Geophysical and Geotechnical Assessment of Subsurface Characteristics Upstream and Downstream Otamiri Watershed Owerri Southeastern Nigeria

Authors : Ezenta N. K; Nwachukwu M. A; Umunnakwe J. C

Volume/Issue : Volume 8 - 2023, Issue 1 - January

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This investigation is motivated by the concern over the siting and stability of engineering structures and anthropogenic activities situated within the watershed area of the Otamiri River in the Imo River basin in Nigeria. Using electrical resistivity inversion and soil sampling, this study research is anticipated to look into the differences between the soil properties and water table in upstream and downstream watershed areas of the Otamiri river. This will lead to different approaches in engineering practices land use planning, watershed management, and better management of water resources within the study area. In order to create a topographic map of the area, GPS was utilised to measure spot elevation and coordinates. Four VES stations were set up to map the subsurface tomography using the electrical resistivity inversion technique. Two composite soil samples that were taken from the VES stations at a depth of 5 metres were examined in a lab. Geotechnical properties measured include shear strength and bearing capacity, liquid and plastic limits, grain size distribution and compaction density. Through the geophysical modelled data we obtained three types of curves, A curve type for VES 1 & 4, QH curve type for VES 2, and UK for VES 3. Common lithology discovered within the study area is sandy-clay, siltstone, sand and gravel. The topsoil from VES 1, 2, 3, and 4 ranged from 1.3-ohm meter to 1223.4-ohm meter. The water table ranged from 19.57m to 54.39m. The geotechnical results revealed a plasticity index of 13.3% to 19.3%, optimum moisture content at 14.0% to 14.4%, and maximum dry density of 1.85mg/m3 to 1.86mg/m3. The trial test showed shear strength at 99.5KN/m2 to 109.8KN/m2 indicating high shear strength and a great tendency to withstand the load. Most measured parameters fell within recommended standard of the federal ministry of Works and housing.


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