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Nigeria, Abia State, Aba

Geotechnical Mapping of Aba and environs

Fifty-four (54) bulk samples were taken from test pits dug to a minimum depth of 1.5 metres for Atterberg Limits Test, Grain Size Distribution analysis, Compaction, and California Bearing Ratio determination.\r\n\r\nTwelve (12) shell and auger boreholes were drilled using the Pilcon Wayfarer 1500 percussion rig. The boreholes were advanced to 10 metres depth. Undisturbed samples were collected in Shelby tubes at selected intervals of 1.5metre from each other for Triaxial Compression Test, Consolidation test and Grain Size Distribution analysis.\r\n\r\nTwelve (12) Dutch Cone Penetrometer Sounding Tests were carried out at 10 metres depth or to refusal. The resistance of the soil was measured by means of a dial guage attachment to the penetrometer machine. Readings obtained and the graphical plots of the soil resistance were used to determine the allowable bearing pressure of the soil.\r\n\r\nResults of the grain size distribution analysis show a consistent increase in grain size with increasing depth. The soils classify as very fine sands, silty to clayey fine sands, or clayey silts with slight plasticity (ML), and gravelly clays, sandy and silty clays (CL) according to the Unified Soils classification scheme (USC).They have good to poor, and good to fair compaction characteristics respectively. They show high compressibility, moderate permeability and are excavatable and stable as fill material, but would be unsuitable as base course.\r\nTheir bulk density showed a range from 1.95Mg/m3 to 2.19 Mg/m3, their natural moisture, a variation from 12% to 21%, while their optimum moisture content ranges between 9.5% and 19.8%. Cohesion values range from 28 kN/m2 to 60 kN/m2, and maximum dry density ranging between 1.09 Mg/m3 and 2.17 Mg/m3 were recorded. The soaked and unsoaked California Bearing Ratio (CBR) values of 5.1% to 11.2% and 7.6%to18.9% were registered respectively. No ground water was encountered during the operation.\r\nFor design of strip/square footing 1.5metres wide founded at 1.5metres depth, the average allowable bearing capacity of soil would be 25 kN/m2 from cone, while the sleeve gives 50 kN/m2, allowing maximum differential settlement of 25mm (1 inch), using a factor of safety of 3.\r\nFor the same footing, under same conditions, the average allowable bearing capacity would be 125 kN/m2 for Standard Penetration Test, while the average allowable bearing capacity of 279 kN/m2 was computed for same footing under same conditions from the Undrained Triaxial compression Test. \r\n\r\nTherefore, it is recommended that the Dutch Cone Penetration Test average bearing capacity of 25 kN/m2 be used for design. However, it must be ensured that no ingress of water be allowed into the foundation. Alternatively, a raft foundation may be necessary for design. If a pile foundation is adopted, it must be imperative to carry out a pile loading test.\r\n

Inserted in the reports are the following maps (1:50,000): Location map.