The coefficient of restitution (COR) is an important input parameter in the numerical simulation of granular flows, as it governs the travel distance, the lateral spreading and the design of barriers. In this study, a new custom-built micro-mechanical impact loading apparatus is presented along with impact experiments on engineered and natural materials. The COR and energy loss of various grains and base block combinations are studied, including fairly regular shaped Leighton Buzzard sand (LBS) grains as a natural soil and granite/rubber as base blocks, apart from the use of engineered materials for the grains (chrome steel balls, glass balls) and blocks (stainless steel, brass). The repeatability of the new micro-mechanical impact loading apparatus was checked by impacting chrome steel balls on stainless steel block. In all the test combinations, the higher and lower values of the COR are found for granite block (ranging between 0.75-0.95) and rubber block (ranging between 0.37-0.44) combinations, respectively. For the tested grain-block combinations, lower values of COR were observed for impacts between materials of low values of composite Young’s modulus.
Coarse grains accumulate in geophysical flow fronts and have high solid fractions. Such fronts may arch in slit structures such as baffles and slit dams, leading to the rapid trapping of particles and potentially high-energy overflow. Existing empirical slit-structure design recommendations are limited and inadequate since they only focus on the slit size to particle diameter ratio (s/δ) and neglect the pileup height and pre-impact flow energy. Flume modelling was thus adopted to study coarse flow fronts impacting a slit structure. The characteristic Froude conditions, flow particle diameter and the ratio s/δ were varied. Results have shown the pileup height, and hence the confining stress is dependent on Froude conditions, but is not strongly influenced by s/δ. The flow particle diameter influences collisional and frictional stresses and hence the mean outflow rate, which is correlated with pileup height. Grain-trapping efficiency depends on both s/δ and Froude conditions. In contrast to existing continuum-based theory for slit-structure interaction, frictional contacts should be considered for coarse-grained flow fronts. High-energy supercritical flows lead to low trapping efficiency since stable arches cannot form at high shear rates. This implies that multiple slit structures may be more appropriate for attenuating high-energy supercritical flows.
Bi-dispersity is a prerequisite for grain-size segregation, which transports the largest particles to the flow front. These large and inertial particles can fragment upon impacting a barrier. The amount of fragmentation during impact strongly influences the force exerted on a rigid barrier. Centrifuge modelling was adopted to replicate the stresses for studying the effects of bi-dispersity in a granular assembly and dynamic fragmentation on the impact force exerted on a model rigid barrier. To study the effects of bi-dispersity, the ratio between the diameters of small and large particles (δs/δl), characterizing the particle-size distribution (PSD), was varied as 0.08, 0.26, and 0.56. The volume fraction of the large particles was kept constant. A δs/δl tending towards unity characterizes inertial flow that exerts sharp impulses, and a diminishing δs/δl characterizes the progressive attenuation of these sharp impulses by the small particles. Flows dominated by grain-contact stresses (δs/δl < 0.26), as characterized by the Savage number, are effective at attenuating dispersive stresses of the large particles, which are responsible for reducing dynamic fragmentation. By contrast, flows dominated by grain-inertial stresses (δs/δl > 0.26) exhibit up to 66% more impulses and 4.3 times more fragmentation. Dynamic fragmentation of bi-disperse flows impacting a rigid barrier can dissipate about 30% of the total flow energy.
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