The study area is located in the Far North Region of Cameroon, between latitudes 10˚N - 13˚N and longitudes 14˚E - 16˚E, along the Mora-Maroua axis within the Diamaré Division (Figure 1). This area represents a key socio-economic corridor connecting Cameroon with Chad and Nigeria. Investigations were mainly carried out in Maroua I, II, and III subdivisions. The climate is Sudan-Sahelian, characterized by a long dry season (October-May) and a short rainy season (June-September) [16]. The average annual rainfall ranges between 700 and 900 mm, with a mean temperature of about 28˚C [16]. These climatic conditions significantly influence weathering processes and the geotechnical behaviour of soils [14]. Geomorphologically, the area consists of low-relief plains and discontinuous highlands (inselbergs). The hydrographic network is mainly seasonal, dominated by the Mayo Tsanaga, Mayo Kaliao, and Mayo Mizao rivers, which belong to the Lake Chad basin [17]. The soils are predominantly hydromorphic vertisols associated with ferruginous and alluvial soils. These materials are known for their high clay content and shrink-swell behaviour, which can affect road performance [17]-[19]. Geologically, the area belongs to the Precambrian basement and is composed mainly of gneisses, granites, and gabbros [20]. The weathering of these rocks leads to the formation of lateritic soils widely used in road construction. Their properties vary depending on mineralogical and geochemical composition, which justifies their detailed characterization in this study.

The eight sampling sites were selected in order to represent the main lateritic formations developed along the Maroua-Mora corridor under varying geomorphological and geological conditions. The selected sites cover materials derived from different parent rocks and weathering environments distributed across the Meri, Maroua, and Mora subdivisions. At each site, disturbed bulk samples were collected from representative lateritic horizons between 0.5 and 1.5 m depth. Each sample corresponds to a composite material obtained from several closely spaced points within the same borrow area to minimize local heterogeneity. In addition, these eight soil samples were collected along the Maroua-Mora axis in the Far North Region of Cameroon, covering the subdivisions of Meri (Mambang, Mogordom, Djoulgouf), Maroua I (Meskine, Pont Sava), and Mora (Doulo, Guédéré, Makalingai) as presented in Table 1. Samples were taken at depths between 0.5 and 1.5 m and coded (MAM, MOG, DJO, MES, PSA, DOU, GUE, MAK) for traceability. Field observations indicate predominantly reddish-brown lateritic soils (2.5YR 5/4), with sandy clayey texture and coarse-grained structure. This relative homogeneity suggests similar weathering conditions, with minor variations likely related to local mineralogical differences [14].

Figure 1.Location map of the case study area.

Table 1.Macroscopic characteristics of the studied soils.

2.2.1. Geotechnical and Mechanical Properties of Lateritic Soils

The experimental analysis consisted of a series of standard geotechnical, mineralogical, and geochemical tests aimed at comprehensively characterizing the physical, hydric, mechanical, and compositional properties of the studied soils. These analyses made it possible to evaluate their particle size distribution, plasticity, natural water content, as well as their compaction and bearing performance, while also integrating the study of their mineralogical composition and geochemical signature, in order to assess their suitability for civil engineering applications, particularly road construction.

Particle size distribution was determined using a combination of sieving and sedimentation techniques. Dry and wet sieving were applied to particles larger than 80 μm, depending on soil cohesiveness, while finer fractions were analyzed by sedimentation. This combined approach enabled the establishment of granulometric curves and the classification of soil particles into distinct size fractions [21][22].

The Atterberg limits, including liquid and plastic limits, were determined on the fine fraction passing the 0.4 mm sieve, in accordance with [23]. These parameters are essential for assessing soil consistency and its sensitivity to moisture variations [24][25].

The natural water content was measured by oven-drying samples at 105˚C - 110˚C until constant mass was achieved. The water content was calculated as the ratio of water mass to dry mass, expressed as a percentage [26].

The specific gravity of soil particles was determined using the pycnometer method, based on the displacement of a liquid of known density. This method allows accurate estimation of the density of solid particles [27].

Compaction characteristics were evaluated using the Modified Proctor test [28], which establishes the relationship between water content and dry density. This test allows the determination of the optimum moisture content and maximum dry density under conditions representative of road construction.

The bearing capacity of the soils was evaluated using the California Bearing Ratio (CBR) test [29]. This method determines the resistance of compacted soils to penetration under standardized conditions and yields key parameters for pavement design. CBR tests were performed on specimens compacted at 95% of the Modified Proctor optimum under unsoaked conditions, in accordance with NF P 94-078 [29]. For each sample, at least three replicate specimens were tested, and the average CBR value was retained. The penetration resistance was measured after standard compaction and curing procedures. The interpretation of the results was carried out in accordance with data reported in Table 2, which presents the CBR classification and corresponding recommendations for road construction [30].

Table 2.CBR classes and recommended use in road construction [30].

2.2.2. Geochemical Analyses

The chemical composition of the samples was determined by X-ray fluorescence spectrometry (XRF) at the Bureau Veritas Commodities Laboratory. This technique enables both qualitative and quantitative determination of major and trace elements based on the emission of characteristic radiation following atomic excitation. Geochemical data are essential for assessing the origin, weathering degree, and potential engineering applications of lateritic materials [31]-[33]. Prior to XRF analysis, samples were oven-dried, crushed, and finely ground to obtain homogeneous powders. Loss on ignition (LOI) was determined after heating at 1000˚C. Analytical quality control included duplicate analyses and calibration using certified reference materials at the Bureau Veritas Commodities Laboratory.

3.1.1. Particle Size Analysis

Particle size distribution indicates a predominance of coarse fractions (65% - 97%), confirming a sandy-to-sandy-clayey texture, while the fine fraction (<0.080 mm), ranging from 23% to 42%, remains sufficiently significant to confer an intermediate behavior between granular and cohesive materials (Figure 2). The generally continuous grading curves fall within the CEBTP [30] envelopes for subbase and, locally, base layers, suggesting an overall acceptable particle size distribution for road applications.

Figure 2.Position of granulometric curves on the CEBTP [30] spindles for base and subbase layers: (a) sample from Mayo Sava Division, (b) sample from Diamare Division.

These findings are consistent with the work of Kamtchueng et al. [12], who demonstrated that lateritic soils containing moderate fines (20% - 30%) exhibit improved compaction and higher CBR values. However, when the fine content exceeds this range, a progressive decline in bearing capacity is observed. Kagonbé et al. [13][14] reported comparable trends on lateritic materials from Garoua, where fine contents between 20% and 40% were associated with increased plasticity and a reduction in mechanical performance. More broadly, the inverse relationship between fine fraction and bearing capacity, coupled with increased moisture sensitivity, is well established in lateritic soil mechanics [2]. This tendency is further supported by recent results obtained on Meiganga soils, where an average fine content of about 52% is associated with predominantly plastic behavior and reduced surface bearing capacity [34]. Therefore, despite a globally favorable grading, the relatively high proportion of fines (23% - 42%) may limit mechanical performance, indicating that stabilization remains necessary for optimal use in base layers.

3.1.2. Atterberg Limits and Swelling Potential

The Atterberg limits reported in Table 3 indicate liquid limits ranging from 35% to 50% and plasticity indices between 9% and 20%, reflecting overall low to moderate plasticity. The MAM sample (PI = 20%) stands out with the highest plasticity, consistent with a greater proportion of active clay minerals, whereas MAK (PI = 9%) and GUE (PI = 10%) correspond to less plastic and more stable materials. Positioning on the Casagrande chart (Figure 3) confirms that all samples fall within the ML-CL domain, indicative of silty to clayey soils with moderate plasticity, in agreement with typical lateritic soils described by Gidigasu [10] and regional studies in northern Cameroon [13]-[15]. Compared to the highly plastic and swelling lateritic fine soils reported by Hyoumbi et al. [5], these materials exhibit significantly lower plasticity and very limited linear swelling (Ɛs = 0.001 - 0.008), indicating reduced shrink-swell potential and improved dimensional stability. The plasticity modulus (207 - 848) further highlights this variability: the high value for MAM (848) reflects marked water sensitivity and behavior approaching more plastic lateritic facies, whereas the lower values for MAK (207) and GUE (243) confirm more stable and less moisture-sensitive soils, consistent with the interpretation of Lérau [24].

Table 3.Summary of geotechnical for the studied soil.

Figure 3.Position of the investigated materials on the Casagrande plasticity chart.

3.1.3. Mechanical Characteristics

The mechanical characteristics derived from Table 4, supported by the compaction and strength trends in Figure 4 and Figure 5, indicate a heterogeneous but globally favorable behavior of the studied lateritic soils for road applications. Maximum dry densities (2.060 - 2.157 g/cm³) and optimum moisture contents (6.8% - 12.3%) are consistent with sandy clay laterites and comparable to values reported in northern Cameroon [35]. The Proctor curves exhibit the classical bell-shaped pattern, confirming well-defined compaction optima and efficient particle rearrangement. The weak inverse relationship between MDD and OMC, together with local variability, reflects differences in granulometry and soil fabric, while lower fines content favors higher densities and reduced water demand [30][36].

Figure 4.Variation of dry density with water content.

Figure 5. Variation of CBR index values (CBRi) with dry density.

Table 4.Mechanical parameters.

CBRi values at 95% OPM range between 15 and 40, with an average around 30, placing most materials within S4-S5 classes according to CEBTP [30]. Based on standard specifications, S4 materials (15 - 30) are suitable as foundation layers for low traffic (T1), whereas S5 materials (30 - 60) can be used as foundation layers for moderate traffic (T2-T3) and even as base layers for low traffic roads (T1). This classification confirms that the studied soils are generally adequate for subgrade and foundation applications, but remain marginal for base layers under higher traffic without improvement. The positive, though weak, correlation between CBR and dry density (Figure 5) highlights the dominant role of compaction in strength development, consistent with observations by Kamtchueng [12]. Linear swelling remains low (0.002 - 0.019), confirming limited expansiveness and good dimensional stability. Overall, these soils, classified as sandy clays [37], exhibit mechanical performance controlled by moisture-density conditions and fines content; although suitable for form and foundation layers across traffic classes (T1-T3), stabilization is recommended to meet the requirements of higher traffic levels or base course applications.

3.1.4. Analysis of Relationships between Particle Size Distribution, Plasticity, Compaction Parameters, and Soil Bearing Capacity

The correlations presented in Figure 6 highlight the interdependence between geotechnical parameters governing the mechanical performance of lateritic soils.

The inverse relationship observed between fines content and CBR values confirms that an increase in fine particles, particularly clay fractions, leads to a reduction in bearing capacity due to higher plasticity and moisture sensitivity. Similarly, the negative correlation between plasticity index and CBR further supports the detrimental effect of soil plasticity on strength characteristics. The swelling-CBR relationship emphasizes the influence of volumetric instability on load-bearing performance, particularly in clay-rich materials. In contrast, the relationship between maximum dry density and optimum moisture content reflects typical Proctor compaction behavior, controlled by particle arrangement and soil fabric. The positive correlation between plasticity index and liquid limit indicates a coherent evolution of consistency limits, reflecting mineralogical control, especially the presence of kaolinite and iron oxides in lateritic systems. Overall, these trends are consistent with previous studies on tropical and lateritic soils, which demonstrate that granulometry, plasticity, and compaction characteristics are key determinants of their suitability for road construction applications [2][4][10][38]-[41].

Figure 6. Correlations between: (a) fines content/CBR value, (b) maximum dry density/optimum moisture content, (c) plasticity index/CBR value, (d) swelling/CBR value, (e) maximum dry density/fines content (%), and (f) plasticity index/liquid limit.

The geochemical composition of the studied lateritic soils (Table 5) is dominated by SiO₂ (44.43 wt.% - 89.77 wt.%), followed by Al₂O₃ (3.85 - 18.62 wt.%) and Fe₂O₃ (2.39 wt.% - 26.76 wt.%), confirming their typical lateritic nature under tropical weathering conditions. These ranges are consistent with those reported in the Far North Cameroon, where quartz, aluminosilicates, and iron oxides control soil composition [42]. High SiO₂ contents in DJO (89.77 wt.%), MAK (78.06 wt.%), and PSA (75.26 wt.%) indicate quartz-rich, weakly cohesive materials, as confirmed by their high SiO₂/(Al₂O₃ + Fe₂O₃) ratios (4.5 - 13.2).

Table 5.Chemical composition of the studied soils.

These materials are typically weakly laterized, with good compaction potential but limited interparticle bonding, leading to moderate to low bearing capacity (CBR) without stabilization [19]. This interpretation is further supported by their position in the SiO₂-Al₂O₃-Fe₂O₃ ternary diagram (Figure 7), where they cluster toward the silica apex, reflecting residual quartz enrichment and limited pedogenetic transformation. In contrast, MAM (SiO₂ = 44.43 wt.%; Al₂O₃ = 18.62 wt.%; Fe₂O₃ = 18.54 wt.%) and MES (SiO₂ = 49.59 wt.%; Fe₂O₃ = 26.76 wt.%) show enrichment in sesquioxides and low SiO₂/(Al₂O₃ + Fe₂O₃) ratios (≈1.20 - 1.26), indicating advanced laterization. These compositions may indicate the probable presence of kaolinite and iron oxides (goethite/hematite), which promote natural cementation, enhancing cohesion and expected CBR values, as commonly observed in ferruginous laterites (5). Intermediate materials (DOU, MOG) present SiO₂/(Al₂O₃ + Fe₂O₃) ratios between 2.4 and 3.6, reflecting moderately laterized soils with mixed granular and cohesive behavior, associated with intermediate compaction and bearing properties.

Al₂O₃ contents (up to 18.62 wt.% in MAM) indicate the development of clay minerals through hydrolysis, contributing to plasticity and moisture retention [42][43]. Fe₂O₃, particularly in MES (26.76 wt.%) and MOG (14.30 wt.%), enhances mechanical strength through natural cementation and is often associated with goethite, as indicated by TiO₂ presence [44]. LOI values (0.89 wt.% - 9.39 wt.%) reflect variations in clay content and hydrated phases, with higher values in MAM and MES indicating greater weathering intensity and cohesion, but also increased moisture sensitivity [43][45]. Overall, MAM and MES are the most suitable for road base applications due to strong sesquioxide content and cementation effects, while MOG and DOU show intermediate performance. PSA, MAK, and DJO, being silica-rich, require stabilization to achieve adequate mechanical performance. These results confirm that the balance between silica and sesquioxides, particularly Fe₂O₃ and Al₂O₃, is the main control factor of the geotechnical behavior of lateritic soils, in agreement with previous studies [19].

Correlation betweenGeochemical Indices and Engineering Properties:

The correlations between the geochemical ratio SiO₂/(Al₂O₃ + Fe₂O₃) and the engineering properties highlight the influence of geochemical composition on the behavior of the studied lateritic soils (Figure 7). The Plasticity Index (PI) displays a moderate negative correlation (R² = 0.3209), indicating that increasing silica content is associated with lower plasticity (Figure 7(a)). This behavior reflects the reduced influence of clayey and ferruginous constituents in silica-enriched soils. Although the correlations remain moderate, the results confirm that geochemical composition contributes significantly to the geotechnical performance of lateritic materials. In contrast, the relationship with the California Bearing Ratio (CBR) shows a weak positive correlation (R² = 0.129), suggesting that silica-rich materials tend to exhibit slightly improved bearing capacity due to the predominance of sandy and quartz-rich fractions (Figure 7(b)). Figure 8 shows that the studied lateritic soils are mainly distributed within the kaolinisation and weak laterisation domains of the SiO₂-Al₂O₃-Fe₂O₃ ternary diagram, indicating a moderate degree of weathering and lateritic evolution.

Figure 7.Correlation between geochemical indices and engineering properties of the lateritic soils: (a) correlation between the geochemical ratio SiO₂/(Al₂O₃ + Fe₂O₃) and plasticity index (PI) values; (b) correlation between the geochemical ratio SiO₂/(Al₂O₃ + Fe₂O₃) and California Bearing Ratio (CBR) values.

Figure 8. POSITION of studied lateritic soils in SiO₂-Al₂O₃-Fe₂O₃ (Wt. %) ternary diagram [46].