Creep is relaxation

Over 200 years of inquiry tells us that if left to rest, a pile of sand will sit still - rendered immobile by static friction. As such, formulations of soil flux on hillslopes, continuum soil mechanics and constitutive relations of granular rheology invoke (bio)physical perturbations or an excess of a critical friction criterion for flow to manifest beneath the angle of repose. Here, we provide observations of granular materials (glass beads, angular sand, kaolinite, sand/kaolinite mixtures, Wissahickon river valley soil) creeping relentlessly below the Coulomb limit in the (near-)absence of disturbance - dynamics that violate all established frameworks for soil creep. Grains sit on a vibration-isolating table, incubated from breezes, fluctuations in temperature/humidity and other environmental noise. Creep is measured via DWS, a light scattering technique capable of resolving motions on the order of the optical wavelength (10^-6 m) and on experimental timescales of minutes. Creep in these materials is manifest as quasi-random hot spots of plastic rearrangements, occurring throughout the bulk of the pile. Averaging over these fluctuations produces exponentially decaying velocity profiles that match the shape and magnitude of field measurements. Creep rates decay logarithmically in time as the initially loose bed relaxes into a lower-energy state. We find that tapping the system further reduces creep rates by compaction. Nevertheless, creep persists for all materials and packing states. We postulate that this behavior is a kind of 'aging' - and in nature, these granular relaxation dynamics also exist, while other perturbations (which create void space) act to rejuvenate disturbances and accelerate the creep rate. Phenomenologically, the picture is consistent with aging in glassy materials, in spite of the absence of thermal agitations. Thus, we propose that hillslope soil creep be modeled as deformation of an amorphous solid -- this framework explicitly connects rheology to soil microstructure, granular friction and mechanical disturbances.