Micro-mechanical investigation of the instability of pure sand and sand-rubber mixtures

Abstract

Soil instabilities caused by either increase of pore water pressure in undrained loading conditions or water infiltration in drained loading conditions, can lead to natural hazards such as landslides. New geo-materials, for instance, mixtures of sand with rubber have become a popular approach in ground improvement acting as a remedial against unstable soil behaviour. The Discrete Element Method (DEM), which offers particle-scale information, was employed to model granular systems comprised of sand (stiff) and rubber (soft) particles under different loading conditions and to provide fundamental explanations to the failure mechanisms within granular systems.

The conditions required for quasi-static shearing in DEM simulations were explored. The inertial number (𝐼) was used to ensure that the shearing rate applied to simulated samples was quasi-static. Clear trends were observed in macro and micro-mechanical quantities at the critical state as 𝐼 varied. A value of 𝐼 that divided the quasi-steady from a dynamic state was identified.

A study of the influence of the initial void ratio (𝑒0) on the coefficient of lateral pressure at rest (𝐾0) for sand samples was carried out. It was found that higher density resulted in lower values of 𝐾0. Denser states presented a bias towards vertically-oriented contacts, resulting in lower stresses transmitted in the horizontal direction. Contrastingly, contacts were oriented more isotropically in looser states, allowing for a more isotropic stress transmission, inducing more critical conditions for geotechnical structures such as retaining walls.

Sand samples were prepared under different consolidation paths to explore stress-induced anisotropy. The small strain stiffness was affected by different degrees of induced anisotropy. However, when sheared to larger strains, the influence of induced anisotropy diminished, and unique macro and micro-scale critical state relationships were attained. Higher degrees of induced anisotropy on loose specimens helped increase the liquefaction resistance.

The instability of granular materials under drained conditions was simulated by conducting Constant Shear Drained (CSD) tests. Regardless of the initial packing, all samples attained an onset of instability that coincided with fluctuations in the particle-scale second-order work. Macro and micro quantities experienced drastic changes, showing either a sharp increase or decrease. It was shown how constant shear drained loading conditions may result in more unfavourable situations than undrained shearing conditions.

A series of DEM simulations, consisting of mixtures of sand and rubber particles, were conducted under different loading conditions. The content and size of rubber particles had a direct impact on the peak strength and shear modulus of the mixtures. Micro-scale information was obtained for all tests, and the contribution of each type of contact in the overall response was analysed and quantified. Different internal structures were encountered at different test stages and linked to the macro-mechanical response. Rubber content contributed little to prevent unstable behaviour under CSD conditions. In constant volume tests, as rubber content increased, less susceptibility to liquefaction was achieved, with the help of a sufficiently stable contact structure aided by high-friction rubber particles.