Description of new dry granular materials of variable cohesion and friction coefficient: Implications for laboratory modeling of the brittle crust
Résumé
Cohesion and friction coefficient are fundamental parameters for scaling brittle deformation in laboratorymodels
of geological processes. However, they are commonly not experimental variable, whereas (1) rocks range from
cohesion-less to strongly cohesive and from low friction to high friction and (2) strata exhibit substantial
cohesion and friction contrasts. This brittle paradox implies that the effects of brittle properties on processes
involving brittle deformation cannot be tested in laboratory models. Solving this paradox requires the use of
dry granular materials of tunable and controllable brittle properties. In this paper, we describe dry mixtures of
fine-grained cohesive, high friction silica powder (SP) and low-cohesion, low friction glass microspheres (GM)
that fulfill this requirement. We systematically estimated the cohesions and friction coefficients of mixtures of
variable proportions using two independent methods: (1) a classic Hubbert-type shear box to determine the
extrapolated cohesion (C) and friction coefficient (μ), and (2) direct measurements of the tensile strength (T0)
and the height (H) of open fractures to calculate the true cohesion (C0). Themeasured values of cohesion increase
from 100 Pa for pure GM to 600 Pa for pure SP, with a sub-linear trend of the cohesion with the mixture GM
content. The two independent cohesion measurement methods, from shear tests and tension/extensional tests,
yield very similar results of extrapolated cohesion (C) and show that both are robust and can be used independently.
The measured values of friction coefficients increase from 0.5 for pure GM to 1.05 for pure SP. The use
of these granular material mixtures now allows testing (1) the effects of cohesion and friction coefficient in
homogeneous laboratory models and (2) testing the effect of brittle layering on brittle deformation, as demonstrated
by preliminary experiments. Therefore, the brittle properties become, at last, experimental variables.