Abstract
Abstract Repeated mining is the main factor that leads to development, propagation, and eventual deformation of the overlying strata fissures in the steeply inclined and extra thick coal seams (SIETCS). The evolution of the overlying strata structure is closely related to microseismic events in a mine. As the mining depth increases, the evolution rules of the overlying strata structure become more complicated and can easily induce dynamic disaster accidents. To solve these problems, this paper established a physical similarity simulation model. Microseismic monitoring equipment was used to study the relationship between the evolution of the overlying strata structure and the energy-frequency of microseismic events. On the basis of the principle of quantitative seismology, the response relationship between the overlying strata structure and the microseisms at different mining stages was compared and analyzed from a quantitative perspective. The characteristics of cumulative apparent volume, energy index, and microseismic b value were used to reveal the precursor characteristics of overburden instability and failure. The results showed that due to the occurrence characteristics of coal seam, the distribution characteristics of rock stratum stress, and the effects of mining disturbances, the energy accumulation-release period after instability failure of the overlying strata induced by shallow mining was longer than the energy accumulation-release period induced by deep mining. And the deep coal and rock mass had a periodic “balance-instability-rebalance-instability again” dynamic evolution process under the disturbance of repeated mining. In the working face mining, the slope of the accumulative apparent volume ΣVA curve suddenly increased, and the energy index EI gradually decreased at the late peak period, which indicated the deformation and failure of overburden. However, the b value of the microseismic event presented the precursory characteristics of rock stratum fracture that gradually increased and then changed drastically.
Highlights
With the continuous increase in coal mining depth and the complex of mining conditions, the coal production had been increased, but it incurred a series of potential safety hazards [1,2,3]
For steeply inclined and extra thick coal seams (SIETCS), due to the selected mining method and the special occurrence environment [4,5,6], the formation characteristics and dynamic evolution rules of the overlying strata structure on the working face were more complicated with the continuous increase in mining depth and mining intensity
Based on the monitoring results of rock formation fracture and microseismic events during extracting in the similarity simulation test, the microseismic signals in SIETCS were analyzed to study the relationship between the apparent volume, energy index, microseismic event b value of microseismic events, other parameter response characteristics and the evolution of the overlying strata structure
Summary
With the continuous increase in coal mining depth and the complex of mining conditions, the coal production had been increased, but it incurred a series of potential safety hazards [1,2,3]. Xia et al [25] monitored the impact hazards during the extracting of island working face in the Qianqiu Coal Mine using the microseismic and geo-acoustic monitoring system and determined the corresponding early warning indicators, which improved the accuracy of the prediction and forecast of rockbursts. The abovementioned scholars had conducted very useful explorations and studies on the formation of overlying strata structure, the mechanism of instability and disaster, and the early warning techniques of coal rock failure process in the mining of SIETCS. The paper reveals the precursor characteristics of instability and failure of overlying strata by studying the variation rules of the microseismic b value during the process of deformation and failure of overlying strata at different extraction levels, which guided the prevention and control of dynamic disasters in SIETCS as a basis for predicting instability and failure of overlying strata
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