Geotechnical and geophysical investigation of subsoil for construction of a waste water treatment plant in coastal environment, southwestern Nigeria
A combined geophysical and geotechnical studies were conducted in a coastal environment in Lagos, Nigeria in order to delineate subsurface lithologies and evaluate their suitability as foundation for a proposed waste water treatment plant. The geophysical study carried out involved electrical resistivity tomography (ERT) resulting in 2-D resistivity imaging along 3 profiles with spread length ranging from 145 – 190 m. The ERT data obtained along the profiles were analyzed using DIPRO for window, a 2-D resistivity inversion program to provide estimates of vertical and horizontal spatial distribution of resistivity beneath the survey line. A total of eight cone penetration test and four standard penetration borehole logs in were conducted to determine the strength and characteristic nature of the foundation soils. In addition, pH and sulphate concentration of a water sample from one of the SPT boreholes were determined to predict if there is possible infiltration of leachates into the groundwater. The results of the integrated investigation revealed the occurrence of thick peat, and clay (≈ 22.00 m) which prevented the further infiltration of leachates plumes as revealed by the ERT images and sandy units at depths greater than 23 m. The occurrence of thick peat/clay horizons makes the use of shallow foundation infeasible to mobilize the proposed structure. The chemical analyses show pH to be slightly acidic (6.88) and sulphate concentration of 31 mg/l which are both within World Health Organization standard, confirms that there is no infiltration of pollutants/leachates into the groundwater, therefore the water in the area will not be inimical to Portland cement concrete at deeper depths. Both geophysical and geotechnical methods reveal that the subsoil contains clayey material which serves as good containment and that pile foundation be used to mobilise the proposed structure to the deeper sand layer.