The (In)sensitivity of Granular Creep to Materials and Boundaries

Soils around the planet creep, despite wide variations in particle properties and environments. This sub-yield "flow" of soil interacts with a variety of boundaries, in terms of geometry and friction. Here we explore the veracity of recent observations of undisturbed, gravity-driven creep, by testing a suite of materials and boundary configurations in an experimental hillslope. Using an optical interferometry technique, we demonstrate that creep is a generic relaxation process whose qualitative dynamics are insensitive to grain properties. Velocity profiles are exponential, albeit with a defect near the no-slip boundary. Quantitative patterns such as spatial variability and magnitude of strain rates, however, are exquisitely sensitive to the details of the experiment. The emerging picture is that creep is accomplished by localized plastic failure, which induces an elastic redistribution. Similar patterns have been observed in model glasses and on earthquake faults, indicating that sub-yield relaxation in disordered materials may share common physics. A while ago, we poured some glass beads into a box. Even days after we were done pouring, the grains still moved-but really slowly. This is creep. Our observations of "undisturbed" creep have aroused surprise and also suspicion from fellow geoscientists-in particular because experiments were conducted using glass beads, while natural soil particles are more angular and sometimes sticky. To help convince everyone that creep is robust, we dumped a bunch of different materials into an acrylic box over and over again. Sand, espresso grounds, yeast, clay-you name it, we dumped it. It all creeps, and it doesn't really matter whether the boundaries are smooth or rough. Soil slowly creeps downhill due to gravity aloneCreep dynamics are robust across a range of granular materials and boundary roughnessGranular creep is similar to creep in glass and also localized slip along faults