Expansion law of seepage path in the concealed structural floor of coal seam in deep confined water

With the mining depth gradually increases,the coal mines in Hanxing mining area are facing an increasing serious problem of water inrush threat from the ordovician limestone caused by the activation of concealed structures under the influence of mining. In order to determine the water inrush condition and the key treatment areas of coal seam floor,the initial seepage conditions and seepage path expansion laws of high confined water with different con- cealed structural types under the conditions of large deep and high water pressure should be studied. Therefore,the generalized mechanics models were established from the coaction mechanism of mining disturbance,concealed struc- ture,combination of confined water,based on the analysis of geological characteristics and water inrush of typical coal mines in Hanxing mining area. According to the generalized mechanics models, the cracking condition, seepage strength and water-crack expansion criterion expressed by the principal stresses were obtained by the theoretical analy- sis of fracture mechanics. Then a classical solution method of boundary loads propagation in semi-infinite elastic plates was applied to derivate the mathematical expressions of the vertical,horizontal and shear stresses of the concealed structural coal seam floor under the coupling of mining stress and confined water pressure. Based on the obtained stress components,the stress state equation was solved by the invariants of the stress tensor to obtain the principal stress ex- pression in the above water-crack expansion criterion. Taking the geological and mining parameters of the typical water inrush mine area in Hanxing area as an example,the case study was carried out and the extended path of coal seam floor crack was calculated using the Matlab program. Furthermore,the RFPA numerical simulation software,which can effectively simulate rock rupture and make fracture extension process visual,was applied to analyze the mechanical be- havior of floor with different types of concealed structure and reconstruct the evolution process of confined water seep- age path. The results of theoretical calculation and numerical analysis showed that the local stress disturbance of the concealed structure affected the expansion direction of the seepage path,so that three types of coal seam floor respec- tively showed different seepage paths. For the intact type of coal seam floor,the seepage crack extended to the bounda- ry of both sides of the goaf with a “positive eight-character” shape from ordovician confined water layer. For the coal seam floor with concealed collapsed column,the seepage crack extended to the boundary of both sides of the goaf with an “inverted eight-character” shape from the top interface of the collapsed column to the depth of 28 m. For the coal seam floor with concealed fault,the seepage crack extended along the opposite direction of the fault extension line and stopped at depth of 30 m. The floor water-bearing fracture and the mining fracture expanded to each other and finally formed a seepage channel from ordovician confined water layer to the coal mining face. The result of seepage crack ex- tension law of coal floor with different types of concealed structure obtained by theoretical analysis and numerical simu- lation could be used for predicting the confined water inrush path and targeted treatment of ordovician limestone kaster water disaster,in combination with microseismic monitoring technology which real-timely and dynamically catch the precursor information on concealed structure activation.