Insights from the Engineering Geological Mapping of Four Basement Rocks Derived Soils

Abstract

Due to the rapid expansion and associated construction of civil engineering structures on the Federal University of Technology, Akure (FUTA) campus, there arose an urgent need for an engineering geological mapping of the underlying soils (residual soils). Generalized geological mapping revealed four types of basement rocks namely migmatite-gneisses, granites, quartzites and charnockites. Results from the fifty (50) soil samples from twenty-five test pits collected all over the spread of the campus coverage of 6.4 km2revealed that the campus is underlain by soils of granular and clayey composition, generally lateritic, having reddish to brownish colour. Engineering geological tests such as natural moisture content, particle size analysis, consistency limits, California bearing ratio and consolidation were carried out on the soils following standard procedures revealed that the values of natural moisture content do not generally follow a consistent pattern and varied from location to location. The grain size characteristics curve, displayed 84% and 16% subsoils are of well graded and poorly graded type respectively. The soils were grouped into CL (low plasticity), CI (medium plasticity) and CH (high plasticity) from consistency limits results. Compaction characteristics of the subsoils revealed 36% and 64% representative of fair to good and poor to very poor foundation materials respectively. Soils with settlement rates greater than 1mm/year were designated as high settlement subsoils. 72% and 28% of the subsoils fell into hard to stiff and soft categories from the shear strength characteristics respectively, and classified as c-ø soils. California Bearing Ratios values range from 10 – 70, indicating their suitability for pavement construction. Conclusively, areas underlain by migmatite-gneiss and charnockite-derived soils, and granite and quartzite-derived soils possessed low and high strength characteristics respectively which can be attributed to their textural characteristics. The subsoils of the entire campus spread are however capable of bearing very substantial loads.