Effect of Fibre Orientation on the Mechanical Response of Reinforced Sand: A Multiscale Study Using X-ray Tomography.

The diffuse inclusion of fibres to infer tensile resistance to construction materials has proved its effectiveness in a variety of civil engineering applications. Extending the same principle to natural soils seems a logical promise, thinking of the potential benefit that could stem from fibre reinforcement on the mechanical performance of geotechnical systems, from slope reinforcement to foundations. However, the full potential of this technique remains underexploited, mostly due to an incomplete understanding of the interactions between fibres and soil. The present study aims to explore with a comprehensive experimental campaign the basic mechanisms that rule the interaction between fibres and sand. While traditional studies follow a phenomenological approach, comparing the global mechanical responses of samples with and without fibre reinforcement and drawing rules to incorporate observation into modelling, the crucial role of grain-scale mechanisms on the evolution of soil deformation is herein pointed out. Bearing this goal in mind, laboratory tests at different scales have been combined to investigate the mechanisms of fibre reinforcement from different perspectives. Firstly, the effects of fibre reinforcement have been investigated with several triaxial tests on large samples to observe the change of mechanical response induced by fibres of different length and orientation. The observed results highlight the paramount role of strain localisation that naturally occurs on sand upon shearing. Then, to better explore the role of fibres on this mechanism, a second experimental campaign has been performed with direct shear tests, i.e., forcing strain localisation along preferential directions. The analysis has then moved on the microscale, performing X-ray tomography on the samples subjected to direct shear and on miniature triaxial tests, to detect how fibres influence strain localisation, porosity evolution, and shear band formation. Results demonstrate that well-oriented fibres not only enhance strength and ductility but also modify the progressive development of deformation patterns, improving load distribution and delaying failure. The analysis has then proceeded at a more detailed scale pointing out the interaction between fibres and surrounding grains. To this aim, specific tests have been performed on samples with a limited number of fibres placed along prescribed directions and loading them with tensile forces. The processing of tomographic images enables a very precise identification of fibres and grains position in the different loading phases and to link the kinematics of both systems. Combining macroscopic behaviour and microscale interactions, this study provides essential insights to optimize Fibre-Reinforced Sand (FRS) applications, enabling more efficient and reliable reinforcement strategies in geotechnical engineering.