Progressive Microscopic Damage and the Development of Macroscopic Fractures in Porous Sandstones

The precursory phenomena associated with dilatancy have been extensively studied as a potential means of earthquake prediction. It is known that microstuctural damage induced dilatancy precedes macroscopic failure of a rock. However, the quantitative relationship between microcrack damage and fault development is not clearly understood. To better understand the process of fault growth, a detailed microstructural study was conducted on porous sandstones deformed to various post-failure stages. A lateral relaxation loading configuration was adopted in which a cylindrical sample is deformed under decreasing radial stresses while the axial load remains constant. Compared to conventional triaxial deformation testing, the relaxation loading configuration greatly increases the stability of fault growth. Fault formation and propagation within Berea and Darley Dale sandstones (21% and 14% initial porosity, respectively) were examined. The effective confining pressure ranges from 80 to 200 MPa, and the failure mode of all samples are in the brittle regime. A suite of samples were deformed and subsequently unloaded at different post-failure stages, before macroscopic faulting occurs. Progressive microstructural damage was investigated. Quantitative characterization of crack damage indices, crack density, grain size distribution, and porosity reduction provides characteristic failure patterns. This study leads to an improved comprehension of the relationship between microscopic damage and macroscopic fracture development and a better insight into the brittle failure process.