Abstract
We investigate the origin of fast shear strength healing induced by mechanical perturbations during slide-release-slide (SRS) experiments using a ring shear apparatus (ACSA, Navier/CERMES, Ecole des Ponts ParisTech, France). A 100-mm-thick annular sample of siliceous sand (0.6 mm mean diameter) is submitted to shear by the mean of a rotating cylinder in a semi-Couette geometry. We explore the role of shear stress perturbations related to small reverse offsets of the loading interface. We show that controlled releases of the shear stress induce shear strength increases when resuming shear load (i.e. the Tightening-up effect of unloading or Tu-effect). However, a threshold of the shear stress perturbation amplitude to get a significant restrengthening is observed. The shear strength increase is shown to be logarithmically related to the amount of imposed reverse offset and linearly to the induced volumetric strain. These results suggest that small perturbations of the contact status (i.e. inelastic strain) in the granular assembly of the gouge interface, have a major influence on the fault restrengthening.
Highlights
Mechanical responses of faults are generally described by friction constitutive laws which are classically expressed in terms of slip, sliding rate and state of shear zone (Scholz 1992)
Classical rate and state friction (RSF) laws account for the effect of stationary contact conditions (Karner & Marone 1998), as shown by laboratory experiments where the friction strength of simulated faults increases with time of contact or with decreasing slip rate (Dieterich 1979a; Beeler et al 1994)
Using doubledirect shear apparatus with bare surfaces, Nakatani & Mochizuki (1996) found a composed effect: a time-dependent increase in static friction and a time-independent increase in dynamic friction caused by lowering shear stress; the latter was called the ‘Tightening-up effect of unloading (Tu effect)’
Summary
Mechanical responses of faults are generally described by friction constitutive laws which are classically expressed in terms of slip, sliding rate and state of shear zone (Scholz 1992). Using doubledirect shear apparatus with bare surfaces, Nakatani & Mochizuki (1996) found a composed effect: a time-dependent increase in static friction and a time-independent increase in dynamic friction caused by lowering shear stress; the latter was called the ‘Tightening-up effect of unloading (Tu effect)’ They underlined that this effect is more important in magnitude than time- and slip-rate-dependent effects described by RSF laws. Karner & Marone (2001) brought out the significant difference between shearing within granular materials and shearing of bare surfaces They investigate under a wider range the effects of the shear load, hold time, loading rate and initial layer thickness on frictional healing using double-direct shear
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