Subsoil Foundation Support Assessment in Owo Area of Ondo State, Southwestern Nigeria

Subsoil foundation study was carried out in Owo Local Government of Ondo State, Southwestern Nigeria with an aim to assess the soil geophysical and geotechnical properties for design of shallow foundation structural support. The geophysical method which consists electrical resistivity [involving vertical electrical sounding (VES) and combined profiling field techniques] and very low frequency electromagnetic. The geotechnical method involved field cone penetrometer test and laboratory analysis of eight soil samples, collected at a depth between 1 and 3 m. These were complimented by hydrogeological measurements (static water level and hydraulic head determinations) and borehole drilling. The result showed that the upper 3 m is competent to support structural foundation elements with an allowable bearing capacity ranging from 291 to 293 kN/m(2) (resistivity values of 59-569 omega-m) with low settlement values of 0.72-0.75 mm. These bearing pressures are considered appropriate for use in design of bases, strip or raft foundations. The AASHTO classification of A-2-4/A-5 constitutes about 62.9%. The average static water level (5.6 m) will not or seriously affect footing structures even in wet season. However, the result of the borehole drilling and VES/Resistivity structures revealed the presence of clayey weathered layer at shallow depth less than 5 m in some places, therefore relatively high consolidation settlement is expected in those places upon loading. The depth to basement is 35 m, this overburden thickness would assist in evenly distribution of the foundation/structural load to the basement rock. All the determined geotechnical parameters of the subsoil fall within the specification recommended for foundation material by Federal Ministry Works and Housing of Nigeria. The quartz-schist and schistose quartzite are the most prominent rocks observed in the field. The Schist tends to slide/split along their planes of schistocity while the quartzite show significant fractures and joints which might require grouting/backfilling during construction activity.