Abstract:
The geotechnical properties of Olusosun landfill site were investigated. Ten samples were taken from ten test pits at the depths varying from 1.85m to 2.0m. The samples were taken in accordance with BS 1377 of 1990. The collected soil samples were stored air-tight inside polythene bags to maintain and preserve the natural moisture content before being taken to the Geotechnical Laboratory of Civil Engineering Department of The Federal University of Technology, Akure for laboratory testing and analysis. The following geotechnical tests were carried out at the laboratory: Natural Moisture Content, Specific Gravity, Atterberg Limits, Particle Size Distribution, Hydrometer, Permeability, Compaction, California Bearing Ratio (CBR), Direct Shear and Consolidated Undrained with pore pressure measurements. Out of the ten samples collected for investigation, four samples namely; TP5, TP6, TP7 and TP8 were classified as sandy soil as the percentage passing 75 micron were below 30%. Only TP1, TP2, TP3, TP4, TP9 and TP10 that had the percentage passing sieve No.200 greater than 30% were used for further investigations. The following geotechnical properties were obtained: Natural Moisture Content ranged between 7.50% and 11.90%, Specific Gravity 2.60 and 2.68, Sand content between 69.40% and 50.20%, for Silt content, 26.30% and 39.40% while that of Clay content was between 3.70% and 10.20%. The Liquid Limit values ranged between 39.00% and 45.00%, Optimum Moisture Content (OMC) 18.00% and 17.20%, Maximum Dry Density (MDD) 15.38 kN/m3 and 16.75 kN/m3, while the CBR values ranged from 3.90% to 18.30%. The Coefficient of Permeability of all the test pits have their permeability values from 2.64x10-3 cm/sec to 5.56x10-3 cm/sec. Other properties include, the angle of internal friction (effective,𝜙′) which ranged from 4.2o to 20.2 o, total 𝜙 t from 5.0 o to 13.4 o while effective cohesion, c′ ranged from 2.1kN/m2 to 19.8kN/m2 and total cohesion c from 0kN/m2 to 13.7kN/m2 at 2.5mm, 3.0mm, 3.5mm and 4.0mm failure point. The results obtained at 3.0mm and 4.0mm failure points gave the best consistencies. The work shows that Olusosun “landfill” in Lagos is only a dumpsite and the groundwater is not protected in this “landfill”. The permeability of the site is 10-3 cm/sec while recommended recommendation is 10-7 cm/sec, hence polluting shallow wells around the area. Preventive engineering measure is recommended to prevent likely future health hazards. Proper design of landfills in Nigeria is strongly recommended.
