Petrographic and geotechnical properties of Lateritic Soils developed over different parent rocks in Ago-Iwoye area, Southwestern Nigeria

Lateritic Soils derived from banded gneiss, quartz– schist, granite gneiss, biotite gneiss and porphyroblastic gneiss on the Olabisi Onabanjo University permanent site, Southwestern Nigeria were investigated with a view of establishing extent to which the parent rock factor has controlled some engineering index properties of the derived soils. Field investigations include field mapping and collection of rock and soil samples while the laboratory investigations methods include grain size analysis, consistency limits, California Bearing Ratio, unconfined compressive strength and linear shrinkage determinations. Findings from grain size analysis shows that soil samples derived from quartz schist, porphyroblastic gneiss and granite gneiss belong to group A–7–6 while soils derived from banded gneiss and biotite gneiss belong to group A-2-7 and A-3 (0) AASHTO Classification System respectively. However, a rather weak influence of the genetic factor on the linear shrinkage of the studied soils was noticed. The compaction at Modified AASHTO level was found to have varying influence on the plasticity, moisture-density relationship, CBR and unconfined compressive strength of the genetically different soils. A rather weak influence of the linear shrinkage of 12%, 11% and 9% for soils derived from quartz schist, porphyroblastic gneiss and banded gneiss respectively was noticed. The study has proved that the influences of the parent rock factor on engineering index properties such as grain size distribution, plastic index, California Bearing Ratio (CBR), unconfined compressive strength, dry density determination of the soils studied were significant. It is thus necessary to take proper cognizance of the parent rock characteristic prior to an adequate understanding of engineering properties and behaviour of residual soils.