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Effects of torpedo blasting on rockburst prevention during deep coal seam mining in the Upper Silesian Coal Basin
Accepted Manuscript Effects of torpedo blasting on rockburst prevention during deep coal seam mining in the Upper Silesian Coal Basin Ł. Wojtecki, P. Konicek, J. Schreiber PII:
S1674-7755(17)30089-6
DOI:
10.1016/j.jrmge.2017.03.014
Reference:
JRMGE 357
To appear in:
Journal of Rock Mechanics and Geotechnical Engineering
Received Date: 8 September 2016 Revised Date:
9 January 2017
Accepted Date: 20 March 2017
Please cite this article as: Wojtecki Ł, Konicek P, Schreiber J, Effects of torpedo blasting on rockburst prevention during deep coal seam mining in the Upper Silesian Coal Basin, Journal of Rock Mechanics and Geotechnical Engineering (2017), doi: 10.1016/j.jrmge.2017.03.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of torpedo blasting on rockburst prevention during deep coal seam mining in the Upper Silesian Coal Basin Ł. Wojtecki1, P. Konicek2, J. Schreiber2,* Polish Mining Group, Powstańców 30, Katowice 40-039, Poland
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Department of Geomechanics and Mining Research, Institute of Geonics, Czech Academy of Sciences, Studentska 1768, Ostrava-Poruba 708 00, Czech Republic
Received 8 September 2016; received in revised form 9 January 2017; accepted 20 March 2017
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Abstract: In the Upper Silesian Coal Basin (USCB), coal seams are exploited under progressively more difficult geological and mining conditions (greater depth, higher horizontal stress, more frequent occurrence of competent rock layers, etc.). Mining depth, dislocations and mining remnants in coal seams are the most important factors responsible for the occurrence of rockburst hazards. Longwall mining next to the mining edges of
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neighbouring coal seams is particularly disadvantageous. The levels of rockburst hazards are minimised via the use of rockburst prevention methods. One active prevention method is torpedo blasting in roof rocks. Torpedo blastings are performed in order to decrease local stress concentrations in rock masses and to fracture the roof rocks to prevent or minimise the impact of high-energy tremors on excavations. The estimation of the effectiveness of torpedo blasting is particularly important when mining is under difficult geological and mining conditions. Torpedo blasting is the
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main form of active rockburst prevention in the assigned colliery in the Polish part of the USCB. The effectiveness of blasting can be estimated using the seismic effect method, in which the seismic monitoring data and the mass of explosives are taken into consideration. The seismic effect method was developed in the Czech Republic and is always being used in collieries in the Czech part of the coal basin. Now, this method has been widely adopted for our selected colliery in the Polish part of the coal basin. The effectiveness of torpedo blastings in the faces and galleries of the assigned longwall in coal seam 506 has been estimated. The results show that the effectiveness of torpedo blastings for this longwall was significant in light of the seismic effect method, which corresponds to the in situ observations. The seismic effect method is regularly applied to estimating the blasting effectiveness in the selected colliery.
1. Introduction
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Keywords: rockburst prevention; torpedo blasting; seismic effect; Upper Silesian Coal Basin (USCB)
previously mined coal seams. The probability of seismic activity and rockburst is high in such mining. Torpedo blasting is the main form of active rockburst prevention method used during the longwall mining of
Silesian Coal Basin (USCB) is rockburst. This kind of disaster has been
coal seam 506 in our selected colliery in the Polish part of the USCB.
continuously investigated for many years (e.g. Budryk, 1938; Pelnar,
Disadvantageous geological and mining conditions, especially the large
1938; Parysiewicz, 1966; Straube et al., 1972; Konopko, 1984;
depth of exploitation, the mining edges of neighbouring coal seams 418
Holecko et al., 1999; Dubiński and Konopko, 2000; Takla et al., 2005;
and 502 and the fracturing of thick sandstone layers deposited above
Drzewiecki and Kabiesz, 2008; Holub et al., 2011).
coal seam 506, are the main factors responsible for the high level of
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One of the natural hazards occurring in collieries in the Upper
The occurrence of rockburst during underground mining process
seismic and rockburst hazards. To mitigate the above-mentioned hazards, active rockburst
methods are generally divided into two types: passive and active.
prevention method, mainly in the form of torpedo blasting, was applied
Within active rockburst prevention methods, torpedo blasting (long-
in the faces and galleries of the assigned longwall in coal seam 506.
hole destress blasting) in roof rocks plays an important role. The main
The effectiveness of the torpedo blastings for rockburst prevention has
purpose of torpedo blasting in roof rocks is to reduce stress
been estimated via the seismic effect method (Konicek et al., 2013).
concentrations occurring in these rocks, although the generation of rock
2. Geological and mining conditions
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leads to the development of rockburst prevention methods. These
fractures is also important due to the creation of zones in which the energy of strong tremors can be dissipated. This type of destress
Coal seam 506 is deposited at a depth between 1018 m and 1057 m
blasting has been widely used in the USCB for many years (e.g.
below the surface. Longwall mining of coal seam 506 was performed at
Dvorsky et al., 2003; Dvorsky and Konicek, 2005; Przeczek et al.,
the extracting level. The thickness of the coal seam near the longwall
2005; Konicek and Przeczek, 2008; Konicek et al., 2011).
cross-cut was about 2.5 m, due to the joint with coal seam 505/1. The
The estimation of the effectiveness of torpedo blasting is an critically
thickness of coal seam was finally measured to be 1.4-1.65 m. Coal
important issue, especially in mining close to the mining edges of
seam 506 is separated from coal seam 505/1 (with thickness of 0.650.95 m) by a layer of shale with thickness of 0.8 m. The dip angle of
*Corresponding author. E-mail: [email protected]
coal seam 506 equals 5°–12°, generally to the south. The lithological structure of the rock mass in the area is shown in Fig. 1. The direct roof
Tel.: +420 731 589 423
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of coal seam 506 consists of shale and sandy shale rocks. Locally in the
with “S” represent the seismic stations. By using the seismic network, a
direct roof, a layer of sandstone exists. In the roof, three thick layers of
dataset to study the site was obtained.
sandstones are present, 39 m, 60 m and 104 m above coal seam 506,
The seismic energy EICM of tremors was calculated using numerical
respectively (see Fig. 1). Fracturing of these thick layers of sandstone
integration method. The square of the amplitude in the following
was mainly responsible for the occurrence of high-energy tremors. The
samples, sampling rate, distance between focus and seismic station,
floor of coal seam 506 encompasses thin layers of shale and sandy
density and attenuation coefficient of rock mass, seismic wave velocity,
shale; below them, a sandstone layer is deposited (Fig. 1).
and the calibration factors, were the parameters for energy calculation
The selected longwall had been designed with caving, mainly between galleries 2 and 3, which is shown in Fig. 2. The longwall
on each seismic station. Each tremor had a specific seismic energy EICM by averaging the calculated values of all seismic stations.
initially advanced from the east, near fault I (throw h = 50 m), and then
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advanced to the west along the diagonal fault (throw h = 110 m). The end of the longwall was arranged to the east of the protecting pillars for drifts on the levels of 840 m and 1000 m. Mining edges of coal seams 418 and 502 (130–141 m and 60–82 m above seam 506, respectively) were present above the longwall field. The longwall mining of coal seam 506 lasted for nearly two years. Longwall face advances are Other disadvantageous factors affecting the hazard level of rockburst are the depth of exploitation (up to 1057 m) and the corresponding high in situ stress level (24.5–31 MPa), the tendency of coal seam 506 to of local faults with maximum throw of 2.2 m.
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burst (uniaxial compressive strength Rc = 28 MPa), and the occurrence
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shown in Fig. 2.
Further exploitation of other coal seams in this part of the coal bed will continue, according to exploitation range of coal seam 506. From this point, clear longwall mining has strategic importance. 3. Seismic monitoring method
The seismic network consisted of 16 seismic stations, located in
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underground excavations with depths ranging from 320 m to 1000 m. Vertical-component sensors including SPI-70 seismometers and DLM2001 geophones composed the network; however, seismometers were
the main component. The sampling rate was 5000 samples per second, with the time provided by global positioning system (GPS). The
to
average error of epicentral location ranged from 35 m to 53 m, while
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the average error of hypocentral location ranged from 63 m to 71 m.
Generally, the error of source location increases from the west to the east, because the seismic stations are located mostly in the west. The configuration of the seismic network used for the seismic monitoring of
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studied longwall is presented in Fig. 3, in which the squares marked
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Fig. 1. Lithological structure of rock mass in the area of longwall in coal seam 506.
Fig. 2. Map of coal seam 506.
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given by Dubiński and Wierzchowska (1973). The strongest tremors (3
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× 107 J and 2 × 108 J) were associated with the activation of the diagonal fault. The occurrence of the other high-energy tremors was a consequence of fracturing of the thick layers of sandstone deposited in the roof of coal seam 506. Also, the influence of mining edges of coal seams 418 and 502 (130–141 m and 60–82 m above seam 506, respectively) on the occurrence of high-energy tremors was significant. The concentration of high-energy tremor sources in the vicinity of mining edges of coal seams 418 and 502 was clear, which is shown in Fig. 4. Besides, most high-energy tremors occurred ahead of the
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longwall face.
Because of those high-level seismic activities and associated high intensity of rockburst hazards during longwall advance, active rockburst prevention method should be applied. 4. Active rockburst prevention for longwall in coal seam 506
Fig. 3. Configuration of the seismic network.
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Active rockburst prevention method for the assigned longwall was implemented basically in the form of torpedo blasting in the roof rocks.
indicated that high-level rockburst hazards were observed in this
The diameter of each blasthole was 76 mm. Special methane explosive
excavation. The aforementioned difficult geological and mining
Emulinit PM was used in each torpedo blasting. The heat explosion of
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The intensity of seismic activities recorded during longwall advance
conditions were reflected in the observed seismic activities. The total
this explosive material is 2278 kJ/kg, and the specific volume of
number of recorded seismic events during the study period was 2190,
gaseous products of explosion is 767 dm3/kg (NITROERG, 2016). The
with a total released tremor energy of 3.8 × 10 J. A total of 95 high-
minimum velocity of detonation is 4000 m/s (NITROERG, 2016).
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energy tremors occurred: 73 events with energy in the order of 10 J
Immediate methane electric detonators
(1.68 ≤ ML < 2.21; ML is the value of local magnitude), 20 events with
explosives. All blastholes within one blasting stage were detonated
were used to initiate the
energy in the order of 106 J (2.21 ≤ ML < 2.74), one tremor with energy
simultaneously without delay.
of 3 × 107 J (ML = 2.99) and one tremor with energy of 2 × 108 J (ML = 3.42). The values of ML have been calculated according to the formula
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Fig. 4. Location of high-energy tremor sources induced during longwall mining of coal seam 506. As mentioned previously, most induced high-energy tremors
occurred ahead of the longwall face, so the main purpose of the torpedo
loaded. The first blasting provoked a tremor with energy of 2 × 104 J,
and the second blasting provoked a tremor with energy of 8 × 103 J. When the longwall approached the mining edge of coal seam 502,
working face of the longwall. Seismic tomography was also applied
blastings in front of the longwall face restarted. The blastholes deviated
frequently for stress level confirmation in the rock mass ahead of the
from the direction perpendicular to the longwall face to the north west
longwall face. The blasting parameters were determined according to
and south west at an angle of about 20°. These blastholes were inclined
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blastings was to destress the surrounding rock mass in front of the
both seismological and tomographical observations. Analyses of
upwards at an angle of 45° to the horizontal, and were 45 m long
geological and mining conditions were performed as well.
approximately. During each destress blasting stage, 288 kg of explosives were detonated. In total, 10 blastings were performed, which provoked tremors with energy from 6 × 103 J to 9 × 104 J. These
loaded into four boreholes (arranged in two pairs) were detonated. The
blastings were executed along the advancing direction of the longwall,
blastholes deviated from the longwall face to the north west and south
with an average distance of 22 m. Besides, one blasting was performed
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Before the first longwall face advance, two blasting stages were performed in the longwall cross-cut. Each time, 288 kg of explosives
west at an angle of about 20°. The blastholes were 30 m long and
in gallery 2. In two boreholes arranged as a pair that deviated from the
inclined upwards at an angle of 30° to the horizontal direction.
direction perpendicular to the gallery 2 to the south west and south east at an angle of about 30°, 144 kg (72×2=144 kg) of explosives were detonated. This blasting provoked a tremor with energy of 2 × 104 J.
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Explosive material occupied around 15 m of each borehole, with the rest being filled by stemming. These blastings provoked tremors with energy of 4 × 104 J and 6 × 104 J.
The results of seismic investigations, recorded seismic activities and
After the longwall started its advance, two other blasting stages were
analysis of current mining conditions indicated that one pair of
performed within the start-up period. During the six phases of blastings
blastholes should be located closer to gallery 3, and the other pair
(arranged in three pairs deviating from the longwall face to the north
should be located in the middle of the longwall face. This location
west and south west at an angle of about 30°, inclined upwards at an
arrangement would be the most suitable for destressing rock mass and
angle of 45° to the horizontal), 576 kg of explosives were detonated.
dissipating energy resulting from strong tremors. The deviation and
Using these blastings, two tremors with energy of 9 × 10 J and 8 × 10
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inclination of the blastholes remained the same. The blastholes deviated
J were provoked. The purpose of active rockburst prevention was to
from the direction perpendicular to the longwall face to the north west
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destress the roof rocks to make caving easier.
and south west at an angle of about 20°, and were inclined upwards at
According to the seismic tomography results and the concentration
an angle of 45° to the horizontal. These blastholes were about 45 m
of high-energy tremors sources near gallery 2, two other blasting stages
long (see Fig. 5). Explosives weighting 72 kg were loaded into each
were designed and executed in this excavation. In the first stage, two
blasthole, which means during each blasting, 288 kg (72×4=288 kg) of
blastholes were used, while in the second one, four were employed.
explosives were detonated. The explosive material occupied nearly 15
These blastholes were 40 m long and inclined upwards at an angle of
m of blasthole (see Fig. 5). The column of explosive material was
65° to the horizontal. Into each blasthole, 72 kg of explosives were
located between coal seams 504 and 503, in alternating layers of sandstone, sandy shale and shale. Blastings with such parameters were
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performed until the end of the longwall. It should be noted that
to gallery 3. They were inclined upwards at an angle of 45° to the
blastings to fracture the thick sandstone layers probably should give
horizontal. Explosives weighting 72 kg were loaded into each
better results, but at this time the system for pneumatic loading of
blasthole. Blastings in gallery 3 were performed to destress the area
boreholes longer than 70 m had only just been tested in assigned
where the distance between the longwall and the diagonal fault was the
colliery. These parameters obtained from the system were then used as
smallest. Tremors provoked by the 29 torpedo blastings, where the
the optimal values concerning the difficult geological conditions and
blastholes were drilled only in the longwall face, had energy from 1 ×
technical possibilities. Torpedo blastings in layers of sandstone were
104 J to 6 × 104 J. Tremors provoked by blastings executed together in
performed in the second longwall, designed in the latter stage, of the
the the longwall face and gallery 3 had energy from 2 × 104 J to 9 × 104
excavation of coal seam 506.
J. All 34 aforementioned blastings were performed in average steps of 12 m along the advancing direction of the longwall.
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Near the entrance to gallery 2, mining edges of coal seams 418 and 502 as well as borders of protecting pillars for drifts coexisted at the level of 840 m and 1000 m, which greatly influenced the stress level in this area. To destress the rock mass and to protect gallery 2, torpedo blastings in this excavation were designed. Explosives weighting 48 kg in each blasthole were loaded. These blastholes were 35 m long, and
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were inclined upwards at an angle of 55° to the horizontal. A total of 4 blastings were performed, with the number of blastholes for each blasting case of 5, 5, 4 and 4, respectively. The provoked tremors had energy from 2 × 104 J to 3 × 104 J. The locations of all blastholes of the Fig. 5. Side view of blasthole.
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torpedo blastings performed for the longwall in coal seam 506 in association with the sources of the provoked tremors are shown in Fig. 6.
A total of 34 blastings applied to the longwall face were carried out with the aforementioned parameters, and 5 of 34 blastings were
The effectiveness of the torpedo blastings in the longwall face and
galleries has been calculated via the seismic effect method.
executed together with torpedo blastings in gallery 3. Blastholes in
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gallery 3, drilled to the south, were about 45 m long and perpendicular
Fig. 6. Location of blastholes and sources of provoked tremors during longwall mining of coal seam 506. 5. Evaluation of effectiveness of torpedo blastings
energy released in the rock mass when blasting to the energy of the particular detonated explosives (Konicek et al., 2013). It can be
The effectiveness of torpedo blasting is connected to the stress calculated seismic effect (SE). This methodology was established in the
calculated according to the following formula: E SE = ICM K ICMQ
Czech part of the USCB by Knotek et al. (1985), and subsequently
where EICM is the seismic energy calculated by the seismic network in
release in the rock mass. This stress release is evaluated using the
(1)
verified by Konicek et al. (2013), and adjusted according to the
the investigated coal mine (J), Q is the mass of the explosive charge
conditions of the assigned colliery in the Polish part of the USCB
(kg), and KICM is the coefficient characterising the conditions of the
(Wojtecki and Konicek, 2016). SE is defined as the ratio of seismic
assigned mine (J/kg).
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KICM must be determined by the conditions under which the seismic
value (i.e. Q). Based on this analytical procedure, a linear dependence
monitoring is carried out, and the seismic energy of the recorded
between the transformed seismic energy data (lnEICM) and the non-
seismic events is calculated in the same way. KICM is determined
transformed weight of explosive charge data (Q) was identified (see
according to the method proposed by Konicek et al. (2013). In this
Fig. 7). This dependence relation is represented by the regression line
method, generally, the statistical data analyses of seismic energy and
(Wojtecki and Konicek, 2016):
weight of the explosive charge obtained from in situ monitoring are
lnEICM = 9.7925 + 0.0022Q
(2)
performed. To determine KICM of the selected colliery, the dataset from nine longwalls had been taken into consideration (a total of 256 torpedo
The significance of this dependence relation between two variables can be generally evaluated using various correlation coefficients (e.g.
The basis of this methodology in light of the energy balance after destress blasting can be found in Konicek et al. (2013), as well as more details about necessary statistical analyses and recommended approaches (Konicek et al., 2013). The results for the conditions of our
Pearson R, R 2 , Spearman, Kendal tau, Gamma). Pearson R correlation coefficient is a widely used type of correlation coefficient in parametric
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blastings in the roof rocks).
statistics methods; it is also called linear or product-moment correlation coefficient. Pearson R correlation assumes that the two variables are measured on at least interval scales. Pearson R is 0.46 and R 2 is 0.21
Due to the fact that the applied methodology (Knotek et al., 1985, 2013) is based on linear regression, it must be proven that the data for analyses are normally distributed. Our statistical analyses, including exploratory analysis, were aimed at determination of data distribution characteristics, error elimination and correlation analysis for the confirmation of the dependence between variables, as well as
linear dependence between lnEICM and Q is relatively high according to the Person R of 0.46, despite the fact that the R 2 is relatively low. The main reason is that the investigated data do not come from laboratory measurements but from in situ measurements, which are affected by significant inhomogeneity and variability of natural and mining conditions.
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dispersion analysis (Wojtecki and Konicek, 2016). In the exploratory
for the evaluated dataset in this case. We consider that the identified
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selected Polish colliery are now described as follows.
analysis, a logarithmic transformation (i.e. lnEICM) was done to the
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seismic energy, while the weight of the explosive charge kept the origin
Fig. 7. Transformed seismic energy as a function of weight of explosive charge in selected colliery (Wojtecki and Konicek, 2016).
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The standard deviation of the transformed seismic energy in this relationship is 0.633. Data located under the straight line, which was
Very good Excellent Extremely well
3.5 ≤ SE < 5.9 SE ≥ 5.9
25.1 19.5 5.6
parallel to the regression line and shifted by the standard deviation of the transformed seismic energy, were then selected, as depicted in Fig.
6. Results
7. Then the median value of this new dataset was used to determine the coefficient KICM, which turned out to be 59.23 J/kg.
During the longwall mining of coal seam 506, a total of 48 blastings were performed in the longwall face. In five cases, these blastings were
system for the evaluation of SE. This classification was made according
performed together with blastings in gallery 3. For each of the 43 self-
to the probability distribution of the calculated seismic effects
contained blastings in the longwall face, the SE values were calculated.
according to Eq. (1).
The effectiveness of each blasting was estimated on the basis of the
From the entire SE dataset, the first quintile, median, third quintile
calculated SE value (see Table 2).
and maximum values were determined to be 1.4, 2.3, 3.5, and 5.9, respectively. The outliers were also determined. Statistical analysis of
Table 2. Parameters of destress blastings in roof rocks performed in the longwall face.
the SE dataset from the selected colliery is shown in Fig. 8. On the release due to torpedo blasting was distinguished. If the SE is lower than the first quintile (recorded energy released by blasting is less than 1.4 times the explosion energy), the effect of blasting is insignificant from a stress release point of view. A value of seismic effect between the first quintile and median means that the blasting effect can be
Q (kg) 288 288 576 576 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288
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assumed as good. A very good effect is provided by blasting with the
Date (YYMM-DD) 2010-03-18 2010-03-20 2010-04-26 2010-05-10 2010-08-30 2010-09-20 2010-10-18 2010-11-14 2010-12-05 2010-12-19 2011-01-09 2011-01-30 2011-02-14 2011-02-28 2011-03-07 2011-03-13 2011-03-21 2011-03-27 2011-04-03 2011-04-10 2011-04-18 2011-04-26 2011-05-09 2011-05-16 2011-05-22 2011-05-30 2011-06-20 2011-07-03 2011-07-25 2011-08-08 2011-08-29 2011-09-11 2011-09-26 2011-10-16 2011-10-30 2011-11-13 2011-11-27 2011-12-05 2011-12-18 2012-01-02 2012-01-16 2012-02-06 2012-05-14
seismic effect ranging between median and third quintile. Seismic
effect between the third quintile and the maximum can be assumed as
excellent. Seismic effects higher than the maximum (i.e. outliers) are extremely well (recorded energy released by blasting is higher than 5.9 times the explosion energy).
The classification system developed to evaluate SE values, based on
the criteria obtained from data distribution probabilities and Eq. (1), is
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presented in Table 1.
EICM (J)
SE
Evaluation of SE
4 × 104 6 × 104 9 × 104 8 × 104 2 × 104 3 × 104 9 × 104 8 × 104 8 × 104 8 × 104 4 × 104 4 × 104 6 × 103 5 × 104 5 × 104 3 × 104 3 × 104 2 × 104 2 × 104 3 × 104 4 × 104 2 × 104 3 × 104 2 × 104 2 × 104 1 × 104 5 × 104 4 × 104 5 × 104 1 × 104 4 × 104 2 × 104 2 × 104 2 × 104 3 × 104 2 × 104 6 × 104 3 × 104 4 × 104 3 × 104 4 × 104 2 × 104 2 × 104
2.3 3.5 2.6 2.3 1.2 1.8 5.3 4.7 4.7 4.7 2.3 2.3 0.4 2.9 2.9 1.8 1.8 1.2 1.2 1.8 2.3 1.2 1.8 1.2 1.2 0.6 2.9 2.3 2.9 0.6 2.3 1.2 1.2 1.2 1.8 1.2 3.5 1.8 2.3 1.8 2.3 1.2 1.2
Very good Good Very good Very good Insignificant Good Excellent Excellent Excellent Excellent Very good Very good Insignificant Very good Very good Good Good Insignificant Insignificant Good Very good Insignificant Good Insignificant Insignificant Insignificant Very good Very good Very good Insignificant Very good Insignificant Insignificant Insignificant Good Insignificant Excellent Good Very good Good Very good Insignificant Insignificant
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basis of the mentioned statistical parameters, the degree of stress
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The obtained value of KICM was used to establish the classification
7. Discussion Fig. 8. Statistical analysis of the SE dataset obtained from the assigned colliery. Table 1. Classification system for the evaluation of SE in the assigned colliery (Wojtecki and Konicek, 2016). SE value SE < 1.4 1.4 ≤ SE < 2.3
Evaluation of seismic effect Insignificant Good
Percentage of dataset (%) 20.7 29.1
Among the 43 tremors induced by destress blastings in the longwall face, as presented in Table 2, the seismic effect varied from insignificant to excellent. In light of seismic effect method, about 20.9% of the blastings had a good effect, 32.6% had a very good effect,
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and about 11.6% were excellent. The excellent effects were observed
geology, mining system, blasting parameters, and seismic network
when the longwall approached the mining edge of coal seam 502.
parameters) presented in any concrete coal mine.
In general, the designed active rockburst prevention procedure –
The seismic effect method was applied to estimating the torpedo
torpedo blastings in roof rocks in the longwall face – can be considered
blasting (long-hole destress blasting) effectiveness in the roof rocks,
appropriate based on the obtained seismic effect values. In most cases
which were executed in the longwall in coal seam 506, in our selected
(65.1%), the performed destress blastings had impact on the stress field
colliery in the Polish part of the USCB. In light of our results, the
in the area ahead of the longwall face.
effectiveness of the blastings was basically good, very good and excellent, which agreed with in situ observations. The dynamic
provoked geomechanical processes associated with stress release.
influence of high-energy tremors did not induce any damage to the
Generally, the tremors recorded after destress blastings had higher
longwall or its galleries. Longwall mining of coal seam 506 was
energy than it would appear only from the detonation of explosives. It
completed successfully, despite of high level of the seismic and
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Calculations indicate that most destress blastings in the longwall face
can be assumed that most of the destress blastings in roof rocks had
rockburst hazards. In this regard, destressing the roof rocks ahead of
brought new and advantageous stress equilibriums ahead of the
longwall face can be assumed to be effective.
longwall face. The other 34.9% of blastings were insignificant. The recorded
Conflict of interest
energy of the provoked tremors came mostly from the detonation of explosives. Most of these blastings were performed when the longwall
The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no
contained blastings in gallery 2 has been evaluated as well. The results
significant financial support for this work that could have influenced its outcome. References
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are shown in Table 3.
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had almost finished its advance. The effectiveness of seven self-
Table 3. Parameters of destress blasting in roof rocks performed in gallery 2.
Budryk W. Rockburst phenomena and prevention of their effects. Przegląd Górniczo-
Date (YYMM-DD) 2010-04-12 2010-03-20 2010-09-03 2011-12-10 2012-01-14 2012-02-12 2012-03-16
Q (kg)
EICM (J)
SE
Evaluation of SE
120 240 144 240 240 288 192
2 × 104 8 × 103 2 × 104 2 × 104 3 × 104 2 × 104 3 ×x 104
2.8 0.6 2.3 1.4 2.1 1.2 2.6
Very good Insignificant Very good Good Good Insignificant Very good
Hutniczy 1938;12:25-27 (in Polish).
Dubiński J, Wierzchowska Z. Methods for the calculation of tremors seismic energy in the Upper Silesia. Katowice, Poland: Central Mining Institute; 1973.
Dubiński J, Konopko W. Rockbursts – assessment, prediction and control – working rules. Katowice, Poland: Central Mining Institute; 2000 (in Polish).
Dvorsky P, Konicek P, Morkovska E, Palla L. Rock blasting as a rockburst control
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measures in the safety pillar of SW crosscuts at Lazy Colliery in Orlová. In:
The first and the second torpedo blastings in gallery 2, near the
longwall cross-cut, gave divergent results. Despite the fact that the
Proceedings of the 10th International Scientific-Technical Conference-Rockbursts 2003. Ustroń; 2003. p. 37-45.
detonated amount of explosives was doubled in the second blasting, the
Dvorsky P, Konicek P. Systems of rock blasting as a rockburst measure in the Czech
energy of the provoked tremor was lower than that provoked by the
part of Upper Silesian Coal Basin. In: Proceedings of the 6th International
first blasting. Probably, the first blasting destressed the rock mass
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effectively. The third torpedo blasting performed in gallery 2 had a very good effect, similar to the effect of the first blasting.
Most blastings performed in gallery 2 near its entrance, where the mining edges of coal seams 418 and 502 and borders of protecting
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pillars for drifts at the level of 840 m and 1000 m occurred together,
Symposium on Rockburst and Seismicity in Mines. Perth; 2005. p. 493–496. Drzewiecki J, Kabiesz J. Dynamic events in roof strata – occurrence and prevention. Coal Science & Technology Magazine 2008;235:55–57. Holecko J, Ptacek J, Takla G, Konecny P. Rock bursts in the Czech part of the Upper Silesian Coal Basin – Features, theoretical models and conclusions for practice. In: Proceedings of the 9th ISRM Congress. Paris, France: International Society for Rock Mechanics; 1999. p. 1101–1104.
had good and very good effects. One blasting had an insignificant
Holub K, Rušajová J, Holecko J. Particle velocity generated by rockburst during
result, probably due to the high efficiency of previous torpedo blastings
exploitation of the longwall and its impact on the workings. International Journal
in this area. The last torpedo blasting in gallery 2 had a very good effect. Blastholes for this phase were drilled where the stress level was theoretically the highest. 8. Conclusions
of Rock Mechanics and Mining Sciences 2011;48(6):942–949. Konopko W. About rockbursts and associated phenomena in the USCB. Bezpieczeństwo Pracy w Górnictwie 1984;4:18-22 (in Polish). Knotek S, Matusek Z, Skrabis A, Janas P, Zamarski B, Stas B. Research of geomechanics evaluation of rock mass due to geophysical method. Ostrava, Czech Republic: VVUU; 1985 (in Czech).
Torpedo blasting in roof rocks is the main form of active rockburst
Konicek P, Przeczek A. Study of selected cases of local stress reduction due to rock
prevention in the assigned colliery. An evaluation of the effectiveness
blasting. In: Proceedings of the 15th International Scientific-Technical Conference
of torpedo blasting is particularly important for longwalls in difficult geological and mining conditions. This estimation can be made using the seismic effect method. This method gives a rapid answer to the effectiveness of blasting according to weight of explosives used. The advantage of this method is its adaptation to the local conditions (eg.
GZN 2008. Targanice: Główny Instytut Górnictwa; 2008. p. 143–161. Konicek P, Konecny P, Ptacek J. Destress rock blasting as a rockburst control technique. In: Proceedings of the 12th ISRM Congress. Bejing: International Society for Rock Mechanics; 2011. p. 1221–1226. Konicek P, Soucek K, Stas L, Singh R. Long-hole destress blasting for rockburst control during deep underground coal mining. International Journal of Rock Mechanics and Mining Sciences 2013;61:141–153.
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NITROERG. NITROERG, Grupa KGHM. Bierun: NITROERG S.A., [cit. 2016-11-15]. http://www.nitroerg.pl, 2016. [online]
seismicity, rokckburst prevention and destress blasting. Jan Schreiber has been a PhD student and research assistant in the Institute of
Pelnar A. Rockbursts in Ostrava-Karvina coalfield. Hornický Věstník, Hornické a Hutnické Listy 1938;2:25–58 (in Czech).
Geonics of the Czech Academy of Sciences since 2015. The same year, he started earning his PhD degree on the Faculty of Civil Engineering at VSB-Technical
Parysiewicz W. Rockbursts in mines. Katowice: Fa “Śląsk”; 1966 (in Polish).
University of Ostrava. He obtained his MSc degree in Geology in 2010 in the Masaryk
Przeczek A, Dvorsky P, Konicek P. System of rock blasting in boreholes diameter more University Brno. Since 2012 he works as a Geomechanic and from 2016 as a senior than 100 mm as a rockburst measure. In: Proceedings of the 12th International Scientific-Technical Conference-Rockbursts 2004. Ostrava: Green Gas, DPB;
Geophysicist in the Department of Gemechanics and Geophysics in the mining company Green Gas DPB, Inc. in Paskov. Throughout his career, Jan has been part of
2005. p. 253–269.
teams that solves the problems related to mining and ground seismic monitoring
V, Vavro M. Rockbursts in carboniferous rock mass. SNTL 1972;6: 7-12 (in
systems to build up a reliable set of data for civil engineering and geomechanical
Czech) .
evaluations.
Takla G, Ptacek J, Holecko J, Konicek P. Stress state determination and prediction in rock mass with rockburst risk in Ostrava–Karvina coal basin. In: Proceedings of ISRM International Symposium - EUROCK 2005, Brno: International Society for Rock Mechanics; 2005. p. 625–628. Wojtecki Ł, Konicek P. Estimation of active rockburst prevention effectiveness during longwall mining under disadvantageous geological and mining conditions. Journal
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of Sustainable Mining 2016;15(1):1-7.
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Straube R, Brothanek J, Harasek V, Kostal Z, Kovacs Z, Mikeska J, Padara Z, Rozehnal
Lukasz Wojtecki currently works in the Polish Mining Group, Rockburst Department. He obtained his MSc degree in geology in 2005, MSc degree in physics in
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2006 and PhD degree in mining and engineering geology in 2014. He worked as a mine geophysicist in one of the hardest coal mines in the Polish part of the Upper Silesian Coal Basin from 2007 to 2012. From 2012 to present, he is a senior specialist in mine geophysics and head of the Mine Geophysics Station. Lukasz Wojtecki has investigated technical and scientific problems related to induced seismicity, seismic investigation, natural hazards, rockburst prevention and destress blasting effectiveness.
Petr Konicek obtained his MSc degree in Mining Geology in 1990 and PhD degree in Geotechnics in 2009. He worked in coal mining industry in the field of mining geology (1990-1997) and in expert service company for coal mining companies in the field of hydrogeology and geomechanics (1997-2009). He works at the Institute of Geonics of the Czech Academy of Sciences as a senior scientist and project leader in Department of Geomechanics and Mining Research from 2009. Since 2016, he has been an Associate Professor in Geotechnics in VSB–Technical University of Ostrava. Throughout his career, Petr has investigated scientific problems related to mining geology, rock mechanics, induced
9
S1674-7755(17)30089-6
DOI:
10.1016/j.jrmge.2017.03.014
Reference:
JRMGE 357
To appear in:
Journal of Rock Mechanics and Geotechnical Engineering
Received Date: 8 September 2016 Revised Date:
9 January 2017
Accepted Date: 20 March 2017
Please cite this article as: Wojtecki Ł, Konicek P, Schreiber J, Effects of torpedo blasting on rockburst prevention during deep coal seam mining in the Upper Silesian Coal Basin, Journal of Rock Mechanics and Geotechnical Engineering (2017), doi: 10.1016/j.jrmge.2017.03.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of torpedo blasting on rockburst prevention during deep coal seam mining in the Upper Silesian Coal Basin Ł. Wojtecki1, P. Konicek2, J. Schreiber2,* Polish Mining Group, Powstańców 30, Katowice 40-039, Poland
2
Department of Geomechanics and Mining Research, Institute of Geonics, Czech Academy of Sciences, Studentska 1768, Ostrava-Poruba 708 00, Czech Republic
Received 8 September 2016; received in revised form 9 January 2017; accepted 20 March 2017
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Abstract: In the Upper Silesian Coal Basin (USCB), coal seams are exploited under progressively more difficult geological and mining conditions (greater depth, higher horizontal stress, more frequent occurrence of competent rock layers, etc.). Mining depth, dislocations and mining remnants in coal seams are the most important factors responsible for the occurrence of rockburst hazards. Longwall mining next to the mining edges of
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neighbouring coal seams is particularly disadvantageous. The levels of rockburst hazards are minimised via the use of rockburst prevention methods. One active prevention method is torpedo blasting in roof rocks. Torpedo blastings are performed in order to decrease local stress concentrations in rock masses and to fracture the roof rocks to prevent or minimise the impact of high-energy tremors on excavations. The estimation of the effectiveness of torpedo blasting is particularly important when mining is under difficult geological and mining conditions. Torpedo blasting is the
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main form of active rockburst prevention in the assigned colliery in the Polish part of the USCB. The effectiveness of blasting can be estimated using the seismic effect method, in which the seismic monitoring data and the mass of explosives are taken into consideration. The seismic effect method was developed in the Czech Republic and is always being used in collieries in the Czech part of the coal basin. Now, this method has been widely adopted for our selected colliery in the Polish part of the coal basin. The effectiveness of torpedo blastings in the faces and galleries of the assigned longwall in coal seam 506 has been estimated. The results show that the effectiveness of torpedo blastings for this longwall was significant in light of the seismic effect method, which corresponds to the in situ observations. The seismic effect method is regularly applied to estimating the blasting effectiveness in the selected colliery.
1. Introduction
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Keywords: rockburst prevention; torpedo blasting; seismic effect; Upper Silesian Coal Basin (USCB)
previously mined coal seams. The probability of seismic activity and rockburst is high in such mining. Torpedo blasting is the main form of active rockburst prevention method used during the longwall mining of
Silesian Coal Basin (USCB) is rockburst. This kind of disaster has been
coal seam 506 in our selected colliery in the Polish part of the USCB.
continuously investigated for many years (e.g. Budryk, 1938; Pelnar,
Disadvantageous geological and mining conditions, especially the large
1938; Parysiewicz, 1966; Straube et al., 1972; Konopko, 1984;
depth of exploitation, the mining edges of neighbouring coal seams 418
Holecko et al., 1999; Dubiński and Konopko, 2000; Takla et al., 2005;
and 502 and the fracturing of thick sandstone layers deposited above
Drzewiecki and Kabiesz, 2008; Holub et al., 2011).
coal seam 506, are the main factors responsible for the high level of
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One of the natural hazards occurring in collieries in the Upper
The occurrence of rockburst during underground mining process
seismic and rockburst hazards. To mitigate the above-mentioned hazards, active rockburst
methods are generally divided into two types: passive and active.
prevention method, mainly in the form of torpedo blasting, was applied
Within active rockburst prevention methods, torpedo blasting (long-
in the faces and galleries of the assigned longwall in coal seam 506.
hole destress blasting) in roof rocks plays an important role. The main
The effectiveness of the torpedo blastings for rockburst prevention has
purpose of torpedo blasting in roof rocks is to reduce stress
been estimated via the seismic effect method (Konicek et al., 2013).
concentrations occurring in these rocks, although the generation of rock
2. Geological and mining conditions
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leads to the development of rockburst prevention methods. These
fractures is also important due to the creation of zones in which the energy of strong tremors can be dissipated. This type of destress
Coal seam 506 is deposited at a depth between 1018 m and 1057 m
blasting has been widely used in the USCB for many years (e.g.
below the surface. Longwall mining of coal seam 506 was performed at
Dvorsky et al., 2003; Dvorsky and Konicek, 2005; Przeczek et al.,
the extracting level. The thickness of the coal seam near the longwall
2005; Konicek and Przeczek, 2008; Konicek et al., 2011).
cross-cut was about 2.5 m, due to the joint with coal seam 505/1. The
The estimation of the effectiveness of torpedo blasting is an critically
thickness of coal seam was finally measured to be 1.4-1.65 m. Coal
important issue, especially in mining close to the mining edges of
seam 506 is separated from coal seam 505/1 (with thickness of 0.650.95 m) by a layer of shale with thickness of 0.8 m. The dip angle of
*Corresponding author. E-mail: [email protected]
coal seam 506 equals 5°–12°, generally to the south. The lithological structure of the rock mass in the area is shown in Fig. 1. The direct roof
Tel.: +420 731 589 423
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of coal seam 506 consists of shale and sandy shale rocks. Locally in the
with “S” represent the seismic stations. By using the seismic network, a
direct roof, a layer of sandstone exists. In the roof, three thick layers of
dataset to study the site was obtained.
sandstones are present, 39 m, 60 m and 104 m above coal seam 506,
The seismic energy EICM of tremors was calculated using numerical
respectively (see Fig. 1). Fracturing of these thick layers of sandstone
integration method. The square of the amplitude in the following
was mainly responsible for the occurrence of high-energy tremors. The
samples, sampling rate, distance between focus and seismic station,
floor of coal seam 506 encompasses thin layers of shale and sandy
density and attenuation coefficient of rock mass, seismic wave velocity,
shale; below them, a sandstone layer is deposited (Fig. 1).
and the calibration factors, were the parameters for energy calculation
The selected longwall had been designed with caving, mainly between galleries 2 and 3, which is shown in Fig. 2. The longwall
on each seismic station. Each tremor had a specific seismic energy EICM by averaging the calculated values of all seismic stations.
initially advanced from the east, near fault I (throw h = 50 m), and then
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advanced to the west along the diagonal fault (throw h = 110 m). The end of the longwall was arranged to the east of the protecting pillars for drifts on the levels of 840 m and 1000 m. Mining edges of coal seams 418 and 502 (130–141 m and 60–82 m above seam 506, respectively) were present above the longwall field. The longwall mining of coal seam 506 lasted for nearly two years. Longwall face advances are Other disadvantageous factors affecting the hazard level of rockburst are the depth of exploitation (up to 1057 m) and the corresponding high in situ stress level (24.5–31 MPa), the tendency of coal seam 506 to of local faults with maximum throw of 2.2 m.
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burst (uniaxial compressive strength Rc = 28 MPa), and the occurrence
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shown in Fig. 2.
Further exploitation of other coal seams in this part of the coal bed will continue, according to exploitation range of coal seam 506. From this point, clear longwall mining has strategic importance. 3. Seismic monitoring method
The seismic network consisted of 16 seismic stations, located in
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underground excavations with depths ranging from 320 m to 1000 m. Vertical-component sensors including SPI-70 seismometers and DLM2001 geophones composed the network; however, seismometers were
the main component. The sampling rate was 5000 samples per second, with the time provided by global positioning system (GPS). The
to
average error of epicentral location ranged from 35 m to 53 m, while
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the average error of hypocentral location ranged from 63 m to 71 m.
Generally, the error of source location increases from the west to the east, because the seismic stations are located mostly in the west. The configuration of the seismic network used for the seismic monitoring of
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studied longwall is presented in Fig. 3, in which the squares marked
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Fig. 1. Lithological structure of rock mass in the area of longwall in coal seam 506.
Fig. 2. Map of coal seam 506.
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given by Dubiński and Wierzchowska (1973). The strongest tremors (3
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× 107 J and 2 × 108 J) were associated with the activation of the diagonal fault. The occurrence of the other high-energy tremors was a consequence of fracturing of the thick layers of sandstone deposited in the roof of coal seam 506. Also, the influence of mining edges of coal seams 418 and 502 (130–141 m and 60–82 m above seam 506, respectively) on the occurrence of high-energy tremors was significant. The concentration of high-energy tremor sources in the vicinity of mining edges of coal seams 418 and 502 was clear, which is shown in Fig. 4. Besides, most high-energy tremors occurred ahead of the
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longwall face.
Because of those high-level seismic activities and associated high intensity of rockburst hazards during longwall advance, active rockburst prevention method should be applied. 4. Active rockburst prevention for longwall in coal seam 506
Fig. 3. Configuration of the seismic network.
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Active rockburst prevention method for the assigned longwall was implemented basically in the form of torpedo blasting in the roof rocks.
indicated that high-level rockburst hazards were observed in this
The diameter of each blasthole was 76 mm. Special methane explosive
excavation. The aforementioned difficult geological and mining
Emulinit PM was used in each torpedo blasting. The heat explosion of
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The intensity of seismic activities recorded during longwall advance
conditions were reflected in the observed seismic activities. The total
this explosive material is 2278 kJ/kg, and the specific volume of
number of recorded seismic events during the study period was 2190,
gaseous products of explosion is 767 dm3/kg (NITROERG, 2016). The
with a total released tremor energy of 3.8 × 10 J. A total of 95 high-
minimum velocity of detonation is 4000 m/s (NITROERG, 2016).
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energy tremors occurred: 73 events with energy in the order of 10 J
Immediate methane electric detonators
(1.68 ≤ ML < 2.21; ML is the value of local magnitude), 20 events with
explosives. All blastholes within one blasting stage were detonated
were used to initiate the
energy in the order of 106 J (2.21 ≤ ML < 2.74), one tremor with energy
simultaneously without delay.
of 3 × 107 J (ML = 2.99) and one tremor with energy of 2 × 108 J (ML = 3.42). The values of ML have been calculated according to the formula
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Fig. 4. Location of high-energy tremor sources induced during longwall mining of coal seam 506. As mentioned previously, most induced high-energy tremors
occurred ahead of the longwall face, so the main purpose of the torpedo
loaded. The first blasting provoked a tremor with energy of 2 × 104 J,
and the second blasting provoked a tremor with energy of 8 × 103 J. When the longwall approached the mining edge of coal seam 502,
working face of the longwall. Seismic tomography was also applied
blastings in front of the longwall face restarted. The blastholes deviated
frequently for stress level confirmation in the rock mass ahead of the
from the direction perpendicular to the longwall face to the north west
longwall face. The blasting parameters were determined according to
and south west at an angle of about 20°. These blastholes were inclined
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blastings was to destress the surrounding rock mass in front of the
both seismological and tomographical observations. Analyses of
upwards at an angle of 45° to the horizontal, and were 45 m long
geological and mining conditions were performed as well.
approximately. During each destress blasting stage, 288 kg of explosives were detonated. In total, 10 blastings were performed, which provoked tremors with energy from 6 × 103 J to 9 × 104 J. These
loaded into four boreholes (arranged in two pairs) were detonated. The
blastings were executed along the advancing direction of the longwall,
blastholes deviated from the longwall face to the north west and south
with an average distance of 22 m. Besides, one blasting was performed
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Before the first longwall face advance, two blasting stages were performed in the longwall cross-cut. Each time, 288 kg of explosives
west at an angle of about 20°. The blastholes were 30 m long and
in gallery 2. In two boreholes arranged as a pair that deviated from the
inclined upwards at an angle of 30° to the horizontal direction.
direction perpendicular to the gallery 2 to the south west and south east at an angle of about 30°, 144 kg (72×2=144 kg) of explosives were detonated. This blasting provoked a tremor with energy of 2 × 104 J.
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Explosive material occupied around 15 m of each borehole, with the rest being filled by stemming. These blastings provoked tremors with energy of 4 × 104 J and 6 × 104 J.
The results of seismic investigations, recorded seismic activities and
After the longwall started its advance, two other blasting stages were
analysis of current mining conditions indicated that one pair of
performed within the start-up period. During the six phases of blastings
blastholes should be located closer to gallery 3, and the other pair
(arranged in three pairs deviating from the longwall face to the north
should be located in the middle of the longwall face. This location
west and south west at an angle of about 30°, inclined upwards at an
arrangement would be the most suitable for destressing rock mass and
angle of 45° to the horizontal), 576 kg of explosives were detonated.
dissipating energy resulting from strong tremors. The deviation and
Using these blastings, two tremors with energy of 9 × 10 J and 8 × 10
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inclination of the blastholes remained the same. The blastholes deviated
J were provoked. The purpose of active rockburst prevention was to
from the direction perpendicular to the longwall face to the north west
4
destress the roof rocks to make caving easier.
and south west at an angle of about 20°, and were inclined upwards at
According to the seismic tomography results and the concentration
an angle of 45° to the horizontal. These blastholes were about 45 m
of high-energy tremors sources near gallery 2, two other blasting stages
long (see Fig. 5). Explosives weighting 72 kg were loaded into each
were designed and executed in this excavation. In the first stage, two
blasthole, which means during each blasting, 288 kg (72×4=288 kg) of
blastholes were used, while in the second one, four were employed.
explosives were detonated. The explosive material occupied nearly 15
These blastholes were 40 m long and inclined upwards at an angle of
m of blasthole (see Fig. 5). The column of explosive material was
65° to the horizontal. Into each blasthole, 72 kg of explosives were
located between coal seams 504 and 503, in alternating layers of sandstone, sandy shale and shale. Blastings with such parameters were
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performed until the end of the longwall. It should be noted that
to gallery 3. They were inclined upwards at an angle of 45° to the
blastings to fracture the thick sandstone layers probably should give
horizontal. Explosives weighting 72 kg were loaded into each
better results, but at this time the system for pneumatic loading of
blasthole. Blastings in gallery 3 were performed to destress the area
boreholes longer than 70 m had only just been tested in assigned
where the distance between the longwall and the diagonal fault was the
colliery. These parameters obtained from the system were then used as
smallest. Tremors provoked by the 29 torpedo blastings, where the
the optimal values concerning the difficult geological conditions and
blastholes were drilled only in the longwall face, had energy from 1 ×
technical possibilities. Torpedo blastings in layers of sandstone were
104 J to 6 × 104 J. Tremors provoked by blastings executed together in
performed in the second longwall, designed in the latter stage, of the
the the longwall face and gallery 3 had energy from 2 × 104 J to 9 × 104
excavation of coal seam 506.
J. All 34 aforementioned blastings were performed in average steps of 12 m along the advancing direction of the longwall.
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Near the entrance to gallery 2, mining edges of coal seams 418 and 502 as well as borders of protecting pillars for drifts coexisted at the level of 840 m and 1000 m, which greatly influenced the stress level in this area. To destress the rock mass and to protect gallery 2, torpedo blastings in this excavation were designed. Explosives weighting 48 kg in each blasthole were loaded. These blastholes were 35 m long, and
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were inclined upwards at an angle of 55° to the horizontal. A total of 4 blastings were performed, with the number of blastholes for each blasting case of 5, 5, 4 and 4, respectively. The provoked tremors had energy from 2 × 104 J to 3 × 104 J. The locations of all blastholes of the Fig. 5. Side view of blasthole.
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torpedo blastings performed for the longwall in coal seam 506 in association with the sources of the provoked tremors are shown in Fig. 6.
A total of 34 blastings applied to the longwall face were carried out with the aforementioned parameters, and 5 of 34 blastings were
The effectiveness of the torpedo blastings in the longwall face and
galleries has been calculated via the seismic effect method.
executed together with torpedo blastings in gallery 3. Blastholes in
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gallery 3, drilled to the south, were about 45 m long and perpendicular
Fig. 6. Location of blastholes and sources of provoked tremors during longwall mining of coal seam 506. 5. Evaluation of effectiveness of torpedo blastings
energy released in the rock mass when blasting to the energy of the particular detonated explosives (Konicek et al., 2013). It can be
The effectiveness of torpedo blasting is connected to the stress calculated seismic effect (SE). This methodology was established in the
calculated according to the following formula: E SE = ICM K ICMQ
Czech part of the USCB by Knotek et al. (1985), and subsequently
where EICM is the seismic energy calculated by the seismic network in
release in the rock mass. This stress release is evaluated using the
(1)
verified by Konicek et al. (2013), and adjusted according to the
the investigated coal mine (J), Q is the mass of the explosive charge
conditions of the assigned colliery in the Polish part of the USCB
(kg), and KICM is the coefficient characterising the conditions of the
(Wojtecki and Konicek, 2016). SE is defined as the ratio of seismic
assigned mine (J/kg).
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KICM must be determined by the conditions under which the seismic
value (i.e. Q). Based on this analytical procedure, a linear dependence
monitoring is carried out, and the seismic energy of the recorded
between the transformed seismic energy data (lnEICM) and the non-
seismic events is calculated in the same way. KICM is determined
transformed weight of explosive charge data (Q) was identified (see
according to the method proposed by Konicek et al. (2013). In this
Fig. 7). This dependence relation is represented by the regression line
method, generally, the statistical data analyses of seismic energy and
(Wojtecki and Konicek, 2016):
weight of the explosive charge obtained from in situ monitoring are
lnEICM = 9.7925 + 0.0022Q
(2)
performed. To determine KICM of the selected colliery, the dataset from nine longwalls had been taken into consideration (a total of 256 torpedo
The significance of this dependence relation between two variables can be generally evaluated using various correlation coefficients (e.g.
The basis of this methodology in light of the energy balance after destress blasting can be found in Konicek et al. (2013), as well as more details about necessary statistical analyses and recommended approaches (Konicek et al., 2013). The results for the conditions of our
Pearson R, R 2 , Spearman, Kendal tau, Gamma). Pearson R correlation coefficient is a widely used type of correlation coefficient in parametric
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blastings in the roof rocks).
statistics methods; it is also called linear or product-moment correlation coefficient. Pearson R correlation assumes that the two variables are measured on at least interval scales. Pearson R is 0.46 and R 2 is 0.21
Due to the fact that the applied methodology (Knotek et al., 1985, 2013) is based on linear regression, it must be proven that the data for analyses are normally distributed. Our statistical analyses, including exploratory analysis, were aimed at determination of data distribution characteristics, error elimination and correlation analysis for the confirmation of the dependence between variables, as well as
linear dependence between lnEICM and Q is relatively high according to the Person R of 0.46, despite the fact that the R 2 is relatively low. The main reason is that the investigated data do not come from laboratory measurements but from in situ measurements, which are affected by significant inhomogeneity and variability of natural and mining conditions.
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dispersion analysis (Wojtecki and Konicek, 2016). In the exploratory
for the evaluated dataset in this case. We consider that the identified
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selected Polish colliery are now described as follows.
analysis, a logarithmic transformation (i.e. lnEICM) was done to the
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seismic energy, while the weight of the explosive charge kept the origin
Fig. 7. Transformed seismic energy as a function of weight of explosive charge in selected colliery (Wojtecki and Konicek, 2016).
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The standard deviation of the transformed seismic energy in this relationship is 0.633. Data located under the straight line, which was
Very good Excellent Extremely well
3.5 ≤ SE < 5.9 SE ≥ 5.9
25.1 19.5 5.6
parallel to the regression line and shifted by the standard deviation of the transformed seismic energy, were then selected, as depicted in Fig.
6. Results
7. Then the median value of this new dataset was used to determine the coefficient KICM, which turned out to be 59.23 J/kg.
During the longwall mining of coal seam 506, a total of 48 blastings were performed in the longwall face. In five cases, these blastings were
system for the evaluation of SE. This classification was made according
performed together with blastings in gallery 3. For each of the 43 self-
to the probability distribution of the calculated seismic effects
contained blastings in the longwall face, the SE values were calculated.
according to Eq. (1).
The effectiveness of each blasting was estimated on the basis of the
From the entire SE dataset, the first quintile, median, third quintile
calculated SE value (see Table 2).
and maximum values were determined to be 1.4, 2.3, 3.5, and 5.9, respectively. The outliers were also determined. Statistical analysis of
Table 2. Parameters of destress blastings in roof rocks performed in the longwall face.
the SE dataset from the selected colliery is shown in Fig. 8. On the release due to torpedo blasting was distinguished. If the SE is lower than the first quintile (recorded energy released by blasting is less than 1.4 times the explosion energy), the effect of blasting is insignificant from a stress release point of view. A value of seismic effect between the first quintile and median means that the blasting effect can be
Q (kg) 288 288 576 576 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288 288
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assumed as good. A very good effect is provided by blasting with the
Date (YYMM-DD) 2010-03-18 2010-03-20 2010-04-26 2010-05-10 2010-08-30 2010-09-20 2010-10-18 2010-11-14 2010-12-05 2010-12-19 2011-01-09 2011-01-30 2011-02-14 2011-02-28 2011-03-07 2011-03-13 2011-03-21 2011-03-27 2011-04-03 2011-04-10 2011-04-18 2011-04-26 2011-05-09 2011-05-16 2011-05-22 2011-05-30 2011-06-20 2011-07-03 2011-07-25 2011-08-08 2011-08-29 2011-09-11 2011-09-26 2011-10-16 2011-10-30 2011-11-13 2011-11-27 2011-12-05 2011-12-18 2012-01-02 2012-01-16 2012-02-06 2012-05-14
seismic effect ranging between median and third quintile. Seismic
effect between the third quintile and the maximum can be assumed as
excellent. Seismic effects higher than the maximum (i.e. outliers) are extremely well (recorded energy released by blasting is higher than 5.9 times the explosion energy).
The classification system developed to evaluate SE values, based on
the criteria obtained from data distribution probabilities and Eq. (1), is
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EP
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presented in Table 1.
EICM (J)
SE
Evaluation of SE
4 × 104 6 × 104 9 × 104 8 × 104 2 × 104 3 × 104 9 × 104 8 × 104 8 × 104 8 × 104 4 × 104 4 × 104 6 × 103 5 × 104 5 × 104 3 × 104 3 × 104 2 × 104 2 × 104 3 × 104 4 × 104 2 × 104 3 × 104 2 × 104 2 × 104 1 × 104 5 × 104 4 × 104 5 × 104 1 × 104 4 × 104 2 × 104 2 × 104 2 × 104 3 × 104 2 × 104 6 × 104 3 × 104 4 × 104 3 × 104 4 × 104 2 × 104 2 × 104
2.3 3.5 2.6 2.3 1.2 1.8 5.3 4.7 4.7 4.7 2.3 2.3 0.4 2.9 2.9 1.8 1.8 1.2 1.2 1.8 2.3 1.2 1.8 1.2 1.2 0.6 2.9 2.3 2.9 0.6 2.3 1.2 1.2 1.2 1.8 1.2 3.5 1.8 2.3 1.8 2.3 1.2 1.2
Very good Good Very good Very good Insignificant Good Excellent Excellent Excellent Excellent Very good Very good Insignificant Very good Very good Good Good Insignificant Insignificant Good Very good Insignificant Good Insignificant Insignificant Insignificant Very good Very good Very good Insignificant Very good Insignificant Insignificant Insignificant Good Insignificant Excellent Good Very good Good Very good Insignificant Insignificant
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basis of the mentioned statistical parameters, the degree of stress
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The obtained value of KICM was used to establish the classification
7. Discussion Fig. 8. Statistical analysis of the SE dataset obtained from the assigned colliery. Table 1. Classification system for the evaluation of SE in the assigned colliery (Wojtecki and Konicek, 2016). SE value SE < 1.4 1.4 ≤ SE < 2.3
Evaluation of seismic effect Insignificant Good
Percentage of dataset (%) 20.7 29.1
Among the 43 tremors induced by destress blastings in the longwall face, as presented in Table 2, the seismic effect varied from insignificant to excellent. In light of seismic effect method, about 20.9% of the blastings had a good effect, 32.6% had a very good effect,
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and about 11.6% were excellent. The excellent effects were observed
geology, mining system, blasting parameters, and seismic network
when the longwall approached the mining edge of coal seam 502.
parameters) presented in any concrete coal mine.
In general, the designed active rockburst prevention procedure –
The seismic effect method was applied to estimating the torpedo
torpedo blastings in roof rocks in the longwall face – can be considered
blasting (long-hole destress blasting) effectiveness in the roof rocks,
appropriate based on the obtained seismic effect values. In most cases
which were executed in the longwall in coal seam 506, in our selected
(65.1%), the performed destress blastings had impact on the stress field
colliery in the Polish part of the USCB. In light of our results, the
in the area ahead of the longwall face.
effectiveness of the blastings was basically good, very good and excellent, which agreed with in situ observations. The dynamic
provoked geomechanical processes associated with stress release.
influence of high-energy tremors did not induce any damage to the
Generally, the tremors recorded after destress blastings had higher
longwall or its galleries. Longwall mining of coal seam 506 was
energy than it would appear only from the detonation of explosives. It
completed successfully, despite of high level of the seismic and
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Calculations indicate that most destress blastings in the longwall face
can be assumed that most of the destress blastings in roof rocks had
rockburst hazards. In this regard, destressing the roof rocks ahead of
brought new and advantageous stress equilibriums ahead of the
longwall face can be assumed to be effective.
longwall face. The other 34.9% of blastings were insignificant. The recorded
Conflict of interest
energy of the provoked tremors came mostly from the detonation of explosives. Most of these blastings were performed when the longwall
The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no
contained blastings in gallery 2 has been evaluated as well. The results
significant financial support for this work that could have influenced its outcome. References
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are shown in Table 3.
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had almost finished its advance. The effectiveness of seven self-
Table 3. Parameters of destress blasting in roof rocks performed in gallery 2.
Budryk W. Rockburst phenomena and prevention of their effects. Przegląd Górniczo-
Date (YYMM-DD) 2010-04-12 2010-03-20 2010-09-03 2011-12-10 2012-01-14 2012-02-12 2012-03-16
Q (kg)
EICM (J)
SE
Evaluation of SE
120 240 144 240 240 288 192
2 × 104 8 × 103 2 × 104 2 × 104 3 × 104 2 × 104 3 ×x 104
2.8 0.6 2.3 1.4 2.1 1.2 2.6
Very good Insignificant Very good Good Good Insignificant Very good
Hutniczy 1938;12:25-27 (in Polish).
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measures in the safety pillar of SW crosscuts at Lazy Colliery in Orlová. In:
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detonated amount of explosives was doubled in the second blasting, the
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first blasting. Probably, the first blasting destressed the rock mass
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effectively. The third torpedo blasting performed in gallery 2 had a very good effect, similar to the effect of the first blasting.
Most blastings performed in gallery 2 near its entrance, where the mining edges of coal seams 418 and 502 and borders of protecting
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pillars for drifts at the level of 840 m and 1000 m occurred together,
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had good and very good effects. One blasting had an insignificant
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result, probably due to the high efficiency of previous torpedo blastings
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in this area. The last torpedo blasting in gallery 2 had a very good effect. Blastholes for this phase were drilled where the stress level was theoretically the highest. 8. Conclusions
of Rock Mechanics and Mining Sciences 2011;48(6):942–949. Konopko W. About rockbursts and associated phenomena in the USCB. Bezpieczeństwo Pracy w Górnictwie 1984;4:18-22 (in Polish). Knotek S, Matusek Z, Skrabis A, Janas P, Zamarski B, Stas B. Research of geomechanics evaluation of rock mass due to geophysical method. Ostrava, Czech Republic: VVUU; 1985 (in Czech).
Torpedo blasting in roof rocks is the main form of active rockburst
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of torpedo blasting is particularly important for longwalls in difficult geological and mining conditions. This estimation can be made using the seismic effect method. This method gives a rapid answer to the effectiveness of blasting according to weight of explosives used. The advantage of this method is its adaptation to the local conditions (eg.
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NITROERG. NITROERG, Grupa KGHM. Bierun: NITROERG S.A., [cit. 2016-11-15]. http://www.nitroerg.pl, 2016. [online]
seismicity, rokckburst prevention and destress blasting. Jan Schreiber has been a PhD student and research assistant in the Institute of
Pelnar A. Rockbursts in Ostrava-Karvina coalfield. Hornický Věstník, Hornické a Hutnické Listy 1938;2:25–58 (in Czech).
Geonics of the Czech Academy of Sciences since 2015. The same year, he started earning his PhD degree on the Faculty of Civil Engineering at VSB-Technical
Parysiewicz W. Rockbursts in mines. Katowice: Fa “Śląsk”; 1966 (in Polish).
University of Ostrava. He obtained his MSc degree in Geology in 2010 in the Masaryk
Przeczek A, Dvorsky P, Konicek P. System of rock blasting in boreholes diameter more University Brno. Since 2012 he works as a Geomechanic and from 2016 as a senior than 100 mm as a rockburst measure. In: Proceedings of the 12th International Scientific-Technical Conference-Rockbursts 2004. Ostrava: Green Gas, DPB;
Geophysicist in the Department of Gemechanics and Geophysics in the mining company Green Gas DPB, Inc. in Paskov. Throughout his career, Jan has been part of
2005. p. 253–269.
teams that solves the problems related to mining and ground seismic monitoring
V, Vavro M. Rockbursts in carboniferous rock mass. SNTL 1972;6: 7-12 (in
systems to build up a reliable set of data for civil engineering and geomechanical
Czech) .
evaluations.
Takla G, Ptacek J, Holecko J, Konicek P. Stress state determination and prediction in rock mass with rockburst risk in Ostrava–Karvina coal basin. In: Proceedings of ISRM International Symposium - EUROCK 2005, Brno: International Society for Rock Mechanics; 2005. p. 625–628. Wojtecki Ł, Konicek P. Estimation of active rockburst prevention effectiveness during longwall mining under disadvantageous geological and mining conditions. Journal
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of Sustainable Mining 2016;15(1):1-7.
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Straube R, Brothanek J, Harasek V, Kostal Z, Kovacs Z, Mikeska J, Padara Z, Rozehnal
Lukasz Wojtecki currently works in the Polish Mining Group, Rockburst Department. He obtained his MSc degree in geology in 2005, MSc degree in physics in
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2006 and PhD degree in mining and engineering geology in 2014. He worked as a mine geophysicist in one of the hardest coal mines in the Polish part of the Upper Silesian Coal Basin from 2007 to 2012. From 2012 to present, he is a senior specialist in mine geophysics and head of the Mine Geophysics Station. Lukasz Wojtecki has investigated technical and scientific problems related to induced seismicity, seismic investigation, natural hazards, rockburst prevention and destress blasting effectiveness.
Petr Konicek obtained his MSc degree in Mining Geology in 1990 and PhD degree in Geotechnics in 2009. He worked in coal mining industry in the field of mining geology (1990-1997) and in expert service company for coal mining companies in the field of hydrogeology and geomechanics (1997-2009). He works at the Institute of Geonics of the Czech Academy of Sciences as a senior scientist and project leader in Department of Geomechanics and Mining Research from 2009. Since 2016, he has been an Associate Professor in Geotechnics in VSB–Technical University of Ostrava. Throughout his career, Petr has investigated scientific problems related to mining geology, rock mechanics, induced
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