ABSTRACT:

Acoustic Emission (AE) monitoring technique can be used to study the damage initiation and propagation process in rock masses under stressing. During the excavation of the Kannagawa Pumped Storage Powerhouse cavern in Japan, a total of 120 AE sensors were installed in 10 monitoring lines, equally divided in two cross-sections to monitor the excavation induced AE activities. In this study, PFC code is used to simulate AE in large-scale underground excavation, assuming the analogue of bond failure and AE activity. Because the difficulty to model the whole cavern and the surrounding rock mass using PFC model solely, a FLAC/PFC coupled approach is used. In the large-scale FLAC model, in-situ stress, geology, excavation sequences are taken into account. Small-scale PFC models surrounding selected AE sensor locations are built to study AE activity during cavern excavation. The coupling between FLAC and PFC is realized through an exchange of displacements, velocities, and forces in each cycling step. It is found that the simulated relative variation of AE numbers are in very good agreement with the field monitoring data, indicating that the PFC tool, when combined with FLAC, can be used effectively to study AE activity in large-scale underground excavations.

1. INTRODUCTION

Excavation of underground openings disturbs the initial balanced stress state, and stress redistribution around the opening and can lead to crack initiation and propagation in the rock, forming an EDZ (Excavation Disturbed Zone) around the excavation. Understanding the extent and severity of the EDZ is important for safe and effective design of underground excavations. Acoustic emission (AE) and microseismic (MS) events are indicators of rock fracturing or damage as the rock is brought to failure at high stress. By capturing the AE/MS events, underground excavation induced rock mass degradation or damage can be located and evaluated. This monitoring technique has been used in combination with other techniques to assess the rock mass condition in underground construction. For this purpose, the AE/MS mechanism associated with the excavation process must be understood. AE/MS activities are low-energy seismic events associated with a sudden inelastic deformation such as the sudden movement of existing fractures, the generation of new fractures or the propagation of fractures. The main difference between AE and MS signals is that the seismic motion frequencies of AE signals are higher than those of MS signals. In the present study, we executed an extensive AE study program during the construction of the Kannagawa powerhouse cavern in Japan. We understand that it is important to acquire field monitoring data but at the same time how to interpret the obtained data for safe and cost-effective underground construction is more important. Since AE monitoring can only provide limited quantitative measures such as AE counts and center frequencies for the assessment of damage, other methods such as numerical simulation techniques are needed to perform thorough examinations of the rock mass conditions in order to provide quantitative assessments of larger volumes. In the following discussion, we first simulate the AE behavior of rocks using PFC [1] and then proceed to a comprehensive simulation of the observed AE activities around the cavern, using FLAC/PFC coupled simulation method.

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