Mechanisms and risk criteria for water inrushes caused by mining-induced propagation of concealed faults in a coal seam floor

Background Concealed faults in a coal seam floor, characterized by random distribution, are prone to induce water inrushes, which are difficult to prevent and control.

Objective and Method This study aims to investigate the mechanisms behind water inrushes induced by concealed faults in a coal seam floor under the influence of coal mining. Through a theoretical analysis, this study established the mechanical models of the initial cracking and wing crack propagation of concealed faults under seepage pressure. By analyzing the maximum propagation height of concealed faults in the floor and the post-propagation safe thickness of aquicludes, this study proposed criteria for a water inrush risk and validated the criteria using numerical simulation and field data.

Results and Conclusions  Analyses of the initial cracking conditions of open and closed concealed faults yielded the angles (θ₀), stresses (σ_ci), and critical water pressures (p_Ⅰc) at initial cracking. Furthermore, models that considered seepage pressure were developed to illustrate the wing crack propagation of concealed faults. Substituting mining conditions into the models yielded the maximum propagation height (l_max) of concealed faults in the floor. Then, based on the distance between the post-propagation top of concealed faults and the fractured zones in the floor, as well as the safe thickness of effective aquicludes, the criteria for a water inrush risk caused by mining-induced propagation of concealed faults in the coal seam floor were proposed. Field case analysis shows that the propagation of concealed faults in a coal seam floor significantly reduced the thickness of aquicludes. The proposed criteria were applied to mining face 182602 of the Wutongzhuang coal mine. As a result, the maximum propagation height of concealed faults was calculated at 12.4 m, which was consistent with the height of 11.5 m derived from numerical simulation. Since the thickness of the remaining effective aquicludes post-propagation was less than the safe thickness of aquicludes, the concealed faults posed a water inrush risk. After on-site remediation, safe mining was achieved. The criteria were also validated by the water inrush accident along mining face 13151 of the Xinan Coal Mine. The results of this study can provide a theoretical basis for understanding the mechanisms behind water inrushes caused by the propagation of concealed faults in a coal seam floor while also holding great application value for preventing these disasters.