The principal factor to which the observed engineering performance of the soils in the humid tropical zone of Southern Nigeria can be attributed has been determined to be the laterization phenomenon. During the process, oxides and hydroxides of iron and aluminum accumulate as the cementing agents binding the soil particles together. Clay-size particles aggregate into coarse grains giving the impression of a granular texture. The soil mass, in turn, correspondingly exhibits characteristics reflecting this texture when undisturbed in the field. When subjected to mechanical reworking or remolding however, such as in the process of pre-test sample preparation, the grains are disaggregated releasing a large quantity of fines. The laboratory test results therefore usually differ from predictions based on field observations.
This paper presents a broad framework, which can be used for a preliminary estimate of the engineering performance of the soils as well as formulating a generalised soil classification scheme for the area. The framework has been realised from a synthesis of selected previous works on the soils, a task, which included detailed evaluation of all the soil formations in the whole area and correlation of their engineering properties with various physical characteristics. The genetic and climatic factors and the post-formation alterations, which influence the engineering behaviour of the soils, were also investigated, with emphasis on their significance in engineering design and construction.
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