Geomechanical Characteristics of Natural and Stabilised Black Cotton Soils from the Accra Plains of Ghana

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Black cotton soils or tropical black earths or black clays are potentially expansive soils which classify as vertisols in pedological parlance and have been found to occur in all major climatic zones of the world. These soils are considered ―problematic‖ and sometimes as ―potential natural hazard‖ because they are susceptible to seasonal volumetric changes, exhibit severe cracking when dry, swell and yield low bearing strengths when wet, etc. These problems cause extensive damage to light structures founded on them and estimated cost of damage due to expansive soils in general runs into billions of dollars annually. Considering the widespread prevalence of these soils and the problems they pose to structures founded on them, there is the need to conduct studies into the geological and geomechanical characteristics of the black cotton soils to enable effective utilization. Although some works have been done globally on these soils, these rather useful information are scattered in various publications. The need to collect these scattered information, synthesize them and present a hands-on-information regarding the nature, distribution, physical and engineering properties of the black cotton soils for the benefit of the construction industry has long been felt. Secondly, it has also become necessary to conduct studies with the view to enhancing knowledge on the geology and geomechanics of these soils. This study has attempted to address the above mentioned problems through a review of literature on the mode of formation, nature, as well as distribution of these soils around the world and secondly, presents studies on typical black cotton soils from the Accra plains of Ghana. Soil samples were collected from Tsopoli and Doryumu, both in the Accra plains of Ghana and subjected to physical, chemical and mineralogical as well as geomechanical studies. Results of the study reveal that the black cotton soils are formed over the Garnet-Amphibolites Gneisses of the Dahomeyan Supergroup. The chemical compositions of the soils indicate iii that the most abundant oxides are silica, alumina and iron oxide. The mineralogy of the two soils from X-Ray Diffraction analyses are similar and are composed of Quartz and montmorillonite. The geomechanical studies also suggest that the natural black cotton soils are unsuitable for subgrade construction and hence require improvement through stabilization. Attempts at stabilising the soils with quarry dust, pozzolana and pozzolana-cement which are readily available local materials, reveal that although the three stabilizing agents improved some geomechanical parameters of the soils the pozzolana-cement appears to be the most effective for stabilising the black cotton soils.
A thesis submitted to the Department of Geological Engineering of the Kwame Nkrumah University of Science and Technology in fulfilment of the requirements for the award of the degree of Master of Philosophy in Geological Engineering.