GEOPHYSICAL INVESTIGATION OF FAILED DAM AT THE COLLEGE OF EDUCATION, IKERE-EKITI, SOUTHWEST, NIGERIA

Abstract

Geophysical investigation has been undertaken to determine the cause(s) of a dam failure at the College of Education, Ikere Ekiti. The area is underlain by the Precambrian Crystalline Basement Complex rocks of Southwestern Nigeria with the lithology consisting mainly of Older granite and Fine-grained biotite granite. The geophysical methods adopted for the study are magnetic, electromagnetic, electrical resistivity and Spontaneous Potential (SP). Traverses were established both parallel (T1, T2, T6, T7) and perpendicular (T3, T4, T5) to the azimuth of failed dam axis. The field data were processed and filtered using suites of software which include Oasis montage™, 2D Euler deconvolution, Diprowin, Winresist and Karous Hjelt, thus, enabling qualitative and quantitative evaluations/interpretations. The results are presented as profiles, sections, charts, tables and maps. The magnetic profiles delineated various anomalies with notable positive amplitudes ranges from 65 to 400 nT and notable negative amplitudes ranges from -20 to -250 nT. Electromagnetic profiling shows dominant high conductivities on T2, T4, T5 and T7 indicating potential fractures/water saturating media. Dipole-dipole results revealed the existence of some discontinuities along some traverses. The geoelectric layers delineated are the topsoil (57 to 1938 Ω-m), weathered layer (31 to 774 Ω-m), partly weathered/fractured basement (80 to 811 Ω-m), and the fresh basement (1900 to ∞ Ω-m). SP troughs with maximum amplitudes ranging from -6 to -75 mV along T1, T2, T4, and T5 are indicative of some streaming potentials possibly occasioned by fractures/faults. The magnetic, and resistivity studies indicate shallow basement with depth to bedrock ranging from 0.8 to 20 m. The major fractured zone delineated within the study area extends from the northwestern upstream region, transverses T5, T4, and terminates at T2. These identified structures strike parallel to the dam axis and are confined to the central area of the reservoir and apparently cannot constitute the major medium for the reservoir leakage. Since the results obtained at T1 and T6 (dam embankment) do not show signatures indicative of shows that no seepage path exists beneath the dam. This study has shown that the failure of the dam cannot be ascribed to geological factors, thus the failure may have been due to other factors presumably associated with construction.