Stabilization of subgrade of flexible pavement using paper mill waste ash

Abstract

In pavement design, an expansive subgrade requires a buffer layer of thickness ranging from 0.6 m to 1.0 m to be placed over it which makes the pavement thicker and expensive. If the expansive subgrade is stabilized using a non-plastic or less plastic material, the buffer layer could be avoided. The objective of this study is to reduce the expansive nature of the subgrade soil by adding paper mill waste ash. The process of mechanical stabilization is carried out in the current study. The waste material used for this study was the by-product obtained from the paper mill. Various proportions of waste material (non-plastic and classified as ML) are mixed with the virgin soil (Liquid limit 47.9%, Plastic limit 24% and hence classified as CI as per ISSCS and A-7-6 as per AASHTO) and the optimum mix which reduces the expansive nature of the soil was found to be 70:30 (clay soil: ash) and CBR test was carried out on the optimum mix and the CBR value has increased from 1.2% on virgin soil to 2.4% on the optimum mix. Hence addition of paper mill waste not only eliminates the buffer layer but also makes the subgrade stronger thereby reducing the overall thickness of the pavement. Finally, for the virgin soil and the optimum mix, pavements with traffic intensities of 2 msa, 5 msa, and 10 msa have been designed and cost analysis has been carried out to prove that the addition of the paper mill waste ash to the virgin subgrade makes the pavement structure more economical. A cost reduction of 29.0%, 25.8%, and 22.3% for 70:30 mix when compared to flexible pavement designed for 2 msa, 5 msa, and 10 msa on virgin soil respectively was found.