For the purpose of achieving safety design of an underground structure such as a power plant, it is necessary to evaluate initial geostress inside the bedrock. One way of measuring initial pressure is the in-situ overcoring method. In this study it was attempted to obtain initial ground pressure in bedrock by utilizing the Kaiser effect of AE. There is already a case reported in which initial geostress was estimated by utilizing the Kaiser effect (Hayashi). In order to estimate initial geostress by utilizing the Kaiser effect, it is necessary to solve the following problems in advance. Rock in situ has been subjected to the three principal stresses for a long time, and is in a saturated state of strain. (Hereafter, the saturated state of strain is defined as the state in which residual strain is sufficiently generated and in which no further strain can be recognized under a certain stress.) Therefore, in case initial geostress of the bedrock is to be obtained by the Kaiser effect from a rock specimen cored in situ, it is necessary to determine whether the Kaiser effect is being affected by initial geostress from a different direction. Then, it is also necessary to study the Kaiser effect characteristics of a rock specimen in which creep strain is recognized. This article explains the results of the experiments conducted to solve these problems.
2. EXPERIMENTATION APPARATUS AND METHOD OF EXPERIMENT
Fig. 1 is a block diagram of the experimentation apparatus. The quantities measured in the experiments are of the stresses and strains to which the specimen is subjected under monotonic loading and of AE generated when the sample is subjected to stress. These data can be computerized and retrieved as relationships of stress-strains and total AE counts. The resonance frequency of the AE transducer used for this experiment was 140 kHz. The filter used was a band pass filter of 100 to 200 kHz and the threshold level was set at 380 mV for the lower level and 400 mV for the upper. One of the problems encountered in AE measurement concerns the elimination of noise. In the present experiment, electric noise generated by the measuring instrument itself was handled by proper grounding. Further- more, the noise generated by the loading plates coming into contact with the ends of the specimen was eliminated by placing thin sponges at both ends of the specimen (Murayama, 1984). The rock in its original location, which has been subjected to the three principal stresses for a long time, is in a saturated state of strain. Two methods were employed to create a saturated state of strain in the laboratory. These Here, a method of applying load repeatedly until there is no further increase in residual strain and a method of applying a constant stress by creep until a constant strain is obtained. The following experiments were conducted in order to study the Kaiser effect characteristics of the specimen in this saturated state of strain. The specimens used for these experiments were of two kinds of rock, namely, granite and tuff.
Fig. 1 Block diagram of AE apparatus(available in full paper)