Dynamic Instability Mechanism of Water-Saturated Granular Coal Subjected to Different Confining Pressure

Dynamic instability of water-saturated granular coal in tectonic stress zones is a critical safety issue in coal mining. This study adopts raw coal granules from the Daping Coal Mine to investigate the dynamic response and instability mechanisms under coupled confining pressure, median particle size (d50), and water saturation via dynamic impact tests, 2D equivalent modeling, and theoretical analysis. The results indicate that confining pressure and median particle size jointly regulate the dynamic mechanical properties of coal, with liquid bridge volume serving as a key mediating variable. The study reveals a dual-path coupling instability mechanism of “liquid bridge softening and confining pressure strengthening”: a critical confining pressure of 12 MPa divides the dominant force from liquid bridge to friction. Small-particle units show a stronger strengthening effect, and large-particle units have a slightly higher critical confining pressure. Field observation validates the theoretical patterns, identifying areas near faults as high-risk zones for dynamic instability. Accordingly, a three-tier prevention and control strategy of “tectonic stress unloading, flexible support, grouting modification” is proposed. The research findings enhance the theory of water-saturated granular coal instability and provide theoretical and engineering foundations for disaster prevention and control in tectonic stress zones of coal mines.