| Summary: | The purpose of this study was to quantitatively evaluate the state of rocks in load steps by using the low-frequency ultrasonic flaw detection method. The initial Vp-velocities measured with a CND tester were in the order of Z-axis < X-axis < Y-axis, with 1687.5 m/s along the X-axis, 1690.7 m/s along the Y-axis, 1548.3 m/s along the Z-axis, and an average of 1642.2 m/s. The overall average of the Q vlaues, measured with a Silver Schmidt hammer, was 62.6, which corresponds to a uniaxial compressive strength of ~105 MPa. The Vp-velocity, measured with a low-frequency ultrasonic flaw detector at load steps of 50%, 60%, 70%, and 80%, typically decreases in the order of X-axis < Y-axis < Z-axis with increasing load steps. This order contrasts with that of the initial Vp-velocities. As the load step increases the factors that reduce the Vp-velocity in the X-axis direction are more influential than those in the Y-axis or Z-axis directions. This indicates that the initial state of rocks can vary and is dependent on the stress state.
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