Geotechnical Properties and Classification of Some Soils Formed on Two Lithologic Materials in Imo State, Nigeria.
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The study investigated geotechnical soil properties and classified soils derived from two different lithologic materials, namely Coastal Plain Sand (Benin formation) and Imo Clay Shale. Free soil survey technique was used in siting profile pits. Six profil e pits were investigated. The profile pits were geo - referenced using hand held Global Positioning System (GPS) Receiver. Soil profile pits were described according to FAO guidelines . Soil samples were collected from horizons, starting from the deepest hori zon upwards. Samples for the study of geotechnical properties were collected at depth of 100 - 200cm. Soil samples were air - dried and sieved for standard routine analysis . The mean, percentage coefficient of variation and simple correlation tools were used f or soil data analysis. Regression analysis among soil properties with significant relation was investigated. C olour of soils underlain by Coastal Plain Sand ranged from 2.5YR to 5YR while soils underlain by Shale ranged from 2.5YR to 10R. Soil underlain by Shale had higher clay content (159 - 443g/kg) and silt content (40 - 160g/kg) while soil underlain by Coastal Plain Sand had higher sand content (760.8 - 906.8g/kg). The bulk density was also higher in soils underlain by Coastal Plain Sand (up to 1.58Mg/m 3 ). We ak structure was observed in soils underlain by Coastal plain sand while shale has moderate to massive structure. Moisture content up to 353g/kg was observed from soil underlain by Shale. Organic matter content was also higher (1.4 - 3.12g/kg) in soil underl ain by Shale. TEB were higher in soils formed from Shale (2.63 - 5.43cmol/kg) than those from Coastal Plain Sand (1.11 - 3.97cmol/kg). Results of geotechnical properties revealed the presence of gravel (Amuro - 53%, Umuna - 19%) in soil underlain by Shale. Higher Atterberg limits (LL≥65% and PI ≥35%), optimum moisture content (≥20%), COLE (≥0.16), volumetric shrinkage (≥56.1) and cohesion (≥14KN/m 2 ) was observed in soils underlain by Shale. Soils underlain by Coastal Plain Sand have higher maximum dry density (≥1.6 1%), shear strength (≥91KN/m 2 ), angle of internal friction (≥25.4 0 ), and ultimate bearing capacity (≥379KN/m 2 ) than soils underlain by Shale. Soil bulk density, porosity, sand and clay content were good predictors (predictive ability = R 2 ≥0.72) of geotechn ical properties like shear strength, COLE, and ultimate bearing capacity. The s oils on Shale were classified as Vertic Hapludult (Amuro) and Typic Hapludult (Umuna), while the soils on Coastal Plain Sand were classified as Psammentic Hapludult (Ezelu) and Typic Hapludult (Obinze, Umuagwo, Mgbirichi). Keyword s : Geotechnical, lithologic, shale, coastal plai n sand, beari ng capacity.