Assessment of the Physical Properties of Soil Samples from the Basement Complex of Ondo State

The basement complex terrain of Ondo State in South West Nigeria is characterised by Precambrian crystalline rocks including granites, gneisses, and migmatites that weather to produce lateritic and sandy clay soils with distinctive physical properties. Understanding these properties is important for civil engineering foundation design, borehole siting, and agricultural land management. This study assesses the physical properties including grain size distribution, Atterberg limits, specific gravity, and natural moisture content of soil samples collected from 20 profiles across Akure, Owo, and Idanre local government areas. Grain size analysis employs sieve analysis for the coarse fraction and hydrometer sedimentation for the fine fraction. The liquid limit and plastic limit are determined by the Casagrande cup and thread-rolling methods respectively. Results indicate predominantly sandy clay loam textures with gravel fractions reaching 35 percent in residual profiles over quartz veins. Liquid limit values range from 28 to 58 percent, and plasticity indices from 8 to 24 percent, classifying soils as low to intermediate plasticity clays by the Casagrande plasticity chart. Specific gravity ranges from 2.60 to 2.76, reflecting varying proportions of ferruginous minerals. Keywords: soil physical properties, basement complex, Ondo State, Atterberg limits, grain size