Evolution of overlying strata fractures in variable mining heights of deeply buried thick coal seams

• Fractures evolve in four stages; max heights: 80 m (restricted), 98 m (full). • Composite simply-supported beam cuts separation by 86.21% and lowers strain. • Roof structures significantly affect gas migration and storage. • Key drainage zone 11–36 m; on-site experiment showed good drainage effect. To investigate the evolution law of overlying strata fractures in a variable-height working face and its influence on pressure-relief gas extraction, a physical similarity simulation experiment was conducted to examine the dynamic development of overlying strata caving and fractures at different mining stages. The results indicate that the fracture development height exhibits an ``incubation-sudden increase-gradual increase-stabilization'' pattern, reaching 80 m during the restricted-height mining stage and 98 m during the full-height mining stage. The overlying strata separation curve shows an asymmetric distribution. During the restricted-height mining stage, the separation volume and strain values on the open-off cut side were greater than those on the working face side. The existence of a ``composite simply supported beam'' structure on the working face side reduces the overlying strata separation volume by 86.21% and expands the strain zone range, while the strain values drop sharply to between 2.57%-3.95%. In full-height mining, the working face side showed significantly increased separation and strain, surpassing the open-off cut side. The roof fracture structures exert a significant impact on the migration and storage laws of pressure-relief gas: the hinged ``masonry beam'' structure is conducive to gas migration and storage, whereas the ``composite simply supported beam'' structure reduces the degree of fracture development, leading to a substantial weakening of gas storage capacity. Key zones for gas extraction were identified accordingly, and in-situ drilling during full-height mining achieved a maximum single-borehole gas concentration of 64.5%, demonstrating efficient extraction and validating the accuracy of the selected zones.