Acoustic emission and ultrasonic velocity study of excavation-induced microcrack damage at the underground research laboratory

Excavation-induced microcrack damage was investigated in the sidewall of a 420-m-deep tunnel at the Underground Research Laboratory. Sixteen channels of 1.0 MHz, whole-waveform acoustic emission and ultrasonic velocity data were collected over a 3.5 week period, during which the tunnel was extended by two half-metre increments. Acoustic emission locations were concentrated near the sidewall free surface. Shear source mechanisms predominated. Compressional velocities were strongly anisotropic, with the maximum velocity parallel and minimum velocity orthogonal to the tunnel. Both compressional and shear velocities rose with depth into the tunnel wall.