Athermal granular creep in a quenched sandpile

TL;DR

This study investigates slow creep and flow in granular heaps, revealing a transition near a critical inertial number and persistent creep after flow cessation, with implications for understanding soil deformation.

Contribution

It provides experimental insights into athermal granular creep, highlighting the transition between flow and creep states and the persistence of creep in quenched heaps.

Findings

Creep persists beneath the surface flow layer with aging effects.

A transition between flow and creep occurs near inertial number $I = 10^{-5}$.

Creep continues after flow stops, influenced by the heap's history.

Abstract

Creep is a generic descriptor of slow motions -- in the context of materials, it describes quasi-static deformation of a solid when subjected to stresses below the global yield, at which all rigidity collapses and the material flows. Here, we experimentally investigate creep, flow, and the transition between the two states in a granular heap flow. Within the surface flowing layer the dimensionless strain rate diminishes with depth, there is an absence of spatial correlations, and there is no aging dynamics. Beneath this layer, the bulk creeps via localized avalanches of plasticity, and there is significant aging. The transition between fast surface flow and slow bulk creep and aging is observed to be in the vicinity of a critical inertial number of $I = 10^{-5}$. Surprisingly, at the cessation of surface flow and the `quenching' of the pile, creep persists in the absence of the flowing layer; albeit with significant differences for a pile that experiences a long duration of surface flow (strongly annealed) and one where flow during preparation does not last long (weakly annealed). Our results contribute to an emerging view of athermal granular creep, showing similarities across dry and submerged systems. Quenched quiescent heaps that creep indefinitely, however, present a challenge to granular rheology, and open new possibilities for interpreting and casting creep and deformation of soils in nature.