HYDRAULIC CONDUCTIVITY OF SUBSOILS VIS-À-VIS FOUNDATION APPLICATIONS IN AKURE, SOUTHWESTERN NIGERIA

The effect of the movement of groundwater on a foundation can be destructive. If groundwater permeates a foundation subsoil with ease, the possibility of foundation failure is very high. This research evaluated the hydraulic conductivity (HC) of both cohesive/residual/lateritic and cohesionless/transported subsoils in Akure, Southwestern Nigeria. Thirty-five samples {21 from FUTA-Northgate (FNG) [cohesive] and 14 from Isinkan [cohesionless]} were systematically collected from 13 pits for laboratory tests. Standard procedures were followed in determining the sample's natural moisture content (NMC), specific gravity, particle size distribution, and hydraulic conductivity. Isinkan subsoil presented very high percentage of sand necessitating the application of constant-head permeameter while FNG presented high percentage of fines necessitating the use of falling-head permeameter. The calculated HC values range from 1.55 x 10^-7-5.55 x 10^-3cm/s for FNG and from 9.45 x 10^-3-1.58 x 10^-2cm/s for Isinkan. FNG revealed %gravel ranging from 2.1-47.5%, %sand from 18.8-65.1%; and %fines from 17.6-59.9% with averages of 25.74, 39.38, and 34.90% respectively while Isinkan gave %gravel ranging from 1.9-28.9%; %sand from 67.9-91.5%; and %fines from 2.2-12.9% with averages of 10.94, 82.34, and 6.79% respectively. FNG subsoils possess very low HC, while Isinkan subsoils possess medium HC. It was further observed that (i) hydraulic conductivity decreases with depth, (ii) well-graded subsoils are better foundation materials than the poorly-graded, (iii) cohesive subsoils are better foundation materials than the cohesionless, (iv) %fines are negatively strongly correlated with HC, and (v) subsoils with higher HC are less attractive for foundation purposes. Conclusively, FNG subsoil, and by extension cohesive/residual/lateritic subsoil, possesses low HC implying that it is a better foundation subsoil material than Isinkan, the cohesionless/transported subsoil, which possesses higher HC.