Abstract
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 with 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.
Highlights
Granular materials can be considered as complex fluids with a finite yield stress that is associated with the transition between solid- and liquid-like states (Jaeger, Nagel & Behringer 1996; Andreotti, Forterre & Pouliquen 2013)
We experimentally investigate the sampling efficiency of a standard auger drill tool under different drilling conditions and soil properties
The experimental results show that the sampling efficiency decreases monotonically with the growth of the ratio between the rotational and penetration speeds
Summary
Granular materials can be considered as complex fluids with a finite yield stress that is associated with the transition between solid- and liquid-like states (Jaeger, Nagel & Behringer 1996; Andreotti, Forterre & Pouliquen 2013). Recent experiments revealed configurations for a rotating cylinder to drill inside granular materials with surprisingly low torque (Guillard, Forterre & Pouliquen 2013; Liu et al 2017). Tang et al (2018a,b) illustrated the coupling between the granular flow in the auger flight and that in the coring tube These experimental results revealed that the coring results crucially depend on the drilling conditions, characterized by an index called the penetration per revolution (PPR). The three processes are coupled together to affect both drilling loads and coring results This investigation aims at modelling the latter two processes as the efficiency of soil sampling relies predominantly on them.
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