3rd TAGRA

Geotechnical failures in basement terrains frequently result from insufficient subsurface characterisation, leading to differential settlement and structural damage. This study applies Vertical Electrical Sounding (VES) to assess geotechnical suitability at Bolorunduro and environs, Ondo State, Southwestern Nigeria, underlain by the Precambrian Basement Complex. Twenty VES stations were surveyed using the Schlumberger electrode configuration, with half-current electrode spacing (AB/2) of 1–80 m. Field data were processed by partial curve matching and inverted using IPI2Win software to generate layer models defining resistivity, thickness, and depth for each subsurface unit. Interpretation delineated three to five geo-electric layers per station — comprising topsoil, lateritic horizons, clay-rich units, weathered basement, and fresh competent basement — with curve types spanning H, HA, KH, QH, HK, A, and AH forms, reflecting lateral subsurface variability across the study area. Depth to competent basement ranged from approximately 2.7 m in the northwestern zone to over 43 m in the clay-rich eastern region, indicating significant lateral heterogeneity. Resistivity values below 100 Ω·m were diagnostic of clay-rich horizons with high plasticity and low bearing capacity, while values exceeding 1,500 Ω·m indicated competent basement suitable for heavy structural foundations. Three geotechnical suitability zones — high, moderate, and low — were delineated and integrated into a suitability map guiding pre-construction planning. The results confirm that VES is a reliable, non-invasive, and cost-effective geotechnical site investigation tool in crystalline basement terrains, providing spatially continuous subsurface characterisation that complements conventional borehole-based assessments.