An experimental study of the application of the spectral acoustic method to the advance detection of geologic structures in tunnelling working site

ObjectiveThe practice of coal mine production has proved that the vast majority of coal and gas outbursts are controlled by geological structures, and mainly occur in tunnelling working site. Therefore, continuous advanced detection of geological structures is an important technical means to prevent and control coal and gas outbursts in tunnelling working site, and is of great significance to ensure safe production in mines. MethodsBased on the research results of spectral acoustic method in Russia for many years, the resonance phenomenon generated at the weakened contact surface of the roof rock layer by the artificial acoustic signal excited by the coal cutting machine is utilized to construct the prediction coefficient of geological structure based on the stress index and frequency index. Through the monitoring and early warning system composed of seismometer, monitoring sub-station, industrial ring network, and monitoring host, the relative stress coefficient, spectral maximum frequency and geological structure prediction coefficient are calculated and analyzed in real time to realize the real-time monitoring and early warning of the geological structure of the tunnelling working site. Through cooperation between Chinese and Russian teams, an experimental study was conducted on the tunnelling working site of ventilation tunnel J15-15080 in the Eighth Coal Mine of Pingdingshan Tianan Coal Mining Co., Ltd. During the experiment, the tunnelling working site advanced continuously for 756 m, with seven faults being exposed.Results and ConclusionsThe experimental results demonstrate the high consistency between the anomaly areas of the monitoring indicator (i.e., the geologic structure prediction coefficient) of geologic structures and distribution of geologic structures in the mining face, with all geologic structures in the mining face identified within the anomaly areas of the monitoring indicator. At a certain distance in front of the exposed geologic structures, the geologic structure prediction coefficient inevitably increased and exceeded its critical value, which can be set at 7 to ensure 100% reliability for the monitoring and early warning of geologic structures. The advance warning distances were calculated at 6.5‒27.3 m based on the normal distance from the mining face to the fault strike and determined at 13.5‒44 m based on the heading direction of the mining face. The geologic structure prediction coefficients exhibited two distribution morphologies on both sides of a fault, i.e., single- and double-wing morphologies. The former refers to the case where the coefficients exceed the critical value in either the hanging wall or the footwall of the fault, while the latter denotes that the coefficients exceed the critical value in both the hanging wall and footwall of the fault. Based on the distribution of relative stress coefficients on a fault plane and the fault's hanging wall and footwall, the stress-strain zones near the fault can be categorized into zones with unilateral high stress, bilateral high stress, and overall high stress. The distances from high-stress zones to fault planes typically ranged from 3.2 to 28.0 m. The findings of this study will provide a new monitoring indicator and method for continuous online early warning in the advance detection of geologic structures in tunnelling working site.