ASSESSMENT OF SUBGRADE BEARING CAPACITY FOR FLEXIBLE PAVEMENT DESIGN USING DYNAMIC CONE PENETROMETER AND CALIFORNIA BEARING RATIO TESTS IN CALABAR, NIGERIA

The properties and bearing capacity of subgrade soils in selected parts of Calabar, Cross River State, Nigeria, to determine their suitability for flexible pavement design. Field investigations comprised drilling and Dynamic Cone Penetrometer (DCP) testing for two borehole locations, while representative soil samples were subjected to laboratory analyses including particle-size distribution, Atterberg limits, Standard Proctor compaction, and California Bearing Ratio (CBR) tests following ASTM and AASHTO standards. The soils were predominantly classified as clayey sand (SC) with low to moderate plasticity, exhibiting Plasticity Index values below 15% and Liquid Limit values below 40%, indicative of good subgrade characteristics. Particle-size analysis revealed fines content of less than 35%, while the Standard Proctor compaction test produced an Optimum Moisture Content of approximately 14% and a Maximum Dry Density of 1.93 gm/cm³. Laboratory soaked CBR values averaged 14.4%, whereas DCP-derived CBR values averaged approximately 15%, demonstrating good agreement between field and laboratory assessments. The DCP profiles identified three distinct subsurface layers, with the second layer, located approximately 0.5 m below the ground surface, constituting the principal pavement foundation. Based on the evaluated subgrade conditions, flexible pavement thicknesses of approximately 270 mm for express roads and 375 mm for airport taxiways were found to be adequate under the specified traffic loading conditions. The close correlation between DCP and laboratory CBR results demonstrates that DCP testing provides a rapid, reliable, and cost-effective method for subgrade evaluation and pavement design in tropical environments. engineering performance of flexible pavements is strongly influenced by the strength and deformation characteristics of the underlying subgrade soils. This study evaluates the geotechnical.