GEOTECHNICAL INVESTIGATION OF SOME SUBGRADE SOILS IN IBADAN, SOUTHWESTERN NIGERIA

Incessant road failure and damage to public utilities are very common in Nigeria. These are usually caused by inadequate geotechnical investigation prior to engineering construction. We carried out geotechnical investigation of some soils in part of Ibadan metropolis, southwestern Nigeria, to determine suitability of the soils as sub-base/base course materials for road and sewage-drain construction. The area is underlain by ancient metasediments which form prominent topographical features that stand about 75 m above the surrounding plains. Fifteen boring points were established with 40 soil samples collected. Also, Dutch cone penetrometer (DCP) was used to determine penetration resistance at different locations. The soil samples were subjected to laboratory analyses to determine optimum moisture content (OMC), maximum dry density (MDD), Atterberg limits (Liquid limit (LL) and plasticity index (PI)), and compaction and unsoaked California Bearing Ratio (CBR). The OMC and MDD for the soils ranged from 9.4 to 12.12% and 1.90 to 2.02 gcm^-3, respectively, whereas the LL and PI ranged from 20.9 to 27.2% and 2.15 to 4.0, respectively. The CBR values which ranged from 40 to 44% was considered fair for the soils. Statistical analyses showed variable correlation between various parameters. The penetration resistance tests were terminated at refusal level when gauge reading of 250 km/cm² was observed at each location. Generally, appreciable bearing capacity was attained at 0.75 m in the area. Using the Unified Soil Classification System (USC), the soils in the area fall into SC and SM groups, and within A-1-9 and A-2-4 in the AASHTO classification system. Based on the field and laboratory results, the materials are rated good sub-base/base course materials for the proposed project. It is demonstrated that detailed and thorough investigation is important for proper evaluation of subgrade soils prior to engineering construction.