Geoengineering Assessment of Subgrade Highway Structural Material along Ijebu Owo – Ipele Pavement Southwestern Nigeria

Abstract: Geochemical, geotechnical, and geophysical investigations were carried out along Migmatite gneiss underlain Ijebu Owo - Ipele pavement (under construction) in order to assess the suitability of the imported subgrade soil. Also Static Water Level (SWL) was obtained from five open wells along the highway. The geotechnical tests include specific gravity, grain size analysis, consistency limits, consolidation, compaction, and California Bearing Ratio (CBR), and shear strength; while geochemical tests were analyzed using X-ray fluorescence and Atomic Absorption Spectrophotometer (AAS). The results show that SiO2, Al2O3 and Fe2O3 constitute an average of 46.2%, 23.30% and 26.54% respectively of the soils chemical composition. The silica: sesquioxide ratio of the samples ranges from 0.89 to 0.96 and classified as true laterite. The topsoil along the highway is made of moderately competent/competent soil with resistivity ranging from 298 - 924 ohm-m and thickness of 3.1 - 8.2 m. The SWL varies from 3.3 - 5.5 m with an average of 4.3 m (moderately low). The soils are characterized by high California Bearing Ratio values, moderate shear strength (150 - 200kpa), and unconfined compressive strength (300 - 420kpa) with predominant kaolinite clay mineralogy group. The AASHTO and USCS classification system rate the soils as good and fair/good respectively.

Keywords: Silica-sesquioxide, Shear strength, Highway, AASHTO, Geotechnical investigation, Bearing capacity