The granular column collapse experiment is an important benchmark case for the physical and numerical study of transitional mass flows. Unlike columns of dry granular materials, the presence of a relatively incompressible fluid, such as water, in the voids of saturated columns complicates the shear behavior of the column by becoming a function of the coupled shear and volumetric behavior of the grain–fluid system. Dilative or contractive behavior at the pore level will cause a decrease or increase, respectively, in the pore fluid pressure. These changes in effective stress, in turn, will define stability or instability and length of runout. Here we use the new opportunity provided by transparent soil to observe air entry within saturated columns to explore the hypothesis that the entry pressure provides the maximum contribution of capillary pressure at incipient failure, thereby providing a quantitative control on the stability of dilative granular columns. Furthermore, the mobility of densely packed saturated columns subject to collapse was significantly influenced by air entry. An analytical model, based on this assumption of limiting capillary pressure, is able to describe the stability of the experimental columns as well as the larger dataset from the literature, reframing the previous empirical stability threshold using limit equilibrium and soil material parameters. Our results demonstrate the importance of stress-dilatancy and air-entry phenomena on the rapid shear behavior of saturated granular materials.@en

Landslide generated tsunamis are a primary natural hazard to coastal communities and infrastructure, but the present state of knowledge is limited to the specific details of momentum transfer from the landslide to the water body for either subaerial or fully submerged conditions. Here we report a series of novel experiments and an analytical model that bridges this gap, by exploring the collapse of a granular column for a wide range of water depths. We show that the maximum seaward wave amplitude is governed by a single dimensionless ratio, the relative depth of submergence. Based on the experimental observations, we propose a continuous function that quantifies the maximum wave amplitude by considering the momentum flux from the initially vertical granular column to the initially still fluid. Predictions made using the momentum function are in good agreement with observations of the present study and with other experimental studies of granular column collapse at larger scales. The analytical model allows the prediction of maximum wave amplitude over the full range of submergence conditions from subaerial to partially submerged and fully submerged collapses, with potential applications for tsunami hazard assessment.@en