Microstructural analysis of time-dependent cracking in shale

This paper analyses crack patterns in argillaceous rocks induced by time-dependent mechanical loadings. To achieve this, thin sections were extracted from specimens after strain-rate-controlled compression tests and creep tests. The microstructural observations, made using an optical microscope and a scanning electron microscope, highlighted the fact that the crack patterns and failure modes of the specimens are dependent on the rate of loading. For slowloading rates, the macroscopic deformations appear to be mainly due to the development of a sub-vertical network of persistent macrocracks in the argillaceous matrix, whereas for the fast-loading tests, a second network of microcracks appears, controlled by the rock's structural anisotropy. When the specimens undergo fast loading, the observed failure is more brittle with cataclastic deformation and grain crushing in evidence within the samples. Observations of the crack width and crack orientations clearly show how the crack patterns influence rock properties such as permeability, which is a key parameter in the damage zone that develops around many geo-engineering structures.