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.
<div>The granular column collapse experiment, which consists of the rapid removal of lateral support to a column of granular material, is an important benchmark case for the physical and numerical study of transitional mass flows. While other researchers have focussed on the link between the aspect ratio of the column to mobility of the flow, these experiments are also an important platform to evaluate frameworks for triggering of slope failures.</div><div>&#160;</div><div>Critical state soil mechanics centers around the theory that initially dense soils will dilate, and initially loose soils will contract upon shearing. If the soil is sheared at a rate which exceeds the rate which fluids can be expelled or drawn into the pore space between particles, the shearing is considered to be occuring at constant volume and termed &#8220;undrained&#8221;. This state is associated with a rise in pore fluid pressure and a reduction in intergranular normal effective stress. The authors have conducted experiments varying the time scales of the volume change and dissipation processes. In these experiments, a novel transparent soil mixture comprised of quartz and mineral oil was utilized to visualize the saturation regime of soils during the granular column collapse experiment. Particular attention was paid to triggering mechanisms and the transition between the metastable state and avalanche regimes. The transparent material allowed visual confirmation of the volume change during shearing and important insights were gained into the role of the unsaturated soil condition in temporary strength. These observations have implications beyond the column collapse experiment, including the initiation of debris flow experiments as well as analysis of triggering mechanisms of unstable slopes in the field.</div>
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