Linking precursory fracture damage to heterogeneity and anisotropy in rock masses: experimental insights

Abstract

Crustal rock fractures may be formed from a wide range of both natural and engineering processes. Resources are distributed at depth within natural rock porosity (fractures) and may be extracted during mining, oil and gas recovery and heated fluid extraction (to generate geothermal energy). In order to extract these resources, it is necessary to generate new fractures and remove rocks using engineering methods and structures in the form of mine access tunnels, pits, paths and boreholes. Determining the amount, style and form of new fractures is complicated by both heterogeneities and anisotropy in rocks and anisotropic stresses. There is still a lack of understanding in the relation between anisotropic rock physical properties and the development of rock fracture. Here we introduce new experimental insights into how anisotropic rock physical properties alter the stress field and development of fracture damage preceding failure. The results are useful in determining new, physics based, failure criteria for slope and subsurface excavation stability. We present results from a series of indirect tensile, uniaxial compression and triaxial compression loading tests where stress, strain and acoustic emission (micro-seismicity) were monitored simultaneously. Rocks were sampled from the mining districts of El Teniente, in the O’Higgins region, and Tiltil, in the Metropolitan Region, since the Tiltil area hosts several tens of small and medium-sized mines, is easily accessible, and offers exposure to multiple types of country rocks with varying degrees of alteration, fracture damage and fracture anisotropy.