Experiments and simulations of an intruder dragged by a spring through a two-dimensional annulus of granular material exhibit robust force fluctuations. At low packing fractions ($\varphi <{\varphi }_0$), the intruder clears an open channel. Above ${\varphi }_0$, stick-slip dynamics develop, with an average energy release that is independent of the particle-particle and particle-base friction coefficients but does depend on the width $W$ of the annulus and the diameter $D$ of the intruder. A simple model predicts the dependence of ${\varphi }_0$ on $W$ and $D$, allowing for a data collapse for the average energy release as a function of $\varphi /{\varphi }_0$. These results pose challenges for theories of mechanical failure in amorphous materials.

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• Confinement
• Creep
• Earthquake statistics & scaling
• Granular flows
• Granular materials
• Jamming
• Dry granular materials
• Granular solids
• Molecular dynamics
• Tension tests