Effect of Poor Drainage Design on the Structural Deterioration of Roads in Minna, Niger State

Inadequate road drainage design is widely recognised as one of the most consequential causes of premature pavement deterioration in Nigerian urban centres. This study evaluates the effect of poor drainage design on the structural deterioration of selected road pavements in Minna, Niger State, in the North Central geopolitical zone. Fifteen road sections across Minna were assessed using a pavement condition index survey, and drainage features including kerb channels, cross-drainage culverts, and side drains were inspected for design adequacy, blockage, and condition. Subgrade moisture content was measured using a time-domain reflectometry probe at intervals along each road section, and readings were compared between road sections with functioning drainage and those with blocked or absent drainage. Surface drainage flow paths were observed during a simulated rainfall event. Results reveal a strong correlation between subgrade moisture content and pavement distress severity, with sections having non-functional drainage exhibiting California bearing ratio values forty-three percent lower than sections with intact drainage systems. Pothole density was 3.2 times higher on sections with drainage deficiencies. Two road sections showed evidence of subbase washout beneath the pavement surface, confirmed by ground-penetrating radar profiling. The study attributes the drainage deficiencies to undersized inlet capacity at junctions, inadequate cross-fall gradients, and lack of routine drain maintenance. Recommendations include retrofitting of additional kerb inlets, steepening of cross-fall gradients to a minimum of two percent, and inclusion of drainage maintenance in all road management contracts in Minna. Keywords: road drainage, pavement deterioration, Minna, subgrade, condition index.