Geophysical Evaluation of Effectiveness of Blasting for Roof Caving During Longwall Mining of Coal Seam

Abstract

Deep longwall mining of coal seams is made in the Upper Silesian Coal Basin (USCB) under complicated and mostly unfavourable geological and mining conditions. Usually, it is correlated with rockburst hazard mostly at a high level. One of the geological factors affecting the state of rockburst hazard is the presence of competent rocks in the roof of extracted coal seams, so rock falling behind the longwall face does not occur, and hanging-up of roof rocks remains. The long-lasting absence of caving may lead to an occurrence of high-energy tremor in the vicinity of the longwall face. Roof caving behind the longwall face may be forced by blasting. The column of explosives is then located in blastholes drilled in layers of roof rocks, e.g. sandstones behind the longwall face. In this article, a characterization of tremors initiated by blasts for roof caving during underground extraction of coal seam no. 507 in one of the collieries in the USCB has been made using three independent methods. By the basic seismic effect method, the effectiveness of blasting is evaluated according to the seismic energy of incited tremors and mass of explosives used. According to this method, selected blasts gave extremely good or excellent effect. An inversion of the seismic moment tensor enables determining the processes happening in the source of tremors. In the foci of provoked tremors the slip mechanism dominated or was clearly distinguished. The expected explosion had lesser significance or was not present. By the seismic source parameters analysis, among other things, an estimation of the stress drop in the focus or its size may be determined. The stress drop in the foci of provoked tremors was in the order of 105 Pa and the source radius, according to the Brune’s model, varied from 44.3 to 64.5 m. The results of the three mentioned methods were compared with each other and observations in situ. In all cases the roof falling was forced.

The Seismic Source Parameters of Tremors Provoked by Long-Hole Destress Blasting Executed During the Longwall Mining of a Coal Seam Under Variable Stress Conditions

“…The moment magnitude M w is usually smaller than the local magnitude M L (Baruah et al 2012;Lizurek et al 2015). The moment magnitude M w equalled from 0.45 to 1.78 for tremors provoked by the long-hole destress blasting in roof rocks of a coal seam (Wojtecki et al 2017a, b), and from 0.67 to 1.29 for tremors provoked by the blasting for roof falling (Wojtecki et al 2017a(Wojtecki et al , 2019.…”

“…It can be assumed that the entire frequency band of seismic waves, emitted from the area of blasting works, has been correctly recorded, and the signal didn't have amplitude distortion. When using the numerical integration method, the square of the amplitude in the following samples between the start and end markers, the sampling rate (200 Hz), the density of the rock mass (2600 kg/m 3 ), the attenuation of the rock The Seismic Source Parameters of Tremors Provoked by Long-Hole Destress Blasting Executed mass-quality factor (with a typical value of 30), the calibration factor calculated for each station, the distance between the focus and the seismic station, and directional seismic wave velocity (mostly of about 3800-4100 m/s) were taken into account for the calculation of the seismic energy at each of the seismic stations (Wojtecki and Gołda 2019). The energies of all the seismic stations were arithmetically averaged providing the final seismic energy of each tremor.…”

“…These tremors were provoked by the firing of 42-576 kg of explosives (Wojtecki et al 2017a;. The scalar seismic moment M 0 of tremors provoked by the blasting for roof falling equalled from 1.44 9 10 10 Nm to 1.47 9 10 11 Nm (Wojtecki et al 2017a(Wojtecki et al , 2019. The mass of explosives fired was between 24 and 120 kg (Wojtecki et al 2017a(Wojtecki et al , 2019.…”

“…It equals 2.34 (Brune 1970) or 1.32 (Madariaga 1976). The source radius r ranged between 20.1 and 65.8 m for tremors provoked by the long-hole destress blasting in roof rocks (Wojtecki et al 2017a, b), and from 30.2 to 64.5 m for tremors provoked by the blasting for roof falling (Wojtecki et al 2017a(Wojtecki et al , 2019.…”

“…Seismic source parameters give more complete information about processes occurring in the foci of tremors and their scale. These parameters, commonly calculated for natural seismic events or tremors induced by mining, can be determined for tremors provoked by long-hole destress blasting (Srinivasan et al 2005;Wojtecki et al 2017a, b) or blasting for roof falling (Wojtecki et al 2019). These parameters provide additional information about blasting effects.…”