The Behaviour of the Loosened Zone Around Rock Caverns Monitored By Acoustic Emission

The Behaviour of the Loosened Zone Around Rock Caverns Monitored By Acoustic Emission

Monitoring the stability of rock pillars is important for underground storage caverns for crude oil, liquified gas and compressed air. The behaviour of the loosened zone around rock caverns plays an important role in the mechanical stability of rock pillars during construction and also after completion. The authors have developed an AE (acoustic emission) monitoring system. By using this system, the excavation of rock caverns for the storage of crude oil has been monitored. This paper describes the results of these experiments and discusses the practical procedures to predict the initiation of the loosening of the rock mass around the cavern, based on the deformation characteristics of the rock and the distribution of the pre-existing fractures.Underground storage caverns for such products as crude oil, liquified gas, natural gas and compressed air are being constructed or studied in various countries. Such facilities will consist of many storage caverns with large cross-sections and their layout will be complicated compared to tunnels. Therefore, the monitoring of rock pillar between caverns is of extreme importance during construction and also after completion. The behaviour of the loosened zone around rock caverns plays an essential role in the mechanical stability of rock pillars. It is important to evaluate the actual mechanism and progress of the loosened zones.However, drilling many boreholes from inside of the storage caverns for installation of the apparatus for stability monitoring is impossible because of cost and time considerations.The authors considered that acoustic emission (AE) techniques would be adopted because of the above considerations, and to detect the spatial distribution of loosened zones in the long term. Thus, in order to develop a suitable acoustic emission monitoring technique, the AE measurements in the laboratory, in situ and during tunnel excavation were carried out. From the results of these measurements, it was concluded that using three indicators described below, the continuous AE monitoring at several points seemed to be useful to detect the spatial progress of the loosened zone which is formed by the excavation.The AE frequency: The rate of occurrence of acoustic emission per hour or minute.The m-value: The ratio of amplitude to the frequency of AE. The m-value is a known factor in the earthquake engineering which is expressed as non-dimensional value of the ratio of maximum amplitude to their frequency (Ishimoto & Iida 1939, Mogi 1962a, Mogi 1962b, Scholz 1968).(Equation in full paper)Figure 1 shows the results of measurement during tunnel excavation. It was found that the loosening of the rock mass around the tunnel initiated at Point-A,B, C, because the decrease and rapid roll-over of the m-value and the peak of the AE energy were recognized at these observation points(Aoki et al 1989). Taking into account this knowledge, we have developed an AE monitoring system which can perform data acquisition and processing in real time, and using this system the measurements during excavation of the crude oil storage caverns have been continuously monitored.(Figure in full paper)

1990-09-10

Aoki, Kenji

Toida, Masaru

Koshizuka, Kenichi

conferencePaper

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