Subgrade soil evaluation using integrated seismic refraction tomography and geotechnical studies: A case of Ajaokuta-Anyigba Federal highway, North-Central Nigeria

ABSTRACT Integrated seismic refraction tomography (SRT) and geotechnical tests were conducted on failed and stable sections of the Ajaokuta – Anyigba Highway, North-central Nigeria. This study was aimed at characterising the soil profile, determining the competence of the subgrade soils, and establishing relationships between geophysical and geotechnical parameters. Primary wave velocities (Vp) were calculated by using SeisImager/2D (V.2.9.1). The subgrade soils of the unstable segment were characterised by low Vp (358–607 m/s) while higher Vp (762–1417 m/s) characterised the stable segment. The geotechnical results confirmed the seismic refraction results. Geotechnical results revealed that the subgrades of the unstable section are generally classified as A-6 and A-7-6 clayey soils, with low to high plasticity, and a fair to poor subgrade rating. While the subgrades of the stable section are grouped as A-2-4 silty sands with low compressibility and good to excellent subgrade ratings. Simple linear regression analysis showed fairly strong positive correlations between Vp and sand with CBR (r = 0.81 and 0.78, respectively). On the other hand, fairly strong negative correlations were obtained between Vp and the plasticity index with fines (r = −0.63 and −0.82 respectively).