Mechanical Response of Supporting Unit with Continuous Mining and Continuous Backfilling Method in Close Distance Coal Seams

In the process of continuous mining and continuous backfilling (CMCB) in close-distance coal seams, the supporting unit (CMCBSU), composed of coal pillar and filling body, is affected by mining-induced disturbances from adjacent coal seams. This study establishes a mechanical model for the CMCBSU, revealing that the coordination of the CMCBSU depends on the similarity degree of elastic modulus of the components. Subsequently, numerical simulations were conducted to analyze the stress conditions. The results showed that the σ1 and σ3 exhibited cyclic loading and unloading characteristics. Based on the stress paths, conventional triaxial compression tests were performed on coal (CTC-coal), filling body, and the CMCBSU, as well as triaxial cyclic loading and unloading tests on coal (TCLU-coal). The results indicated that coal exhibited significant brittleness, the filling body demonstrated strain-softening characteristics, and the CMCBSU showed strain-softening behavior. Hysteresis loops were observed in the elastic region of the TCLU-coal. The failure characteristics of the specimens indicated that the shear stress was the primary cause of specimen failure. After testing, the filling body exhibited radial fish-scale-like wrinkles on the specimen surface, the coal and the CMCBSU showed primary shear cracks. In the CMCBSU, the primary shear crack generated on the filling body side relates to that on the coal side. In contrast, secondary cracks on the filling body side rarely penetrate the coal side. Excluding the influence of internal weak planes on specimen failure, cyclic loading and unloading within the elastic region of the coal reduced its internal friction angle. Mechanical parameters indicate that the weaker load-bearing medium determined the load-bearing capacity of the CMCBSU, the medium with a higher elastic modulus primarily determined the CMCBSU’s resistance to elastic deformation, and the cyclic loading and unloading caused by CMCBSU in close-distance coal seams had minimal impact on the coal’s resistance to elastic deformation.