Occurrence Mechanism and Safety Identification of the ‘Muffled Thunder’ Phenomenon in Coal Mine Roadway

The term ‘muffled thunder’ denotes a persistent low-frequency acoustic phenomenon observed in Yangquan coal mines, distinct from conventional coal burst signatures (‘coal cannon’). Documented over the past decade within the No. 15 coal seam, this sound occurs predominantly in roadways proximal to active working faces. It serves as an early indicator of stress accumulation in brittle strata. Although non-destructive to roadways, monitoring its frequency and energy facilitates assessment of overburden stability without disrupting operations and provides precursor data for localised rock instability. Therefore, understanding the acoustic mechanisms and identifying the associated risks are essential for enhancing the safety of coal mining operations. This study focussed on the 15# coal seam of the Yangsheng Coal Industry in China, utilising a combined monitoring approach of acoustic waves and micro-seismic methods to locate the acoustic sources within the K2 and K3 limestone beds, situated 16.10 and 40.57 m, respectively, from the coal seam roof. Preliminary acoustic emission mechanics experiments indicated that the ‘low strength’ and ‘high brittleness’ characteristics of the K2 and K3 limestone strata are intrinsic factors contributing to the ‘muffled thunder’ phenomenon. Through numerical simulations analysing the overburden fracture development, acoustic emission patterns, and energy storage and release mechanisms of the surrounding rock during mining, it was determined that the K2 limestone layer serves as the primary accumulation area for acoustic emissions and elastic strain energy. The shear-slip model of the composite stratum revealed that the K2 limestone stratum first reached its ultimate strength and subsequently failed. As the azimuth angle of the maximum critical stress increased, the shear stress required for failure also increased, leading to increasingly pronounced brittle failure characteristics. The energy released during the tensile–shear brittle failure of the K2 limestone stratum was the main contributor to the ‘muffled thunder’ phenomenon. Integrating mechanical testing with numerical simulations, a risk discrimination mechanism for the ‘muffled thunder’, based on elastic strain energy, has been established. This study concludes that ‘muffled thunder’ is a typical acoustic dynamic phenomenon, which is further supported by the in situ monitoring results of the surrounding rock.