Abstract
<title>Abstract</title> <p>Highwall solid backfilling in open-pit coal mines effectively mitigates surface subsidence and ecological degradation, promoting green mining and sustainable development. This study proposes a novel coal pillar–granular material composite body based on the spatial distribution of residual coal pillars and backfill material after highwall mining, and designs a testing apparatus simulating granular confinement on both sides of the pillar. Under loading, the composite body exhibited a distinctive stress–strain response with two pronounced peaks. Mechanical strength models for three bearing stages were established to clarify the strength evolution process and underlying mechanisms. The results indicate that the α-strength (the peak strength at initial failure) first increases and then decreases with increasing coal pillar width, reflecting the combined effects of width and scale. In contrast, the β-strength (the peak strength at subsequent failure) increases with pillar width according to a power-law relationship, and this effect is enhanced by greater granular material height. Granular material height shows a linear correlation with both strength parameters, although β-strength is more sensitive to height variations. This study reveals the evolution mechanism and influencing characteristics of composite body strength under varying mining parameters, providing fundamental insights for the green and safe recovery of end-slope coal resources.</p>