Acoustic emission behavior of rock materials containing two preexisting flaws and an opening subjected to uniaxial compression: insights into self-similarity, chaotic, and fractal features

To investigate the acoustic emission (AE) self-similarity, chaotic, and fractal features and precursory anomalies of the surrounding rock of underground caverns, uniaxial compression tests were performed on prepared granite specimens containing two preexisting flaws and an opening. Combined with AE observations, and the variations in the self–similarity coefficient, the entropy value, and correlation dimension were analyzed. The results show that the minimum value of the self–similarity coefficient probably indicates the macroscopic fracture of the sample, and the critical recognition mode before specimen failure can be expressed as a “declining-minimum value-rising” type of the self–similarity coefficient. Entropy values ranging from 0.8 to 1.2 can be used as the critical interval to evaluate the mechanical state of the specimen. When the entropy is greater than 0.8, the possibility of sample failure increases, and an entropy larger than 1.2 implies that the sample is at risk of failure at any time. The failure of samples is a process of increasing order and decreasing correlation dimension. The samples have the same identification pattern in the critical fracture state, namely, the correlation dimension “fluctuating-continuing decrease (or drop to the minimum and then rise slightly)”. The continuous decline stage of the correlation dimension can be regarded as the precursor information of sample failure, and the decline to the lowest value indicates that the specimen is in a subcritical or critical failure state. The change in flaw angle affects the curve shape of the three characteristic parameters but does not change their overall evolution pattern.