Multi-scale studies of particulate-continuum interface systems under axial and torsional loading conditions
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The study of the shear behavior of particulate (soil) – continuum (man-made material) interfaces has received significant attention during the last three decades. The historical belief that the particulate – continuum interface represents the weak link in most geotechnical systems has been shown to be incorrect for many situations. Namely, prescribing properties of the continuum material, such as its surface roughness and hardness, can result in interface strengths that are equal to the contacting soil mass internal shear strength. This research expands the engineering implications of these findings by studying the response of interface systems in different loading conditions. Specifically, the axial and torsional shear modes are studied in detail. Throughout this thesis it is shown that taking an engineering approach to design the loading conditions induced to the interface system can result in interface strengths that exceed the previously considered limiting shear strength of the contacting soil. Fundamental experimental and numerical studies on specimens of different types of sand subjected to torsional and axial interface shear highlighted the inherent differences of these processes. Specifically, micro-scale soil deformation measurements showed that torsional shear induces larger soil deformations as compared to axial shear, as well as complex volume-change tendencies consisting of dilation and contraction in the primary and secondary shear zones. Studies on the global response of torsional and axial shear tests showed that they are affected differently by soil properties such as particle angularity and roughness. This difference in global behavior highlights the benefits of making systems that transfer load to the contacting soil in different manners available for use in geotechnical engineering. Discrete Element Modeling (DEM) simulations allowed for internal information of the specimens to be studied, such as their fabric and shear-induced loading conditions. These findings allowed for the development of links between the measured micro-scale behavior and the observed global-scale response. The understanding of the behavior of torsional and axial interfaces has allowed provides a framework for the development of enhanced geotechnical systems and applications. The global response of torsional shear found to induce larger cyclic contractive tendencies within the contacting soil mass. Therefore, this shear mode is more desirable than the conventional axial shear for the study of phenomena that depend on soil contractive behavior, such as liquefaction. A study on the influence of surface roughness form revealed that surfaces with periodic profiles of protruding elements that prevent clogging are capable of mobilizing interface friction angles that are 20 to 60% larger than the soil friction angle. These findings have direct implications in engineering design since their implementation can result in more resilient and sustainable geotechnical systems.