Basic index properties, mineralogy and microstructure of an amphibolite derived laterite soil

Variability in the nature, over a small area, of soils derived from coarse-grained rocks have been widely reported. Such reports concerning soils from fine-grained rocks are however sparse. Some geotechnical properties of a laterite soil derived from a fine-grained amphibolite have been investigated with a view to ascertain its variability. Samples of the soil were taken from the same horizon within an area of about one square metre. The basis index properties, chemistry, mineralogy and microstructure of the sol samples have been investigated. Despite the similarities in the grain-size distribution. Atterberg limits and chemistry of the samples, large variations were obtained in the mineralogy as evidenced by the goethite content, as well as in the micro-structure. The variation in the microstructure is attributed to the inconsistent cementation of the soil particles by non-uniformly distributed goethite which itself is believed to be a result of the variations in the Eh-pH conditions within the soil. The implications of this microstructural variation on the geotechnical properties of the undisturbed soil is assessed.