Preparation mechanism of water inrush channels in bottom floor of Dongjiahe Coal Mine based on microseismic monitoring

To overcome the water inrush problem of working face at Dongjiahe coal mine,a microseismic monitoring system was installed to analyze the microcrack data from baseplate of fault zone. The initiation and evolution process of microcracks in baseplate was reconstructed during the working face passes through the fault. Also,the microcrack data was combined with Realistic Failure Process Analysis (RFPA2D) system to study the variation law of stress fields dur- ing the evolution process of water-conductive fissure zone in wall rocks of baseplate fault. The results showed that:① according to the analysis of microseismic event distribution,microcracks occurred when working face is 85 m ahead of the fault. The depth of microcracks in baseplate reached 25 m before working pace passing through the fault and the baseplate has a sectioned local failure characteristics. After working face passing through the fault,the maximum depth of microcracks reached 35 m and microcracks are gradually transfixed. ② According to the analysis of microseismic energy density distribution,the concentration zone of high energy density before passing through the fault was about 15 m in strike length,and located in 5-25 m beneath the coal layer. Meanwhile,the fault located within 5 m range of coal layer remained stable. After passing through the fault,the high energy zone expanded to upper goaf and deeper zone,to around 80 m range into the upper goaf along the working face,and around 35 m range beneath the coal layer. ③ Ac- cording to the microseismic monitoring and numerical simulation results,the expansion process of baseplate water in- rush tunnel could be divided into two phases as before passing the fault and after passing the fault. Before passing the fault microcracks occurred initially in the wall rocks of fault around 25 m beneath the coal layer,and expanded through the fault. There were microcracks occurred in 5-25 m beneath coal layer but not yet transfixed. After passing the fault, the microcracks expanded from top to bottom and formed a water inrush tunnel. ④ According to the analysis on the evolution law of fault stress based on numerical simulation,before passing the fault,the shear stress of the fault was a negative value and gradually decreasing. The upper part of the fault tended to slip downwards. The greater the depth the faster the shear stress reached its extremum,resulting in a compression and shear failure in wall rocks from bottom to top. Upon the working face reached near the fault,the shear stress reversed rapidly and reached its maximum. The upper part of the fault tended to go upwards under the action of confined water. The surrounding rocks had a pull fail- ure from top to bottom,and cracks gradually transfixed,forming a water conductive tunnel.