The Geotechnical Investigation of Subsoil Materials in a Typical Basement Terrain, Southwestern Nigeria- A Case study

Abstract

Geotechnical investigation of the subsurface materials of a proposed Primary Health Center (PHC) in Modeke Oyo East Local Government Area (OELGA). of Oyo State has been conducted. The investigation is aimed to determine the suitability of the site for the proposed Bungalow building by assessing the subsoil strata that underlie the area. Three (3) points were tested to depth of 4.75m using a 2.5-ton DCPT equipment, while three (3) trial pits were dug to a depth of 1.5m with the aid of hand auger, and various lab analysis were conducted on selected samples from the trial pits. The result obtained from the DCPT indicates that subsurface is made up of stiff to very stiff sandy lateritic clay underlain by very stiff to hard sandy lateritic clay with hard pans layers. The allowable bearing pressure was observed to increase with depth to 4.75m further confirmed the subsurface lithologies. The Allowable Bearing Pressure (qa) in kN/m2) with a range of values 81 – 256 kN/m2, 54 – 155 kN/m2 and 162 – 338 kN/m2 for DCPT points 1, 2, and 3 respectively, indicate a Safe Bearing Pressure that can support load up to 45 kN/m with factor of safety of 2.5. Sieve analysis results revealed that the soil is well graded with dominant gravel which show a well-drained soil with good strength characteristics. Liquid limits ranged from 26.7% to 43.3%, plastic limits from 15% to 27% and plasticity index from 9.7% to 27.3%, thus indicating low to medium compressibility. The natural moisture contents values range between 4% and 15%, indicating lateritic clay of low to medium plasticity with low moisture content. Based on the above deductions, it can be concluded that, the factor of safety of 2.5 calculated from the allowable bearing pressure from the CPT, low to medium compressibility from Atterberg limits test and moisture content below 15% show that the investigated site is good and could safely support a bungalow building as proposed.