Engineering and environmental evaluation of silty clay modified by waste fly ash and oil shale ash as a road subgrade material

Abstract

The objective of this study was to evaluate the viability of utilizing waste fly ash (FA) and oil shale ash (OSA) to modify silty clay (SC) as a subgrade material. First, the modified soils at five different ratios of OSA/FA/SC were prepared. Second, the compaction, Atterberg limits and CBR (California bearing ratio) tests were conducted to determine the optimum ratio of OSA/FA/SC. Third, the static and dynamic mechanical characteristics of the modified soil at the optimum ratio of OSA/FA/SC were measured. Subsequently, the environmental impact of the modified soil subgrade leachates on water was assessed. The results show the physical properties of SC are improved by the addition of FA and OSA. The modified soil at the OSA/FA/SC ratio of 40:20:40 possesses the highest CBR value, propriate physical properties and the balanced usage ratio of FA, OSA and SC. The triaxial tests demonstrate that the static strength parameters and dynamic fracture strength of SC are significantly increased by FA and OSA. The obtained dynamic modulus could be valuable for evaluating the deformation and stability of this modified soil in geotechnical engineering. The main ions and trace elements concentration analysis results indicate that the modified soil leachates have no pollution on Class II surface water and Class III ground water. Based on these results, SC modified by FA and OSA has the potential to be used a viable subgrade material.