G. VI@INTIN et al.: ROCK BURST DEPENDENCY ON THE TYPE OF STEEL ARCH SUPPORT IN THE VELENJE MINE
11–18
ROCK BURST DEPENDENCY ON THE TYPE OF STEEL ARCH
SUPPORT IN THE VELENJE MINE
HRIBINSKI UDARI V ODVISNOSTI OD VRSTE JEKLENIH
PODPORNIH LOKOV V PREMOGOVNIKU VELENJE
Goran Vi`intin1, Janez Mayer2, Bojan Lajlar2, @eljko Vukeli}1
1University of Ljubljana, Faculty of Natural Sciences and Engineering, A{ker~eva Street 12, SI-1000 Ljubljana, Slovenia
2Velenje Colliery, Partizanska 78, 3320 Velenje, Slovenia
goran.vizintin@guest.arnes.si
Prejem rokopisa – received: 2016-05-11; sprejem za objavo – accepted for publication: 2016-07-01
doi:10.17222/mit.2016.084
The Velenje Colliery is operating in the largest coal deposit in Slovenia and at the same time in one of the thickest known coal
seams in the world. The lignite seam is at one point up to 165 m thick. Until now more than 220 million tons of lignite coal were
extracted from this seam. The colliery is situated in the so-called Velenje depression, which is a component part of the Southern
Karawanke geotectonic unit. Considering the coal mining we can claim that rock bursts are a kind of constant that accompany
the mining activities and represent occasional difficulties during production and at the same time disturb the local inhabitants.
Based on complaints and at the same time because of the need for a thorough understanding of the causes of ground shaking, the
Velenje Colliery decided for a systematic monitoring and evaluation of the seismic events. Based on these measurements the
seismic events were precisely located and correlated to the technological process itself. Seismic events analysis indicates that
these events are instantaneous stress relaxations piled up for a longer time because of the lignite extraction. This last statement
can be confirmed by an analysis of the longitudinal seismic waving first arrivals (P) to seismological stations of the state
monitoring network of Slovenia, Austria and Italy, even though such events are rare because of their weakness. The same
analysis showed that the magnitudes rarely reach over 1.5 of local magnitude after Richter. Simultaneously with the seismic
events analysis we analyzed the way of support relaxation. Tests on the existing support type K24 showed that this support is
capable of withstanding large stresses without deformation. From these tests we found that the stress relaxation at currently used
support type K24 can occur over relatively large deformations. The relatively simple stress-relaxation analysis at existing
support shows it surprisingly well match to the energy relaxed at rock bursts in Velenje Colliery. Based on research done
between 2003 in 2012 it is possible to conclude that the existing support type must be replaced with one that can relax the stress
more frequently and with less deformation, consequently reducing rock bursts and increasing safety at work.
Keywords: rock burst, steel arch, induced seismicity
Premogovnik Velenje deluje na najve~jem nahajali{~u premoga v Sloveniji, ki je hkrati eno takih, ki ima najbolj debele plasti
premoga na svetu. Plast lignita je na enem mestu debela tudi do 165 m. Do sedaj je bilo izkori{~enih 220 milijonov ton lignita iz
te plasti. Premogovnik se nahaja v velenjski depresiji, ki je sestavni del geotektonskega dela Ju`nih Karavank. Pri rudarjenju
premoga so hribinski udari konstanta, ki spremlja rudarske aktivnosti in ob~asno predstavlja te`ave pri proizvodnji ter isto~asno
moti lokalno prebivalstvo. Zaradi prito`b in hkrati zaradi potrebe po razumevanju vzrokov tresenja tal, so se v Rudniku Velenje
odlo~ili za sistemati~no registriranje in oceno seizmi~nh pojavov. Na osnovi teh meritev so bili seizmi~ni dogodki natan~no
locirani in povezani s samim tehnolo{kim procesom. Analiza seizmi~nih dogodkov ka`e, da gre za trenutne sprostitve napetosti,
ki so se nakopi~ile v dalj{em ~asu zaradi ekstrakcije lignita. To trditev je mogo~e potrditi z analizo longitudinalnih seizmi~nih
valovanj (P), ki pridejo prva do seizmolo{kih postaj dr`avne kontrolne mre`e v Sloveniji, Avstriji in Italiji, ~eprav so taki
dogodki redki zaradi {ibkosti valov. Analiza je pokazala, da lokalna magnituda redko prese`e 1,5 po Richterju. Isto~asno z
analizo seizmi~nih pojavov smo analizirali na~in relaksacije podporja. Preizkusi na obstoje~em podporju vrste K24 so pokazali,
da to podporje lahko prenese ve~je napetosti brez deformacije. Iz teh preizkusov sledi, da se pri trenutni uporabljeni podpori
vrste K24 lahko pojavijo sprostitve napetosti preko relativno velikih deformacij. Relativno preprosta analiza spro{~anja
napetosti je pokazala dobro ujemanje z energijo spro{~eno pri hribinskem udaru v premogovniku Velenje. Na osnovi raziskav,
opravljenih med 2003 in 2012, je mogo~e zaklju~iti, da je obstoje~o vrsto podporja potrebno zamenjati s tako, ki lahko bolj
pogosto spro{~a napetosti in z manj deformacije, kar omogo~a manj hribinskih udarov in pove~a varnost pri delu.
Klju~ne besede: hribinski udari, jekleni lok, povzro~ena seizmi~nost
1 INTRODUCTION
Velenje Colliery is situated in the northern part of
Slovenia in the so-called Velenje depression, which is a
component part of the Southern Karawanke geotectonic
unit. Southern Karawanke form the northern part of the
Velenje depression and the northern part of its basement
reaching from the Smrekovec fault in the north to the
Velenje fault in the south. On the northern margins of the
depression the layers of Triassic dolomite are slightly
folded, but from the depression margin the Triassic dolo-
mite is steeply inclined towards the southeast. North of
Topol{ica we can find the outcrops of Upper Carboni-
ferous and Lower Permian age. The black shale, the
quartz sandstone and conglomerate and the grey lime-
stone are typical. In the vicinity of the Velenje fault there
is a minor parallel fault along which the zone of
Paleozoic husks is present. This zone consists of quartz
conglomerate, shale and limestone.1 The Velenje depres-
sion is a component part of a larger Velenje–Dobrna
depression. The depression is placed between the Velenje
fault in the north and the [o{tanj fault on the southern
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Materiali in tehnologije / Materials and technology 51 (2017) 1, 11–18 11
UDK 622.272.6:620.17:622.012.2(497.4) ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)11(2017)
edge following the quite right shape of the depression or
a basin. The origin of the depression dates into the
Styrian orogenic phase (Lower Miocene). At that time of
general uplift of the territory, the Badenian lithotamnic
limestone was sedimented upon the Egerian and
Eggenburgian basement made of andesitic tuff and marly
clay. North of the Velenje fault the Karawanke were
ashore yet. At the end of Badenian period the sea with-
draws. In the terrestrial phase in the Athic orogenic
phase (lower Pliocene) the majority of Miocene sedi-
ments were eroded. The depression started to sink in the
Rodian tectonic phase (middle Pliocene) as the upper
Pliocene strata are proved yet in the roof of the coal
seam.2 As shown by boreholes that were bored through
the Pliocene coal-bearing strata, the depression conti-
nued to sink through the complete Upper Pliocene.1 The
Gorenje–[o{tanj block is situated south of [o{tanj fault.
This block is built mostly of Oligocene andesitic tuffs
and marly clay. Nearby these volcanic and clastic sedi-
ments there are some outcrops of Triassic dolomite and
upper Permian black limestone found western and
northwestern from [o{tanj1 (Figures 1 and 2).
The coal seam of Pliocene age is syncline shaped and
as such adapted to the paleomorphological and geolo-
gical characteristics of the observed area. Along the
[o{tanj fault the coal seam is thicker, but gets thinner
towards the northeast. The thickening is also attributed to
major sinking in the area around the [o{tanj fault. The
deposit, represented with this single coal seam is 8.3 km
long and round 1.5 km wide. In some areas around the
[alek valley there can be found other coal appearances in
the shape of high ash coal layers, black organic clays,
brown organic silt and clay. The depression is filled up
with the sediments of Pliocene and Pleistocene age. The
component parts of the depression fill are alternating
layers of clays, silts, sands, pebbles, claystones, silt-
stones, marls and conglomerates. In-between the Velenje
coal seam is placed.3 The sedimentary fill of the depres-
sion represents the complete sedimentation cycle,
reaching from the terrestrial phase over marshy phase to
the lacustrine phase. The floor strata are of Pliocene age
and up to 400 m thick. In the lower part the floor is made
of particles from surrounding rocks but higher the clay,
silt and sands are present. These materials represent the
first, lacustrine fill of the depression, but further on the
sinking continued much more slowly and in the marshy
environment a lot of organic substance has been depo-
sited. Above the floor strata there are organic clays and
high ash coal that pass into more pure coal. The coal
seam thickness reaches up to 165 m at one point, but is
on average around 60 m. After the marshy phase the
sinking of the depression continued in the way that the
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12 Materiali in tehnologije / Materials and technology 51 (2017) 1, 11–18
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 2: Cross-section through the Velenje depression oriented NE-SW, with underground outline of the coal seam3
Slika 2: Presek skozi velenjsko depresijo, orientirano SV-JZ s podzemnim prikazom le`i{~a premoga3
Figure 1: Geologic map of the Velenje depression and the surround-
ings1
Slika 1: Geolo{ka mapa velenjske depresije in okolice1
lake formed again and the roof sediments slowly filled
the lake. Also above the coal there are Pliocene and
Pleistocene sediments represented mainly by clays,
sandy clays, mudstones and sands (Figure 2).
2 THE MINING METHOD
After 1970 the specially adapted longwall mining
method has been developed in the Velenje Colliery. This
method is tailored to great seam thickness and coal seam
geometry, using a mechanized hydraulic shield roof
support. Besides coal extraction the fill-in and hardening
of the goaf behind the support is enabled. The coal is
extracted in levels that follow each below other from the
roof towards the floor. The longwall face is kind of
divided into the upper and lower sectors. Longwall faces
in the Velenje Colliery are 160 m to 200 m wide. The
mine design imposes the extraction from edge of the
seam towards the centre in slices that are from 400 m to
800 m long and from 3 m to 15 m high. The working
process is completely mechanized using the hydraulic
shield support and the shearer that cuts the coal. There is
a chain conveyor installed to transport the coal from the
lower and the upper sector of the face. Coal extraction
follows the retreating method where the designed block
is developed to its boundary first and then mined back.
Development means to elaborate the supply roadway
(travel road or main gate or head gate) and the haulage
roadway (belt road or tail gate), make the connection
between them with the face roadway and enable the
ventilation. As the chocks are installed the face advances
and the shearer and shield support move forward and the
goaf increases. This goaf collapses under the weight of
the overlying strata. The upper sector of the face is won
by slow movements of the canopy as the stress crushes
the coal. The working cycle is complete when all the
coal from the upper sector comes down. The working
height of the face can be limited by calculation involving
the thickness of the isolating clay layer above the coal
seam. This clay layer protects the face against inrushes
of water bearing sands.4 Considering the Velenje mining
method the most important parameters are fractures in
the rock and a roof strata collapse. This has to do with
excavation principles and advancing techniques of
mining. Because of the use of natural forces in the
mining process the upper sector is a basis of high
productivity of the face (Figures 3 and 4). Subsidence of
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 4: Layout of the longwall face with separate phases of coal
extraction4
Slika 4: Izgled {irokega ~ela z lo~enimi fazami izkopavanja premoga4
Figure 3: Cross-section of the hydraulic shield support with the
principle of mining in the Velenje Colliery4
Slika 3: Presek hidravli~ne varovalne podpore z na~inom izkopavanja
v rudniku Velenje4
the surface above excavated longwall faces resulted in
formation of three subsidence lakes.
3 INDUCED SEISMICITY
Each change of natural stress condition in the Earth’s
interior can result in a rock relaxation. The most
common are the natural changes of the stress-state condi-
tions, but starting with the extraction of raw materials the
induced seismic events can be very frequent too. Seismi-
city in mines depends on mining depth and mining
geometry, production percentage, geological anomalies
(faults, dykes) and tectonic pressures. In an underground
roadway the deformation and stress energy rise with
depth and at the same time the possibility of a rock burst
rises too, applied to all the roadway cross-section
shapes.5 Rock burst by definition is a powerful collapse
of the rock, occurring quite periodically in underground
mining. Rapid excavation is followed by sudden rock
collapse parallel to the face, inducing rock bursting.
Most often the rock bursts are observed in hard and
deformable rocks6, at processes of space enlargements
caused by rapid excavation in mines and collieries. The
development of better excavation methods of raw
materials enabled the rise of production, but also caused
rapid adjacent rock instability and seismic events occur-
rence. Due to seismic events increase, the systematic
monitoring begun and resulted the finding of rock bursts
mechanisms. J. C. Johnston and H. H. Einstein7 sug-
gested six of them:
• Ore excavation at high vertical roof pressure;
• Roof collapse in underground roadway;
• Slipping along existing fault;
• Rock collapse in front of the advancing face;
• Fractured rock at the active face and on the opposite
side of the face due to pressure growth;
• Bearing pillar collapse.
Based on further geological research and seismic
record analyses, J. C. Johnston and H. H. Einstein7 de-
fined two rock burst types shown in Table 1. Relation at
Type I depends primarily on mining, longwall face rating
and blasting schedule. Upper magnitude limit of the
Type I depends on the rock strength, but the epicenter
location depends on the position of the mining site and
its geometry. Rock burst intensity at Type II depends on
thge interaction between induced stress in mine area,
tectonic stress area and fault possibility. Excavation
depth is another one of the important criteria. Shallow
mining can induce seismicity, but it is depending on a
large number of parameters, like face depth, face rate,
face geometry, tectonic stress, mining stress, fault zone
proximity regarding face position7 (Table 1).
Table 1: Rock burst types7
Tabela 1: Vrste drobljenja kamenine7
TYPE I TYPE II
In general the relation is a
function of mining activity.
There are not enough data to
define the relationship to
mining relations.
Location is round 100 m
inside of the face or inside
existing weakness zone or the
geological anomaly is close
to the mine.
Location is set on the existing
fault plane that can be up to 3
km away from the mine.
The rock can crack as the
induced mining stresses
exceed the shear strength of
the rock. Crack planes can be
variously oriented.
Occurs on existing tectonic
faults. Mining can induce the
events on faults with
advantageous orientation.
High stress drops are often
observed.
Stress drop is similar to
natural earthquakes.
Magnitudes are low to
medium.
High magnitudes are very
likely.
4 SEISMICITY IN THE COLLIERY AREA
In 1999 the Velenje Colliery decided to set up a local
observation site network in the [alek valley. The deci-
sion was taken because of rock burst events in the mine
and with the goal to locate the hypocenters of quakes
better. The strongest quake reached intensity of IV–V
after EMS-988. Until then such measurements were only
made in the state-owned observational network. The
measurements in mine’s local network enable us to
locate the weak seismic events in [alek valley, but only
in a range of the coal deposit dimensions. Underground
observation sites system setup and calibration has been
done by company K-UTEC Sondershausen.9 This digital
seismic observation sites system for monitoring of rock
bursts at longwall faces enables a precise location of
seismic wave’s source and intensity.9,10,11 The surface
system setup has been constructed by the Velenje
Colliery staff.9,10,11 Such a network setup with regular
sensor layout can ensure the systems accuracy from ±1
m to ±10 m.9 Based on such a network setup we recorded
289 seismic events, which were also recorded on the
state network in 2004. Considering the underground and
the surface system of Velenje Colliery there were 106
matches that undoubtedly belong to the Velenje Colliery
area. Only four stronger seismic events met the require-
ments for kinematic analysis of focal quake mecha-
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14 Materiali in tehnologije / Materials and technology 51 (2017) 1, 11–18
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Graph 1: Number of events of specific magnitudes in the years 2003
and 2004
Graf 1: [tevilo dogodkov s specifi~no magnitude v letih 2003 in 2004
nism.12 Figure 5 shows a certain part of localized events
and the four analyzed too.
From analysis of four focal mechanisms in Figure 5,
taking into consideration the classification13 shown in
Figure 6, we can conclude that in all four events it is
about the predominant normal component of the fault.
We can explain this with sudden stress relaxation
together with displacement in a crushed zone corres-
ponding to subsidence due to mining.
The magnitude measurements showed practically
insignificant magnitudes between 0 and 0.1 for most
events. The next most frequent magnitudes were between
0.5 and 0.9 in 2004 and 2005. The highest magnitude
was 2.4 in 2003 and 1.6 in 2004. From Graph 1 we can
conclude that the majority of the seismic events are not
detected by the [o{tanj inhabitants. In the [alek valley
area the state seismic observational network did record
one larger event not recorded by the local colliery net-
work. We are not able to locate this event precisely
enough without data from local network. Many recorded
events in both networks have the dominant event fre-
quency larger than 3 Hz. According to the size and
localities of seismic event occurrences it is possible to
conclude that all of them are Type-I rock bursts. The
only event that could be classified as Type II is the larger
one, but there are not enough data to localize it more
precisely.
5 STEEL ARCH SUPPORT AND ROCK BURSTS
At the moment of rock burst occurrence a powerful
collapse of the rock takes place. Rapid excavation is
followed by sudden rock collapse parallel to the face,
inducing rock bursting.6 From experience we know the
support damages occur as a consequence of rock bursts
(Figure 7). Rock burst reduction can be achieved
through some specific measures, one of these being
slowdown of the coal production speed, but such a
measure is a very undesirable one as the production
speed is one of the key parameters of the colliery’s
economics.14,15
Following the reports made in department for safety
at work we can find that support damages in the mine
rarely exceed 50 m. In the case of shorter damage
segments the displacement at joints is larger, but in case
of longer damage segments the displacement at joints is
smaller. Observations show the possibility of combina-
tions at damage sections up to 50 m where certain joints
get larger and others get smaller deformations or some-
where deformations are not noticeable at all. Figure 7
shows deformations after the rock burst where at the
middle two joints the shift is 0.37 m while the other two
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 5: Focal mechanisms of four seismic events of which the
signals could be interpreted for first moves
Slika 5: @ari{~ni mehanizmi {tirih seizmi~nih dogodkov, katerih
signal je mogo~e razlagati za prve premike
Figure 7: Damages on K24 support after a rock burst
Slika 7: Po{kodbe na podporju K24 po hribinskem udaru
Figure 6: Faults with the corresponding focal mechanisms13
Slika 6: Po{kodbe z ustreznim `ari{~nim mehanizmom13
joints remained practically without any deformation. In
case of this specific rock outburst the damaged section
reached 50 m in length but not all of the joints were
equally damaged. Resulting from the Velenje Colliery
local seismic network and seismic stations in Slovenia,
Italy, Austria, Hungary and Croatia measurements we
know that such events rarely exceed 1,5 magnitude after
Richter.
In 2011 and 2012 the mechanic characteristics of
existing support have been analyzed in context of rock
burst exploration. The K24-type steel arch support is
currently in use in the Velenje Colliery. This support type
is thermally treated (hardened) meaning the arch is more
flexible. In cases where larger pressure acts on steel arch
support K24 this support reacts in a discontinuous yield-
ing manner (sudden relaxation). The support has been
sent to a certified laboratory in Oprava, Czech Republic
for a comparative test. For comparison the support type
TH29 has been chosen while relaxing the stress in
significantly shorter deformation intervals.16 Results of
the loaded support joint deformations comparative tests
are shown in Graph 2. As seen from Graph 2 the majo-
rity of TH29 support joints relax the stress in short
deformation intervals while this is not the case for joints
of the K24 support. Also seen from Graph 2, that com-
pares clamp samples of K24 andTH29 support, the K24
clamp yields at 380 kN load resulting in deformation of
0,031 m momentary displacement. Knowing the defor-
mation displacement on the clamp at given force we can
calculate the deformation work with the following
Equation (1):
Wdef = F × s (1)
where Wdef – deformation work (J), F – force of load
(N) and s – deformation displacement (m), resulting for
one clamp 11.78 kJ. Following the reports made in
department for safety at work we can find that support
damages in the mine rarely exceed 50 m. Considering
the fact that each frame has got seven joints and three
frames are installed per one meter of the underground
roadway we can calculate the number of joints that yield
on the length of 50 m with the next Equation (2):
Stot = (L × (N – 1) + 1) × S (2)
where Stot – number of all support joints, L – length in
meters, N number of frames per meter and S number of
clamps. In case of rock burst in length of 50 m the num-
ber of clamps is 707. We can make a calculation of
common deformation energy needed for deformation of
707 clamps by multiplication of number of clamps and
deformation work: 707 clamps × 11.78 kJ/clamp results:
8.34 MJ. Regarding the Equation (3):17
ML = (lg(Wdef × 10
7) – 11.8)/1.5 (3)
where ML – local magnitude of seismic waving, this
energy is equivalent to Richter local magnitude ML =
1.41. Graph 3 shows relation between support deforma-
tion length and local magnitude ML. From Graph 3
taking into account Graph 1 follows that sudden
support yielding rarely exceeds 50 m length and this
complies with rock burst observation experience.
6 DISCUSSION
Results of our multiannual research show that it is
possible to relate the seismic events and the activities in
the Velenje Colliery. Majority of seismic events is very
weak and therefore not detectable and do not represent
any disturbance to local inhabitants. In spite of the
previous fact such events are uncomfortable or even fatal
for miners or other workers in the mine. In 1970 the spe-
cially adapted longwall mining method was developed
that is tailored to great seam thickness and coal seam
geometry using mechanized hydraulic shield roof
support. This method significantly improved the econo-
mics of underground coal production. From the other
side, the excavation speed increases the imbalance in the
stress state of the fractured and damaged rock behind the
longwall face. As a consequence of that inside the rock
the stress is accumulated and can be suddenly relaxed,
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16 Materiali in tehnologije / Materials and technology 51 (2017) 1, 11–18
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Graph 3: Local magnitude ML size in dependence of damaged section
length
Graf 3: Velikost lokalne magnitude ML v odvisnosti od dol`ine
po{kodovanega odseka
Graph 2: Comparison of stress relaxation on K24 and TH29 support
Graf 2: Primerjava spro{~anja napetosti na K24 in TH29 podporju
causing a rock burst. The coal production speed is
economically very important and therefore not to be
decreased, but there are some other specific measures
that can help to reduce the stress relaxation in the zone
of the fractured and damaged rock. Seismic events
analysis indicates the possibility to determine the focal
points of the most seismic events. It is also possible to
establish that most seismic events are under the ML = 1.5
value. Regarding their dynamics and localities of seismic
events occurrences, it is possible to conclude that all of
them are Type-I rock bursts. Until now, the events of
Type II were not recorded with the local Velenje Colliery
seismic system. Some larger events recorded only in the
state network of seismological observation stations indi-
cate the possibility of their presence but without any
detection in the local network the precise localization is
not possible. Further analysis of satisfactory recorded
seismic events in both networks indicates the probability
of these seismic events to be a consequence of stress
relaxation in fractured rock. As well, from the other side,
based on underground observation we know that during
rock bursts the deformation of steel arch support is
usually not longer than 50 m. In terms of energy this
corresponds to measured seismic events in the Velenje
Colliery area. It is also true that rock bursts leave effects
on shorter distances too but we have to bear in mind that
in such cases deformations in clamp areas are larger. In
spite of the fact that these statements are initial findings
of multiannual research, in our opinion the way of joint
deformation on steel arch support is the reason for rock
burst occurrence in the Velenje Colliery. Therefore, we
suggest that Velenje Colliery as a measure for rock burst
reduction starts to substitute the existing K24 support
with another one which enables the gradual stress rela-
xation together with gradual deformation as for example
the TH29 support.
Our next suggestion to Velenje Colliery is to consider
activities for stress relaxation in spots more exposed to
rock bursts. One of these activities might be blasting in
flank boreholes filled with minor explosive amounts.
Before boring the flank boreholes a very precise micro
locations map of potential rock bursts is required.
Elaboration of such map requires a fixed underground
monitoring system so the existing mobile system needs
to be replaced. In addition to improved monitoring sys-
tem the stress measurements on steel arch support need
to start. In case of reaching the limit yield values, a clear
danger of rock burst is indicated. Based on better locali-
zation and monitoring of stress increase at joints, the
precise procedure of preventive blasting will be pre-
scribed.
7 CONCLUSIONS
From the research we can see the connection between
support type and rock bursts. Support type that blocks
continuous stress relaxation in form of minor deforma-
tions can cause stress accumulation and in case of over-
burden the support deformation is large. Such event
results the rock burst, hurting people and damaging mine
facilities. In order to reduce the rock bursts we suggest to
Velenje Colliery the following:
• Start to introduce the steel arch support capable of
stress relaxation in form of constant short deforma-
tions.
• Set up a fixed rock burst occurrence monitoring
system.
• Develop in-seam stress monitoring system (pressure
cells installed in rock).
• Test the loaded arch support frames for in-arch stress
measurements development.
• Develop relaxation blasting procedures using bore-
holes.
• Set up rock burst description database.
Ongoing research should be directed into possibility
of rock burst Type-II occurrence. These rock bursts can
cause more severe seismic events along otherwise stable
fault structures that become active because of excavation
conditioned disturbed spatial geophysical equilibrium.
Acknowledgments
This study was financially supported by the research
project L1-5451 funded by Slovenian Research Agency
(ARRS).
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