Rock burst process of limestone and its acoustic emission characteristics under true-triaxial unloading conditions

The rock failure process is associated with acoustic emission (AE). Hence, violent rock failure processes such as rock burst under different stress paths in laboratory can be studied using the AE monitoring technique. In this paper, single-face dynamic unloading tests under true-triaxial condition were carried out for Paleozonic marine sedimentation limestone samples produced by blocks cored from 1140 m depth in Jiahe coal mine of China. Frequency-spectra analysis was conducted for the full-wave AE data and figures showing the relationship of frequency-amplitude of AE signals in rock burst stages were obtained. The dynamical damage process and characteristics of limestone under the test condition are summarized. It is found from the fast Fourier transformation (FFT) analysis of the AE signals that there exist two main frequency ranges, i.e. lower (60-100 kHz), and higher (170-190 kHz), for the samples. When the limestone samples are under relatively lower stress, the AE signals are characterized by high frequency and low amplitude. As the stress increases, the AE signals tend to shift more towards a signature of high amplitude. Near the bursting failure of the rock samples, it is observed that there are much higher amplitude and lower frequency events. The AE accumulated energy release rapidly increases from the unloading state of the rock samples to its failure. The dynamic unloading test facility provides a useful tool to gain an insight into the characteristics of rock bursts. (C) 2009 Elsevier Ltd. All rights reserved.