Experimental study on the fragmentation properties and energy dissipation of coal-rock composite under impact loading

Abstract

Tunnel construction frequently traverses coal-rock composite strata, whose mechanical heterogeneity and complex energy evolution complicate ground control and elevate the risk of dynamic disasters. This study investigates the fragmentation characteristics and energy dissipation mechanisms of coal-rock composites through dynamic impact tests using a Split-Hopkinson Pressure Bar system. Specimens with varying coal height ratios, structural configurations, and impact loadings were systematically analyzed. Results demonstrate that energy dissipation and fragmentation behavior are highly sensitive to both coal height ratio and composite structure. A quadratic polynomial effectively describes the coupling relationship among dissipated energy density, average particle size, and incident energy. The particle size distribution is bounded by a pure coal specimen (upper limit) and an R-C-20% composite (lower limit). While C-R and R-C structures exhibit an “arch-bridge” trend in average particle size with increasing coal height ratio, R-C-R structures show a gradual decrease. Fractal dimensions follow an initial increase, then decrease or N-shaped trends under low loading, becoming strongly influenced by interface effects at higher loads. Coal components exhibit dissipated energy density that increases with loading but decreases linearly with coal height ratio, approaching the values of a pure coal specimen. Rock components maintain a lower dissipated energy density than pure rock specimens, remaining largely insensitive to coal height ratio, consistent with their primary role as energy transfer media. These findings clarify the scale and structural dependencies of fragmentation and energy dissipation mechanisms in coal-rock composites, providing theoretical and experimental support for dynamic disaster prevention in deep engineering projects.