ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ/MINERAL WEALTH 166/2012 47 FAULT PRESENCE UNDER A FAILING BUILDING COMPLEX MAPPED BY ELECTRICAL RESISTIVITY TOMOGRAPHY Geophysics A.F. FOLORUNSO 1 * E.A. AYOLABI 2 S.O. ARIYO 3 I.O. OYEBANJO 3 AbsTRAcT An electrical resistivity tomography supported with Vertical Electrical sounding (VEs) survey was car- ried out within the precincts of a Faculty building at Olabisi Onabanjo University Main campus, Ago- Iwoye, Nigeria. The building is intensively affected by dangerous cracks that cause structural instabil- ity a few years after its construction. The survey aimed to image the shallow subsurface structures and determine the possible cause of the structural instability. 2D Resistivity imaging using Wenner array with smallest electrode spacing of 10m and VEs using schlumberger electrode coniguration were carried out along four traverses extending about 130 meters at the precinct of the building complex. The acquired data were processed and interpreted spatially to expose the shallow struc- tural setting of the site. Integrated interpretation led to the delineation of hazard zone in the area. This zone is interpreted as fault with vertical and sub-vertical linear features having moved both ver- tically and horizontally with associated resistivity values as low as 5Ωm. This feature is suggested as the main reason of the rock instability that resulted in potentially dangerous cracking of the buildings. Inferred lithologic units which include silty/clayey soil, sandy clay and the basement were mapped out at the site. compacted lateritic (hardpan) topsoil along traverse 4 (backside of the building), though not a rock unit, constitutes an important factor to the re-construction of geologic unit from ERT model. The possible cause of failure of the building was proposed to result from the underlying fault on which the whole weight of the building rests. It was also discovered that the pre-construction geo- technical investigations possibly failed to capture the zones because they are point investigations that lack continuous imaging of subsurface condition. Key words: 2-D electrical resistivity tomography, structural instability, Fault zone, synclinal fea- tures, compressible soil and springs INTRODUcTION Foundation investigation requires the characterization of subsurface soil and determination of soil strength as prerequisites for the foundation design of important civil engineering structures. The usual practice in geo- technical studies is the use of Standard Penetrometer Test (SPT) to determine the resistance of soil to pen- etration used to evaluate the soil strength in terms of number of blows (N-values). The N-values are deined as the number of blows per 30 cm of penetration into the soil. N-values can be used to obtain the bearing capacity of soils following the procedure of IS 6403 - (1981) code (Sudha et al., 2009). In Dynamic Cone Penetration Test (DCPT), the resistance (i.e. N-value) to penetration of the cone in terms of the number of blows per 30 cm of penetration could be correlated with the bearing capacity of the soil. Over the year, the ineficacy of this geotechnical approach to the char- acterization of subsurface soil became pronounced through colossal of building failures and collapse in so much that the integrity of engineering soil tests be- 1 Ocean University of China, College of Marine Geosciences, Qingdao 266003, China. E-mail: detayof@yahoo.gr * Corresponding author 2 Department of Geosciences, University of Lagos, Nigeria, E-mail: eojelabi@yahoo.com 3 Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye, Nigeria.