Laboratory investigation on slip transition of rock fractures

• Main Author: Mei, Cheng
• Other Authors: Wu Wei (CEE)
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Civil engineering::Geotechnical
• Online Access: https://hdl.handle.net/10356/155837
• Institution: Nanyang Technological University

Shear failure of rock fracture includes a wide spectrum of slip behaviors ranging from stick-slip to stable sliding, and the fracture slip near the stability transition offers new insights into the physics of rock friction. The slip transition is often explained by the ratio of the stiffness of loading system to the critical rheologic stiffness of fracture using a spring slider system. However, the theoretical analysis shows that the fracture slip transition may be governed by more controls associated with the rate-and-state constitutive friction laws. Here a series of stress-unloading friction experiments are conducted on granite and polycarbonate fractures to investigate the slip transition under controlled laboratory conditions. The results of the stress-unloading friction experiments reveal that the fracture instability can be reasonably characterized based on a spectrum of slip behaviors (e.g., stick-slip cycles, period-multiplying cycles, chaotic cycles, and stable sliding). Both polycarbonate and granite fractures exhibit fast ruptures in stick-slip cycles under a high normal stress. Period-multiplying cycles with slow and fast ruptures and chaotic cycles occur when the normal stress gradually decreases. Finally, the fractures slide in a stable manner under a very low normal stress. The source characteristics and seismic radiations near the stability transition indicate the proportional correlation among AE energy, amplitude, and duration, as well as shear stress drop, recurrence interval, and slip deficit. The period-multiplying cycles with slow and fast ruptures are used as a sensitive indicator of slip transition. First, the load-point velocity leads to the gradual variation of frictional properties, and the stiffness ratio and the ratio of frictional properties control the fracture stability and rupture dynamics. Second, the chemically treated rock experiences a severe damage on fracture surfaces, resulting in variation of rock components, gouge particles, and frictional properties. The recurrence pattern and source characteristics of slip events in the slip transition are greatly affected by the static friction coefficient and frictional properties. Last, the gouge accumulation on the granite fracture over an accruing load-point displacement leads to changes in the stiffness ratio and frictional parameters, affecting the recurrence pattern and energy release of slip events. These experimental findings confirm the impact of creep rate and gouge materials on the bifurcations near the stability transition and reveal the physics controlling of period-multiplying cycles of the Parkfield tremors following a slip-predictable sequence. Additionally, the support vector machine model trained by the AE amplitude performs better than that by the elapsed time, and the prediction accuracy presents a slight decrease with more irregular slip events in the slip transition. The AE signal in the slow rupture is more sensitive to the AE attenuation, resulting in a relatively worse performance of the model than the fast rupture. The study provides a systematic and comprehensive platform to better characterize the slip transition including period-multiplying and chaotic cycles. However, there are still unresolved problems which need further study. The ductile rheology due to high temperature is expected at natural faults in deep rock, which may pose an impact on the recurrence pattern of fault slip. The rupture nucleation process is vital to understand the physics of slow and fast ruptures, which can also be investigated in the future work.