Force chain dynamics in a quasistatic granular pile.

Area of Science:

• Physics
• Geophysics
• Materials Science

Background:

• Granular materials exhibit hazardous failures (avalanches, earthquakes) and slow creep.
• Failure mechanisms are often friction-limited, but friction dynamics in granular systems are complex.
• Understanding granular dynamics is crucial for predicting natural hazards.

Purpose of the Study:

• To investigate granular controls on failure and creep dynamics.
• To image and analyze contact forces and force chain evolution in granular materials over extended periods.
• To explore the relationship between force chain changes, material aging, and failure events.

Main Methods:

• Utilized quasi-2D photoelastic disks to image inter-particle contact forces.
• Conducted long-term experiments (weeks to a month) with imaging at 15-min intervals.
• Analyzed force chain network evolution, event frequency, and correlation with environmental factors like temperature and ground motion.

Main Results:

• Force chain networks formed along principal stress directions.
• Discrete force chain shifts followed an age-weakening Weibull distribution initially, with deviations during quiescent periods.
• Steeper slopes increased failure likelihood, and events correlated with localized stress strengthening.
• Temperature changes showed potential links to events, but seismic ground motions did not correlate with particle or force chain changes.

Conclusions:

• Local, grain-scale force chain dynamics persist long after apparent mesoscale stability.
• These dynamics can occur without obvious external triggers, suggesting intrinsic mechanisms.
• Force chain evolution may explain transitions between large-scale failures and creep in natural granular systems.