Abstract

Clay-sized particles have been shown to control the engineering performance of lateritic soils, while the mode of formation and mineralogical composition of parent rocks in evaluating properties peculiar to clay-sized particles are yet to be a subject of serious research. Fresh Gneiss (GN), Quartz-schist (QS) and Granite (GR) were sampled in parts of Southwestern Nigeria. Thirty samples each of disturbed and undisturbed soils were also obtained at depths of 1.0, 1.5, 2.0, 2.5 and 3.0 m from profiles over GN, QS and GR. X-ray fluorescence and X-ray diffraction were employed to determine the major oxide geochemistry and clay mineralogy respectively, while grain-size distribution, plasticity characteristics, undrained cohesion (Cu) and volume compressibility (Mv) were determined following the British Standards (BS-1337).

Parent rocks petrography reveals quartz and muscovite in QS, and quartz, alkali feldspars and biotite in GN and GR. SiO₂/Fe₂O₃+Al₂O₃ indicates that soils form GN and GR fall into a class different from QS. Kaolinite (52.3-75.5%) formed the dominant clay mineral in the soils with subordinate amount of illite (2.3-17.6%), while 1.9 and 0.9% of smectite occurred at 3.0 m depth in soils over GN and GR respectively. The relationship between I_p and Fe₂O₃ taking cognizance of parent rock factors reveals the form of iron oxide that reduces the plasticity of lateritic soils. The mode of formation and mineralogical composition of parent rocks caused variation in cohesion and compressibility characteristics of lateritic soils.

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