Subsurface characterization for foundation valuation of existing engineering structures in basement complex of southwestern Nigeria

The subsurface characterization for foundation evaluation of existing engineering structures aims to investigate the structural competence of selected high-rise buildings on the University of Ilorin campus, located in the basement complex of north- central Nigeria. Very Low Frequency Electromagnetic (VLF–EM) and Vertical Electrical Sounding (VES) techniques were used to explore the subsurface geological sequence. Twenty-one VLF-EM traverses were established in an E-W orientation, following the prominent NE–SW geological trend. The 32 anomalous zones identified from the filtered EM data were fur- ther examined using the Schlumberger VES technique. The EM responses ranged from -40% to 45%, showing alternating positive and negative peaks, which indicate contrasting near surface formations, identified as vadose zones (associated with lateritic clay and/or clayey sand) and water-bearing weathered layers extending to an estimated depth of about 8 meters. The vertical geoelectrical sequence revealed 3 to 5 electrical horizons, comprising topsoil made up of clay lenses, sandy-clay, lateritic hard-pan, and clayey-sand, along with a highly weathered basement and fractured to fresh bedrock. The basement is predominantly weathered and lies at a shallow depth, ranging from 1.0 to 10.6 meters. The overburden is relatively thin, composed of some clay lenses, sandy clay, and a dominant lateritic hard-pan or clayey-sand, which are considered compe- tent subsoils for high-rise buildings. However, thick clay layers in the southern and northern parts of the area could pose a risk to high-rise structures if they extend continuously across other profiles. The predominantly fractured bedrock is also notable for groundwater development, but if seismically active, it could lead to building failures. Overall, the study area has a subsurface sequence and structurally competent bedrock capable of supporting the selected high-rise structures, as evidenced by several traverses and VES points, including Traverses 1 (VES1, VES2, and VES3), 2, 5 (VES7, VES8, and VES9), and 20 (VES30, VES31, and VES32).