ABSTRACT
The foundation studies of Ondo south and its environments were carried out in sedimentary basin of Ondo State, underlained by the pure sedimentary basin. This constitutes part of the system of West African precratonic basin developed during the commencement of rifting, associated with the opening of the Gulf of Guinea in the early Cretaceous to late Jurassic. Ondo south studies involve Irele, Igbekebo, Igbokoda and Okitipupa. The areas were studied using integrated geophysical, geotechnical and geochemical methods with a view to investigating the image of the subsurface geology, to characterize the soil in terms of their engineering geological properties so as to determine their suitability for building the foundation and assessing the two methods for determining building foundation requirements in the study areas. Eighty (80) Vertical Electrical Soundings (VES) were quantitatively interpreted using the partial curve matching technique and xxii computer assisted 1-D forward modeling and Fourty (40) 2D Electrical Imaging were conducted for the geophysical survey using Electrical Resistivity Method. Cone Penetration Tests (CPT) were performed, a total of six (6) CPT was carried out in each location and a total twenty four of (24) CPT were carried out within the four locations of the study areas using the Dutch static penetrometer, and borehole lithological logs were used to generate columnar sections of the soil layers. Free soil survey technique was used in citing profile pits. Four profile pits were investigated. The profile pits were geo-referenced using hand held Global Positioning System (GPS) Receiver. Soil profile pits were described according to Food and Agricultural organization (FAO) guidelines. Soil samples were collected from horizons, starting from the upwards horizon to the deepest. Samples for the study of geotechnical properties were collected at a depth of about 1.0-2.5m. Soil samples were air-dried and sieved for standard routine analysis. From standard routine analysis this research work established the nature and the properties of the soil, in each location to correlates the four stations and determine the suitable foundations. Soil texture were examined from each location, hence colour and the nature of samples were analyzed for the stations. Particle Size Distribution test was examined for the four stations using some selected samples of different depths in metres for both disturbed and undisturbed soil samples, Atterberg Limit Tests were conducted for Irele, Igbekebo and Okitipupa only using some selected samples of different depths in meters for both disturbed and undisturbed soil samples where Natural Water Content (Wc%), Liquid Limit (L.L%), Plastic Limit (P.L%) and Plasticity Index (P.I%) were determined. Quick Undrained Tri - axial was conducted for Irele, Igbekebo and Okitipupa only using some selected samples of different depths in meters for undisturbed soil samples where Natural water content (Wc%),Bulk density b b(Mg/m3 ), Undrained cohesion Cu(KN/m2 ), Angle of Internal friction(0), Cell pressure 3 (KN/m2 ), and Compressive Stress (KN/m2 ) Compression xxiii Tests were determined. Oedometer Consolidation Tests were conducted for Irele, Igbekebo and Okitipupa only using some selected samples at different depths in meters for undisturbed soil samples where Pressure Range (KN/m2 ), Coefficient of compressibility, Mv (m2 /MN), and Coefficient of consolidation Cv (m2 /year) were determined. Direct Shear Box Tests Was conducted for only Igbokoda location in respect the bulk samples, natural water content (Wc%), Normal stress (KN/m2 ), Strain at failure %, Bulk density b (Mg/m3 ), Dry density d (Mg/m3 ), Apparent cohesion (KN/m2 ), Angle of Internal friction (0), and Maximum shear stress (KN/m2 ). The soil and groundwater can contain constituents detrimental to pile materials. Electrochemical classification tests can be used to determine the aggressiveness of the subsurface conditions and the potential for pile deterioration. Chemical analysis of water samples were carried out to test for the following: Determination of metals iron (conductivity), PH, Chloride, and Sulphate ratio of the water samples at river Oluwa that bounded Irele and Okitipupa, likewise Igbekebo and Igbokoda Rivers respectively. Geoelectric sections were generated from the VES interpretation results, constrained by borehole lithological logs, for foundation level identification and geoelectric parameter determination. Depths to top and thicknesses of the layers within each geologic unit were determined. 1D, 2D and cone penetration test were also correlated to show the relationship between the three methods for the foundation study so as to identify the nature and the properties of sub-surface soil in the studied locations. In Irele area reveal the lateral and the vertical subsurface information with resistivity values ranging from 544 to 3506 Ωm. The resistivity values corresponding to the topsoil, clay, clayey sand, sandy clay, and sand/sandstone. In Igbekebo area reveal the lateral and the vertical subsurface information with resistivity values ranging from 302 to 9257 Ωm. The resistivity values corresponding to the topsoil, clay, clayey sand, sandy clay, and sand/sandstone. xxiv In Igbokoda area reveal the lateral and the vertical subsurface information with resistivity values ranging from 4.5 to 8207 Ωm. The resistivity values corresponding to the topsoil, peat, clay, clayey sand/sandy clay and sand. In Okitipupa area reveal the lateral and the vertical subsurface information with resistivity values ranging from 298 to 2036 Ωm. The resistivity values corresponding to the topsoil, clay, clayey sand, sandy clay, and sand/sandstone. The distribution and thickness of the sand zones are more pronounced in Ilaje, Igbokoda compared to Irele, Igbekebo and Okitipupa area. Conclusively, the distribution of depth and thickness of the sand zones from VES (1D), 2D, CPT and SPT are more pronounced in Ilaje, Igbokoda compared to Irele, Igbekebo and Okitipupa area. The trial-pit, the distribution of sand is more pronounced in Ilaje Igbokoda than other locations of the study, and as a result of shallow water table of the rang of about 2- 6 m, the PH and chloride contents are very high as a result of this, the water in the study area is corrode and have capacity to bleach. Hence the foundation design of the locations must be treated according to the specialized British Standard (BS 1377:1990).
