Frictional behavior and micro-damage characteristics of rough granite fractures

Abstract

<p>Natural faults or discontinuities in general are usually rough at different scales, and the roughness of fault surfaces plays a dominant role in determining the mode of sliding (stable or unstable). Understanding the frictional behavior of rough fractures is important for investigating the mechanics and nucleation of earthquakes, and other dynamic geo-hazards such as <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/induced-seismicity" title="Learn more about induced seismicity from ScienceDirect's AI-generated Topic Pages">induced seismicity</a> and fault slip <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/rockburst" title="Learn more about rockburst from ScienceDirect's AI-generated Topic Pages">rockburst</a>. We perform direct shear tests on rough granite fractures to investigate the shear behavior, microscopic asperity damage and surface wear characteristics under different normal stresses (10 to 40 MPa). We then compare our test results with those obtained using ground saw-cut fault surfaces. Results indicate that stick-slip occurs on all the rough fractures during the sliding stage, and the amount of stress drops and <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/fault-gouge" title="Learn more about fault gouges from ScienceDirect's AI-generated Topic Pages">fault gouges</a> tends to increase with normal stress. The peak <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/coefficient-of-friction" title="Learn more about friction coefficient from ScienceDirect's AI-generated Topic Pages">friction coefficient</a> is much higher than that of ground fault surfaces, while the steady-state <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/coefficient-of-friction" title="Learn more about friction coefficient from ScienceDirect's AI-generated Topic Pages">friction coefficient</a> is comparable with that of the ground surfaces. We identify three different scales of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/brittle-fracture" title="Learn more about brittle fractures from ScienceDirect's AI-generated Topic Pages">brittle fractures</a>, which are considered to be closely related to the stick-slip sliding, namely shear-off of asperities, fracturing of the survived asperities and off-fault tensile fractures propagating into the host rock. The micro-damage of the post-shear surface occurs in the form of powder-sized gouge, micro-cracks and grain edge wear; and the micro-crack number, maximum length and aperture tend to increase with increasing normal stress. Influence of different fault types (rough fractures, ground saw-cut surfaces and gouges) on the stress drop is not very discernable, which is partly because that stick-slip is highly susceptible to different loading conditions. Friction drops for different fault types are found to be clustered around 0.1 but lower than 0.3 below 50 MPa normal stress, which also depend on the loading conditons (e.g., stiffness). Our study contributes to a better understanding of the mechanics underlying the dynamic geohazards (earthquakes, <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/induced-seismicity" title="Learn more about induced seismicity from ScienceDirect's AI-generated Topic Pages">induced seismicity</a>, fault slip rockburst) which are associated with the unstable shear failure on rough faults or fracture surfaces.<br></p>