Creep motion in a granular pile exhibiting steady surface flow

Area of Science:

• Physics
• Geophysics
• Materials Science

Background:

• Granular materials are ubiquitous in nature and industry.
• Previous models assumed a static, "frozen" bulk beneath the flowing surface of granular piles.
• This assumption lacked experimental verification for deep bulk regions.

Purpose of the Study:

• To experimentally investigate the flow behavior in the bulk of granular piles.
• To determine if a "frozen" bulk region truly exists below the surface flow.
• To characterize the nature and depth dependence of particle motion in the granular bulk.

Main Methods:

• Experimental investigation of granular piles under steady surface flow conditions.
• Measurement of particle motion at various depths within the granular bulk.
• Quantitative analysis of velocity decay with depth.

Main Results:

• No evidence of a completely "frozen" bulk region was found.
• Particles at all depths, even deep within the bulk, exhibit slow creep motion.
• The mean velocity of this creep motion decays exponentially with depth.
• The characteristic decay length is approximately equal to the particle size and independent of flow rate.

Conclusions:

• The "frozen bulk" model for granular piles is inaccurate.
• Slow creep motion is a fundamental characteristic of deep granular systems.
• This phenomenon is expected to be present in all sheared granular systems, with implications for geophysics and material handling.