From Powders to Collapsing Soil/Living Quicksand: Discrete Modeling and Experiment.

The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e., at the particle scale) of granular/soil materials. We present a variant of the contact dynamics method, originally developed to model compact and dry systems with lasting contacts. This variant accounts for the cohesive nature of fine powders and soils. The attractive force plays an important role in the stabilization of large voids, leading to highly porous systems as e.g. in fine cohesive powders. Using this model we can explain how large pores are stabilized, and which contact laws are crucial. The model is applied for compaction of fine powders and compared to experiments, both showing a power law behavior in the high stress regime. We use a modification of the model to investigate the “quicksand” behavior of a collapsing soil material. Our contact dynamics model as a microscopic description is adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of the system. Our physical model is validated with real data obtained from in situ measurements performed with a specific type of natural quicksand at the shore of drying lagoons. Cyanobacteria form an impermeable crust, giving the impression of stable ground. After breaking the crust a person rapidly sinks to the bottom of the field. We measured the shear strength of the material before and after perturbation and found a drastic change. We show that the shear strength behavior of our collapsing soil model is consistent with the behavior of this quicksand, for both the unperturbed and the collapsed phases of the material. We also investigate how deep the object can be pushed in and how well the intruder is captured by the material after it collapsed above the intruder. During the penetration process we measured the relation between the driving force and the resulting velocity of the intruder. We also investigated the influence of different strength of viscous drag acting on the grains. [ABSTRACT FROM AUTHOR]