Engineering properties of eucalyptus ash cement gravel columns reinforced with fibres from plastic bottles

Construction of embankments on soft clay foundations often encounters excessive settlement and low stability due to the low shear strength and high compressibility of the soil. Ground improvement techniques using columns such as concrete columns, soil–cement columns, and gravel columns are commonly adopted to enhance subsoil performance. However, these columns generally provide strength higher than required for embankment applications, resulting in increased construction costs and resource consumption. This study proposes the development of a semi-rigid column by integrating the characteristics of flexible and rigid columns, namely a eucalyptus ash–cemented gravel column reinforced with plastic bottle fibers. The objective is to investigate its engineering properties for construction applications while simultaneously reducing material costs and environmental impacts. Ordinary Portland cement was partially replaced with eucalyptus ash, a by-product from biomass power plants with suitable chemical composition for pozzolanic reactions, to enhance bonding performance and reduce cement consumption. In addition, recycled plastic bottles were processed into fibers to reinforce the column material, aiming to improve crack resistance and mechanical performance while mitigating plastic waste. The materials consisted of coarse aggregates with sizes of 4.76–9.51 mm and 9.51–19.00 mm, hydraulic cement content of 340 kg/m³, and a water–cement ratio of 0.32. Cement was replaced with eucalyptus ash at 5, 10, 15, 20, and 25% by weight. Recycled plastic fibers with a width of 2 mm and length of 70 mm were incorporated at 0.5, 1.0, 1.5, and 2.0% by volume. Tests were conducted to determine porosity, compressive strength, splitting tensile strength, and coefficient of permeability at curing ages of 28 and 90 days. The results showed that aggregate size, eucalyptus ash content, and plastic fiber content all affected the strength and drainage capacity of the columns. Well-graded aggregate provided the highest strength due to better particle arrangement, while single-sized aggregate provided better porosity and water permeability. Replacing cement with 10% eucalyptus ash resulted in the highest strength, along with the lowest porosity and coefficient of permeability. Similarly, a plastic fiber content of 1.0% resulted in the highest strength, along with the lowest porosity and coefficient of permeability. These findings can serve as a guideline for designing the mix proportions of eucalyptus ash–cemented gravel column reinforced with plastic bottle fibers to suit specific application objectives—whether emphasizing strength or drainage performance—thereby promoting efficient resource utilization and reducing environmental impact.