We have attempted to monitor the progress of breakage in a rock mass by using an acoustic method. For this purpose, experimental studies had been performed both in laboratory and in situ. The changes in both the propagation velocity and the attenuation of acoustic wave with the progress of breakage in rocks were observed in both experiments. The observations show that the change in attenuation caused by the existence of cracks is more sensitive than that in the velocity. Finally, we concluded that the attenuation measurement is a useful tool for inspecting the stability of rock structures.
Nous avons essaye de detecter le progrès de la destruction dans la roche par une methode acoustique utilisant l'onde elastique. Dans ce but nous avons fait des experiences dans le laboratoire et in situ. Dans les deux experiences, il a ete observe que la velocite de la propagation ainsi que l'affaiblissement de l'onde elastique changent tous les deux avec le progrès de la destruction dans la roche. D'après le resultat des experiences, le changement de l'affaiblissement est plus sensible à l'existence de fentes que celui de la velocite de la propagation. Nous concluons donc que le measurement le l'affaiblissement est un procede utile pour detecter la stabilite de la structure de roche.
Zur Überwachung des Verlaufs des Bruchs in Felsen wurde der Versuch nach einem akustischen Verfahren durchgefuhrt. Dafuer wurden Experimente sowohl im Labor als auch an Ort und Stelle angestellt. Daraus ergibt sich, dass sich die Wellengeshwindigkeit und die Schwachung im Verlauf des Bruchs im Felsen verandern. Die Ergebnisse zeigen, dass die Änderung des von der Existenz der Risse verursachten Schwachung empfindlicher als die der Geschwindigkeit ist. Aus obigen Versuchen kann gefolgert werden, dass die Vermessung der Schwachung ein wirksames Mittel ist, die Stabilitat des Felsens zu pruefen.
It is commonly known that the propagation velocity of the acoustic wave decreases with the progress of breakage in rocks. Several investigators have detected that there is the changes in acoustic wave velocity when rock specimens are loaded in laboratory up to the failure under triaxial as well as uniaxial compression (Matsushima, 1960; Gupta, 1973). To know the stress condition in a rock mass, in situ measurement of acoustic wave velocity had often been made (Gladwin, et al., 1974). However, the detection of the change in acoustic wave velocity is not always easy in situ, therefore, a more simple system is required for this purpose. We noted that the signal from the acoustic wave propagating in rocks gives us at least two informations, one is the wave velocity and the other is the attenuation of the wave. At first, two informations shown above were monitored in laboratory for several rock specimens which were loaded uniaxially by using a stiff loading machine up to the failure. Considering these results, we designed an in situ measurement system of the amplitude in order to monitor the change in attenuation of the acoustic wave propagated through a rock mass. In situ tests had been made at a mine using that system and the results showed that the system might be useful. Thus, the motivation of our study to develop a simple measurement system which can be easily handled by anyone for monitoring the stability of rock structures is accomplished.
Uniaxial compression tests were conducted by using a closed loop servo-controlled testing machine. Simultaneous monitoring of attenuation and propagation velocity of the longitudinal wave, AE (Acoustic Emission) count rates and the strains were made throughout the test.
Five kinds of rocks were selected for this study. Only, Akiyoshi marble (a coarse-grained marble) was less brittle than the others at room condition. And those were very compact. These samples were cut in the form of rectangular prism of about 6x6x12cm and were dried in a desiccator before exper1ment.. physical properties of rocks studied are listed in Table 1.
Attenuation and velocity of longitudinal wave propagating perpendicular to the loading axis are measured by using the ultrasonic pulse transmission technique. The experimental setup used 1S shown schemat1cal1y in Fig.l. The cubic type piezoelectric transducers (PZT-7) which had a resonant frequency of 390kHz were cemented on two opposite faces of the specimen by CN-adhesive and were used as a transmitter and a receiver. At each stress level a square pulse was applied from the pulse generator to the transmitter and also to the transient recorder for triggering. The signal from the receiver was amplified and fed into the transient recorder. The delay time between the source and the received signals and the amplitude of the first peak in the received signal were recorded. The shape of these first peaks was stable throughout the experiment.
