Granular compaction under confinement.

Successive vertical vibrations can cause a granular pack to rearrange and move to more and more dense configurations. The dilation of the pack that occurs during the vibrations is important to these rearrangements and the resulting compaction of the system. This paper presents an experimental study in which a confining force is used to limit the amount of dilation of the pack. These experiments show that the compaction is greatly reduced in the presence of a confining force. Systems with different shaking accelerations and different confining forces compact in the same way if the amount of dilation is the same. Under large confining forces, there is very little dilation. The resulting compaction when dilation is inhibited is very slow and may approach a final packing fraction of approximately 0.6, consistent with ideas of a critical packing fraction for the onset of dilation.