Evaluation of Fly Ash and Ground Granulated Blast Furnace Slag on Consistency Limits of Black Cotton Soil
6 Pages Posted: 3 Feb 2014
Date Written: February 2, 2014
Black cotton soil (BCS) is a class of expansive soil which is unsuitable as pavement material due to its swelling and shrinkage characteristics. The sample site location is on the north west of South Africa. The approach that is adopted when such material are encountered is to remove the soil and replaced with quality materials. Cost of replacement is very high and stabilization of the soil is another alternative method of construction. Ground granulated blast furnace slag (GGBFS) is a by-product in the manufacturing of steel while fly ash (FA), a residue generated in combustion of coal is industrial waste that is readily available in the country. To sustain the environment by reducing the material to land fill sites, this paper examines the effect of FA and GGBFS respectively on consistency limits of BCS. Consistency limit is an indicator of clay behaviour and its evaluation provides basic mechanical data about the soil. Samples of the soil were obtained at a depth of 0.5m below the subgrade level. Sampling and tests of the soil was done according to South Africa standards to characterize BCS, FA and GGBFS. The percentages of FA and GGBFS were varied from 1-14% and 1-9% in steps of 1% respectively. Particle size distribution, Atterberg limit, standard proctor and California bearing ratio tests were conducted on the stabilized soil. The particle size distribution of BCS showed that 77% is clayey and silt; according to American Association of State Highway and Transportation official classify the soil as A-7-6(15) while the material falls outside G7-G8 envelope according to standard methods of testing road construction materials (TMH 1), making it unsuitable as subgrade material. Liquid limit (LL) and plastic index (PI) values of the virgin soil are 58% and 30% respectively. It was observed that PI decreases while LL increases as the percentage of the stabilizers increases; this could be attributed to cation exchange reaction that resulted in an increase inter-particle distribution. There was about 43%, 40% reduction in the value of PI for FA and GGBFS at 12% and 8% respectively when compared with the control. The maximum dry density (MDD) and optimum moisture density (OMC) of the virgin material are 1704 kg/m3 and 15.3% respectively. MDD and OMC for FA & GGBFS modified BCS increases as the percentages of the stabilizers increases. Both the maximum dry density and optimum moisture content increases for FA, GGBFS from 1–12% and 1–6% respectively and then decreased. Increase in OMC is necessary for dissociation of admixtures with Ca2 and OH- ions to supply more Ca2 for the cation exchange reaction. The stabilizers have significantly improved the index properties of the soil.
Keywords: black cotton soil; consistency limits; fly ash; ground granulated blast furnace slag; stabilization
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