Use of Coal Bottom Ash as Mechanical Stabiliser in Subgrade Soil

This paper presents the laboratory investigation work which forms part of a full scale research road project in Mauritius where coal bottom ash is used as mechanical stabiliser in a saprolitic subgrade soil. Three mixtures of subgrade soil and CBA were investigated in the laboratory, each containing varying percentages of coal bottom ash by weight (15%, 30%, and 40%, resp.). The laboratory research indicated that the mechanical properties of the subgrade soil are improved with the addition of bottom ash. Highest values for soaked and unsoaked CBR values were obtained for the mixture containing 30% by weight of bottom ash, which were 145% and 95%, respectively, as compared to 40% and 55% for the subgrade soil alone. Upon addition of coal bottom ash, a considerable decrease in swelling potential during soaking was observed for the mixture containing 40% by weight of CBA. The swell decreased from 0.17% for the subgrade soil alone to 0.04% for the mixture containing 40% by weight of CBA. Moreover, a CBA content of 30% resulted in a mix of intermediate plasticity as compared to the subgrade soil which is highly plastic. It is concluded that coal bottom ash can be used successfully as a mechanical stabilizer in the experimental subgrade soil by addition of 30 to 40% of CBA. 1. Introduction In Mauritius, in the year 2008, thermal power generation accounted for 96% of the total energy produced, out of which the contribution of coal as fuel was 45% and over 105 000 tonnes of coal ash were generated [1]. In view of the expected increase in utilization of coal for power generation in the forthcoming decades in Mauritius, ash disposal may become a major environmental problem unless appropriate measures are adopted. In order to address this issue, the government of Mauritius has set up a Technical Advisory Committee (TAC) in 2007 to investigate on the disposal of coal ash in an integrated environment strategy [2]. The TAC concluded that coal ash can be defined as a recyclable by-product of energy production that requires environmental supervision in its use and recommended both encapsulated and unencapsulated applications. The encapsulated uses of coal ash include partial substitute for cement (blending with cement), aggregates for concrete products, mineral filler in asphalt, and controlled low strength materials. The unencapsulated uses of coal ash are fills below roads/buildings/pavements/parking areas, soil stabilization, and material for pipe bedding [3]. Physically, CBA has a semispherical to spherical particle shape and a complex morphology with a rough