Discontinuity detection ahead of a tunnel face utilizing ultrasonic reflection: Laboratory scale application

Predicting ground conditions ahead of the tunnel face has been one of the most important requirements of tunnel construction. This study investigates the development and application of a high resolution ultrasonic wave imaging system, which captures the multiple reflections of ultrasonic waves at the interface, to detect discontinuities at laboratory scale rock mass model. Ultrasonic wave reflection imaging based on A- and B-modes is obtained through stacking, signal compensation, demodulation, and display. Experiments are carried out by using horizontal scanning and new rotational scanning devices. Experimental studies show that the rotational devices are able to identify horizontal and inclined discontinuities and the cavity on the plaster block at a fixed location. Furthermore, two discontinuities including horizontal and inclined discontinuity planes are detected. The rotating scanning technique produces images similar to those obtained by the typical horizontal scanning technique. This study suggests that the new rotational technique can be an economical and effective tool for the detection of discontinuity on a rock mass for the investigation of the ground condition in front of the tunnel face.