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Browsing Faculty of Technology by Author "BOBOLA Damilola Gbadebo"

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    Investigation of groundwater seepages using geotechnical and ground penetrating radar techniques
    (Department of Civil Engineering, Falculty of Technology, Obafemi Awolwo University, Ile Ife, 2022) BOBOLA Damilola Gbadebo
    This study evaluated the geotechnical properties of the soil in the study area, determined the stratigraphy, zone of fractures (micro and macro) and cavities with ground penetrating radar, correlated results obtained from the ground penetrating radar investigation with the geotechnical tests of the soil and developed a basemap for the study area for exposition of fractures and cavities zones using information obtained from the geotechnical and ground penetrating radar investigations. These were with a view to determining the causes of groundwater seepages in the study area using geotechnical and ground penetrating radar techniques. Five trial pits, each 3.6m deep were dug in the study area and soil samples were collected at 3.6m from each pit. Sieve analysis, natural moisture content, specific gravity, Atterberg’s limits, compaction, shear strength, permeability, and consolidation laboratory tests were carried out on them to determine their geotechnical properties. The Ground Penetrating Radar (GPR) survey data were acquired using a Geophysical Survey System Incorporated Sirveyor-3000 (GSSI SIR-3000) equipment with a central antenna frequency of 100, 200 and 400 MHz along 10 traverse lines and the data were subjected to processing by the Radpro software. The properties of the soil samples were then statistically analysed and correlated using Microsoft Excel Software Package. The vertical and horizontal stresses acting close to the fractured zone were also analyzed. Critical fracture delineated by GPR was then subjected to further geophysical investigation using Puqi Water Trace Detector (PQWT) instrument and a basemap showing zones of fractures and micro-fractures was generated. Three types of soils were identified based on Unified Soil Classification System (USCS): Silty Sand (SM), Clayey Sand (SC) and Clay with low plasticity (CL), with plasticity for all the soil samples ranging from low to intermediate plasticity from the result of the geotechnical investigation. Also the shear strength of the soils were low to medium ranged with low permeability. The GPR radargrams delineated four subsurface layers. These include the top soil, weathered layer, partly weathered/fractured bedrock and the basement bedrock. Along traverses 1, 6, 8 and 10, geological structures (fractures and micro-fractures); which facilitated groundwater seepages were delineated from the radargrams. A critical fracture denoted as F1 with width of atleast 2 metres was delineated by GPR in traverse line 1 along with one other fracture (with width close to 1.2 metres) and one micro-fracture (few centimetres wide). Several other micro-fractures were also delineated in other traverse lines. Correlation of some of the geotechnical properties of the soil samples show that, soils in the study area substrata with lower percentage of fines, are more prone to groundwater seepage, as they are more permeable and have high shear strength. Analysis of vertical and horizontal stresses of the subsurface close to fracture F1 showed that the weathered and partially weathered layer in the substrata of the study area are more prone to easy flow of groundwater seepage due to high vertical and horizontal stresses. Furthermore, mapping of the critical fracture F1 with PQWT instrument gave a profile map that delineated five distinct layers with varying density up to a depth of 150 m. Also, another fracture of 3 m width was observed directly under the fracture F1 delineated by GPR starting at a depth of 125 m below the ground surface. This wide fracture serves as a conduit for movement of groundwater to the subsurface as the fracture cut across the other subsurface layers. The study concluded that the basement floor was constructed close to a fractured bedrock leading to the incessant seepage of groundwater into the structure’s basement