The influence of pre-drying on tropical soil testing

Abstract

Paper presented at the 31st Annual Southern African Transport Conference 9-12 July 2012 "Getting Southern Africa to Work", CSIR International Convention Centre, Pretoria, South Africa.

Large parts of Western and Central Africa fall within the tropics, a humid, temperate region known for rapid and intense weathering of rock. These tropical soils develop due to a complex weathering process, mainly as the result of chemical breakdown. Accordingly tropical soils exhibit different engineering properties and a clear understanding of these properties are required in the design of road pavements. Standard testing used to characterize soils in Southern Africa, in particular the determination of Atterberg limits and the grain size distribution of particles less than 0.075mm, influence bonded and structural water present in and around a soil particle through drying, mixing and dispersion with the use of flocculants. This paper discusses some of the basic concepts of tropical soil testing and presents laboratory test results used to establish and characterize the properties of the soils encountered during project investigations in West and Central Africa. Testing findings are supplemented by visual examination of soil particles under high magnification as well as XRD and XRF analysis. Standard drying methods affect clays and clayey materials the most, and the drying temperature should be kept as low as possible. Both Atterberg limits and hydrometer results are affected by drying temperature. Manipulation prior to testing needs to be carefully controlled as it leads to breakdown of the soil structure resulting in varying Atterberg limit test results.