IN Part I of this pa pert, Professor Morrison suggested that our technique of causing cracks to develop around openings in a sheet of photoe1asüc plastic is, in effect, a laboratory-scale rockburst. Our investigations have indicated that the location and direction of these cracks are predictable, and that their growth can be controlled. On several occasions, we have been able to cause these cracks to propagate sufficiently slowly to be followed , with the naked eye. In practice, the rate of growth, gradual or sudden, would depend upon rock properties.
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