ENGINEERING GEOLOGICAL AND GEOPHYSICAL EVALUATIONS OF SUBSOIL COMPETENCE AT ALAGBAKA EXTENSION, AKURE, SOUTHWESTERN NIGERIA

Abstract

Engineering geological and geophysical investigation were carried out at Alagbaka extension of Akure, Ondo State with the aim of assessing the characteristics of the subsoil in order to identify competent zone that would support any proposed structures in the area. The study area is presently undergoing developmental projects such as erection of buildings of different designs and telecommunication mast. The need for both engineering geological and geophysical investigation of the subsoil became imperative. A variety of twenty six disturbed soil samples were collected from different pits at a depth of about 2m. The samples were analyzed for natural moisture content, grain size analysis, Atterberg limits, standard compaction, unconfined compressive strength and consolidation tests. The Schlumberger configuration was carried out for data acquisition, the halfcurrent electrode (AB/2) was used and the quantitative interpretation of the vertical electrical sounding (VES) data which involved partial curve matching and computer iteration. A total of twenty six soundings were occupied across the study area and geo-electrical section were constructed across the geology of the study area. A model using Consolidation, Atterberg limits, Unconfined compressive strength, Resistivity value at 2m, and Resistivity of the bedrock (CAURR) approach was introduced in decision making. The results of the laboratory tests reveals that moisture content range from 6.1% to 18.5%, liquid limit from 25.1% to 48.6%, plastic limit from 17.6% to 29.5%, linear shrinkage from 2.9% to 7.7%, and plasticity index from 5.2% to 19.5%. The grain size analysis showed that the amount of fines range from 20.6% to 47.4%. The specific gravity range from 2.65 to 2.8, maximum dry density from 1820Kg/m3 to 1998Kg/m3 and optimum moisture content from 13.2% to 19.7%. Coefficients of consolidation range from 0.02019 to 0.02944 and the shear strength values varied from 74.1kPa to 103.8kPa. The geo-electrical section reveals a total of four geo-electric layers which comprise of the top soil, weathered layer, partially weathered/partially fractured bedrock and the fresh Basement. The superimposing of the competency map generated from consolidation, Atterberg limits, unconfined compressive strength, resistivity and resistivity of the bedrock parameters divided the study area into extremely high competent zone, very high competent zone, high competent zone and moderately competent zone. Similarly, the CAURR model classified the study area extremely high competent zone, very high competent zone, high competent zone and moderately competent zone. It can be concluded from the study that the map generated from superimposing the five competence parameters and the CAURR model generated four similar zones in line with the previously established zonation.