Variation of Compressibility Characteristics with Depth of Residual Soils on Basement Complex of Central Nigeria

Abstract

Disturbed and undisturbed soil samples, collected from six bore holes within the study area were used to evaluate variation of compressibility characteristics with depth of overburden over Basement Complex of central Nigeria. Tests conducted included index properties, mineralogical, shear strength and one dimensional consolidation. Results of the study showed similarity in lithology of the overburden, to residual soils of area underlain by Migmatite–Gneiss Complex in southwestern Nigeria. Mineralogy of the soils generally revealed dominance of kaolinite mineral, with ocassional 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 soils with depth, which must have resulted from the varied extents 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 preconsolidation pressure, as in the literature, do not perfectly hold for tropical residual soils.