Sediment creep triggered by porous flow

The surface of our planet is covered by a disordered granular layer. When tilted under gravity, the majority of the time it creeps: particles slowly move downward, despite being under the critical angle (or critical stress) for avalanching. This study contributes to the fundamental understanding of granular creep, investigating how a particular mechanism -- a gentle porous flow -- enhances sediment creep under gravitational stress. In a quasi-2D microfluidic apparatus, sediment layer creep experiments were performed under sub-yielding angle and porous flow conditions. Logarithmic decay rates of the deformation are observed, with the rate increasing with both the tilt angle and porous flow rate. We identify a new dimensionless parameter, $P^*$, that accounts multiplicatively for the porous flow and sediment layer slope effects on particle motion; this allows a rescaling of all the widely dispersed creep deformation results on a single curve. This curve presents two very distinct creep regimes whose natures are not fully understood. However, observations of the void size distributions during the creep experiments also show a systematic change of the microstructure between the two regimes.