Geophysical and Geotechnical Evaluation of Erosion Sites in Ebem-Ohafia Area of Abia State, Southern Nigeria

Abstract

This work is an integrated evaluation of the external and internal structures of an erosion site in Ebem-Ohafia area of Abia state, Nigeria using the geophysical and geotechnical methods of investigation. The geophysical method used was the electrical method which employed the Schlumberger electrode configuration with maximum half current electrode spacing of AB/2 = 165 m, and 4 vertical electrical sounding (VES) data were acquired. Results show that the top soil resistivity values vary from 58.8 Ωm – 886.6 Ωm, that of the weathered layer vary from 100 Ωm - 3586.6 Ωm; and the maximum depth of each sounding location varies from 33.4 m - 59.6 m. In the geotechnical approach, four soil samples from each of the sounding locations were used for the study. The geotechnical results show that the soil has relatively high clay content with plasticity index ranging from 6.0% -12.0%. The consistency limits of the soils generally indicate low to medium plasticity. The natural moisture content varies from 5.3% to 9.4%; while the liquid limit ranges from 27.4% - 41.1%. By using the resistivity values together with plasticity index in the evaluation, it is established that the higher the value of layer resistivity, the lower the plasticity index of the layer. This indicates that the vicinity of VES 1 is the most erosion-prone locality in the study area, while the vicinity of VES 4 remains stable. The plastic index of the soils within the area is adjudged to be of low to medium plasticity (,20%); hence, the soils are expected not to exhibit high cohesion potential. It was however concluded that geomorphologic and anthropogenic factors are the major causes of the erosion menace in the area. Subsequently, good agricultural practices and regulars monitoring of the area is recommended.