Abstract
In both nature and the laboratory, loosely packed granular materials are often compacted inside confined geometries. Here, we explore such behaviour in a quasi-two dimensional geometry, where parallel rigid walls provide the confinement. We use the discrete element method to investigate the stress distribution developed within the granular packing as a result of compaction due to the displacement of a rigid piston. We observe that the stress within the packing increases exponentially with the length of accumulated grains, and show an extension to current analytic models which fits the measured stress. The micromechanical behaviour is studied for a range of system parameters, and the limitations of existing analytic models are described. In particular, we show the smallest sized systems which can be treated using existing models. Additionally, the effects of increasing piston rate, and variations of the initial packing fraction, are described.
Highlights
When granular materials are placed in confined geometries, we often observe a significant portion of the stress being redirected toward the confining boundaries
This paper is an investigation into the micromechanics of a system which is highly constrained by external boundaries
For x ≤ L we find that the major principal stress is collinear with the system geometry, and the Janssen stress model fits well
Summary
When granular materials are placed in confined geometries, we often observe a significant portion of the stress being redirected toward the confining boundaries. Granular material is compacted near the piston, and with increasing displacement of the piston, the size of the packing increases Such an accumulation process is known to occur in the petroleum industry, where sand is liberated from the host rock during extraction, altering the underground morphology of cracks [5, 6]. This may be relevant for understanding proppant flowback in propped fractures [7]. This geometry is representative of a number of recent experimental studies in Hele-Shaw cells [8,9,10,11] where the validity of Janssen stress redirection has not been ascertained
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