Geotechnical implications of sand–rubber mixture in transportation infrastructure: assessing shear strength and compressibility characteristics

Abstract The increase of discarded tires in urban environments has emerged as a pressing environmental concern. This study explores the potential of incorporating scrap tire particles into sand matrices as a sustainable solution to diminish tire stockpiles and decrease environmental pollution. The main focus of this research is to investigate the mechanical properties of loose sand–rubber mixtures (SRM) characterized by a void ratio of 0.86, with varying rubber-to-sand particle size ratios (SR) of 0.25, 1, and 4. An extensive set of 300 direct shear tests was conducted using normal stresses (NS) of 50, 100, and 150 kPa. These tests were supplemented by 110 Oedometer tests using constant NS of 60 kPa for three days, 60 kPa for 1.5 days with an additional 140 kPa for 1.5 days, and 200 kPa for three days. Analysis of shear stress and deformation characteristics reveals that mixtures with different size ratios show similar trends but different values, which means characteristics of SRM depend not only on rubber content but also on size ratio. The addition of rubber particles to the mixtures makes the material more deformable and alters its softening behaviour. Specifically, adding up to 20% rubber content increases the mixture's friction angle, while higher rubber percentages cause it to decrease. A critical transition point is identified at approximately 20% rubber content, where the sand component begins to mimic rubber behaviour. Additionally, mixtures with SR = 0.25 exhibited a lower dilation angle compared to those with higher SR values, indicating that smaller rubber particles contribute to reduced dilation. Furthermore, the compressibility tendency of SRM escalates with higher rubber proportions, with mixtures featuring an SR of 0.25 exhibiting the most pronounced compressibility under equivalent NS conditions.