Engineering geological evaluation of transported tropical red soils

Abstract

Tropical soils include a wide variety of pedogenic materials common to the tropical regions of the world, of which the material properties and engineering behaviour may vary significantly. These soils in many ways have unique characteristics that can mainly be ascribed to the compositions and micro-structures of a material developed under hot, wet soil-forming conditions. Due to the variation in their properties and behaviour, the classification of soils formed under such conditions as distinct and well-defined soil types appears to have been avoided in the past. Unfortunately, this has brought about the grouping and classification of materials of significantly varying chemical, geological and/or geotechnical properties and engineering behaviour under a single, all-encompassing term of tropical red soils . The unique pedogenic environment and soil characteristics further mean that conventional soil mechanics concepts and geotechnical investigative procedures, developed almost exclusively from work completed on temperate soils, do not necessarily apply to soils formed in tropical environments. This research reviews the typical and unique characteristics and behaviour of tropical red soils as summarised in the existing literature, including the application of existing soil classification systems and the possible ambiguity existing in the classification and nomenclature pertaining to soils formed in tropical environments. Significant inconsistencies in the identification and classification of tropical red soils and laterites were observed to exist throughout the available literature, with frequently overlapping ranges of geological, geotechnical and chemical properties reported for both lateritic and tropical red soil types. Much has been written in the existing literature regarding the unique material characteristics and engineering behaviour of tropical red soils and the inapplicability of conventional soil mechanics concepts. The majority of the available results, conclusions and recommendations in the existing literature refer to residual tropical red soils only, with very little information regarding the characteristics and behaviour of transported tropical red soils being reported. The characteristics and behaviour of a number of transported tropical red soil horizons, from a selected area in the Mayoko region of the Republic of the Congo, were evaluated through conventional and specialised testing regimes during the completion of this study. A large number of disturbed and undisturbed samples were collected from the study area and submitted to geotechnical laboratories in South Africa to establish the chemical-, mineralogical-, index and mechanical properties of these soils. The test results, sample preparation and test procedures and effect of these on the engineering behaviour of the material were evaluated and compared to the typical results reported for residual tropical red soils in the literature. A number of the findings from this study, particularly the sensitivity of tropical red soils to change in moisture content and effect of the dispersing agent on the reported particle size distribution of such soils, confirmed the findings which have been reported in the literature for residual tropical red soils. However, the transported tropical red soils were also found to differ markedly from residual tropical red soils in certain aspects, most notably in terms of the apparent effect of the lack of remnant structure on the material mechanical properties, permeability, sample disturbance and relevance of tests completed on remoulded test specimens.