Geotechnical and Engineering Geological Investigation…
The phenomenal rise in the number of reported cases of collapsed buildings in the country with its attendant loss in revenue and human lives has been a cause for concern to well-meaning Nigerians. As part of its constituted mandate, the Nigerian Geological Survey Agency in 2006 commenced a systematic engineering geological mapping of selected state capitals and major urban centres. In 2007, this exercise was extended to Aba and its environs.
The objective of this project is to determine the following:
(i) type, physical, and engineering characteristics of the subsoil,
(ii) suitability or otherwise of the subsoil for foundation of civil structures,
(iii) Depth to bedrock, if any, and groundwater conditions.
Fifty four 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.
Twelve 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.
Twelve 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.
Results 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.
Their 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.
For 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.
For 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. Therefore, 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.