Geological implication of grain-size segregation in dense granular matter

Abstract

To the current common belief, grain size segregation in granular matter requires sufficient porosity. Therefore, grain size segregation found in a natural fault gouge could imply elevated fluid pressure and the reduced normal stress on fault, possibly caused by the frictional heat during an earthquake. To clarify whether fluidization is essential to grain size segregation, we conduct numerical simulation on a simple model of fault gouge in a plane shear geometry under constant volume condition: the volume fraction is fixed at 0.6, at which the granular system possesses yield stress. We observe apparent grain size segregation at this volume fraction, meaning that grain size segregation alone does not imply fluidization of granular matter. We also show that segregation is driven by the nonlinear velocity profile, and that the gravity is not essential to segregation. The physical condition tested here may be relevant to earthquake faults: the normal stress of 1 MPa, the sliding velocity of 1 m s−1, and the duration of 0.1 s.This article is part of the theme issue ‘Statistical physics of fracture and earthquakes’.

Particle Size Segregation in Granular Mass Flows With Different Ambient Fluids

“…A similar correlation is observed in our results (Figure 12b). It is also consistent with previous results that the segregation rate correlates with the velocity difference between layers (Fan & Hill, 2011a, 2011bItoh & Hatano, 2019) and that large particles in chute flows are segregated toward the "cooler" regions (near the free surface) of the flow (Dahl & Hrenya, 2004;Staron & Phillips, 2014).…”

A unified description of gravity- and kinematics-induced segregation forces in dense granular flows

“…In the absence of gravity (e.g. lateral segregation in vertical silo flows), large particles tend to migrate toward high shear rate regions (Fan & Hill 2011;Itoh & Hatano 2019), but the tendency reverses when the flow becomes dilute (Fan & Hill 2011). Although different mechanisms including geometric effects (Savage & Lun 1988), mass effects (Félix & Thomas 2004) and shear gradient dependence (Fan & Hill 2011) have been proposed, a unified picture remains elusive.…”

Statistical physics of fracture and earthquakes

“…Such an effect requires a significant porosity of the system and gravity. In [35], however, the authors show that neither fluidization nor gravity is essential for the segregation effect. In their simulation of a dense bi-disperse system, the authors show that segregation occurs both with and without gravity, and that the relative positions of larger and smaller grains are interchanged in the presence and absence of gravity.…”