Evaluation of Sub-Base Material Using California Bearing Ratio and Triaxial Compression Tests: A Case Study of High Court Road Kabale, Uganda

2.1 Sample collection

The study samples were collected from High Court Road in Kabale District, Uganda. This location was chosen due to its unpaved condition and relatively high traffic volume, which make it an ideal site for evaluating sub-base material performance. The coordinates of the site are Latitude: -1.27168 (S1°16′18.04296″) and Longitude: 29.98424 (E29°59′3.24744″), with the specific address marked as PXHM+PXF, Kabale, Uganda as depicted in Figure 2.

Figure 2. Maps of Kabale High Court Road (Google Maps, 2025).

Soil samples were collected from a depth of 1.0 meter at selected points along High Court Road in Kabale. The sampling process employed both hand tools and sampling tubes to ensure the retrieval of representative sub-base materials. Disturbed samples were obtained through manual excavation, whereas undisturbed samples were carefully extracted using sampling tubes to maintain their in-situ structure. To ensure the reliability of subsequent laboratory analyses, each sample was immediately sealed, clearly labelled, and transported under controlled conditions. In the laboratory, samples were properly prepared for testing in accordance with standard procedures. Strict protocols for handling and packaging were observed throughout the process to avoid contamination and preserve the integrity of the samples. The excavated pit used for sample collection and the prepared samples ready for transportation are shown in Figure 2. The field test standards adopted during the investigation are summarized in Table 1.

Field investigations were carried out along a road corridor characterized by varied topography, including climbing, sloping, and relatively flat sections. Borehole investigations confirmed the absence of groundwater across all segments of the site, an important observation for understanding the soil’s behavior under unsaturated conditions. The surface soils throughout the area were predominantly reddish-brown, indicating lateritic characteristics. No evidence of structural settlement, flood marks, or cracking was observed in adjacent buildings, suggesting a history of geotechnical stability. Furthermore, there were no drainage ditches, dumping yards, or visible signs of springs or swamps, all of which support the suitability and safety of the location for construction. Historical and observational assessments revealed no prior occurrences of landslides, shrinkage cracks, or flooding, further reinforcing the site’s long-term stability. This comprehensive field assessment offers a standardized and reliable geotechnical profile of the site’s climbing, sloping, and flat sections, providing critical insights that support informed decisions for future construction and infrastructure development. Five test pits were manually excavated to a depth of 1.0 meter along the alignment of the existing High Court Road to allow for the observation of subsurface stratification as shown in Figure 3. A. Table 2 shows the locations of the test pits and their respective sampling points. The chosen depth was sufficient to capture the characteristics of the sub-base material, which is relatively thick along this road segment. Both disturbed and undisturbed soil samples were collected at alternating intervals from the Left-Hand Side (LHS), center, and Right-Hand Side (RHS) of the roadway. This sampling strategy ensured spatial representation across the road cross-section. Each sample was carefully extracted, packaged in airtight bags, clearly labelled, and sealed to preserve its physical and mechanical properties during transport. All samples were then delivered to the soil mechanics laboratory for further geotechnical testing and analysis.

Figure 3. Excavated pits.

2.2 Laboratory testing

Laboratory testing was conducted to evaluate the engineering properties of the collected sub-base soil samples. All tests were performed in accordance with established ASTM standards to ensure consistency, accuracy, and reliability of results. The range of tests carried out included moisture content determination, particle size distribution, Atterberg limits, compaction tests, California Bearing Ratio (CBR), and Triaxial Shear Strength tests. These tests were selected to comprehensively assess the mechanical and strength characteristics of the sub-base materials. A summary of the laboratory tests conducted, along with their corresponding ASTM and AASHTO standards, is presented in Table 3.

2.2.1 Assessment of sub-base material effectiveness

To evaluate the sub-base material’s contribution to the structural integrity of the road, a comprehensive methodology was adopted. This methodology integrated traffic loading calculations, subgrade evaluation, and climate adaptation factors to determine the overall effectiveness of the material in supporting the road structure over time.

2.2.1.1 Pavement design

The pavement design was primarily based on traffic loading, which was quantified using Equivalent Single Axle Loads (ESALs). The Average Annual Daily Traffic (AADT) and Load Equivalency Factor (LEF), derived from local traffic data, were employed to calculate the cumulative ESAL over a 20-year design period. The ESAL (W18) was computed using Equation (1) and (2).

Where: AADT is the Average Annual Daily Traffic, LEF is the Load Equivalency Factor, G is the traffic growth factor t is the analysis period (20 years), r is the annual growth rate (%).

3.1 The index test results

The results of particle size distribution and Atterberg limit tests are presented in Figure 4 and Table 4, which summarizes the soil index properties and soil classification.

Figure 4. Particle size distribution chart.

The particle size distribution curve presented in Figure 4 reveals that the soils are predominantly sandy clays with considerable plasticity. Over 70% of the fine particles (silt and clay fractions) pass through the 0.075 mm sieve, indicating a high proportion of fines. According to the AASHTO classification system (AASHTO, 2021), soils with more than 35% passing the 0.075 mm sieve are categorized as clayey. Based on this criterion, the majority of the soil samples fall within the A-7-6 classification group, which corresponds to highly plastic clayey soils. Complementary classification using the Unified Soil Classification System (USCS) classifies the soils as CH—high plasticity clays. These soils are typically characterized by low permeability, high compressibility, and significant shrink-swell potential in response to moisture variations. The Atterberg Limits and gradation test results corroborate this classification. As shown in Table 4, the soils exhibited a Plasticity Index (PI) greater than 10% and a Liquid Limit (LL) slightly above 40%, consistent with the characteristics of high-plasticity clays.

3.2 Grading Modulus (GM)

The grading modulus (GM) values presented in Table 4 range from 0.15 to 0.32, indicating that the tested soils are poorly graded and predominantly fine-grained. These low GM values reflect a narrow and uniform particle size distribution, with a significant proportion of particles passing through the finer sieve sizes. Such a distribution suggests limited variability in grain sizes, resulting in inadequate interlocking between particles. Poorly graded soils those with grading modulus values below 1.5 or above 3.5 are characterized by either a predominance of similar-sized particles or a lack of intermediate-size fractions. This leads to poor compaction behavior, low shear strength, and increased permeability, making them less suitable for load-bearing sub-base applications. In contrast, well-graded soils, with GM values between 2.5 and 3.5, possess a more balanced particle size distribution that facilitates better compaction, lower void ratios, and improved mechanical stability.

These findings are consistent with previous research by Alzara et al. (2024), who observed that low GM values often correlate with compaction difficulties and reduced engineering performance. Additionally, the classification criteria align with the work of Das (2019), who emphasized the importance of grading characteristics in determining soil suitability for geotechnical applications. Therefore, the low GM values observed in this study highlight the need for soil improvement measures, such as blending or stabilization, to enhance the sub-base performance of the road materials.

3.3 Soil compaction, California Bearing Ratio (CBR), and triaxial compression tests results

Table 5 provides a summary of soil compaction, CBR, triaxial compression, and natural moisture content test results.

3.3.1 Soil compaction results

The moisture-density relationship which obtained from soil compaction test was illustrated in Figure 5. The summary of results presented in Table 5 showed that the maximum dry densities range from 16.05 kN/m³ to 18.03 kN/m³, with corresponding Optimum Moisture Content (OMC) values between 18.9% and 23.3%. These values are consistent with soils that possess a significant proportion of fine particles.

Figure 5. Moisture density relationship.

The relatively high OMC values indicate that the soils require substantial water to achieve maximum dry density. This is typical of fine-grained soils, where moisture serves to reduce inter-particle friction, enabling soil particles to realign and pack more densely under compactive effort. The natural moisture content of the soils presented in Table 5, ranging from 14.5% to 15.6%, is below the OMC, suggesting the necessity of moisture addition during field compaction. Proper moisture conditioning enhances workability, reduces cohesion, and lowers the energy required for compaction, ultimately resulting in a denser, more stable, and durable sub-base layer. Based on AASHTO classification, the soils fall within the A-6 and A-7 groups, indicating high plasticity and significant moisture sensitivity. In the ASTM Unified Soil Classification System (ASTM D2487), the soils are classified as either CL (lean clay) or CH (high-plasticity clay), depending on the specific Atterberg limit values. These classifications are associated with soils that have a large specific surface area and a strong affinity for water. This result showed that such soils typically retain more water and require additional moisture during compaction to achieve their target densities, which are in line with the findings of Wu et al. (2025).

3.3.2 California Bearing Ratio (CBR)

The results of the California Bearing Ratio (CBR) tests, as illustrated in Table 5, show that the CBR values for the analysed soil samples range between 2% and 13%, depending on the level of compaction applied. These findings clearly demonstrate the positive correlation between increased compaction effort (at 93%, 95%, and 98% relative compaction levels) and soil strength, with higher compaction yielding correspondingly higher CBR values. Despite this improvement, the obtained CBR values fall significantly below the minimum recommended threshold for sub-base materials. According to Rolt et al. (2022), a CBR value greater than 30% is generally required for sub-base layers in roads subjected to medium to heavy traffic loading. The low CBR values observed suggest that, in their natural condition, the soils under investigation exhibit insufficient strength and are highly susceptible to deformation under applied loads. Das (2020) also noted that optimizing moisture content and utilizing advanced compaction techniques can substantially improve the effectiveness of soil densification, thereby enhancing the soil’s bearing capacity and structural resilience. Consequently, these findings underscore the necessity of implementing soil stabilization or modification measures such as the incorporation of chemical stabilizers, mechanical reinforcement, or blending with higher-quality materials to enhance the strength, durability, and overall performance of the sub-base for road construction applications.

3.3.3 Triaxial compression test results

The triaxial compression test results on sub-base soil samples, collected from depths of 0.5 to 1.0 meters, provide critical insights into the shear strength parameters necessary for evaluating roadbed stability. As shown in Table 5, the measured cohesion (c) values ranged from 67.5 kPa to 95.2 kPa, reflecting notable variability in the internal bonding of soil particles, which directly influences their resistance to shear strength. The angle of internal friction (φ) varied between 9.00° and 19.71°, with higher values indicating improved resistance to sliding under repeated loading conditions. Axial strain values at failure presented in Figure 6 ranged from 0.60% to 1.80%, suggesting that the materials possess moderate stiffness and can accommodate deformation under traffic loads without undergoing immediate failure. The results indicate that soils exhibiting lower cohesion and friction angles are more vulnerable to shear failure, particularly under increased loading conditions. This highlights the necessity for stabilization measures to enhance their structural performance—consistent with findings by Kafle et al. (2024) and Kang et al. (2022). Observed failure modes, such as barrelling deformation, are characteristic of plastic soil behavior preceding ultimate failure. Moreover, the Mohr-Coulomb failure envelope clearly illustrates the increased susceptibility of soils with lower friction angles to shear-induced deformation, reinforcing the recommendation for soil modification to ensure long-term stability.

Figure 6. Deviator stress vs axial strain graph for pits.

The stress-strain behavior under confining pressures of 50 kPa, 100 kPa, and 150 kPa presented in Figure 6 exhibited a typical pattern: an initial linear increase in deviator stress with strain, representing elastic behavior, followed by peak stress and a gradual decrease as the soil undergoes softening and plastic flow. This transition from strain hardening to softening corresponds with shear band formation, a common failure mechanism in granular soils under axial loading. These patterns corroborate the findings of Li et al. (2024). Furthermore, the effect of increased confining pressure was evident in delaying the onset of failure, in agreement with observations made by Huang et al. (2021) and Zhang & Sun (2023). Additionally, recent studies (e.g., Yang et al., 2024) have shown that repeated freeze–thaw cycles significantly degrade soil mechanical properties, with cohesion decreasing by up to 56.66% after 10 cycles at −20 °C. Such deterioration underscores the need to consider environmental impacts when designing sub-base layers, especially in regions susceptible to seasonal freezing. For soils with inadequate shear strength parameters, stabilization techniques such as cement or lime treatment are strongly recommended to enhance load-bearing capacity and durability. Sub-base materials must possess adequate strength and stiffness to effectively distribute traffic-induced stresses and minimize permanent deformation under repeated loading. To improve the engineering performance of marginal soils for sub-base applications, chemical stabilization techniques such as the incorporation of cement, lime, or fly ash are widely adopted. These additives promote pozzolanic reactions that enhance the soil’s microstructure, thereby increasing unconfined compressive strength (UCS) and improving durability against moisture-induced degradation (Mostafa A. E. A. et al., 2024). Additionally, optimizing compaction parameters particularly by applying higher compactive effort and maintaining optimum moisture content has been shown to significantly improve density, strength, and load-bearing capacity of treated sub-base layers (Alzara M. et al., 2024).

3.4 Effectiveness of the Sub-base material in contributing to the structural integrity and longevity of the road

The sub-base layer plays a crucial role in ensuring the structural integrity and durability of pavement systems by distributing traffic loads, reducing stress transmitted to the subgrade, and minimizing differential settlement and moisture infiltration. To assess the effectiveness of the selected sub-base material, the estimated traffic loading was calculated based on the cumulative number of Equivalent single-axle loads (ESALs) as presented in Eq. 1. By incorporating traffic data parameters specifically, the Load Equivalency Factor (LEF) values from Table 6, along with the Directional Distribution Factor (DD) and the Lane Distribution Factor (DL) obtained from Table 7, the cumulative ESALs were determined. The result of this computation yielded a total traffic load of 0.444 × 10⁶ ESALs.

This cumulative ESAL value as shown in Table 6 serves as a baseline for evaluating whether the strength and stiffness properties of the sub-base material are adequate for the anticipated service life of the pavement. The material’s performance, particularly its resistance to deformation and moisture-induced weakening, directly influences the road’s longevity. Laboratory tests such as the California Bearing Ratio (CBR) and triaxial compression, along with in-situ density measurements, provide insight into the sub-base’s ability to sustain the projected load levels with minimal deterioration.

Table 7 provides guidance for determining the percentage of the total 18-kip Equivalent Single Axle Loads (ESALs) expected to be carried by the design lane, based on the number of lanes in each direction. This factor is critical in distributing the cumulative ESALs appropriately among lanes to ensure an accurate representation of the design traffic load on the critical (design) lane. Given the calculated cumulative traffic loading of 0.444 × 10⁶ ESALs, and applying the appropriate Lane Distribution Factor from Table 7, the pavement falls within Traffic Class T2. According to standard traffic classification criteria (e.g., AASHTO or equivalent national road design standards), Traffic Class T2 typically corresponds to a cumulative traffic load range of 0.3–0.7 × 10⁶ ESALs over the design life of the pavement. This classification is essential for selecting appropriate structural layer thicknesses and materials. It indicates that the road is subject to moderate traffic loading, and the sub-base material must be sufficiently stable and durable to withstand the expected service conditions without undergoing significant deformation or loss of bearing capacity.

Table 8 defines traffic loading categories based on the cumulative number of Equivalent Standard Axles (ESAs or ESALs) expected over the design life of the pavement. Based on the cumulative traffic loading calculated as 0.444 × 10⁶ ESALs, the pavement falls within Traffic Class T2, which corresponds to a moderate traffic category ranging from 0.3 to 0.7 million ESAs. This classification is critical for selecting suitable pavement layer types and thicknesses, especially when dealing with weak subgrade conditions. The soaked California Bearing Ratio (CBR) value of the subgrade soil was found to be 2%, which indicates very poor subgrade strength. According to established pavement design standards (e.g., AASHTO and TRL guidelines), a CBR value below 5% is generally considered inadequate for supporting pavement layers directly and necessitates the use of substantial sub-base and base layers or subgrade improvement techniques such as chemical stabilization or geosynthetic reinforcement.

This combination of moderate traffic loading (T2) and very weak subgrade strength (CBR = 2%) highlights the importance of using high-quality, well-compacted sub-base materials and possibly incorporating stabilization measures to enhance the pavement’s structural capacity and long-term performance.

Table 9 categorizes subgrade soils based on their California Bearing Ratio (CBR) values: Based on this classification, the soaked CBR value of 2% places the subgrade within Class S1, which denotes very weak subgrade conditions. This classification implies that the subgrade has minimal load-bearing capacity and is highly susceptible to deformation under traffic loads and moisture variation. Therefore, it necessitates appropriate subgrade treatment and reinforcement to ensure adequate pavement performance and structural integrity throughout its service life. To accommodate the calculated traffic loading of 0.444 × 10⁶ ESALs (Traffic Class T2) and to address the weak subgrade, the AASHTO (2020) design adopted the W1 – Granular Base/Granular Sub-base Design Guide. This guideline was specifically selected due to its relevance for wet climate conditions, which are prevalent in the project area.

The W1 design approach integrates comprehensive measures to address moisture-related challenges such as subgrade swelling, saturation, and strength reduction, issues likely to intensify due to climate change-induced hydrological variability. To mitigate these risks, carefully selected granular materials are employed in conjunction with optimized compaction techniques and effective drainage strategies. This combination enhances load distribution, minimizes moisture retention, and preserves pavement integrity under elevated moisture conditions, thereby supporting the road’s long-term durability and functional performance in adverse environmental settings.

Figure 7 illustrates the current state of the existing roadway, highlighting the need for structural intervention. Figure 8 presents the proposed W1 design, engineered to accommodate the anticipated traffic loads throughout the pavement’s projected service life. As a cost-effective alternative, Figure 9 showcases a design incorporating a 225 mm granular layer. This reduced-thickness option maintains performance and serviceability, particularly in areas where granular or crushed stone materials are readily available, without significantly compromising structural integrity or longevity. The pavement’s classification within Traffic Class T2 aligns with findings from comparable studies in regions characterized by moderate traffic volumes. According to Rolt et al. (2022) and the U.S. Department of Transportation (2021), pavements subjected to ESALs ranging from 0.3 to 0.7 million typically necessitate structural reinforcement to ensure enduring performance under repetitive loading conditions.

Figure 7. Existing roadway.

Figure 8. Designed pavement section in existing subbase soils.

Figure 9. Designed pavement section in granular sub-base.

A California Bearing Ratio (CBR) of 2% classifies the subgrade as an S1 type, which is highly susceptible to moisture-induced deformation. This observation is supported by Arshad et al. (2021), who emphasized the need for stabilization, such as cement treatment, for subgrades with low CBR values to improve bearing capacity. Similarly, Majumder and Venkatraman (2022) demonstrated that hydrated lime treatment can significantly enhance the strength and stiffness of weak subgrade soils. The consistently wet climatic conditions of the Kabale region further underscore the importance of employing a robust sub-base to counteract the detrimental effects of moisture fluctuations on subgrade performance.

This study adopts a climate-resilient pavement design framework by referencing wet-climate pavement design catalogues, as advocated by Sharifi et al. (2019), to improve structural resilience under varying moisture regimes. Furthermore, traffic loading assumptions are aligned with AASHTO’s (2020) lane distribution factors, assigning 100% ESAL loading to single-lane roadways and proportionally distributed loading for multi-lane configurations, thereby ensuring an accurate representation of in-service traffic conditions

3.5 Pavement design specifications with Geogrid/Reinforcement for 0.444 × 10⁶ ESALs

The pavement structure has been designed to accommodate a cumulative loading of 0.444 million Equivalent Single Axle Loads (ESALs), incorporating geosynthetic reinforcement for enhanced performance and longevity. The surfacing layer comprises a 50 mm thick dense-graded asphalt concrete. This surface material is designed with a penetration grade binder of either 60/70 or 80/100 and a bitumen content ranging from 4.5% to 6% by weight of the total mix. Proper compaction of this layer is ensured at 95% of the maximum dry density (MDD) to achieve optimal performance in terms of resistance to cracking, rutting, and weather-induced degradation.

Beneath the surfacing is a 150 mm thick base layer constructed using crushed stone or crushed gravel. This layer is expected to possess a soaked California Bearing Ratio (CBR) of at least 80% and a Unconfined Compressive Strength (UCS) of no less than 200 kPa. Its primary function is to distribute traffic-induced loads efficiently and minimize structural deformations. The sub-base layer, with a thickness of 225 mm, is also composed of crushed stone or gravel and must meet a minimum soaked CBR of 30% and a UCS of 150 kPa. This layer serves to further distribute loads and provide foundational support to the upper pavement structure.

To enhance structural integrity and reduce material usage, geosynthetic reinforcement is incorporated. Two main options are recommended: geogrids and geocells. Geogrids are strategically placed at the interface of the sub-base and subgrade to improve load transfer efficiency and restrict lateral movement of materials. Their use can result in a thickness reduction of approximately 25–30% while maintaining structural performance. Alternatively, geocells may be placed within the sub-base layer and filled with aggregate, offering stabilization and control of lateral displacement, thereby significantly reducing the potential for rutting.

The imported subgrade material consists of well-graded granular soils such as crushed stone, gravel, or stabilized soil with minimal fines, ensuring that the silt and clay content does not exceed 12%. This 300 mm thick layer must achieve a minimum soaked CBR of 15%, with unsoaked values of at least 10% prior to stabilization. To limit deformation under moist conditions, the plasticity index (PI) should not exceed 6%. Compaction of the subgrade is targeted at a minimum of 95% MDD, with a desired CBR range of 5–7% and a UCS threshold exceeding 75–100 kPa, ideally achieving at least 150 kPa for improved subgrade stability and performance.