In order to research microseismic (MS) activity rules in the process of rockburst in deep tunnel, the immediate rockburst induced by TBM and D&B excavation are measured in the field. The attention law of microseismic waveform shows that frequency is a better parameter to reflect the real feature of microseismic source. Then, a calculated method of the main frequency of MS event and its fractal dimension is introduced. The study on the evolution of fractal dimension of the main frequency in the process of immediate rockbursts shows that the distribution of main frequency of MS events is a fractal and has statistical self-similarity. Immediate rockburst induced by different excavation method has different precursory. The lowest fractal dimension of main frequency occurred on the day before major rockburst in the TBM tunnel. But in the D&B tunnel, the fractal dimension gets lowest on the right day of major rockburst. These results can provide references for warning and mechanism study of rockburst in deep tunnel.
Big excavation section, large buried depth and long length is the trend of tunnel engineering. As one of the geological hazards, rockburst is often met in deep tunnel under excavation unloading conditions of high geostress zones. According to the location and time of rockburst is in the range of excavation influence zone or not, the type of rockburst can be divided into immediate (Feng et al. 2012) and time delayed (Chen et al. 2012). The frequencies, strength and damage to engineering of immediate rockbursts are often much bigger than time delayed rockbursts. Therefore, it is important to make certain the evolution law and mechanism of immediate rockbursts for reducing the risk of rockburst in deep tunnel.
The damage evolution process of small scale fracture (micro fracture, mm) and big scale fracture (earthquake, km) are fractal. So, for the middle scale fracture (m), the damage and fracture of rockburst is fractal also (Xie 1992). The studies show that time and location distribution of microseismic events is a clustering fractal (Xie & Pariseau 1993).
The characteristic of waveform recorded by the sensors provide a wealth of information for the study of rockburst. In this paper, the attenuation of microseismic waveform is studied firstly to determine the main frequency of MS events as an analysis parameter. Then, the fractal features of main frequency in the process of immediate rockburst induced by TBM and D&B excavation are got.