ROCK BURST AND COAL BUMP

Pankaj Kumar Das

B.Tech From IIT(ISM) Dhanbad
 ROCK BURST and COAL BUMP
• Sudden & violent failure of rock that occur near
openings and that are of such magnitude that
they expel large amount of rock from it’s
surrounding mine working into the opening.
When it occurs in hard rock mining it is known
as Rock Burst.
• Similar phenomenon in (Coal Mine) coal and
coal measure rock is known as Coal
Bump/Mountain Bump/Outburst/Bounce.
Coal bump is usually accompanied by air blast
and greater clouds of coal dust in suspension.
 ROCK BURST and COAL BUMP
• The seismic event is caused by the mining of
underground or surface openings in a high-
stress environment, resulting in the
instantaneous release of large amounts of
accumulated energy.” Underground, a rock
burst sounds like a loud blast. On the
surface, a rock burst sounds like a sonic
boom, while the ground and structures on
the surface shake as if a small earthquake
had occurred.
• It releases tremendous amount of strain
energy stored in rock mass (10-5 to 109 j/
Richter scale -6 to 5).
Damage of mine gallery due to Rock Burst
 ROCK BURST and COAL BUMP
This has been recognized as a sudden
catastrophic failure of rock and causes serious
problems to underground mining worldwide in the
past 100 years.
First occurrence of rock burst was reported in
gold mines of Witwatersrand (South Africa) and
Kolar Gold Fields (India).
Rock bursts/Coal bumps in mines are safety
concern in many countries such as India, USA,
Germany, United Kingdom, Poland, France,
Mexico, People's Republic of China, Czech
Republic,Australia and Republic of South Africa.
EFFECT OF ROCK BURST/COAL BUMP

Rock burst is a typical dynamic

disaster in mining activity. The sudden
occurrence and the severe destruction
is a great threat to mine safety.
Particularly, with the increasing depth
of mining, rock burst is becoming more
and more obvious. Rock burst is a
dynamic phenomenon of mine pressure
whose occurrence mechanism is very
complex.
 EFFECT OF ROCK BURST/COAL BUMP
• DEMAGE OF EXCAVATION & SURFACE
STRUCTURES DUE TO GROUND VIBRATION
• HINDERS SMOOTH WORKING IN MINES
• HAZARD TO PERSONS & EQUIPMENT IN MINES
• AFFECT MINE VENTILATION
• INCEASE IN COST OF PRODUCTION
/EXCAVATION
• SUBSTANTIAL PRODUCTION DELAYS AND
LOSSES
• COSTLY REHABILITATION PROGRAMS
 CLASSIFICATION OF ROCKBURST
ROCK BURST POSSIBLE SOURCE OF RICHTER SCALE
TYPE MECHNISM (MAGNITUDE)
STRAIN BURST SUPERFCIAL SPALLING
-0.2 to 0
BUCKLING OUTWARD EXPULSION OF
LARGE SLAB 0 to 1.5
PILLAR/FACE SUDDEN COLLAPSE OF PILLAR
BURST 1 to 2.5
SHEAR REPTURE VIOLENT REPTURE OF SHEAR
FRACTURE THROUGH INTACT 2 to 3.5
ROCK
FAULT SLIP SUDDEN MOVEMENT ALONG
EXISTING FAULT 2.5 to 5.0
 CLASSIFICATION OF COAL BUMP

• SHOCK BUMP
Occurs where strong massive strata lying
immediately above/near mine opening caves induce
shock wave to the surrounding strata/pillar which
cause bump.
• PRESSURE BUMP
Occurs when strong and brittle strata/pillar is
stressed beyond its strength and fails suddenly and
violently.
CAUSES OF ROCK BURST & COAL BUMP
There are many factors contributing to rock
burst/coal bump
DEPTH OF MINE WORKING
Increasing mining depth, gravitational
stress increases and the coal/ore body and
surrounding rocks can store more strain
energy, which always induce destructive
bumps. The periodic failure of thick and stiff
roofs may cause sudden release of strain
energy to trigger bursts/bumps.
 CAUSES OF ROCK BURST & COAL
BUMP
STRESS CONDITIONS
High stress is one of the primary causes of
rockbursts in mines. High stress conditions
are typically a combination of the natural pre-
mining in-situ stress and stress changes
induced by mining.
Excavations in rock induce stress
concentrations and stress relief, creating
biaxial or near uniaxial loading conditions near
openings. This reduction in confining stresses
can initiate rock mass degradation and brittle
failure of intact rock.
 CAUSES OF ROCK BURST & COAL
BUMP
STRESS CONDITIONS
Depending on the nature of the loading system
and the properties of the rock, failure may
occur in a stable yielding manner, or in an
unstable dynamic manner. Both stable and
unstable failure conditions result in energy
release and the generation of seismicity.
However, it is unstable failure that has the
greater potential to release significant energy
release, causing large seismic events and
potentially, rockbursts.
 CAUSES OF ROCK BURST & COAL
BUMP
GEOLOGIC CONDITIONS
a) Type of Rock
Igneous & Metamorphic rocks are more prone to rock burst
b) Mineral Composition of Rock
Siliceous rocks and those contain hard mineral are more
prone to rock burst. Fine grained rocks are more prone to
rock burst
c) Strong Roof and Floor Strata
Strong floor strata immediately below the coal seam and
strong roof strata within 30 to 50 feet of the seam have long
been recognized as major contributors to coal. In fact, the
confinement offered to the coal seam by these stronger,
stiffer strata appears necessary to generate levels of stored
energy sufficient to cause bumps within and immediate to
the coal seam structure.
 CAUSES OF ROCK BURST & COAL
BUMP
GEOLOGICAL DISCONTINUITIES
Geological discontinuities often play an equally
important role. Many types of geological
discontinuities including faults,dykes,pegmatite,
shear zones, joints or bedding, and local
variations in rock stiffness contribute to mine
seismicity and rockbursting. In reality, geological
discontinuities play a dominant role in the
deformation and failure of a rock mass under
load.
 CAUSES OF ROCK BURST & COAL BUMP
GEOLOGICAL DISCONTINUITIES
• The location, continuity, orientation and
material properties of the geological features
are significant factors in rock mass failure,
and these characteristic dictate how energy
is stored and released in a deforming rock
mass.
• It is important to note that while geological
features play a role in the release of energy
and the generation of seismicity and
rockburst damage.
CAUSES OF ROCK BURST & COAL BUMP

ORIENTATION OF WORKING FACE

The angle between the working face and
geological features such as dykes, faults,
or dominant joint sets plays in causing
stress concentration around the face and
thereby influence the occurrence of
rockburst.
CAUSES OF ROCK BURST & COAL BUMP
LAY OUT & MINING SEQUENCE
Improper mine lay out and sequence can induce an
extremely high stress concentration on a localised
area such as one or more pillars. The pillar which is
out of sequence are most prone to rockburst.
PRESENCE OF MAJOR FAULT
The stress concentration is evident on the structural
planes/fault and weak interfaces long the excavation.
This region is obviously the location where
rockbursts are most likely to occur. The locations of
structural planes such as faults are also the
dangerous zones for sudden stress change.
 CAUSES OF ROCK BURST & COAL BUMP
MECHANICAL PROP. OF ROCK
Bursting rocks characterised with their high strength,
higher deformation modulus and brittle behaviour. The
violent detachment of rock fragments during rock busting
is associated with how the stored mechanical/strain energy
is dissipated during the entire deformation process. A
certain part of stored energy would be transformed into
kinetic energy. The kinetic energy results in the
detachment, which may be thrown into the opening with a
certain velocity depending on the overall stiffness of the
surrounding system and deformation characteristics of the
busting material.
The uniaxial compressive strength, Modulus of Elasticity
and brittleness of rock plays an dominant role storage of
strain energy in rock.
STRAIN ENERGY DENSITY= σ2/2E
ROCK BRITTLENESS = {(σc- σt)/ (σc+ σt)} or Sin ø or
σC/σt or qσC (Where q= % fine in Protodyakonov Index test)
 CAUSES OF ROCK BURST & COAL BUMP
For rocks exhibiting bursting phenomenon
the following identity must hold:
ES=EK+ET+EP+EO
Where, Es- Stored mechanical energy
EK- Kinetic energy
ET - Thermal energy
EP- Plastic work done
EO-Other energy
C
VELOCITY OF ROCK FRAGMENTION  V 
H E
EJECTION DISTANCE  d  V
g
Where, H=Height of the opening
Assumed that the stored mechanical energy is totally transformed into kinetic
energy
CAUSES OF ROCK BURST & COAL BUMP
MECHANICAL PROP. OF ROCK
Based on energy balance around excavations,
two energy parameters, namely the energy
release rate (ERR) and the energy storage rate
(ESR) are defined to help estimate rockburst
potential due to mining in deep underground
mines. The magnitude of ERR and ESR depends
on geomechanical properties of rock mass, in
situ stress conditions, the mine geometry and the
layout of the mining excavation.
Wk = Kinetic Energy in rock Wr = Release Energy from rock,
Um = Stored Strain Energy in rock
 ERR Vs NO. OF DAMAGING ROCKBURST
NO. OF DAMAGING
ROCKBURST

ERR (MJ/m2)
 METHODS OF ASSESSMENT OF
ROCKBURST & COAL BUMP PRONENESS
Many existing studies attempted to monitor and
predict rockburst/bump:
 BURST PRONESS INDEX METHOD
 EXTENSIONAL STRAIN METHOD
 DRILLING YIELD METHOD(COAL BUMP PRONENESS)
 MICROGRAVITY METHOD
 RHEOLOGIC METHOD
 REBOUND METHOD
 MICROSEISMIC METHOD
 ELECTROMAGNETIC EMISSION SYSTEM (EME)
 BURST PRONESS INDEX METHOD

PERMANENT STRAIN
ENERGY (EP)

STRAIN ENERGY
RETAINED (ER)

BPI RB
PRONENESS
>5 HIGH
STRAIN MEDIUM
2
ER
Burst Pr oneness Indx   
TO
5
EP <2 NOT
PRONE
 METHODS OF ASSESSMENT OF
ROCKBURST & COAL BUMP PRONENESS
 DRILLING YIELD METHOD (COAL BUMP
PRONENESS)
It involves drilling of a borehole in coal seam ahead
of a working face. The quantity of cuttings ejected
and any dynamic effects such as audible noise,
grating and jamming of the drill in the hole are
observed. Highly stressed zones characterised by
increasing activity and zones of decreased and
increased can be determined. The closer a highly
stressed zone is to the working face/abutment, the
greater is the danger of bump.
 METHODS OF ASSESSMENT OF
ROCKBURST & COAL BUMP PRONENESS
 DRILLING YIELD METHODMETHOD(COAL BUMP
PRONENESS)
The relation between stress and drill cuttings(Haramy et al,1988)

 VC 
V  123log    18
 Ve 
Where σV = Vertical abutment stress (MPa)
VC = Vol.of drill cuttings, Ve = Vol.of borehole
It has been found that if the highly stress is located at a
distance between 1.5H to 3H in the face/abutment than there
is possibility of bump, where H is height excavation.
 METHODS OF ASSESSMENT OF
ROCKBURST & COAL BUMP PRONENESS
MICROSEISMIC METHOD
The origin of microseismic (MS)/acoustic emission
(AE) activity in rock is related to process of
deformation and failure which are accompanied by
sudden release of stored strain energy that generates
an elastic stress wave which travels from the point of
origin within the rock to the boundary. Using a
suitable sensor, it is observed a microseismic signal
or as a discrete microseismic event. The fundamental
frequency character of an observed microseismic
signal depends on the characteristic of the source,
and the distance between the source and the
monitoring transducer.
 ACOUSTIC EMISSION

ACOUSTIC EMISSION
STRESS
STRESS

DEFORMATION
Cumulative Acoustic Emission
Failure of rock

Unstable micro- fracture

formation

Semi-linear
New micro- fracture
Deformation
formation

Time or Stress or Strain

Acoustic emission response characteristics

associated with the fracture process
ACOUSTIC EMISSION RATE
ROCKBURST

ELAPSED TIME,
Microseismic Network
Microseismic Network
 MICROSEISMIC METHOD
Compared with the traditional displacement or stress
monitoring techniques, the microseismic monitoring
technique has the following distinctive features:
(1) The monitoring range can be very wide. The time,
location and magnitude of microcracking events in
rock mass can be determined directly. It overcomes
the drawbacks of the traditional “point” monitoring
techniques, which are localized, discontinuous,
laborintensive and poor in safety.
(2) It realizes automation, informationization and
intelligentization of monitoring, which represents the
development trend of stability monitoring for deep
underground structures.
(3) The monitoring instruments are being developed
towards highly integrated, small-sized, multi-channel
and highly sensitive devices.
 MICROSEISMIC METHOD

(4) It supports automatic monitoring and

remote information transmission. The
monitoring data can be sent to the
microseismic data analysis center through
wireless GPRS.
(5) As it receives information of seismic
waves, the sensors can be installed in the
region far away from the failure-prone
area, which is advantageous for ensuring
long-term operation of monitoring system.
 METHODS OF ASSESSMENT OF
ROCKBURST & COAL BUMP PRONENESS
Electromagnetic Emission (EME) (for Coal Bump)
• Electromagnetic emission (EME) always occurs
along with coal rock deformation and fracture. It
is closely related to the load on coal rock and to
the degree of coal rock deformations and
fractures under load. In nature, the deformation
and fracture of coal rock mass are the results of
formation, expansion, combination, integration
of some micro-defects or micro-cracks, and the
internal damages of coal rock under stress.
 METHODS OF ASSESSMENT OF
ROCKBURST & COAL BUMP PRONENESS
Electromagnetic Emission (EME) (for Coal Bump)
• EME is related to the features of coal rock
material, such as the damage parameters and
the constitutive relation, and directly associated
with the evolution and propagation of the
internal defects.
• EME graded warning criteria against coal rock
dynamic collapses by determining static critical
coefficient and dynamic trend coefficient has
been used in recent years for predicting coal
bump in mines.
 PREVENTION & MINIMIZATIONS OF
ROCKBURST EFFECTS

• Uniform and quick placement of support and use of

rapid yielding hydraulic support is more effective in
dynamic loading situation.
• Higher rate of extraction of ore/coal from a
comparatively small face and immediate placement of
supports resulting in better ground control.
• A longwall sequence of stope faces helps to avoid
formation of pillars with their extreme stress build up
along the length of face such that the superincumbent
load is distributed more less uniform.
• Strict sequence of stoping with adequate leg and lead
when pallel loads in close vicinity.
 PREVENTION & MINIMIZATIONS OF
ROCKBURST EFFECTS
• Stoping away from the zone of weakness
such as faults, dykes etc.
• By avoiding sharp corners that cause or
enhance stress concentration.
• Use of high density fill rockburst is
minimized.
• Additional fracture/ de-stressing could be
induced in the rock ahead of the face to
extend the fracture zone by
• Volley Firing
• Hydraulic Fracturing
Geomechanics effects of destress blasting
 PREVENTION & MINIMIZATIONS
OF ROCKBURST EFFECTS
Volley Firing: In this method, explosives are
used to fracture the working face to a certain
depth before mining. The method is used prior
to face advance or entry development to
advance the high stress zone away from the
working face.
Hydraulic Fracturing:This method involves the
injection of fluid under pressure to cause
material failure by creating fractures or fracture
systems. Hydraulic fracturing is most effective
in the roof and coal seam ahead of the longwall
face.
 PREVENTION & MINIMIZATIONS
OF ROCKBURST EFFECTS
• Remnants pillars: The formation of remnants pillars
should be avoided.
• Pillar dimension: Width to height ratio should be
treated with caution. Problems arise where pillar sizes
are too small or too large. These improperly sized
pillars are termed “critical pillars” and their use can
result in the most extreme hazard possible. To mitigate
the frequency of bumps yielding-pillar in gate road is
useful.
• Face orientation: The angle between the face and
geological features such as dykes, faults, or dominant
joint sets must be carefully considered. Experience
shows that an angle greater than 30°between the
feature and the orientation of the face is desirable.
 PREVENTION & MINIMIZATIONS
OF ROCKBURST EFFECTS
• In situ stress : The in situ stress is an important
factor in designing underground excavations, and
cannot simply be assumed. Measurements should be
determine stress state exists and location of its
anomaly
• Width of face: The use of shorter panels should be
made to limit the extent of ruptures along face-parallel
shears and the consequent damage
• Seismic network: Use of mine-wide seismic network
helps to facilitate the identification of hazardous areas
and influencing parameters as well as it aids to the
back analysis of rockbursts.
Methods to reduce damaging effects of excessive stresses