Abstract
Geo-electrical resistivity survey has been used for geotechnical investigation at a site proposed for solar power installations in Ajowa Akoko, Nigeria, to assist its foundation prognosis. Vertical electrical soundings (VES) were conducted in 24 locations spaced 50 meters apart with maximum AB/2 spread of 160 meters covering an area of 2km 2 . Electrical characterization of subsurface layers was done to identify a maximum of four geo-electric subsurface layers comprising the top soil (clayey sand and laterite), weathered layer (zone of capillary rise), and highly weathered (saturated) layer and fresh basement. The concept of electrical resistivity variation with subsurface formations - related to degree of saturation, specific gravity of solids, void ratio and unit weight of water - has been used to calculate the effective stress of each soil layer obtained from pore water pressure and total stress data. The effective stress estimated over the surveyed area for the various depths of 1.5m, 4.0m and 19m corresponding to the base of each characterized subsurface were 26.13kN/m 2 - 44.00kN/m 2 , 73.80kN/m 2 and 181.19kN/m 2 respectively. The effective stress plot shows that the soil stability increases with depth in the surveyed location. The study demonstrates the usefulness of VES method in estimating geotechnical attributes for efficient and precise information.
Highlights
Geophysical exploration techniques can serve as tools in various fields of studies including foundation assessment, stress estimation and lithological classification in geotechnical studies
The effective stress is more suitable for long-term soil analysis because after a relatively long period of time, the fully drained stage of the soil would have been reached as deformation of soil in the long term is a function of effective stresses and not total stress
The vertical electrical sounding done did not probe to the fresh basement
Summary
Geophysical exploration techniques can serve as tools in various fields of studies including foundation assessment, stress estimation and lithological classification in geotechnical studies. The total stress estimation is suitable for low permeability soils such as clay for long-term geotechnical analysis, and suitable for freedraining materials like sand and gravel on a short-term basis This is because it is assumed that a long time will elapse before there is significant dissipation of pore pressure water in the case of cohesive soils. According to Okoli (2004), earth subsurface resistivities are related to important geologic parameters viz type of rocks and soils, porosity and degree of saturation These findings can necessitate possible hydrogeological interpretations of the measured electrical resistivity at the survey area such as; determination of depth to and thickness of potential aquifer horizon, establishment of groundwater formation and mapping of desirable points for optimal groundwater recharge. The objectives of this study was to assess geotechnical conditions and estimate total stress, pore water pressure and effective stress parameters
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