3]	Brinkgreve, R. B. J., & Vermeer, P. A. (2003). PLAXIS Version 8 material model manual. Delft University of Technology.
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Shitu, E., Gebremariam, A. S., Ahmad, S., Sheob, M. (2026). Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia. American Journal of Civil Engineering, 14(2), 67-81. https://doi.org/10.11648/j.ajce.20261402.13

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ACS Style

Shitu, E.; Gebremariam, A. S.; Ahmad, S.; Sheob, M. Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia. Am. J. Civ. Eng. 2026, 14(2), 67-81. doi: 10.11648/j.ajce.20261402.13

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Shitu E, Gebremariam AS, Ahmad S, Sheob M. Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia. Am J Civ Eng. 2026;14(2):67-81. doi: 10.11648/j.ajce.20261402.13

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@article{10.11648/j.ajce.20261402.13,
  author = {Ermias Shitu and Aklilu Shitu Gebremariam and Sufiyan Ahmad and Mohd Sheob},
  title = {Reliability-Based Slope Stability Analysis Along the 
Bonga-Felegeselam Road, Ethiopia},
  journal = {American Journal of Civil Engineering},
  volume = {14},
  number = {2},
  pages = {67-81},
  doi = {10.11648/j.ajce.20261402.13},
  url = {https://doi.org/10.11648/j.ajce.20261402.13},
  eprint = {https://article.sciencepublishinggroup.com/pdf/10.11648.j.ajce.20261402.13},
  abstract = {Slope instability is a common problem in mountainous road corridors, particularly in regions characterized by steep terrain, weak soil formations, and intense seasonal rainfall. The Bonga-Felegeselam road in southwestern Ethiopia has experienced several slope failures during construction due to these unfavorable geological and hydrological conditions. This study investigates the causes of instability and evaluates appropriate stabilization measures for two critical cut slopes located between Stations 16+900 to 16+980 (left-hand side, LHS) and Stations 35+200 to 35+300 (both LHS and RHS) of LOT I: Bonga Asphalt Road Project. Detailed site characterization was conducted through field observations, geometric configuration, soil classification, and laboratory testing to determine the relevant geotechnical properties. Slope stability was assessed using both deterministic and probabilistic approaches based on the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The analyses considered different groundwater conditions to evaluate the influence of rainfall-induced saturation on slope stability. The results indicate that the slopes become highly unstable under fully saturated conditions, with factors of safety ranging from 0.657 to 0.916 and failure probabilities between 41.8% and 95.3%. Sensitivity analysis further showed that slope stability is more sensitive to variations in the friction angle than cohesion. To mitigate the instability, a combination of masonry retaining walls, surface drainage, and subsurface drainage systems was proposed. Post-remediation analyses demonstrated a significant improvement in slope stability, increasing the factor of safety to values between 1.57 and 1.90. The findings highlight the importance of integrating deterministic and probabilistic approaches to develop reliable stabilization strategies for rainfall-prone mountainous regions.},
 year = {2026}
}

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TY  - JOUR
T1  - Reliability-Based Slope Stability Analysis Along the 
Bonga-Felegeselam Road, Ethiopia
AU  - Ermias Shitu
AU  - Aklilu Shitu Gebremariam
AU  - Sufiyan Ahmad
AU  - Mohd Sheob
Y1  - 2026/03/27
PY  - 2026
N1  - https://doi.org/10.11648/j.ajce.20261402.13
DO  - 10.11648/j.ajce.20261402.13
T2  - American Journal of Civil Engineering
JF  - American Journal of Civil Engineering
JO  - American Journal of Civil Engineering
SP  - 67
EP  - 81
PB  - Science Publishing Group
SN  - 2330-8737
UR  - https://doi.org/10.11648/j.ajce.20261402.13
AB  - Slope instability is a common problem in mountainous road corridors, particularly in regions characterized by steep terrain, weak soil formations, and intense seasonal rainfall. The Bonga-Felegeselam road in southwestern Ethiopia has experienced several slope failures during construction due to these unfavorable geological and hydrological conditions. This study investigates the causes of instability and evaluates appropriate stabilization measures for two critical cut slopes located between Stations 16+900 to 16+980 (left-hand side, LHS) and Stations 35+200 to 35+300 (both LHS and RHS) of LOT I: Bonga Asphalt Road Project. Detailed site characterization was conducted through field observations, geometric configuration, soil classification, and laboratory testing to determine the relevant geotechnical properties. Slope stability was assessed using both deterministic and probabilistic approaches based on the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The analyses considered different groundwater conditions to evaluate the influence of rainfall-induced saturation on slope stability. The results indicate that the slopes become highly unstable under fully saturated conditions, with factors of safety ranging from 0.657 to 0.916 and failure probabilities between 41.8% and 95.3%. Sensitivity analysis further showed that slope stability is more sensitive to variations in the friction angle than cohesion. To mitigate the instability, a combination of masonry retaining walls, surface drainage, and subsurface drainage systems was proposed. Post-remediation analyses demonstrated a significant improvement in slope stability, increasing the factor of safety to values between 1.57 and 1.90. The findings highlight the importance of integrating deterministic and probabilistic approaches to develop reliable stabilization strategies for rainfall-prone mountainous regions.
VL  - 14
IS  - 2
ER  -

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Author Information

Ermias Shitu

Independent Researcher, Addis Ababa, Ethiopia

Contact Email

http://orcid.org/0009-0005-0240-6439
Aklilu Shitu Gebremariam

Department of Civil Engineering, Addis Ababa Science and Technology University (AASTU), Addis Ababa, Ethiopia

Contact Email

http://orcid.org/0000-0002-8187-0096
Sufiyan Ahmad

Department of Civil & Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan

Contact Email

http://orcid.org/0009-0009-1922-8199
Mohd Sheob

Department of Civil, Environmental and Architectural Engineering, University of Colorado Boulder, Boulder Colorado, United States

Contact Email

http://orcid.org/0000-0001-9358-3871

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Shitu, E., Gebremariam, A. S., Ahmad, S., Sheob, M. (2026). Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia. American Journal of Civil Engineering, 14(2), 67-81. https://doi.org/10.11648/j.ajce.20261402.13

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ACS Style

Shitu, E.; Gebremariam, A. S.; Ahmad, S.; Sheob, M. Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia. Am. J. Civ. Eng. 2026, 14(2), 67-81. doi: 10.11648/j.ajce.20261402.13

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AMA Style

Shitu E, Gebremariam AS, Ahmad S, Sheob M. Reliability-Based Slope Stability Analysis Along the Bonga-Felegeselam Road, Ethiopia. Am J Civ Eng. 2026;14(2):67-81. doi: 10.11648/j.ajce.20261402.13

Copy | Download

@article{10.11648/j.ajce.20261402.13,
  author = {Ermias Shitu and Aklilu Shitu Gebremariam and Sufiyan Ahmad and Mohd Sheob},
  title = {Reliability-Based Slope Stability Analysis Along the 
Bonga-Felegeselam Road, Ethiopia},
  journal = {American Journal of Civil Engineering},
  volume = {14},
  number = {2},
  pages = {67-81},
  doi = {10.11648/j.ajce.20261402.13},
  url = {https://doi.org/10.11648/j.ajce.20261402.13},
  eprint = {https://article.sciencepublishinggroup.com/pdf/10.11648.j.ajce.20261402.13},
  abstract = {Slope instability is a common problem in mountainous road corridors, particularly in regions characterized by steep terrain, weak soil formations, and intense seasonal rainfall. The Bonga-Felegeselam road in southwestern Ethiopia has experienced several slope failures during construction due to these unfavorable geological and hydrological conditions. This study investigates the causes of instability and evaluates appropriate stabilization measures for two critical cut slopes located between Stations 16+900 to 16+980 (left-hand side, LHS) and Stations 35+200 to 35+300 (both LHS and RHS) of LOT I: Bonga Asphalt Road Project. Detailed site characterization was conducted through field observations, geometric configuration, soil classification, and laboratory testing to determine the relevant geotechnical properties. Slope stability was assessed using both deterministic and probabilistic approaches based on the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The analyses considered different groundwater conditions to evaluate the influence of rainfall-induced saturation on slope stability. The results indicate that the slopes become highly unstable under fully saturated conditions, with factors of safety ranging from 0.657 to 0.916 and failure probabilities between 41.8% and 95.3%. Sensitivity analysis further showed that slope stability is more sensitive to variations in the friction angle than cohesion. To mitigate the instability, a combination of masonry retaining walls, surface drainage, and subsurface drainage systems was proposed. Post-remediation analyses demonstrated a significant improvement in slope stability, increasing the factor of safety to values between 1.57 and 1.90. The findings highlight the importance of integrating deterministic and probabilistic approaches to develop reliable stabilization strategies for rainfall-prone mountainous regions.},
 year = {2026}
}

Copy | Download

TY  - JOUR
T1  - Reliability-Based Slope Stability Analysis Along the 
Bonga-Felegeselam Road, Ethiopia
AU  - Ermias Shitu
AU  - Aklilu Shitu Gebremariam
AU  - Sufiyan Ahmad
AU  - Mohd Sheob
Y1  - 2026/03/27
PY  - 2026
N1  - https://doi.org/10.11648/j.ajce.20261402.13
DO  - 10.11648/j.ajce.20261402.13
T2  - American Journal of Civil Engineering
JF  - American Journal of Civil Engineering
JO  - American Journal of Civil Engineering
SP  - 67
EP  - 81
PB  - Science Publishing Group
SN  - 2330-8737
UR  - https://doi.org/10.11648/j.ajce.20261402.13
AB  - Slope instability is a common problem in mountainous road corridors, particularly in regions characterized by steep terrain, weak soil formations, and intense seasonal rainfall. The Bonga-Felegeselam road in southwestern Ethiopia has experienced several slope failures during construction due to these unfavorable geological and hydrological conditions. This study investigates the causes of instability and evaluates appropriate stabilization measures for two critical cut slopes located between Stations 16+900 to 16+980 (left-hand side, LHS) and Stations 35+200 to 35+300 (both LHS and RHS) of LOT I: Bonga Asphalt Road Project. Detailed site characterization was conducted through field observations, geometric configuration, soil classification, and laboratory testing to determine the relevant geotechnical properties. Slope stability was assessed using both deterministic and probabilistic approaches based on the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The analyses considered different groundwater conditions to evaluate the influence of rainfall-induced saturation on slope stability. The results indicate that the slopes become highly unstable under fully saturated conditions, with factors of safety ranging from 0.657 to 0.916 and failure probabilities between 41.8% and 95.3%. Sensitivity analysis further showed that slope stability is more sensitive to variations in the friction angle than cohesion. To mitigate the instability, a combination of masonry retaining walls, surface drainage, and subsurface drainage systems was proposed. Post-remediation analyses demonstrated a significant improvement in slope stability, increasing the factor of safety to values between 1.57 and 1.90. The findings highlight the importance of integrating deterministic and probabilistic approaches to develop reliable stabilization strategies for rainfall-prone mountainous regions.
VL  - 14
IS  - 2
ER  -

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