Study on the initiation and propagation of mining induced fractures by underground coal mining in the loess gully region.

Mining-induced fractures resulting from shallow coal seam extraction in the loess gully region of China are complex and variable, presenting critical hazards including water and sand inrush, as well as air leakage, all of which severely endanger operational safety. This study aims to examine the spatiotemporal evolution characteristics of mining-induced fractures in shallow coal seams within China's loess gully region. Focusing on the Shenfu mining area, a combination of theoretical analysis, numerical modeling, and physical similarity simulation is employed to investigate mining processes and fracture development in shallow coal seams within loess gully. Findings indicate that the loess gully surface contains highly concentrated stress regions, distributed both horizontally and vertically. Mining-induced fractures along the slope primarily experience tensile failure, and most are ultimately closed due to the rotary extrusion exerted by the slope; The evolution of mining-induced fractures exhibits three distinct periodic development patterns: "half cycle" (inoculation-expansion-stationary), "single cycle" (inoculation-expansion-semi-closed or closed), and "double cycle" (inoculation-initial expansion-initial closure-secondary expansion-secondary closure) within the loess gully region. Through the development of a "rock chain structure" model for overlying strata, the dynamic mechanisms driving the three periodic mining-induced fracture types are individually elucidated. Analyzing the stress state of the "rock chain structure" model under linear loading reveals the instability conditions of the bedrock structure and the fracture types resulting from linear loading. These findings offer both theoretical insights and practical guidance for further research and engineering applications in shallow coal seam mining within loess gully regions.