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Effect of volume fraction on granular avalanche dynamics
Nick Gravish1, Daniel I Goldman1
1School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|October 15, 2014
Summary
Granular slope stability depends on initial volume fraction. Below the critical state, compaction precedes avalanches; above it, dilation occurs first, impacting flow dynamics.
Area of Science:
- Physics
- Geophysics
- Materials Science
Background:
- Granular materials exhibit complex behaviors under stress.
- Understanding granular slope stability is crucial for various applications, from civil engineering to planetary science.
- The role of initial packing density in granular flow dynamics remains an active area of research.
Purpose of the Study:
- To investigate the influence of initial volume fraction on the evolution and failure mechanisms of granular slopes.
- To differentiate the flow dynamics of granular materials prepared below and above the critical state.
- To quantify the critical angles and flow characteristics associated with granular avalanches.
Main Methods:
- Prepared granular slopes with varying initial volume fractions (ϕ(0)) using 0.3-mm glass spheres.
- Tilted slopes to 45° while monitoring grain motion with high-speed video cameras from side and top views.
- Defined the granular critical state (ϕ(c)) as the onset of dilation with increasing volume fraction.
- Measured initial angles (θ(0)), maximal avalanche angles (θ(m)), and resting angles (θ(R)).
- Analyzed the velocity fields of granular flow.
Main Results:
- For ϕ(0) < ϕ(c), slopes showed precursor compaction events before avalanches at θ(m)=28.5±1.0°.
- For ϕ(0) > ϕ(c), slopes exhibited dilational motion before avalanches at θ(m)=35.9±0.7°.
- Both θ(0) and θ(m) increased with ϕ(0), approaching random close packing values.
- The resting angle θ(R)=22±2° was independent of ϕ(0).
- Flow depth and duration were affected by ϕ(0), with precursor flow being deeper and faster for ϕ(0) < ϕ(c).
Conclusions:
- Initial volume fraction is a critical determinant of granular slope stability and avalanche onset.
- The pre-avalanche behavior (compaction vs. dilation) is strongly dependent on whether the initial state is below or above the granular critical state.
- These findings provide insights into the fundamental mechanics governing granular avalanches and material failure.
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