  When a jet of gas impinges vertically on a granular bed and forms a crater,
the grains may be moved by several different mechanisms: viscous erosion,
diffused gas eruption, bearing capacity failure, and/or diffusion-driven
shearing. The relative importance of these mechanisms depends upon the flow
regime of the gas, the mechanical state of the granular material, and other
physical parameters. Here we report research in two specific regimes: viscous
erosion forming scour holes as a function of particle size and gravity; and
bearing capacity failure forming deep transient craters as a function of soil
compaction.
