Geophysical Investigation into the Cause(S) of Structural Failure within Bacosa and Faculty of Science Buildings, Bowen University Temporary Site, Iwo

Abstract

Cracks on concrete and walls could be bothersome for quality of life and for property claims. Geophysical investigations was carried out around Faculty of Science and BACOSA buildings of Bowen University Temporary site, Iwo with a view to establish the possible cause(s) of failure of buildings within the study area.

The geophysical methods used for the investigation were the Very Low Frequency Electromagnetic (VLF-EM) and the Electrical Resistivity (ER) methods. The VLF-EM measurements were taken at an interval of 10m along six traverse lines in the E-W and S-N directions. From the result of the VLF-EM, six VES stations were occupied in the study area involving Schlumberger configuration with half current electrode separation (AB/2) varying from 1 to a maximum of 65m. The 2-D VLF-EM models generated showed a network of suspected linear (geological) structures such as fractures, faults and contacts within the study area.

The VES curve types obtained in the area include HA, KH, QH and AA with HA type curve dominating. Four subsurface geologic layers were delineated in the study area. These include the topsoil, weathered layer, clayey sand and fresh basement with resistivity ranging from 121.3 to 771.2 ohm-m, 134.4 to 595.8 ohm-m, 81.6 to 951.9 ohm-m and 1474.7 to 5134.4 ohm-m respectively. The thickness range from 0.5 to 2.4m, 1.8 to 9.8m and2.7 to 24.5m. Depths to the bedrock are generally less than 40m. The resistivity of the weathered layer beneath the topsoil in which the foundation is seated was found to be low within the study area, ranging from 134.4 to 595.8 ohm-m typical of clayey materials.

From the models generated from VLF-EM and resistivity data, the distressed structures were found to be situated within the areas with a fairly high concentration of fractures, faults and contacts and relatively low resistivity values(less than 1000 ohm-m) typical of incompetent clayey formation.

Keywords: Geophysical investigation, Structural failure, Electrical resistivity, Depth to bedrock, Foundation, Very low frequency electromagnetic, VLF-EM