Monitoring of crack propagation in anisotropic rocks during compressive strength testing using gas permeability measurements

Porosity and related fluid permeability are key parameters that influence a number of other physical and mechanical properties of rocks and therefore have an impact on their specific engineering uses. Porosity and permeability depend on the microstructure of the rock, mainly on the size distribution, shape, and mutual connectivity of the pores in the rock matter, but also on the degree of rock failure caused by the presence of microcracks. This close connection between the rock's permeability and the presence of a microcrack network in the rock means, in other words, that changes in permeability during loading can serve as a measure for monitoring crack initiation and propagation. In the presented research, biotite migmatite, a metamorphic rock with a well-developed plane-parallel structure, was used for this purpose with the aim of simultaneously assessing the influence of textural anisotropy on the process of crack formation and development. It was found that an increase in gas permeability due to the formation of microcracks occurs when ca. 60 to 90 % of the maximum loading force is achieved, mostly depending on the spatial relationship between the orientation of anisotropy planes and the direction of the applied loading force.