Geological, Geophysical and Geotechnical Investigations of Road Failure along Keffi – Nasarawa Road, Northcentral, Nigeria

Abstract

Road failure remains a major infrastructural challenge in Nigeria, particularly along the Keffi–Nasarawa road, where persistent pavement deterioration has resulted in economic losses and safety risks. This study integrates geological, geophysical, and geotechnical investigations to evaluate the factors responsible for pavement failure along the route. Geological mapping revealed that the road traverses diverse lithologies, including schist, granite-gneiss, migmatite-gneiss with varying degrees of weathering. A geophysical survey employing electrical resistivity methods (Schlumberger array) comprised 35 Vertical Electrical Soundings (VES) with data processed using WinResist software,. The results identified four to five subsurface lithologic layers characterized by varying thicknesses and resistivity values: topsoil (1.0–1.5 m; 102–800 Ωm), clayey sand/lateritic clay (7–19 m; 90–400 Ωm), weathered bedrock (13–25 m; 150–376 Ωm), and fresh bedrock. These conductive subsurface zones indicate structurally weak layers that contribute to pavement instability. Geotechnical investigations on 24 soil samples conducted in accordance with British Standards 1377, included compaction, particle size distribution, Atterberg limits, and California Bearing Ratio (CBR) tests. Soaked CBR values ranging from 0.46% to 21.13% and unsoaked values from 0.93% to 52.4%, while plasticity indices ranged between 17.5 and 30. The failed sections are characterized by high plasticity , clay rich soils with low bearing capacity, poor drainage, and seasonal flooding, which collectively reduce subgrade strength and pavement stability. The failure is primarily attributed to weak, clay-rich subsoils (silty clay), poor drainage conditions, and seasonal flooding, which collectively reduce soil strength and pavement stability.