STRENGTH CHARACTERISTICS OF RESIDUAL SOILS ON THE FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE (FUTA) CAMPUS, SOUTHWESTERN NIGERIA

Abstract

The strength characteristics of residual soils formed over the basement rocks of the Federal University of Technology Campus, Akure Southwestern Nigeria were investigated. This was done with a view to know the influence of the local geology on the residual soils and predict the engineering behavior of the soils as foundation materials for civil engineering structures. Fifty (50) soil samples (both disturbed and undisturbed) were collected from twenty five test pits at depth of about 1m. The samples were subjected to natural moisture content, particle size analysis, consistency limits, linear shrinkage, specific gravity, standard compaction, triaxial shear strength, consolidation and unsoaked California Bearing Ratio (CBR) tests. The study area was underlain by Charnockites, Migmatite-Gneisses, Granites and Quartzites. The natural moisture content of the soils ranged from 8% to 26%, liquid limit ranged from 32.3% to 65.8%, plastic limit ranged from 12.9% to 41.6%, while plasticity indices ranged from 5.84% to 37.49%. The soils were grouped into CL (low plasticity), CI (medium plasticity) and CH (high plasticity). Linear shrinkage values ranged from 1.40 to 13.60. Grain size analysis gave coarse contents varying from 13.1% to 75.7% and fine contents from 24.3% to 86.9%. The unsoaked CBR ranged from 10% to 70%, and the compaction characteristics gave maximum dry density (MDD) values rangeing from 1403kg/m3 to 1977kg/m3 at optimum moisture contents (OMC) of 7.71 to 26.58%. The values of the angle of internal friction ranged from 10.6o to 36.4o. The coefficient of volume compressibility ranged from 0.190 to 0.458, pre-consolidation pressure ranged from 0.0236 to 0.0589 and the rate of settlement ranged from 0.596 to 1.614mm/yr. Bulk density varied from 1.673 to 2.441g/cm3 and undisturbed dry density values ranged from 1.524 to 2.221 g/cm3. Similarly, void ratio ranged from 0.11 to 0.67 and porosity was from 0.123 to 0.407. It can be concluded that the study area is underlain by soils that are of silty sand and clayey composition, generally lateritic, having reddish to brownish colour. Migmatite-gneiss and charnockite-derived soils possessed low strength characteristics while the granite and quartzite-derived soils possessed high strength characteristics which can be attributed to their textural characteristics. There exist positive and strong correlations between fine contents with MDD and CBR, while having negatively strong correlation with settlement. Also, strong positive correlation existed between clay content and MDD while weak correlations existed with CBR, angle of internal friction and settlement. The tests showed that the subsoils were generally made of cohesive-frictional particles and have appreciable amount of 3 cohesive (clay) binders to resist substantial shear force. These also confirm that the soils ranged from fairly competent to competent materials that are capable of bearing very substantial loads.