Subgrade Evaluation and Pavement Failure Investigation Along Rafin Dan-Nana Road, Off Gwarzo Road, Kano State, Nigeria

Abstract

This study evaluated the subgrade strength of a failed pavement segment of approximately 1 km along Rafin Dan-Nana Road, off Gwarzo Road, Kano State, Nigeria. Eighteen disturbed soil samples were collected from nine trial pits at 100 m intervals at depths of 1.0 and 1.5 m. Laboratory tests, including particle size distribution (PSD), Atterberg limits, standard Proctor compaction, and the unsoaked California Bearing Ratio (CBR), were conducted in accordance with BS 1377:1975. AASHTO classification placed all samples between A-2-4 and A-7-6; 14 of the 18 samples were classified as A-4 or better, with five rated very good (A-2-4) and nine rated fair (A-4). Despite these mixed classifications, 16 of 18 samples recorded CBR values at or above the 10% minimum subgrade threshold; only two samples at 1.0 m depth (TP7 S1: CBR = 5.0% and TP9 S1: CBR = 9.0%) fell below this limit. These results indicate that subgrade inadequacy was not the principal cause of the observed pavement failure. Direct visual inspection identified completely blocked drainage channels and drain inverts elevated above road surface level as the predominant factors responsible for pavement distress. The study concludes that adequate laboratory subgrade strength does not guarantee field performance without effective drainage control, and recommends urgent rehabilitation of the drainage infrastructure as a priority intervention. This study and many others have prompted the concerned authority to act by reconstructing the failed pavement.