Mining induced stress surge and drop mechanisms in backfilling panel of a coal burst mine

High stress and multi faults lead to rising burst frequency in deep coal mine. Though backfilling mining serves as the most straightforward method for stress-decreasing and burst-preventing, it fails to eliminate coal burst completely. In order to decrease the influence of coal burst on safety mining of deep-buried seam with bursting liability, stress surge and drop mechanisms emerging in backfilling panel are studied with field measurement, theoretical analysis and laboratory test, and destressing effect of roof pre-blasting and large borehole drilling methods is analyzed. High stress, multi faults, width-changing pillar and mining disturbance leads to the emergence of stress concentration, stress surge and stress drop phenomenon in backfilling panel, but the concentration coefficient is smaller than 2.0. At the normal region, influence range reaches 30 m. It increases to 50 m when fault influence is considered and the value grows to 70 m if both fault and pillar influences are added to the panel. Fault activation and rock failure results in sudden release of strain energy, the mechanisms underlying the transition between strain energy and kinetic energy as well as dynamic stress wave are revealed. Note transition ratio reaches 17%. The superposition between dynamic stress wave and static stress field leads to sudden change in mining induced stress. Surrounding rock presents consolidation hardening behavior if it fails before load superposition, resulting in stress surge. Brittle caving behavior emerges if surrounding rock transits from intact into broken state, leading to stress drop. If surrounding rock still keeps intact after stress superposition, it presents elastic rebound behavior and experiences dynamic load. Large-scale rupture of hard roof is not observed due to supporting effect provided by backfilling materials and thus, microseismic energy associated with single event is smaller than 10⁵ J. But microseicmic events presents asymmetrical distribution due to fault influences. They present high frequency and low energy mode at maingate side while low frequency and high energy mode is observed at tailgate side. As a result, burst monitoring pre-warning happens two times in the latter roadway and one time is observed in the former one. Hard roof pre-blasting and large borehole drilling methods are used to decrease mining-induced stress. Large-scale blasting fractures are formed in roof strata, which control stress concentration degree and stress increase speed effectively. Thus, coal burst danger is significantly decreased in deep mining.