Rock avalanche deposits store quantitative evidence on internal shear during runout

• Published in: Geophysical research letters Vol. 44; no. 17; pp. 8814 - 8821
• Main Authors: Zhang, M., McSaveney, M. J.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-09-2017
• Subjects: Avalanches; Breakage; Graders; Grading; grain fragmentation; Grain size; Interactions; Mechanical stimuli; normal stress; Particle size; ring‐shear test; rock avalanche; Rocks; Shear; shear rate; Shear strain; Shearing
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL073774

We investigated the quantitative effect of internal shear on grain breakage during rock avalanche runout, by means of 38 ring‐shear experiments on identical sand samples at different normal stresses, shear strains and shear strain rates. We compared sample grain‐size characteristics before and after shearing. We found that grain size decreased with increase in normal stress and shear strain. Reduction in grain size was inferred to occur through grain breakage associated with grain interactions in strong force chains during strain. The results were consistent with observations of both inverse‐grading structure in deep rock avalanche exposures, and fining and grading of particles with increasing rock avalanche travel distance. Our study suggested that with appropriate calibration, variations in grain‐size distributions within a rock avalanche deposit would provide quantitative information on the distribution of internal shear during its runout.
Key Points
Internal shear of granular debris during rock avalanche runout was simulated in a simple ring‐shear device
Normal stress and shear strain were key factors controlling grain‐size reduction in the simulation
Grain‐size variations in rock avalanche deposits hold quantitative information about internal shear during runout