Mechanical behavior of "living quicksand": Simulation and Experiment

The nature and danger of quicksand has been disputed since a long time. Despite widespread belief that humans can be swallowed or even sucked in, engineers of soil mechanics have typically asserted that, since the density of sludge is larger than that of water, a person cannot fully submerge. We investigated a specific type of 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. The initial structure cannot be restored once it had collapsed, i.e. the material investigated shows a strong memory effect. We simulated a model for this type of quicksand in which we constructed a tenuous granular structure representing the unperturbed soil. The initial structure consists of cohesive disks put together by ballistic deposition and settled by gravity using Contact Dynamics. We study the material behavior by determining the shear strength of the model material and by penetration tests, i.e. pushing in an object, which leads to breaking of cohesive bonds. We 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.