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

Résumé

This study evaluated the performance of the sub-base material of High Court Road, Kabale, Uganda, using the California Bearing Ratio and Triaxial Compression Tests. Optimizing sub-base material selection is expected to enhance road longevity, minimize maintenance costs, and ensure sustainable, value-for-money construction, ultimately improving infrastructure reliability and reducing long-term performance. Field samples were collected at one-meter depth and tested for California Bearing Ratio and shear strength parameters of cohesion, angle of internal friction, and stress-strain behavior of the soil under varying axial loads, to evaluate the overall suitability under estimated loading conditions. Laboratory tests indicate that the sub-base consists predominantly of fine-grained soil, classified as high-plasticity clay based on the Unified Soil Classification System. Compaction results show maximum dry densities between 16.05 kN/m 3 and 18.03 kN/m 3 , with Optimum Moisture Content between 18.9% and 23.3%. Shear strength parameters indicate cohesion values ranging from 67.5 kPa to 95.2 kPa, which fall below the recommended minimum of 100 kPa for stable sub-base performance. The angle of internal friction ranges from 9.00° to 19.71°, lower than the 26°–29° range typically required for stability and high bearing capacity. The CBR values range from 2% to 13%, significantly below the recommended minimum of 30% for sub-base layers. This confirms that the current sub-base material has poor load-bearing capacity and is unsuitable for supporting expected traffic loads. To support the anticipated traffic load, the study recommends construction using a 450 mm thick sub-base material embedded in the 950 mm thick pavement, attaining a minimum of 98% compaction, stabilization using lime/cement, and reinforcement of the sub-base with geosynthetics. An alternative pavement design comprising a 225 mm sub-base, in a 725 mm thick pavement, is also recommended, particularly in instances where granular material and crushed stones are more readily available

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