INTEGRATED GEOPHYSICAL AND GEOTECHNICAL CHARACTERIZATION OF SUBSOIL IN PART OF THE NEW STADIUM, AKURE, SOUTHWESTERN NIGERIA

Abstract

An Integrated geophysical and geotechnical subsoil characterization of parts of the new stadium complex, Akure, southwestern Nigeria had been carried out with a view to characterize the subsoil materials for stability of proposed engineering foundation. The VLF-EM and magnetic profiles and the 2-D images delineated conductive zones that are diagnostic of geologic structures (faults, fractures, joints, shear zones and geologic contacts) along Traverses 1-5. The fractured/fault zones delineated by the VLF-EM and the magnetic methods constitute weak geologic zones that are considered to be inimical to civil engineering structures. The 2-D geomagnetic section delineated overburden thicknesses which vary from 5-15 m along the Traverses. The subsurface geologic units delineated by the geoelectric sections consisted of the topsoil, weathered layer, partlyweathered/fractured basement and fresh basement bedrock. The topsoil resistivity values ranged from 18 – 537 ohm-m while the thickness varied from 0.6 – 2.5 m. The weathered layer resistivity values range from 20 – 458 ohm-m while the thickness varied from 1.3 – 13.0 m. The partly weathered/fractured basement resistivity generally varied from 25 – 520 ohm-m with thicknesses ranging from 27.3 – 46.0 m.The fresh basement bedrock resistivity values ranged from 300 - ∞ ohm-m. Depth to fresh basement bedrock ranged from 2.1 – 14.5 m which correlated with the 15.0 m overburden thickness delineated by the 2-D geomagnetic section. Part of the topsoil and the weathered layer are characterized by low resistivity values suggestive of the presence of clay and or weak geomaterials at a depth range of 1-4 m within which civil engineering foundation are usually placed. These zones are suspected toconstitute a major threat to the stability of any proposed civil engineering structures within the study area. The geotechnical classification of the soil samples in the study area using plasticity indices, adjudge samples A, C and D to be of high index and are characterized by high plasticity/high compressibility and consequently of low engineering competence. Samples B and E are classified as low-medium plasticity/low-medium compressibility, and are rated moderate–high engineering competence. However, it was observed that soil with higher liquid limits or plasticity index have lower electrical resistivity and hence are adjudged low engineering foundation competence. It can therefore be concluded that the subsoils in the investigated area are generally of low and moderate civil engineering competence.