The authors have been conducting research using AE for estimating initial stress and predicting failure of rock masses. It is suggested that there exists a time dependence in the Kaiser effect of AE, and in case of estimating initial stress using the Kaiser effect, it is necessary to perform testing as soon as possible after sampling rock in-situ. It was also found that there are great differences between rock types in AE frequency distribution and m-value distributions measured in processes up to failure, and this would be an effective means of predicting failure.
Acoustic Emission (hereinafter abbreviated to "AE") has been drawing attention in the field of civil engineering in recent years and many kinds of investigations have been made. Hardy (1981) put together a study of AE concerning rock masses. AE, which is called "Microseismic Activity" in the field of mining, is the phenomenon of a part of the energy stored inside a substance being released and propagated taking the form of elastic wave motion. This is a phenomenon which is generally observed in practically all solid materials in cluding metals. ceramics, concrete, glass, and ice. Ordinarily, the objects of investigation in AE research are faint signal waves in the inaudible range that cannot be heard as sound. When constructing underground structures such as tunnels and powerhouses, it would be possible for rational designs to be made if the initial stress of the ground were to be known. Consequently, numerous methods of estimating initial stress were devised in the past. At present, however, only the stress relief method (Merrill et al.. 1961) and the hydraulic fracturing method (Mizuta et al.. 1983) from among those methods are frequently used. A phenomenon called Kaiser effect (Kaiser, 1953) has been confirmed to exist in AE. This is a condition where in case a material to which a load had been applied in the past is reloaded, hardly any AE occurs up to the maximum stress produced by the load applied in the past. Goodman (1963) was the first to distinguish the Kaiser" effect in rock, following which studies concerning the Kaiser effect in rock were made in many Quarters (Yoshikawa et al. 1981). Hayashi et al. (1979) were the first to apply the Kaiser effect in estimating initial stress of a rock mass. The authors cited the various problems involved in estimating initial stresses of rock mass using the Kaiser effect and have carried out work to resolve these problems (1984, 1985, 1986). Meanwhile, since AE occurs accompanying formation of minute cracks and release of energy, it is conceivable to apply it in prediction of failure. Actually, in the field of seismology, the earthquake mechanism has been explained through model experiments on rocks (Mogi, 1962: Scholz, 1968). The authors have been thinking of applying AE to prediction of failure in underground caverns and evaluation of long-term stabilization.
An outline of the AE measurement system used in experiments is shown in Fig. 1.