Soil creeps imperceptibly but relentlessly downhill, shaping landscapes and the human and ecological communities that live within them. What causes this granular material to ‘flow’ at angles well below repose? The unchallenged dogma is churning of soil by (bio)physical disturbances. Here we experimentally render slow creep dynamics down to micron scale, in a laboratory hillslope where disturbances can be tuned. Surprisingly, we find that even an undisturbed sandpile creeps indefinitely, with rates and styles comparable to natural hillslopes. Creep progressively slows as the initially fragile pile relaxes into a lower energy state. This slowing can be enhanced or reversed with different imposed disturbances. Our observations suggest a new model for soil as a creeping glass, wherein environmental disturbances maintain soil in a perpetually fragile state.

Soil-mantled hillslopes owe their smooth, convex shape to creep; the slow and persistent, gravity-driven motion of grains on slopes below the angle of repose. Existing models presume that soil creep occurs via mechanical displacement of grains by (bio)physical disturbances. Recent simulations, however, suggest that soil can creep without these disturbances, due to internal relaxation dynamics characteristic of disordered and fragile solids such as glass. Here we report experimental observations of creeping motion in an undisturbed sandpile, at micron resolution over timescales of 10^0-10^6 s, for a variety of natural and synthetic granular materials. We observe two behaviors typically associated with creeping glass: strain occurs as localized and spatially-heterogeneous grain motions; and creep rates decay as a power-law function of time. Further, creep can be accelerated or suppressed by thermal cycles and shaking, respectively. Averaged strain profiles decay exponentially with depth, in agreement with field observations of creeping hillslope soils. Our findings demonstrate that soil is fragile in terms of sensitivity to disturbances, but that creep dynamics are robust across grains and glasses. Mapping soil creep to the more generic glass problem provides a new framework for modeling hillslope sediment transport, and new insights on the nature of yield and failure.

<p>It is now well established that many lansdcapes are organized to be close to the threshold of sediment motion: rivers, wind-blown dunes and hillslopes. </p> <p>Whether explicitly or implicitly, this threshold is almost universally treated as a Mohr-Coulomb failure criterion, which is an opaque barrier that prevents us from viewing and understanding motion beneath the yield point. Below-threshold motion is creep, and the dynamics are creepy indeed: typical continuum descriptions break down, and observed behaviors can be counterintuitive. </p> <p>In this talk I present two experiments, using two different optical techniques, that study very slow particle motions below the threshold of motion. Experiments in a scaled-down river use refractive-index matched scanning to image the interior of a sediment bed sheared by a fluid, and track particles over many orders of magnitude in velocity to show that creep is activated deep into the sediment bed. This creep hardens the bed and drives segregation. Tracking creeping grains becomes impractical, however, as it takes several months to measure the slowest particle motions. </p> <p>To overcome these simplifications and expand our study of creep, we examine an apparently static sandpile that is isolated from external disturbance. Instead of particle tracking, we use an optical interferometry technique called Diffusive Wave Spectroscopy (DWS) that allows us to measure creep rates as low as nanometers/second. Viewed through the lens of DWS, the model hillslope is alive with motion as internal avalanches of grain rearrangements flicker throughout the pile. We observe similar dynamics to those observed in the river experiment -- albeit over much shorter timescales -- even though the only significant stress is gravity. What causes these grains to creep below their angle of repose? Observations suggest that minute mechanical noise may play a role, but reducing the noise floor beyond our fairly quiescent conditions is very challenging. Instead, we raise the driving stresses through heating, tapping and flow. </p> <p>The observations lead to new view of sediment creep as relaxation and rejuvenation of a glassy material, where mechanical noise plays a role akin to thermal fluctuations in traditional glass materials. Sub-yield deformation is a new world to explore, for those patient enough to look for it. </p>

Colloquially, a "logjam" indicates a kinematic arrest of movement. Taken literally, it refers to a type of dense accumulation of wood in rivers widely recognized as bestowing numerous biological and physical benefits to the system but also present serious hazards to infrastructure. Despite this, no in-situ field measurements have assessed the degree of arrest in a naturally-formed logjam. Using time-lapse photography, repeat total station surveys and water level loggers, we provide an unprecedented perspective on the evolution of a logjam in central Idaho. Despite the namesake, we find that the logjam is not jammed. The ensemble of logs progressively deforms in response to shear and buoyant lift of flowing water, modulated by the rising limb, peak and falling limb of the snowmelt hydrograph. As water rises and log drag against the bed and banks decreases, they collectively translate downstream, generating a heterogeneous pattern of deformation.As streamflow recedes and the logs reconnect with the bed and banks, the coherent deformation pattern degrades as logs settle opportunistically amongst their neighbors.Field observations of continuous movement at a low rate are qualitatively similar to creep and clogging, behaviors that are common to a wide class of disordered materials. These similarities open the possibility to inform future studies of environmental clogging, woodladen flows, logjams, hazard mitigation and the design of engineered logjams by bridging these practices with frontier research efforts in soft matter physics and granular rheology.