Strain hardening by sediment transport

The critical fluid-shear stress for the onset of sediment transport, theta(c), varies with the history of applied shear. This effect has been primarily attributed to compaction; the role of shear jamming is less explored. We examine the response of a granular bed to fluid-shear stress cycles of varying magnitude and direction, and determine isotropic and anisotropic contributions. Creep and bed-load transport result in direction-dependent strain hardening for theta/theta(c) < 4. Dilation-induced weakening, and memory loss, occur for larger stresses that fluidize the bed. Our findings provide a granular explanation for the formation and breakup of hard-packed riverbed "armor."