Subsoil Condition and Development of Geo-Data Set for Effective Design, Construction and Management of Engineering Structures in Ondo Metropolis, Southwestern Nigeria

Abstract

This study deals with development of engineering geo-data and modelling, as an essential tool for design, construction, and management of civil engineering structures in Ondo metropolis, Southwestern Nigeria. Findings revealed the soils are predominantly of sandy silt to clayey silt, silty sand to sandy silt, and sandy clay, with %fines of 48.25, and 51.75 % sand. The depth to basement rock was 22.2 m (avg.), and evidenced of fractured rock at depth range of 9.9 – 26 m. The average static water levels 2.5 m (in well) – 26 m (in borehole). The average values of plasticity index, soaked CBR, group index, unconfined compressive strength, and permeability are 23.9%, 7%, 6, 186.8 KN/m2, and 1.68E-05 cm/s respectively. The recommended minimum thickness of 79 – 140 mm (avg. 109 mm) obtained from design curves will be sufficient for flexible pavement. The average allowable bearing capacity of the soil for square and round foundations varied from 234 – 297 KN/m2 (avg. 268 KN/m2) and 232 – 298 KN/m2 (avg. 268 KN/m2) with estimated total settlement of 17.69 – 18.88 mm (avg. 18.28 mm) for structural pressure of 100 KN/m2. For embankment, the suitability index of the soil suggests a fair/expanding not collapsible construction material. Rocks of igneous and metamorphic rock of high compressive/shear strength, modulus of elasticity, high crushing strength, low deformability, and presumable bearing capacity of 8, 000 – 10, 000 KPa when fresh, and between 5000 – 7000 KPa when slightly weathered. Therefore are valuable as foundation constructions, aggregate in pavement, building stone, and armourstones. The correlation coefficients give between MDD/PI vs. CBR (0.1113), LL vs. coefficient of consolidation (0.0018), PI vs. undrained shear strength/effective overburden (0.0332), PI vs. angle of shearing (0.013), dry density vs. angle of shearing (0.2131), suitability index vs. CBRs (0.3494), clay contents vs. PI (0.422).