Influence of wave impedance of backfill medium on explosive stress wave propagation and rock mass damage evolution

In engineering practices such as backfill mining, the existence and characteristics of the rock-backfill interface significantly influence the propagation of explosive stress waves and the dynamic response of the rock mass. This study aims to reveal how, among these characteristics, different backfill media and their wave impedance differences with the rock specifically affect these processes. To this end, physical experiments were combined with numerical simulations, utilizing Digital Image Correlation (DIC) technology to capture strain field evolution under blasting, and LS-DYNA was employed for numerical analysis. The research indicates that the wave impedance difference between the rock and the backfill medium is key to controlling stress wave propagation and energy distribution. When the wave impedances are similar, stress wave transmission is dominant, leading to higher peak strain values at various points and a more uniform action of the stress wave on the rock mass, resulting in relatively uniform final failure. When the wave impedance difference is large, interface reflection is enhanced, forming a significant "blocking effect"; in this case, although the initial peak strain may be lower, the continuous action of the interface can lead to greater final strain and plastic deformation in the rock mass. Numerical simulations further show that the greater the wave impedance difference, the more pronounced the "guiding" and "blocking" effects of the backfill on crack propagation. This leads to intensified damage and crack accumulation in the rock mass adjacent to the backfill due to energy concentration, which also results in a higher fractal dimension of the cracks. Therefore, the type of backfill medium directly determines the strain response characteristics and final damage patterns of the rock mass by modulating the reflection and transmission behavior of waves at the interface. These findings have practical guiding significance for optimizing backfill material selection and blasting parameter design in backfill mining.