Multiscale Assessment of the Role of Particle-Scale Attributes on the Crushability of Granular Soils

Sohn, Changbum

doi:https://doi.org/10.21985/n2-fb20-ff68

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Multiscale Assessment of the Role of Particle-Scale Attributes on the Crushability of Granular Soils

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Granular materials are ubiquitous elements of our daily life, representing some of the most manipulated materials on Earth and playing a key role in disparate fields of science and engineering. Considerable research has been performed to explore the factors that impact the mechanical behavior of granular materials. Within this context, major challenges derive from the analysis of granular material behavior at high confining pressure, in that under those conditions particles fracture, leading to localized deformation and major microstructural changes. Despite the importance of these processes, we still do not know how the shape of the grains affect the fracture mechanisms leading to comminution, how these mechanisms evolve during deformation, and whether the same processes triggered during external loading remain active during the spontaneous deformation stages often observed during long-term creep. Further progress is therefore necessary to clarify these open questions and understand the multi-scale mechanics of granular geomaterials. This study has the goal to fill these gaps in knowledge by exploring the role of particle attributes (e.g., size and shape) on the continuum-scale mechanical behavior of these materials, by placing particular emphasis on both instantaneous and delayed comminution, i.e. the fragmentation of the particles that constitute a granular solid. This thesis discusses experiments conducted at particle and assembly scales on coarse-grained materials and interprets them in light of fracture mechanics theories. First, diametral compression tests on particles of varying size have been conducted to measure the energy stored in individual grains at the onset of fracture. Then, oedometric compression tests on samples made of the same particles have been performed to measure the yielding pressure, as well as to track the evolution of breakage. These experiments have been used to test the performance of recently proposed scaling laws bridging the energy released by a single particle with the work input required to comminute an assembly. Additionally, creep tests have been performed on quartz sands to evaluate the role of particle attributes on the accumulation of creep deformation, as well as the delayed breakage growth over time. The viscous breakage theory is further used to simulate the creep stress-strain response, evolution of particle gradation and delayed breakage growth of all tested sands with the purpose of quantifying the model parameters and validating the theoretical hypotheses. Moreover, synchrotron X-ray microtomography is used to directly characterize how the particle shape affects the feedbacks between collective comminution and individual grain fracture. Rounded and angular sands are compressed beyond their comminution pressure and imaged at the microscale. An analytical study is further conducted to quantify grain-scale processes responsible for comminution Lastly, the imaging technique is further used to directly confirm the experimental and numerical findings on time effects. Finally, a discrete element method is used to simulate diametral, oedometric, and creep compression tests. These virtual laboratory experiments have the purpose to demonstrate how the local particle failure impacts to the collective breakage response, considering particle-to-particle interactions. To conclude, the main findings of the thesis are summarized and some directions for future research are discussed.

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