Abstract
An integrated geophysical investigation involving ground magnetic, very low frequency (VLF-EM), and electrical resistivity methods using dipole–dipole array and schlumberger vertical electrical sounding (VES) techniques were conducted around School of Science Area Obanla, Federal University of Technology, Akure, for post construction studies in assessing building foundation integrity. Two traverses were established in approximately E-W direction of length 170 to 200 m and station interval of 10 m, along which VLF-EM, ground magnetic and dipole–dipole measurements were carried out. Sixteen VES stations were occupied within the study area. The VLF- EM data were interpreted using the Karous Hjelt (KH) package and inverted into its 2D Pseudosection. The VES data were quantitatively interpreted using the partial curve matching technique and 1-D forward modelling with WinResist 1.0 version software. The dipole-dipole data were inverted into 2-D resistivity images using the DIPPRO™ 4.0 inversion software. The VLF-EM result mapped three near surface conductive zones suspected to be fractures/faults which are inimical to foundation integrity. The magnetic results delineated series of bedrock ridges and depression. The VES result delineated four major Geo-electric layers within the study area. The topsoil, weathered layer, fractured bedrock and fresh bedrock. The top soil (resistivity varies from 47 to 490 I©m and thickness ranges from 0.7 to 3.9 m); weathered layer (resistivity varies from 13 to 207 I©m and thickness ranges from 1.9 to 22.1 m), fractured bedrock (resistivity varies from 489.3 to 878.8 I©m and thickness ranges from 2.4 to 19.6 m) and bedrock with resistivity 1094 to 96583 I©m and depth to bedrock 2.6 to 24.8 m). The dipole-dipole results also mapped linear features (fracture) at distance 60 to 100 m and 100 to 120 m respectively along the two traverses. Then from the geophysical investigation, three major causes of potential failure in the area were identified, these are; failure due to lateral inhomogeneity of the subsurface layers, failure precipitated by differential settlement and failure initiated by geologic features such as fractures and faults. Key words: Foundation integrity, lateral inhomogeneity, electromagnetic, resistivity, dipole-dipole.
Highlights
The rate of failed structures in Nigeria have increased in recent times (Oyedele et al, 2011)
Figure (10a) shows the summary of profiles and sections oablotnagineTdra(fvbreo)rmse v1a.rioTuhse geophysical magnetic methods intensity employed contrast observed at distance 20 to 30 m coincides with the conductive zones delineated by the very low frequency electromagnetic (VLF-EM) section at distance 10 to 30 m
The linear feature delineated on the dipole-dipole pseudo-section at distance 60 to 100 m (Figure 8a) coincides with depression observed on the geo-electric section at distance between 80 to 140 m and is delineated as conductive zones by the VLF-EM section at distance 60 to 80 m
Summary
The rate of failed structures in Nigeria have increased in recent times (Oyedele et al, 2011). Vertical and near vertical cracks or discontinuities have been noticed in the walls of both old and recent buildings in the school (Bayode et al, 2012) This assertion can be attributed to the minimal attention towards the use of geophysics in foundation studies. There is need to evaluate the foundation integrity of the buildings around School of Science Area at the Federal University of Technology, Akure in terms of the subsurface structures and nature of the soil. The fractured bedrock generally occur in a typical basement terrain (Odusanya and Amadi, 1989) in tropical and equatorial regions, weathering processes create superficial layers, with varying degree of porosity and permeability These geologic events gave rise to such structures as folds, faults and fractures that are geologically associated with zones of weakness. Geophysical methods can map these geologic structures; their application is employed to study the subsurface geology of the area in order to ascertain if there are geologic structure that can affect foundations or cause building collapse
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