Abstract :
Soil domain within Akure–Owo (A–122) highway has been studied in order to understand the causes of incessant failure of the highway structure. The study combined geophysical, geochemical, hydrogeological, and geotechnical investigations. The results revealed that the topsoil/subsoil on which the soil is constructed composed of incompetent/fairly competent clay, sandy clay, clay sand, and laterite. The depth to basement rock ranged between 30.4–42.2 m. The soils are lateritic with silica–sesquioxide ratio of 1.58 (avg.). The clay mineralogy is within the illite (60 %)–illite/montmorillonite group (40 %). The soils are of SC-SM of low–intermediate plasticity and compressibility of moderate to high specific gravity. The avg. GI value of the soils is 6, and adjudged fair subgrade soil material. The in-situ California Bearing Ratio (CBR) (avg. 27 %) and soaked CBR (avg. 38 %) satisfied the 10 % minimum specification for subgrade. The DCPT indicated the soil to be generally of medium/stiff/dense consistencies with penetrative index of 1.33–53.67 mm/blow. It also showed that 378–872 mm depths are the suitable surficial layer to host the road structure based on the CBR and SNG with relative densities of 0.371–0.509. The strength coefficient, SNG, SN, and SNP contributions of the soil are good for subgrade but low for subbase and base courses. The regression models of all parameters gave strong positive correlations for soaked CBR and in-situ CBR, and ER and MR; while weak positive correlation for in-situ CBR and MR, RD and DCPI, RD and in-situ CBR. Based on the GI and CBR values, and the traffic count carried out which placed the highway, the recommended thickness of the highway structure should range from 140 mm (good segment) to 445 mm (for weak segment) (avg. 193 mm) which partly corresponds to 315 mm measured along the highway alignment during reconnaissance survey. This implies that the design thickness of the highway corresponds very well with recommended thickness emanating from this study. Thus the failures in some portions along the highway can be attributed to lack of drainage facility at the shoulders of the highway, topography/basement relief, and usage.
Keyword :
California Bearing Ratio; Penetrative index; Elastic modulus; Geotechnical; Structural number; Subgrade modulus