Laboratory investigation of friction along rock joints and identification of peaks in shear stiffness prior to the joint’s shear failure
Ahmadreza Hedayat, & John HintonPublished August 13, 2016, SCEC Contribution #6730, 2016 SCEC Annual Meeting Poster #028
Laboratory studies of friction along synthetic and natural fault gauge materials have been instrumental in providing a better understanding of the fault deformation mechanisms and when combined with geophysical monitoring techniques, have been very successful in assessing the real (true) contact area. The amplitude and velocity of the waves recorded continuously during friction experiments provide new insights into these dynamic processes occurring along faults.
This study involved performing direct shear experiments on gypsum and Indiana limestone rock joints with simultaneous monitoring of the ultrasonic waves propagated through the rock joints. The specimens were loaded in a direct shear apparatus in which the normal stress across the joint was applied first and then the shear load was increased with a constant displacement rate until shear failure occurred. Two arrays, each with 13 embedded ultrasonic transducers, were placed on the sides of the rock joint during the direct shear testing and the investigation of failure was carried out by acquiring transmitted and reflected shear waves during the experiment.
The contact area and its evolution during friction is a key factor in frictional sliding, and ultrasonic waves provided an estimate of the fracture (fault) stiffness which is affected by the true contact area. At the interfaces between the grains, the ultrasonic wave transmits through the contact points and reflects at air gaps. Thus, the intensity of the wave transmitted through the rock joint is a measure of the contact area and the elastic stiffness of the contacting surfaces. In this study, the fracture shear stiffness is calculated based on the ratio of the transmitted to reflected ultrasonic waves. The fracture shear stiffness measured at different areas along the joint showed an increased with the application of the shear load and reached a maximum (peak) prior to reaching the maximum shear strength of the rock joint. Peaks in the fracture shear stiffness were systematically observed for different areas of the rock joint and were found to be related to the local areas of failure (damage) along the rock joint. Identification of peaks in the fracture shear stiffness prior to the failure of the rock joint was the major finding of this study and this poster provides a summary of the research approach and findings.
Key Words
Rock joint, direct shear experiment, ultrasonic waves, shear failure, peaks in shear stiffness
Citation
Hedayat, A., & Hinton, J. (2016, 08). Laboratory investigation of friction along rock joints and identification of peaks in shear stiffness prior to the joint’s shear failure . Poster Presentation at 2016 SCEC Annual Meeting.
Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)
