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Title: Transmitted, Reflected, and Converted Modes of Seismic Precursors to Shear Failure of Rock Discontinuities
The failure of rock along pre-existing discontinuities is a major concern when building structures on or in rock. A goal is to develop methodologies to identify signatures of imminent shear failure along discontinuities to enable implementation of measures to prevent the collapse of a structure. Previous studies identified precursory seismic signatures of shear failure along rock discontinuities in transmitted and reflected signals. Here, laboratory direct shear experiments were conducted on idealized saw-tooth discontinuities in gypsum to determine the differences or similarities in precursors observed in transmitted, reflected and converted elastic waves. Digital Image Correlation (DIC) was used to quantify the vertical and horizontal displacements along the discontinuity during shearing to relate the location and magnitude of slip with the measured wave amplitudes. Results from the experiments showed that seismic precursors to failure appeared as maxima in the transmitted wave amplitude and conversely as minima in the reflected amplitudes. Converted waves (S to P & P to S) were also detected and their amplitudes reached a maximum prior to shear failure. DIC results showed that slip occurred first at the top of the specimen, where the load was applied, and then progressed along the joint as the shear stress increased. This more » process was consistent with the precursors i.e., precursors were first recorded near the top and later at the center and finally at the bottom of the specimen. Interestingly, precursors from reflected waves were observed first, followed by precursors from transmitted and then by converted waves. Also, the differences in time of occurrence between the three precursor modes decreased along the plane of the discontinuity. The results showed that reflected waves were the most sensitive to damage and slip along a discontinuity and that monitoring for precursors may provide a method for detecting impending failure. « less
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55th US Rock Mechanics/Geomechanics Symposium
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National Science Foundation
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