The Role of Cohesion in Granular Fault Gouges on the initiation of
Sliding: Slip-weakening Mechanisms and Energy Budget
Abstract
Fault zone usually presents a granular gouge, coming from the wear
material of previous slips. Considering a mature fault gouge with
mineral cementation between particles, we aim to understand the
influence of these cohesive links on slip mechanisms. As cohesion is
difficult to follow and to quantify with Lab or in-situ experiments, we
choose to use Discrete Element Method that has already shown its ability
to represent granular gouges with relevant kinematics and rheology. In
this work, we consider a dry cohesive contact model in 2D (2x20mm²)
involving two rough surfaces representing the rock walls separated by
the granular gouge (5000 particles, grain size 27- 260 μm). A step
forward compared to literature is to add cohesion on real angular and
faceted grains that modifies contact between particles. Focusing on
physics of contacts inside the granular gouge, we explore contact
interactions and friction coefficient between the different bodies. To
represent the cementation we set up a Bonded Mohr-Coulomb law,
considering that inter-particular bridges and particles are made with
the same material. This numerical model is displacement-driven and is
implemented to study the peak of static friction (shape, slope,
duration) under a confined pressure of 40 MPa. Depending on the
compacity and on the cohesion level of each model, the peak strength may
be sharp, short, and intense for dense and highly cohesive cases or
smooth, delayed with moderate amplitude for mid-dense and moderately
cohesive cases. Three main behaviors are observed: a non-cohesive regime
where the added cohesion is too small to truly disturb the global slip
mechanism (Couette flow), an intermediate cohesive regime with clusters
of cohesive grains, changing the granular flow and acting on slip
weakening mechanisms (Riedel shear band R1) and an ultra-cohesive regime
where gouge behaves as a brittle material with several Riedel shear
bands emergence. We also investigate the role of cohesive bonds in
energy budget, focusing on fracture energy term. Three mechanisms are
playing a role in fracture energy evolution: the rupture of cohesive
bonds, the dilatancy of the gouge and Coulomb dissipations due to
friction. These factors are linked to the initial percentage of cohesion
inside the sample and help characterize the mechanisms at stake in the
initiation of sliding.