Aoki, K. (Dept. of Urban and Environmental Engineering, Faculty of Engineering, Kyoto University) | Mito, Y. (Dept. of Urban and Environmental Engineering, Faculty of Engineering, Kyoto University) | Mori, T. (Kojima Technical Research Institute, Tokyo) | Maejima, T. (Tokyo Electric Power Company)
In highly stressed rock, the stress-induced cracks are often developed by the excavation of caverns, and EDZ (Excavation Disturbed Zone) will be formed around the caverns. The monitoring and control of EDZ is the one of the most important required technology for the design of caverns such as the high-level radioactive waste repository. Thus the authors are developing the effective design and construction system for underground caverns based on the monitoring of EDZ using AE (Acoustic Emission) that sensitively detects the generation of the stress-induced cracks. In this study, the field measurements of the EDZ, which is developed around the actual large underground cavern in highly stressed rock, is conducted in accordance with the progress of the excavations. The behaviors of AE parameters are compared with the behaviors of stress and displacement. Furthermore, DEM simulation of the bonded particles model of the cavern is carried out using the in-situ rock deformability and strength, and initial rock stress, which are obtained from in-situ tests. The result of the simulation shows good agreement with the actual measurement results on rock stress, crack initiation, etc. As the result, it is clarified that the behavior of AE parameters gives us the very good information on the state of fracturing along the stress path, and this realizes the stress-based observational design and construction system. Further it is also clarified DEM simulation using bonded particle model is a powerful tool to predict the stress change and fracturing.
In general, underground excavation causes crack initiation and propagation, and Excavation disturbed zone, EDZ will be formed around the cavern. Now in several underground projects, to monitor and control the EDZ is the one of the most important technology for the design. Especially in highly stressed rock at great depth such as HLW repository, the formation of EDZ is strongly influenced by stress change in rock mass. So, it is very important to monitor the EDZ with stress change. For this purpose, AE measurement would be a suitable in both of scientific and practical points of view, and this will allows us to perform the rational observational design and construction. In such an observational system, numerical analysis and AE measurement will give the reliable stress and EDZ distribution, which is essential for rational design of rock support. In order to realize this system, it is important to grasp the relationship between AE, stress change, and EDZ.
In this study, examination of field measurement data from a large cavern and its numerical analysis were carried out, and then, the relationship between AE parameters, stress change, and fracturing is investigated.
2 AE PARAMETERS
Acoustic Emission (AE) is a transient elastic wave, which is generated by the rapid release of strain energy within a solid material.
In this study, the behaviors of the following three AE parameters are fully discussed, comparing with the failure behaviors of rocks under the specific stress condition.
2.1 AE counts
AE counts denote the number of AE events with individual destruction of the solid structure.