Ultrasonic wave monitoring methods are among the most promising proven methods in investigating the shear failure of rock discontinuities (fractures or joints). In this study, ultrasonic waves recorded during direct shear experiments on rock joints were employed to investigate the shear failure processes. A series of direct shear experiments was performed on gypsum specimens where shear waves were transmitted through and reflected off the discontinuity as the rock was subjected to shear failure. Distinct peaks in the dominant frequency of transmitted and reflected waveforms were systematically observed prior to the shear failure of the rock and were considered as "precursors" to the shear failure. This study shows that a rock discontinuity behaves as a low pass filter that attenuates the high frequency components of the signal. Such attenuation depends on the stiffness of the rock discontinuity which can be affected by the applied shear stress and the damage at the asperity contact points.
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