Structural characteristics of double layer hard roof and effects on coal pillar and roadway stability

Underground coal mines require coal pillars to support the overlying strata and protect mining roadways. Excessively wide coal pillars waste recoverable coal resources, whereas overly narrow pillars may induce roadway instability. This challenge becomes more pronounced under composite key strata conditions. A coupled mechanical model was developed based on a double-layer hard roof. The fracture characteristics of the double-layer hard roof and the pillar load-bearing response were analyzed, with particular emphasis on force-energy conversion mechanisms. Results show that the fixed-end lengths of both sub-key stratum (SKS) I and SKS II exhibit exponential relationships with coal pillar width. Parametric analyses of the overburden load and pillar bearing capacity indicate that the pillar width can be reduced to 15 m. Regarding energy distribution, SKS I is characterized by energy accumulation at the fixed end, whereas the peak energy density of SKS II shifts toward the rebound zone as the suspended length increases. Field application demonstrated the effectiveness of the proposed approach. These findings provide a theoretical and practical basis for determining coal pillar width under analogous geological and mining conditions.