Auto‐acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration

Abstract

[1] Granular shear flows are intrinsic to many geophysical processes, ranging from landslides and debris flows to earthquake rupture on gouge-filled faults. The rheology of a granular flow depends strongly on the boundary conditions and shear rate. Earthquake rupture involves a transition from quasi-static to rapid shear rates. Understanding the processes controlling the transitional rheology is potentially crucial for understanding the rupture process and the coseismic strength of faults. Here we explore the transition experimentally using a commercial torsional rheometer. We measure the thickness of a steady shear flow at velocities between 10 À3 and 10 2 cm/s, at very low normal stress (7 kPa), and observe that thickness is reduced at intermediate velocities (0.1-10 cm/s) for angular particles, but not for smooth glass beads. The maximum reduction in thickness is on the order of 10% of the active shear zone thickness, and scales with the amplitude of shear-generated acoustic vibration. By examining the response to externally applied vibration, we show that the thinning reflects a feedback between internally generated acoustic vibration and granular rheology. We link this phenomenon to acoustic compaction of a dilated granular medium, and formulate an empirical model for the steady state thickness of a shear-zone in which shear-induced dilatation is balanced by a newly identified mechanism we call auto-acoustic compaction. This mechanism is activated when the acoustic pressure is on the order of the confining pressure, and results in a velocity-weakening granular flow regime at shear rates four orders of magnitude below those previously associated with the transition out of quasi-static granular flow. Although the micromechanics of granular deformation may change with greater normal stress, auto-acoustic compaction should influence the rheology of angular fault gouge at higher stresses, as long as the gouge has nonzero porosity during shear.Citation: van der Elst, N. J., E. E. Brodsky, P.-Y. Le Bas, and P. A. Johnson (2012), Auto-acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration,

Stick-slip instabilities in sheared granular flow: The role of friction and acoustic vibrations

“…The quantity ρ should be a function of the vibrational frequency and amplitude; past studies [14,15,41] suggest that ρ is proportional to the vibrational amplitude, and the square of the frequency. However, the detailed dependence is unimportant for the purposes of this paper which, as in the van der Elst et al experiments [31], focuses on one single amplitude and frequency, unless otherwise specified.…”

“…The basic idea is that the coupling K between the configurational and kinetic-vibrational subsystems consists of additive contributions of the fluctuations Γ induced by shearing, ξ induced by interparticle friction, and ρ induced by external vibrations (when switched on). In our analysis [30] of a series of experiments by van der Elst et al [31], these are referred to as mechanical, frictional, and vibrational noise strengths. The effect of ξ and ρ on granular rheology is clearly illustrated there.…”

Frictional heating lubrication for submarine landslide

“…9, 10, 11 therein). Recently, carrying out an experiment of granular shear flow, Van Der Elst et al (2012) also discovered a "strain-localization-zone contracting" phenomenon that is caused by a granular rheology, not by the "strain-softening" behaviour. In earlier research (Mül-haus and Vardoulakis 1987), the strain localization zone observed for sand had a thickness on the order of tens times the mean particle size, the thickness probably ranging between millimeters and centimeters.…”

Synchronized oscillations and acoustic fluidization in confined granular materials

“…In this way seismic failure is promoted. To investigate this scenario, experimental studies [1,2,6,7] have demonstrated that acoustic perturbations modify granular rheology and lead to auto-acoustic compaction [7]. Recently the AF scenario has been explored in 3D molecular dynamics simulations [23] which have shown that weak external perturbations, at the frequency ω AF , even if increasing the confining pressure or reducing the applied shear, induce slip instabilities.…”

“…Confined granular materials under shear display the typical stick-slip dynamics observed in real fault systems. In the last years this dynamics has been deeply investigated in several experimental settings as well as by means of molecular dynamics simulations [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These studies mostly focus on two central questions: i) Why the stress responsible for seismic failure is usually orders of magnitude smaller than the value expected on the basis of rock fracture mechanics?…”