Studies on the formation of faults from En-echelon fractures in carrara marble
Date of Issue2016-03-31
School of Civil and Environmental Engineering
As a special set of fractures of regular orientation and spacing, en-echelon fractures widely exist in nature. It is worthwhile to understand the interaction among them, and how this interaction finally contributes to a fault. Although relevant studies have acquired instructive results for the interaction of en-echelon flaws, further work is necessary to enhance the understanding about the evolution of en-echelon fractures into a fault in laboratory, especially for brittle natural rocks. In this study, uniaxial compression test is conducted on marble specimens containing en-echelon flaws. The fracturing processes of these specimens leading to the formation of a fault have been generalized. Linking cracks of tensile and shear properties show different development styles at the macro-scale. The flaw-array angle dominates the linking patterns between flaws, geometry of tensile wing cracks (TWCs) developed near the outer flaw tips and the specimen peak stress. Secondly, two series of specimens containing en-echelon flaws are subjected to different loading levels to produce progressive damage zones (linking cracks). After that, microscopic observations have been conducted on these damage zones (linking cracks) to investigate the microscopic characteristics of tensile and shear cracks. Analysis on microcrack density reveals the distinct development of grain boundary cracks and transgranular cracks in tensile and shear cracks. Additionally, abundant cases about the nucleation of transgranular cracks have been examined carefully. Four nucleation mechanisms of transgranular cracks in Carrara marble are thus identified. Finally, a recently reported contact model—flat-joint contact model is applied to the particle flow code (PFC2D) simulation to model the present laboratory experiments. The numerical results provide a quantitative assessment on the interaction of en-echelon flaws. Furthermore, cracking behavior in these synthetic specimens are found to agree well with experimental results obtained in both microscopic and macroscopic observations. This agreement suggests not only the reliability of experimental results but also the capability of the flat-joint contact model of PFC2D in modeling rock fracturing problems.