Site Characterization for Foundation Design in Yenagoa, Nigeria
Abstract
Site investigation is performed at a proposed site location in Yenagoa, Nigeria, to establish the soil profile and determine the engineering geological properties of subsoils in the area. The general profile of the soils at the site consists of medium dense 0.95m to 1.95m thick uniformly graded sand (fill) overlying soft to firm greyish brown sandy clay that extended to depths between 3.75m and 6.90m below ground surface. Assessment of the engineering properties of the soils showed that the top sand layers occupying the entire site are medium dense in compact state and are expected to exhibit dilative behaviour when sheared. Again, the sand deposits found at deeper depths are mostly dense with high shearing resistance. The groundwater depth was measured at depths between 0.98m and 1.0m. The percentage of sand in the clay were found to be quite significant (about clay 11% to 22%). The overconsolidation ratio (OCR) of the soils indicated that are mostly normally consolidated. Also, the material can be classified as highly compressible based on the values of coefficient of volume compressibility which was found to range between 0.06MN/m2 and 0.93MN/m2. The values of the coefficient of consolidation which varied between 1.1m2 /yr and 22.3m2 /yr for the applied range of pressures indicates that the clayey soils occupying the site are susceptible to prolonged consolidation. The range of values obtained as maximum dry density (MDD) was between 1.66Mg/m3 and 1.71Mg/m3 while the optimum moisture content ranged between 12.4% and 15.9%. The second geoelectric layer has the lowest resistivity value among the three geoelectric layers and occur in the depth range of about 1.47m to 31m. The second layer (depth range of 1.47m to 31m) whose resistivity value range is 23-75Ωm is in the range of highly corrosive to moderately corrosive severity rating. The bearing resistance values for shallow foundation are relatively high at about 122kPa, 141kPa, 160kPa and 198kPa respectively for a 1.0m, 1.5m, 2.0m and 3.0m footing size. This is attributed to the shearing resistance values obtained for the sand. The result showed that foundation pressure in the order of 120kPa and placed at 0.5m depth would cause tolerable settlement for footing width not exceeding 1.0m. Also, the medium to dense sand layers encountered from depth of 10m and beyond is recommended as the points of pile termination. The working loads obtainable for 406mm, 600mm, and 800mm diameter pile at 12m depths is 299kN, 620kN, and 1072kN for the referenced borehole point. The study showed that within the sand layers the magnitude of the vertical downward movement experienced by the piles are small. Also, the settlement is less than 10% of the pile diameter under higher load magnitudes. It showed that the bearing resistance of the piles can be adopted as the allowable values. It is envisaged that this work will provide reliable information for foundation design problem in the region.
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