ABSTRACT:

Discontinuities such as bedding planes and natural fractures reduce the strength and stiffness of the rock mass and facilitate failure. In this study we conducted initial tests on the impact of the natural fractures and bedding angle on failure and the generation of acoustic emissions (AE) of Opalinus shale during unconfined compression. Cylindrical shale specimens were prepared with their long axis at varying angles to natural fractures and bedding. First, we measured the velocity, amplitude, and energy of 100 KHz acoustic waves transmitted along the sample axis. Each of these increased strongly as the angle incident to bedding decreased from 0 to 90 degrees. These observations will help to provide useful corrections for source location, focal mechanism studies, and for calculating the radiated seismic energy. The strength of the Opalinus showed a classic “U shape” strength anisotropy as the loading direction varied. During loading, we used wideband sensors to measure acoustic emissions, which became measurable at about 80% of the peak stress. The cumulative energy of acoustic emissions during failure were greatest when s1 was normal to bedding; about 60% less when s1 || bedding; and least for intermediate angles. Based on visual observations of the cracking patterns, tensile cracks were most numerous when the greatest compressive stress was parallel to bedding, fractures in shear orientations dominated when bedding was oriented at 300, 450, 600. When the greatest compressive stress was normal to bedding, the damage had a combination of shear and tensile orientations.

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