Improving Engineering Properties of Expansive Subgrade Soil with Waste Plastic and Bottom Ash: Experimental and Numerical Approach

Abstract

Expansive soils are problematic soil in many parts of the world, including Ethiopia. They are characterized by their high swelling potential, which can lead to damage to lightweight structures such as road embankments, and other infrastructure. The characteristics of the weak subgrade must be changed for the pavement structure built on such soil. The study aims to examine the suitability of bottom ash and waste plastic to enhance the engineering properties of expansive soil and improve the deformation characteristics of soil. Laboratory tests including grain size distribution, specific gravity, California bearing Ratio test, Atterberg limits, swelling pressure, free swell index, consolidation test, triaxial shear strength, X-ray diffraction analysis, and deformation analysis by the finite element method were conducted. The soil in the study was stabilized using different ratios of waste plastic and bottom ash. Waste plastic was used in ratios of 1.5%, 2.5%, 3.5%, and 4.5%, taken to optimum to mix with bottom ash added in ratios of 4%, 8%, 12%, and 16%. The laboratory test results revealed that the optimal ratio of waste plastic and bottom ash was 4.5% and 8%, respectively, which significantly, improved subgrade soil. When the plasticity index of expansive soil was reduced from 61.15% to 26.81%, the optimal ratios of these mixes were used. The subgrade soil was categorized as A-7-5 according to the AASHTO classification system and CH in the USCS classification system. The optimum moisture content at 4.5% of waste plastic and 8% of bottom ash is reduced from 25% to 15%. Additionally, the maximum dry density increases from 1.334 g/cm³ to 1.52 g/cm³. In the California bearing Ratio, test results, optimal percentages of mix significantly improved the CBR values of natural subgrade soil. The CBR value increased from 0.92% to 9.03%, indicating a substantial enhancement in soil strength. Furthermore, the deformation analysis showed that deformation decreased from 52*10 -3 m to 11*10 -3 m for the improved subgrade soil, showing improved stability and reduced susceptibility to deformations. Mineral analysis identified common minerals such as silicon oxide quartz, chloritoid, iron, calcite, aluminum hydroxide, and quartz in the soil samples and bottom ash. The study emphasized that the mixture of waste plastic and bottom ash had a significant positive impact on improving the weak subgrade soil. The experimentally determined optimal ratio of bottom ash and waste plastic improved the weak subgrade soil and the engineering characteristics of expansive soil, demonstrating the potential use of this material in soil improvement applications. Adding these waste materials to subgrade soil not only improved the soil properties but also decreased the environmental impact of waste materials, contributing to environmental sustainability.