Recycled tyre aggregates (soft particles) mixed with common granular material such as crushed rock (rigid particles) are considered effective solutions for a range of applications in transportation geotechnics in recent years. While extensive research has been conducted on the mechanical properties and behaviour of sand-rubber combinations as unbound soft-rigid mixtures, most studies on bound soft-rigid mixtures have focussed on utilizing brittle binders like Portland cement. On the other hand, there have been only a few studies in recent years exploring the behaviour of soft-rigid mixtures bonded with non-brittle binders. This study aims to enhance our understanding of the impact of binder elasticity and stiffness on the compressibility mechanism of soft-rigid granular mixtures. One-dimensional compression tests complemented with shear wave velocity measurements were conducted on bound samples, using different types of binders, to investigate how the characteristics of binders influence the fabric of the mixture and, consequently, its behaviour. The findings indicate a multiphase behaviour of bound mixtures, in contrast to the single-phase behaviour of unbound mixtures, particularly for higher contents of binder and for brittle and semi-flexible binders.

Soft-rigid mixtures (SRMs) are typically pre-mixed before backfilling in engineering applications. However, the properties of soft materials make it particularly challenging to achieve uniform mixing with soils. This study is a pioneer in the exploration of non-uniform mixing in SRMs. A total of 76 quasi-static cubic compression tests were conducted with various mixing forms, mixing degrees, soft particle contents, and confining pressures under a standardized initial state. Based on the numerical results, the macroscopic responses were first quantitatively analyzed to reveal the effects of non-uniform mixing on the micromechanical behaviors of soft-rigid mixtures from compressibility, stress, and volumetric deformation perspectives. Then, the evolution of micro-scale properties, including the internal structure, stress network, internal stability, and fabric anisotropy, was investigated. It was found that the effects of non-uniform mixing on SRMs are considerably more pronounced than on traditional geotechnical binary mixtures. From a macroscopic perspective, non-uniform mixing greatly impacts the critical strength and void ratio of SRMs, with effects comparable to a 10% change in soft content. On a microscopic level, SRMs with higher uniformity exhibit a more stable internal structure, stress network, and enhanced internal stability. The results also show that layering should be avoided during construction. Additionally, the mixing index without considering stress direction is unsuitable for engineering applications. This paper underscores the paramount importance of considering the uniformity in soft-rigid mixtures, providing a robust foundation for further studies in this field.