The study area is located in the West Region of Cameroon (Figure 1), within the Bafoussam III Subdivision, Mifi Division, with Bamougoum as its administrative headquarters. It is bordered to the south by the municipalities of Bamendjou (Hauts-Plateaux Division) and Pete-Bandjoun (Koung-Khi Division), to the west by Penka-Michel (Menoua Division), to the north by Bafoussam II and Mbouda (Bamboutos Division), and to the east by Bafoussam I, all within the Mifi Division. Geographically, the study area extends between latitudes 05˚27'00''N and 05˚30'00''N, and longitudes 10˚22'00''E and 10˚24'00''E.

Figure 1. Topographic map of the study area.

Field investigations included geological reconnaissance, slope delineation, and soil sampling. A representative natural slope was selected, and its geometry (height and inclination) was measured using a clinometer and reference stakes. Two types of soil samples were collected at mid-slope (B horizon):

Disturbed samples: 50 kg from four points for classification tests.Undisturbed samples: Extracted using PVC core samplers (10 cm diameter, 20 cm height) for mechanical testing, sealed with paraffin to preserve moisture.

Laboratory tests determined the physical, mechanical, and index properties of the soil, following French (NF) and/or Cameroonian (BNQ) standards.

Identification tests:

Natural water content (oven drying at 105˚C) ([6]);Particle size distribution (sieving and sedimentation) ([8]);Atterberg limits (Casagrande method) and derived plasticity and consistency indices (NF P94-051);Specific gravity (pycnometer method) ([7]).

Mechanical and compaction tests:

Standard Proctor compaction (OMC and MDD) (NF P94-093);California Bearing Ratio (CBR) on soaked and compacted specimens (NF P94-078);Direct shear test for cohesion (c) and internal friction angle (φ) ([9]);Oedometer test for compressibility parameters (compression index Cc) ([10]);recompression index (Cr), preconsolidation pressure (σ’_P).

Soils were classified using: HRB (Highway Research Board), and LCPC (France) systems based on grain size and plasticity.

Slope stability was assessed using two complementary approaches:

Limit Equilibrium Method (LEM): Performed in GeoStudio Slope/W using the Mohr-Coulomb model. Various slip surface search algorithms and methods (Bishop Simplified, Spencer, Morgenstern-Price) were applied. The minimum factor of safety (FS) was identified for the critical slip surface. Finite Element Method (FEM): Implemented in PLAXIS 2D. The slope geometry was discretized, and the Mohr-Coulomb model was assigned. Initial stresses were generated via the (K0) procedure. Phi-c reduction analysis was performed to determine the FS, which was then compared to the LEM results.

3.1.1. Morphological and Structural Description of Soil Profiles

Four soil profiles were described in the field (Figure 2). Profiles 1 and 2 correspond to Slope 1 at Gouache, the site of the 2019 landslide, while Profiles 3 and 4 were observed on Slopes 2 and 3 in the surrounding areas. Profiles 3 and 4 show zones affected by water infiltration.

Soil Profile 1 (N 05˚29'11.3'', E 010˚22'6.9''; altitude 1342 ± 3 m) is located in a cultivated area. It consists of:Horizon A (0 - 50 cm): fine, pedotubed soil, pale red (7.5 R 6/2), clay-sandy texture, granular structure, with millimetric roots and rootlets. The boundary with the underlying horizon is regular and gradual (Figure 2).Horizon B (50 - 90 cm): mineral horizon, light red (2.5 YR 6/6), massive structure, with rootlets and whitish-reddish spots.Soil Profile 2 (N 05˚29'07.2'', E 010˚22'32''; altitude 1350 ± 6 m), located at the landslide core, comprises:Horizon A (0 - 72 cm): fine, pedotubed soil, dark reddish black (7.5 YR 2.5/1), clay-silty-sandy texture, granular structure, with fine roots and rootlets. The boundary with the underlying horizon is irregular and gradual (Figure 2).Horizon B (72 - 210 cm): mineral horizon, red (10 R 5/8), clay-sandy texture, massive structure.Soil Profile 3 (N 05˚29'04.1'', E 010˚22'06.4''; altitude 1314 ± 3 m) consists of:Horizon A (0 - 20 cm): fine, pedotubed soil, dark reddish black (7.5 R 2.5/1), clay-sandy texture, granular structure, with roots and rootlets. The boundary with the underlying horizon is regular and gradual (Figure 2).Horizon B (20 - 180 cm): mineral horizon, dark red (2.5 YR 3/6), clay-sandy texture, massive structure.Soil Profile 4 (N 05˚28'43.56'', E 010˚22'23.64''; altitude 1389 ± 3 m) includes:Horizon A (0 - 47 cm): fine, pedotubed soil, dark grayish black (5 R 4/1), clay-sandy texture, massive structure, with rock fragments and roots. The boundary with the underlying horizon is irregular and gradual (Figure 2).Horizon B/C (47 - 130 cm): mineral horizon, yellowish-red (5 YR 5/8), with C phase embedded within B.

Figure 2. Soils profile.

3.1.2. Geometrical Characteristics of the Slopes

The geometrical properties of the three studied slopes were determined, including slope height (H), inclination angle relative to the horizontal (β), and horizontal distance (D). The results show that, Slope 1 (Profiles 1 and 2) exhibits a height of 30.44 m, an inclination of 59˚, and a horizontal distance of 18.29 m. Slope 2 (Profile 3) has a height of 25 m, slope angle of 52˚, and horizontal distance of 19.53 m, while Slope 3 (Profile 4) shows a height of 26.38 m, slope angle of 48˚, and horizontal distance of 23.76 m. The average values for the three slopes are H = 27.27 m, β = 53˚, and D = 20.52 m.

3.2.1. Physical Properties

1)Natural Water Content, Specific Gravity, and AtterbergLimits

The physical properties of the soil, including natural water content (W), specific gravity, and Atterberg limits (Figure 3), were determined to characterize the materials. The results are summarized in Table 1. Natural water content ranged from 11.13% (Sample 1) to 23.47% (Sample 4), with an average of 16.7%. The liquid limit (WL) (Figure 3), varied between 44% and 58.2%, while the plastic limit (WP) ranged from 29.37% to 42.85%. The plasticity index (Ip) and consistency index (Ic)) averaged 18.6 and 2.11, respectively. The absolute density of solids ranged from 2.27 to 2.45 g/cm³, with a mean value of 2.35 g/cm³.

Table 1. Physical properties of soil samples.

Figure 3. Liquid limit curve.

2)Particle Size Distribution

The particle size distribution of the soil samples was determined by sieve analysis, and the results were used to construct the grain size curves shown in Figure 4. The curves represent cumulative percentages of soil passing through the sieves as a function of sieve diameter, providing a clear indication of the soil texture and grading for each sample. These distributions help characterize the relative proportions of sand, silt, and clay, which are key parameters for assessing slope stability and mechanical behaviour.

Figure 4. Grain size distribution curves of the different sample.

The particle size composition of the four soil samples (Sample 1 - Sample 4) is summarized in Table 2. Gravel content varied from 21.13% (Sample 1) to 57.24% (Sample 4), sand content ranged from 42.12% (Sample 1) to 66.70% (Sample 2), and fine particles (silt + clay) ranged from 36.75% (Sample 1) to 65.45% (Sample 3). On average, the soil samples contained 39.81% gravel, 51.01% sand, and 51.81% fines. These variations reflect the heterogeneity of soil texture across the slopes, which has important implications for slope stability and mechanical behavior.

Table 2.Particle size composition of soil samples (%).

3.2.2. Mecaniques Parameters

1)Proctor Test Parameters

The Standard Proctor compaction test results for the soil samples (Figure 5) are summarized in Table 3. The optimum moisture content (wopt)ranged from 17% (Sample 2) to 20.4% (Sample 4), with an average of 19%. The corresponding maximum dry density (γ_dmax) varied from 1.61 g/cm³ (Sample 4) to 1.72 g/cm³ (Sample 1), with an average of 1.66 g/cm³. These results reflect the compaction characteristics of the soils, which are essential for understanding their mechanical behavior and assessing slope stability.

Table 3. Proctor test results.

Figure 5. Proctors curves of soils samples E1 at E4.

2)California Bearing Ratio (CBR)

The California Bearing Ratio (CBR) tests were performed on soil samples compacted at 95% of the optimum moisture content. The results, summarized in Table 4, show that CBR values range from 0.41% (sample 2) to 5.97% (sample 1), with an average of 2.53%. These low CBR values indicate weak subgrade strength, highlighting the potential need for soil improvement or stabilization measures to ensure slope stability and support civil infrastructure.

Table 4.CBRvalues of soil samplesat 95% OMC.

3)Oedometer Test Results

The oedometer tests were conducted on the soil samples to evaluate their compressibility and consolidation characteristics. The results are summarized in Table 5. The compression index (Cc) varied from 0.14 (sample 4) to 0.22 (sample 3), with an average of 0.18, while the recompression index (Cr) ranged from 0.02 (sample 4) to 0.05 (sample 3), averaging 0.031. The preconsolidation pressure (σ’p) ranged from 3.30 kPa (sample 1, sample 3) to 3.40 kPa (sample 4), with a mean of 3.32 kPa. The initial vertical effective stress (σ’v0) varied between 2.40 kPa (E1) and 3.33 kPa (E4), averaging 2.75 kPa. The overconsolidation ratio (OCR) ranged from 1.02 (sample 4) to 1.33 (sample 1), with an average of 1.22. The initial void ratio (e0) varied from 1.18 (sample 1) to 1.48 (sample 2), with a mean of 1.39. These parameters indicate the soil’s potential for compression under applied loads, which is critical for slope stability and foundation design.

Table 5. Oedometer test results.

eo:initial void ratio; Cc: compression index; Cg: swelling index; σ’vo:initial vertical effective stress; σ’p: preconsolidation pressure; Roc: overconsolidation ratio.

4)Direct Shear Test Results

Direct shear tests were performed on the soil samples to determine their shear strength parameters. The results are summarized in Table 6. The cohesion (C) values ranged from 18.22 kPa (Sample 1) to 70.44 kPa (Sample 3), with an average of 43.33 kPa. The internal friction angle (φ) varied between 28.74˚ (Sample 4) and 36.7˚ (Sample 3), with a mean value of 32.28˚. These parameters are critical for evaluating slope stability and designing soil reinforcement measures.

Table 6.Shear strength parameters of soil samples.

3.2.3. Soil Classification

The tested soils were classified according to two common geotechnical systems: the Highway Research Board (HRB) system and the Laboratoire Central des Ponts et Chaussées (LCPC) system. The results are presented in Table 7. According to HRB classification, sample 1 is classified as A-7-6, while samples 2, 3, and 4 are classified as A-7-5. Using the LCPC system, all samples are identified as silty sands (SL). These classifications reflect the fine-grained, silty-sandy nature of the soils, which is consistent with the physical and mechanical properties obtained from laboratory tests.

Table 7. Soil classification results.

3.3.1. Limit Equilibrium Method (LEM)

Slope stability was evaluated using the Limit Equilibrium Method (LEM) under different assumptions, including Bishop (Figure 4), Morgenstern-Price, Spencer, Janbu, and Fellenius. These approaches allow identification of the critical slip surface and the corresponding Factor of Safety (FS) for each slope. Slope 1 corresponds to the average cohesion and friction angle of soil profiles 1 and 2. Table 8 presents the FS values under dry conditions, while Table 9 shows the FS under a defined piezometric water level (Table 9). Results indicate that slope 1 has FS values close to 1, suggesting marginal stability, whereas slope 2 shows higher FS values, indicating greater stability. Slope 3 exhibits intermediate stability.

Table 8.Factor of safety of slopes under dry conditions (LEM).

Table 9.Factor of safety of slopes under piezometric water level (LEM).

These results show that piezometric water significantly reduces slope stability, particularly for Slope 1 (Figure 6), which is highly susceptible to failure under wet conditions. Slope 2 remains stable due to higher cohesion and friction angle, while Slope 3 presents moderate stability. According to [2]; [4] and [5], slopes 1 and 3 are unstable.

Figure 6. Factor of safety of slopes.

3.3.2. Slope Stability Using the Finite Element Method (FEM)

The main parameters used for the FEM analysis include the Young’s modulus (E),Poisson’s ratio (ν),oedometer modulus (E_oed),shear modulus (G),internal friction angle of the soil grains (φ),cohesion (C), and dilatancy angle (ψ). The numerical values of these parameters for the studied slopes are summarized in Table 10.

Table 10. Calculation parameters.

3.3.3. Mesh, Boundary Conditions, and Deformations

The boundary conditions are selected in a standard manner. The mesh is of medium size. Figure 7 illustrates the mesh and the deformed mesh.

Figure 7. Meshes and boundary conditions; deformed meshes of the slopes.

The total displacement values for slopes 1, 2, and 3 are shown in Table 11. They are 0.335 m, 26.48 m, and 15.85 m, with corresponding factors of safety of 1.460, 3.815, and 3.146.

Table 11. Factor of safety (FS) and displacement results.

Table 12 presents the results of the factors of safety and displacements for the slopes under a defined piezometric water level.

The slopes 1, 2, and 3 have factors of safety (FS) of 1.460, 3.815, and 3.146, respectively. Slope 1 is considered marginally stable, whereas slopes 2 and 3 are stable, with corresponding maximum total displacements of 0.335, 26.48, and 15.85 m. [1], in his study on the use of numerical methods for assessing the stability of earth dams in Algeria, reported FS values ranging from 1.458 to 54.45 (with displacements between 26.756 m and 50.08 m), which are much higher than those obtained in the present study, particularly with respect to displacements. This discrepancy may be attributed to differences in cohesion and internal friction angles. Regarding Table 11, the factors of safety range from 1.140 to 3.062, with displacements between 0.379 and 7.12 m. [4] and [5] also identifies slope 1 as marginally stable, while slopes 2 and 3 remain stable. The presence of water affects the soil parameters and contributes to a reduction in FS. Given the differences in FS values and the observed instability of slope 1 across both calculation methods, a comparative study of these methods is recommended, along with the construction and geotechnical monitoring of this slope.

Table 12. FS and displacement results with piezometric water level.