Sand-Rubberchips as Retaining Earth Wall Backfill: Permeability Characteristics

Abstract

Backfills are crucial in retaining wall construction, ensuring stability, structural integrity and adequate drainage. Traditionally, sand has been the primary backfill material, but it is a limited natural resource. This study explores the partial replacement of sand with recycled tyre rubberchips to reduce reliance on natural sand while enhancing sustainability. Since permeability governs the drainage capacity and long-term stability of backfills, the effect of rubberchip substitution on permeability was examined using a constant head test at substitution levels ranging from 10% to 80%. The sand had a D50 of 0.35 mm while the rubberchips had a D50 of 0.9 mm, introducing coarser particles into the matrix. Results showed a progressive increase in the permeability coefficient with higher rubberchip content, reaching almost twice the value of pure sand at 100% substitution. Theoretically, this can be explained by the larger void ratios and reduced inter-particle contact in the sand–rubber composite, which create more continuous flow paths. Water absorption by rubberchips was minimal, indicating a negligible risk of additional lateral pressure on the retaining wall. The 80% sand–20% rubberchip mixture yielded permeability values closest to those of sand while maintaining good drainage. These findings suggest that sand–rubberchip backfills can serve as a sustainable, high-permeability material for retaining structures, although further verification of their strength and load-bearing capacity is necessary before field application.