Variation of Compressibility Characteristic wit Depth of Residual Soils of Basement Complex of central Nigeria

Abstract

Disturb and undisturbed samples collected from six boreholes within the study area were used to evaluate variation of compressibility characteristics with depth of overburden over Basement complex of central Nigeria. Tests conducted include Index properties, mineralogical, shear strength and one-dimensional consolidation. Results of the study showed similarity in the lithology of the overburden to residual soil of area underlaid by Migmatite-Gneiss complex in South-west Nigeria. Mineralogy of the soils generally revealed dominance of kaolinite mineral, with occasional traces of illite within the profile. The observed trend in variation of shear strength parameters was attributed to the variation in composition of the soils along the depth of the profile. The observed variability in settlement characteristic was generally attributed to variation in characteristics of the tropical residual soil with depth, which must have resulted from the varied extent of decomposition and laterization process, resulting to mechanical heterogeneity of the sequences. Variation of compressive index (Cc) with depth, showed a deviation from the usual assumption that, due to lithostatic load, Cc decreases with depth. It was therefore concluded that the definition of of preconsolidation pressure as the literature, do not perfectly hold for tropical residual soils