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A Weakening Rheology of Dry Granular Flows With Extensive Brittle Grain Damage in High‐Speed Rotary Shear Experiments
Author(s) -
Hu Wei,
Chang Ching Shung,
McSaveney Mauri,
Huang Runqiu,
Xu Qiang,
Zheng Yangshuai,
Yu Jiefu
Publication year - 2020
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2020gl087763
Subject(s) - thixotropy , geology , brittleness , rheology , shear (geology) , geotechnical engineering , mechanics , shear rate , shear zone , grain size , strain rate , shear stress , shearing (physics) , petrology , materials science , seismology , geomorphology , composite material , physics , tectonics
The puzzle of the unexpectedly high mobility of large geophysical flows has been reported as solved many times since Albert Heim drew attention to it after a disastrous landslide at Elm, Switzerland. Many hypotheses have been proposed to explain the hypermobility; however, no consensus position has emerged in more than a century, and debate rages on. We show a new trend of dense granular flow behavior above average normal stress of 0.3 MPa with weakening at high strain rate, which may be explained by shear‐thinning thixotropy. Experimental results for a given shear rate at normal stresses between 0.3 and 1 MPa show a bifurcation in shear‐resistance behavior. The trend of all our data deviates obviously from the traditional rheological model. We identified three regions of dense granular flow: a static region, an inertial region, and a weakening region at increasing strain rate in grain flows that results in widespread grain breakage. The former two regions agree with the traditional rheological model established at lower normal stresses; however, instead of entering a collisional region, a weakening region appeared. For gravity flows, it is possible for unstable flows to occur for certain combinations of normal stress and shear strain rate. An underlying mechanism of grain crushing and grain crushing‐induced special grain structure may greatly reduce the shear resistance due to thixotropy. The profound weakening provides a natural explanation for the observed high mobility in such geophysical behavior as the long runout of rock avalanches, fault weakening, impact‐crater evolution, and pyroclastic flows.