  From geotechnical applications to space exploration, auger drilling is often
used as a standard tool for soil sample collection, instrument installation,
and others. Focusing on granular flow associated with the rotary drilling
process, we investigate the performance of auger drilling in terms of sampling
efficiency, defined as the mass ratio of the soil sample collected in the
coring tube to its total volume at a given penetration depth, by means of
experiments, numerical simulations, as well as theoretical analysis. The ratio
of rotation to penetration speed is found to play a crucial role in the
sampling process. A continuum model for the coupled granular flow in both
coring and discharging channels is proposed to elucidate the physical mechanism
behind the sampling process. Supported by a comparison to experimental results,
the continuum model provides a practical way to predict the performance of
auger drilling. Further analysis reveals that the drilling process approaches a
steady state with constant granular flow speeds in both channels. In the steady
state, sampling efficiency decreases linearly with the growth of the rotation
to penetration speed ratio, which can be well captured by the analytical
solution of the model. The analytical solution also suggests that the sampling
efficiency is independent of gravity in the steady state, which has profound
implications for extraterrestrial sample collection in future space missions.
