Relaxation Response of Critically Stressed Macroscale Surficial Rock Sheets
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Rock environments both underground and on Earth’s surface show indications of energetic macroscale fracture. In tunnels and excavations, these manifest as rockbursts—energetic explosions of rock that can damage engineering projects, and may pose ongoing financial and safety risk as rock stresses adjust during post-failure relaxation. In natural settings at the surface, evidence for rockbursts exist in the form of tent-like structures of ruptured exfoliation sheets, but few direct observations of such events exist, precluding the analysis of how natural rock formations may evolve after rupture. Here we investigate the post-failure evolution of a granitic rock dome following rapid fracture events (i.e., surficial rockbursts) that occurred in California, USA during 2014–2016. Building upon previous work that showed a thermal stress origin for the observed fracturing, we investigate the return to background stress conditions (i.e., stress relaxation) observed in both short- (week, month) and long-term (multi-year) rock deformation trends. Acoustic emissions, deformation, and environmental monitoring data indicate that partially detached rock sheets forming the surface of the dome undergo fracture aperture closing during cooling periods, concurrent with reduction of rock stress by the source of forcing (i.e., thermal stress). However, with sufficient critical and/or subcritical fracture, our observations also show that rock sheets can become decoupled from the source of stress, resulting in a long-term return to background stress conditions. Our results provide insight into the cyclic and likely ephemeral nature of rock fracture in surficial rock domes, as well as in underground rockburst environments.
KeywordsExfoliation Fracture Rockburst Subcritical Thermal Stress Twain Harte California Granite
We thank Dennis Wykoff and Lauren Gerber, Twain Harte Lake Directors for their assistance with our research efforts. We also acknowledge the contributions of our colleagues S. Lewis of Condor Earth Technologies, and S. Corbett and J. Smith (both U.S. Geological Survey; USGS) who worked with us on other parts of this project, and to R. Putnam (Moorpark College) and H. Wood (Condor Earth) who assisted with installation of instrumentation. Primary funding for this project was provided by the USGS Landslide Hazards Program. Acoustic emissions instrumentation was acquired by M. Eppes for a prior project through funding by the National Science Foundation EAR-0844335. We thank J. Perkins (USGS), R. Schulz (ARMA), and an anonymous reviewer for providing helpful feedback on earlier versions of this work. The data analyzed during the current study are available in the supplemental material accompanying this paper and/or from the corresponding author on reasonable request. Some of the work shown herein was originally presented as part of the 2018 Annual Meeting of the American Rock Mechanics Association held in Seattle, Washington in June 2018. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.
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Conflict of interest
The authors declare that they have no conflict of interest with the research presented herein.
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