Geo-environmental engineering site evaluation for shallow foundation design in Akoko area of Ondo State, southwestern Nigeria

Abstract

Civil engineering structures like buildings, dams, bridges, roads etc. are built or rest on soils, hence the bearing capacity and settlement characteristics of foundation soil to support the structure is important in effective structural design. Consequently, engineering site evaluation was carried out in Akoko area of Ondo State, Nigeria with the aim of investigating the qualitative engineering properties of the subsurface material for shallow foundation design by geological field studies, geophysical, static water level measurement and field/geotechnical laboratory analysis of collected samples. The geotechnical laboratory results show that the soils are of good foundation material with American Association of State Highway and Transportation Officials classification of A-2-4 to A-2-6 and A-2-7 types, with percentage fines less than 35%. The most of VES-curve types are H pattern (which accounts for 58%) which is very resistive at shallow depth. The static water level ranges from 1 to 15 m (generally less than 2 m) while the hydraulic head varies between 300 to 530 m. In the upper 2 m, sediments delineated are clay, silty clay, sandy clay, clayey sand, and basement. All the CPT-plots show the same signature with increasing resistance values with depth. The CPT value of 100 kg/cm² corresponding to 245 KN/m² which signifies moderately competent soil material was obtained at depths of 1.2 to 1.9 m, with immediate settlement values of 0.78 to 21 mm. Consequently, shallow foundation such as simple pad/raft or strip shallow foundation and spread footing are very feasible in the study area. However, substantial additional settlement may occur in the area with high water table, which could exceed tolerable limit and threatens the integrity of the foundation structure.