Geotechnical Assessment of Sub-Grade Soil Conditions for Road Construction in Borno State

Sub-grade soil conditions critically influence the performance of road pavements in arid and semi-arid environments such as Borno State in North East Nigeria, where seasonal temperature variations and occasional rainfall events create challenging engineering conditions. This study geotechnically assesses sub-grade soils along three proposed rural road corridors in Borno State. Soil samples were collected from trial pits at 500-metre intervals along a combined 24-kilometre route in Maiduguri, Bama, and Dikwa LGAs. Standard geotechnical tests including Atterberg limits, particle size distribution, California Bearing Ratio, maximum dry density, and optimum moisture content were conducted. Results indicate that sub-grade soils along the Maiduguri corridor are predominantly A-6 soils under the AASHTO classification, with CBR values ranging from 4 to 9 percent at 95 percent Modified Proctor Dry Density. Soils from the Bama corridor showed expansive characteristics with Liquid Limit values up to 58 percent and Plasticity Index up to 24 percent, indicating potential for swelling and heave under seasonal moisture variation. Dikwa corridor soils showed the best geotechnical performance with CBR values up to 18 percent. The study concludes that sub-grade improvement by lime stabilisation is necessary for the Bama corridor before pavement construction, while select granular capping layers will suffice for Maiduguri and Dikwa. Lime treatment at 4 to 6 percent content by dry weight is recommended.

Keywords: sub-grade assessment, geotechnical investigation, road construction, Borno State, North East Nigeria