Mining-induced fractures resulting from shallow coal seam extraction pose significant hazards, especially in the complex terrain of China's loess gully region. A recent study focusing on the Shenfu mining area explores how these fractures evolve over time, presenting key findings on their characteristics and implications for operational safety.
The loess gully region, known for its unique geological features, faces distinct challenges compared to standard mining areas due to its highly concentrated stress regions both horizontally and vertically. These intense stress concentrations can lead to serious operational hazards, including water and sand influx, alongside air leakage.
To examine these phenomena, researchers employed theoretical analysis, numerical modeling, and physical similarity simulation techniques. Findings from these investigations reveal pivotal information about the mining-induced fractures, indicating not only their tendency to undergo tensile failure but also how they frequently close due to the extrusive forces exerted by the slopes.
The study categorizes the evolution of mining-induced fractures, identifying three primary periodic development patterns. These include the '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). This classification system provides clarity on how fractures behave under varying conditions of stress and geological composition.
One significant component of the research is the introduction of the 'rock chain structure' model, which helps elucidate the dynamic mechanisms behind these periodic fracture types. This model enhances the theoretical framework used to understand fractures as mining progresses, particularly under unique terrain conditions typical of loess gullies.
Historically, the issue of mining-induced fractures has garnered attention from Chinese scholars since the 1990s, recognizing the imperative need for effective management strategies. The study highlights the 135,201 working face within the Nanliang Coal Mine as illustrative of shallow coal seam operations characterized by its substantial strike length of 1233 meters and its average coal seam thickness of 2.3 meters.
Further analysis revealed specific advancements during mining efforts. For example, at 110 meters advancement, fractures at the ditch bottom exhibited tensile failure. After reaching the gully bottom at 180 meters, significant shear failures occurred among the overlying rocks. These occurrences underline the fragility of the geological structure as mining activities escalate.
The research also indicates how mining fractures follow evolutionary patterns; at 370 meters advancement, the surface above the working face displayed noticeable mining-induced damage. Each advancement led to new observations, such as the first periodic weighting observed at 64 meters, which resulted in sliding instability of the stepped rock beam structure formed during the mining process.
This cyclic nature of mining-induced fractures was characterized using monitoring data from physical similarity models, explaining the surface stress evolution and plastic zones developed during various stages of mining activities. For example, during inverse slope mining, fractures demonstrated both 'single-cycle' and 'double-cycle' patterns, reflecting the complex interactions between geological structures and mining processes.
With advancements recorded up to 260 meters, the study encapsulated how sliding instability occurred approximately 4 meters behind the coal wall due to continuous structural shifts. The examination of fracture widths indicated comprehensive evolutionary processes, affirming the report's findings on the genetically controlled behavior of these geological formations.
The combined findings offer imperative insights for future mining operations and related engineering practices, particularly within loess gully regions. By enhancing current theoretical models, these insights may significantly contribute to safer mining practices through improved monitoring and predictive capabilities.
Supported by the Datong City Science and Technology Plan Key Research and Development Project, the research stands as a pivotal contribution toward advancing safety measures and determining the mechanistic underpinnings of mining-induced fractures. This is fundamental to not only ensuring operational safety but also protecting the environmental integrity of the loess gully region as mining activities continue.