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Property determination and wave propagation in a block of Barre granite

COLLEGE OF ENGINEERING UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA 94720

Abstract

The principal axes of a 666.8 by 609.6 by 489.0 mm ( in by 24 in by in) block of Barre granite, treated as an orthotropic elastic material were determined from measured pulse velocities along directions connecting 160 pairs of surface points, encompassing the entire spectrum of possible orientations. The elastic moduli of the rock were ascertained by Hopkinson bar tests involving rods cored from other samples along their principal directions; this was required for the execution of a wave-propagation analysis in the block treated as a half-space.

Construction and insertion techniques were developed for transducers to be embedded in the rock at 14 locations. External and internal calibration procedures were devised to permit interpretation of the data transmitted from the interior of the sample. Transients in the block were generated by the impact of 6.35-mm ( in) diameter steel spheres on loading bars sandwiching a thin quartz disk, serving as an input transducer, against the specimen. The wave patterns sensed by the transducers were displayed and photographed on oscillographic screens.

A finite element program capable of handling arbitrary anisotropy was developed and employed for comparing the experimental results with analytical predictions based on the measured input as the boundary condition. For those stations where computations were performed, the correlation ranged from good to qualitative. It is concluded that better transducer embedment and in situ calibration techniques are required for internal transducers used in hard rocks of this type.