The stability monitoring of large and deep underground openings such as gas storage caverns, compressed air energy storage and high level nuclear waste repository are of extreme importance during construction and also after completion. The behaviour of loosened zone around rock caverns play important role on the stability of caverns, and it is quite important to evaluate the actual mechanism of the occurrence of loosened zones caused by excavation. Authors have carried out the laboratory and in situ experiment to investigate the failure mechanism of the rock around the caverns by AE monitoring technique. The Paper describes the results of experiments that the indicator m (ratio of amplitude to frequency of AE) and AE energy (square of amplitude of AE) are available to detect the initiation of small-scale failure of rock, i.e., the loosening of rock mass around opening.
The construction of large and deep underground openings such as gas storage caverns, compressed air energy storage and high level nuclear waste repository are being executed or under study in various countries. These caverns will be constructed at great depth and their layout (cavern size, location of caverns) will be complicated compared with tunnels and caverns for underground hydropower plants. Consequently, the stability monitoring of a large and deep underground opening is extreme importance during construction and also after completion. Especially, the behaviour of loosened zone around rock caverns play important role on the stability of caverns and it is quite important to evaluate the actual mechanism of the occurrence of loosened zones caused by excavation. Authors have been studying more accurate monitoring technique to detect the behaviour of loosened zone which are formed by the excavation work. In order to develop a suitable monitoring technique, to detect the initiation of small-scale failure of rock is important and in this occasion, two problems should be solved:
The method to detect the phenomena of small-scale failure.
The method to perform the real-time monitoring continuously throughout a long term. Authors considered that acoustic emission (AE) technique was available for above mentioned demands, and have carried out the study on applicability of AE monitoring technique to detect the loosening of rock mass around openings (Fig. 1). This paper describes the results of laboratory and in situ preliminary experiments. Especially, the results of evaluation of the indicator which expresses the initiation of small-scale failure, i.e., the loosening of rock mass around openings is discussed.
As the indicator to detect the initiation of small-scale failure of rock, authors noticed the m-value (the ratio of amplitude to frequency of acoustic emission). The m-value is well known in the earthquake engineering.
The purpose of this experiment is to discuss the effectiveness of the m-value as a indicator to detect the initiation of small-scale failure of rock. Acoustic emission occurring within the sample was detected with attached piezoelectric tranceducers.
