Nonlocal rheology of dense granular flows

Abstract:

Geological granular materials are known to creep below the Mohr-Coulomb failure criterion, a process which provides an important control on the formation of geomorphological features. Such flows cannot be characterized by local rheological models, but several nonlocal rheologies have recently been developed to address these shortcomings. We perform laboratory shear tests on idealized materials to test the efficacy of such models across different particle types, packing fractions and shear rates. Using an annular shear cell that allows for continuous measurements, we measure both the local stress ratio μ and the local inertial number I through the use of a torque sensor, boundary leaf springs, photoelastic particles, and particle-tracking. We observe that across a wide variety of parameters, nonlocal models are able to quantitatively describe slow granular flows, even into the creeping regime. We find that the magnitude of nonlocal effects depends on both the particle shape and stiffness, and that a particular set of particles only needs to be tested under one set of driving conditions to determine their model parameters. We confirm the prediction that there is a growing length-scale over which nonlocal effects are relevant, with the largest effects occurring close to the frictional yield criterion.