Application of Geophysical and Geotechnical Methods for Mapping and Characterization of Sand and Gravel Deposits in Njaba Area of Imo River Basin, Nigeria

Excavation of sand and gravel occurs along Njaba River bank for Civil engineering purposes which occasionally results to landslides that lead to casualties. Hence, Geoelectrical and Geotechnical techniques were adopted for mapping and characterizing the sand and gravel deposits and at the same time assess the slopes stability. Ten vertical electrical soundings (VES) were carried out at different locations, using OHMEGA-500 resistivity meter. The field data were interpreted using Advanced Geosciences Incorporation (AGI) ID resistivity inversion software and Schlumberger Automatic analysis version. The model results revealed that the sand and gravel layers have high resistivity values ranging from 1359Ωm to 7353Ωm. The geologic information, the borehole data and VES model were integrated to map the sand and gravel bed. The thickness of the beds ranges from 15.70m to 67.60m. Four samples collected at different locations were tested and analyzed in laboratory to determine basic geotechnical parameters. Average values obtained were: moisture content (10.7%), bulk density (2.10g/cm3) maximum Dry density (1.73g/cm2), California Bearing Ratio (24%). The particle size distribution obtained revealed that the coarse sand, medium sand, fine sand are 3.7%, 34.52%, 61.26% respectively, implying that they are of good grade and in conformity with civil engineering requirements. The slopes instability in the study area is as a result of the low shear strength.