b-value as a criterion for the evaluation of rockburst hazard in coal mines
Abstract
Every year a few rockbursts with fatalities and a few hundred meters of damaged excavations have occurred in the mines of Upper Silesia Coal Basin (USCB) in Poland. The paper presents one of methods for assessing the level of seismic and rockburst hazard, using continuous seismological observation in Bobrek coal mine. This assessment is based on the study of the Gutenberg-Richter (G-R) relation for the current and past mining in the coal field. According to many laboratory and field studies, the b-value may be an indicator of the fluctuations of stress levels, e.g. Scholz, 1968; Gibowicz, 1974, 1979, and the consequent seismic hazard.
The novelty of the presented research is the development of quantitative criterion based on the b-value changes, calculated in a moving time windows with the 1 day step and the anomalies resulting from the comparison of the temporal values of b with its average value for the past seismicity in the field of coal mining; AG-R =[(bm – b)/bm]·100% (where: bm – average value of b for field of coal mining, b – temporal value of coefficient b calculated in time windows). This approach allows us to standardize the criterion to the current values of b in the investigated area (Table 1). In many measurement examples, high values of the temporal anomaly AG-R and at the same time downward trend and low b-value before the occurrence of the strong mining seismic events, were documented in this paper and in studies by Pierzyna, 2014). Particularly interesting results were obtained for mining and geological conditions that led to the formation of the local zones of stress concentration, e.g. Dubiński, 1989, and an increase in seismic activity and seismic hazard.
The b-value criterion be in agreement with the four basic levels of rockburst hazard used in Polish coal mines, e.g. Mutke et al., 2015.
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