Lecture on rock bursts

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Rock Bursts: Causes, Mitigating Measures and Monitoring
INTRODUCTION
Mining depth is increasing day by day, since the deposits near the surface are mined out.
Going deep in the mine creates different conditions in comparison with the near surface
mining due to the increased stress. As a result, stress-induced rock fracturing is inevitable and
when stored energy is suddenly released, rocks fail violently, leading to seismic events and
rockbursts. A rockburst is defined as damage to an excavation that occurs in a sudden or
violent manner and is associated with a seismic event. Rockbursts are the most serious and least
understood problem facing deep mining operations.
With the advance of the mining depth in higher stress environments, rockbursting is
becoming an increasing problem world-wide. Understanding and prediction of the rockbursts
that may happen during the mining process have got a critical attention. It has been realized
that rockburst hazard must be properly managed as part of a daily ground control decision-
making process since the current mining conditions and techniques cannot be changed to
eliminate rockburst hazard.
The first published information on rockburst was in 1738 from tin mines of England. During
the second half of 19th
century, rockbursts were noted in Western Europe in coal mines, while
in the 20th
century rockbursts occurred in gold mines on Witwaterstrand in South Africa and
on Kolar Gold Field in India. Subsequently many hard rock mines in Canada, China, Chile,
South Africa, Australia, Sweden, and other countries, and some deep tunnels in Switzerland,
China, and Peru have experienced rockbursts to various degrees. Considerable research
effort, at an international scale (e.g. Australia, Canada, South Africa, China), has been
devoted to the understanding of the rockburst phenomenon. Despite significant research and
progress in the last two decades, the problem is still not well understood.
TYPES OF ROCKBURSTS
Ortlepp and Stacey (1994) and Ortlepp (1997) classified rockbursts into five types, see Table
1. Their classification was based on first motions from seismic records, the Richter
magnitude of the events, and the postulated source mechanism.
For brevity of discussion, it is considered to have three rockburst types, i.e. strainburst, pillar
burst, and fault-slip burst. Rockbursts are either mining-induced by energy release causing
damage at the source (e.g. strainburst without significant dynamic stress increase from a
remote seismic event) or dynamically-induced rockbursts with damage caused by energy
transfer or significant dynamic stress increase from a remote seismic event (e.g. strainburst
with dynamic stress increase caused by a remote seismic event).

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Table 1: A suggestion for a classification scheme of seismic event sources in tunnels
(after Ortlepp and Stacey, 1994).
a) Strainburst: In many deep underground excavations, strainbursts are the most common
rockburst type; they can be mining-induced due to static stress change caused by nearby
mining or dynamically induced due to dynamic stress increase caused by a remote
seismic event (called dynamically-induced strainbursts). Two conditions must be met for
a strainburst to occur. First, the tangential stress (the maximum principal stress) must be
able to build up in the immediate skin of the excavation. Second, the rock mass
surrounding the fracturing rock must create a relatively “soft” loading environment such
that the rock fails locally in an unstable, violent manner. The energy released by a
strainburst comes from the stored elastic strain energy in the failing rock and the
surrounding rock mass (not from the seismic source).
In mining, stress changes in the drifts (horizontal tunnels in a mine) may occur after
development due to stopping activities; consequently, mining-induced strainbursts can
happen during the production stage. Strainbursts normally occur within three times the
diameter from the advancing face. Such strainbursts can also occur right at the working
face and in the floor. Delayed strainbursts occur in situations where the maximum
principal stress remains constant but the rock strength degrades over time, or the rock
strength reduces due to loss of confinement. Due to a potentially unstable equilibrium
situation near an excavation, strainbursts may be triggered by a small dynamic
disturbance, a production blast, a remote pillar burst or fault slip event. For such
dynamically-triggered strainbursts, little or none of the released energy stems from the
triggering event. Instead, the stored strain energy at the bursting location and the
surrounding rock constitutes most of the release energy.
b) Pillar burst: As the name implies, it is defined as a violent failure in the pillar core or the
complete collapse of a pillar. Pillar bursts often occur in deep mines when the extraction
ratio is high at a later stage of mining. The volume of failed rock and the affected
surrounding rock mass is usually larger than that involved in a strainburst and hence the

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released seismic energy is much greater. Similar to strainburst, pillar burst can be
classified into mining-induced pillar burst and dynamically induced pillar burst. A
mining-induced pillar burst is caused by static stress increase from increased room span
or nearby stope extraction. The seismic source is in the confined core of the pillar, and
rockburst damage and seismic source are co-located. On the other hand, a dynamically-
induced pillar burst is caused by dynamic stress increase from a remote seismic event. In
this case, the rockburst damage and the seismic source (i.e. fault-slip event) are not co-
located.
c) Fault-slip burst: It is caused by the dynamic slippage along a pre-existing fault or along a
newly generated shear rupture. A critically stressed fault, with shear stresses exceeding
the shear strength, can slip when the degree of freedom is changed as it is intersected by a
mine opening. Alternatively, it may slip when the shear strength is reduced due to a drop
in clamping stress or water infiltration into the fault. Finally, it may slip when the mining-
induced shear stress is increased and exceeds the strength of the fault, which is a function
of the normal stress, the coefficient of friction of the fault surface, its waviness or dilation
characteristics, and, in the case of fracture propagation, the strength of the rock mass.
Similar to pillar burst, fault-slip rockbursts occur in deep mines when the extraction ratio
is high and large closures are allowed to persist over large mining volumes. The most
plausible cause of fault-slip along a pre-existing fault is the reduction of normal stress
acting on the fault as a result of nearby mining, although an increase in shear stress or a
combination of normal stress decrease and shear stress increase can similarly cause a fault
to slip. This type of rockburst may release a large amount of seismic energy, coming from
the instantaneous relaxation of elastic strain stored in a large volume of highly stressed
rock surrounding the slip or rupture area. They may create sufficiently high ground
vibrations or ground motions that can cause damage to excavations (dynamically-induced
strainbursts), cause shake down of loose or insufficiently supported rock, and/or trigger
strainburst and pillar burst at relatively remote locations (hundreds of meters from the
seismic source). Shear rupture type rockbursts have been observed in some mines. Large
rockbursts, with Richter magnitude exceeding 3.5, can result from violent propagation of
shear fracture through intact rocks.
ROCKBURST DAMAGE MECHANISM
Understanding the rockburst source mechanism is critical to deriving strategies to eliminate
and mitigate rockburst hazard, and a thorough understanding of the rockburst damage
mechanism is needed to work out tactics to implement rockburst support. Kaiser et al. (1996)
classified rockburst damage into three types, i.e. rock bulking due to fracturing, rock ejection
due to seismic energy transfer, and rockfall induced by seismic shaking (Fig. 1). Rock
bulking due to rock fracturing can be caused by both a remote seismic event and the bursting
event itself. Brittle rock fracturing occurs as a result of crack and fracture initiation,
propagation, and coalescence. This leads to the generation of new fracture surfaces in a
previously intact or less fractured media and, as a consequence, this rock mass disintegration
leads to rock mass bulking. This bulking process is in large part a result of geometric block

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incompatibilities and thus is much larger than dilation during plastic rock mass yield. Most
importantly, it is directional, perpendicular to the excavation wall. During bulking, the broken
rock volume increases as it is fractured and fragmented.
Rock ejection can be caused by a strainburst event, a pillar burst event, or by a remote
seismic event through dynamic moment transfer. Ejected rock may travel at velocities in
excess of 3 m/s; velocities up to 10 m/s were estimated. The upper end of this ejection
velocity range cannot be explained by the moment transfer damage mechanism alone. When
rock suddenly fractures, part of the stored strain energy in the surrounding rocks can be
transferred to blocks in the form of kinetic energy, causing rock ejection. With high strain
energy stored in the rock near the excavation, the stress wave from a remote seismic event
may add a dynamic stress disturbance and cause a strainburst (“bring the bucket to
overflow”). In this case, the ejection velocity is not directly related to the momentum from
the seismic source but more closely related to the energy stored in the near-wall rock and how
this stored energy is released.
Seismically-induced rockfalls, as the name suggests, are caused by the (low frequency)
shaking of ground due to a large remote seismic event, perhaps induced by a pillar burst or a
fault-slip rockburst. It occurs when an incoming seismic wave accelerates a volume of rock
that was previously stable under static loading conditions, causing forces that overcome the
capacity of the support system. Note that it is also possible that the first incoming seismic
wave may fracture a volume of rock, and subsequent vibration induced by the seismic waves
accelerates the fractured rocks, causing falls of ground. Seismically-induced rockfalls occur
frequently at intersections where the span is large and roof rock confinement is low.
CAUSES OF ROCKBURST
There are many factors that influence rockburst damage and the severity of the damage.
Depth of mine working
Increasing mining depth, gravitational stress increases and the ore body and surrounding
rocks can store more strain energy, which always induce destructive bursts. The periodic
failure of thick and stiff roofs may cause sudden release of strain energy to trigger
bursts/bumps.
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. 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.

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Fig. 1: Rockburst damage mechanism, damage severity, and required support functions
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 ore and strong
roof strata within 30 to 50 feet of the deposit have long been recognized as major contributors
to rockburst. In fact, the confinement offered to the ore body by these stronger, stiffer strata
appears necessary to generate levels of stored energy sufficient to cause rockburst within and
immediate to the ore body structure.
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.
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.
Method of working
Rockburst can be caused due to inappropriate excavation span, excavation ratio, rock support
system and production rate while considering a mining method. Further, 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 is 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.

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The locations of structural planes such as faults are also the dangerous zones for sudden
stress change.
Mechanical properties of rockmass
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
dominant role storage of strain energy in rock.
STRAIN ENERGY DENSITY= σ
2
/2E
ROCK BRITTLENESS = {(σc- σt)/ (σc+ σt)} or Sin ø or σc/σt or qσc (Where q= %
fine in Protodyakonov Index test)
ROCKBURST CONTROL
The problem of rockburst control resolve into two types consideration: that involving factors
which effect the incidence of seismic activity, and that concerned with factors which reduce
the effects of the damaging seismic events. Some of the important factors which are
considered for controlling rockburst or its reducing its effects are discussed below.
Reducing mine induced seismicity
Minimizing the impact of seismic activity on mining operations may take one of several
forms of proactive measures. Several techniques and methods have been developed in an
attempt to assess rockburst potential of underground mine structures. Several of these
techniques are based on the energy balance around excavations. Among those is the Energy
Release Rate (ERR) that was developed in South Africa. ERR is found to have a significant
correlation with the risk or potential of damaging rock bursts due to mining in deep
underground mines. ERR became one of the most used parameters for stope design in deep
underground South African mines.
Based on the energy balance, an incremental approach can be used to follow the changes due
to mining. The mining of an underground orebody usually implies the widening of
excavations by increments. This leads to an energy release rate by unit surface, used when the
opening geometry is regular, or a volumetric energy release rate, used for irregular geometry
openings. The magnitude of ERR depends on geomechanical properties of rock mass, in situ
stress conditions, the mine geometry and the layout of the mining excavation.
The energy that is stored in the part of the rock mass to be excavated, ΔUm/ΔA (see Figure
2), is a measure of the stress concentration at the front if failure does not occur. The quantity
ΔUm/ΔA is called ERR, where ΔA is the change in area defined as the length of the round

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multiplied by the height of the mining stope. The relationship between measured seismic
magnitude and the strain energy released in one mining step with the method longwall mining
has been studied extensively.
Fig. 2: Mining area according to the South African definition
A relationship between ERR, number of damaging bursts, and rock conditions for longwall
mines has been established. The ERR gives a hint of the rockburst potential within the mine
workings. A damage criterion has then been formulated from this relationship, relating the
ERR and the number of seismic events (Figure 3). The diagram shows that damage is
negligible when ΔUm/ΔA is less than 15 MJ/m2
while extreme damage occurs if the release
of energy ΔUm/ΔA is larger than 100 MJ/m2
.
It has been found that when mining takes place in very small steps, no or little seismic energy
is released. ERR is used for designing mining layouts in deep mines with the purposes of
reducing seismicity.
An example has been illustrated here. Considering the excavation of Figure 4 is to be created
in two steps, as illustrated in Figure 5. Since the behaviour of the rock mass is linear elastic,
the state of stress and strain at the end of the two mining steps must be the same as when
mining in one step. Moreover, the total strain energy stored in the rock mass should be the
same. However, it has been found that there is significantly less seismic energy released
when mining in two steps than in one step. It is seen that the reduction in seismic energy
release when mining in two steps, instead of one. Furthermore, the maximum reduction of
seismic energy (maximum area of the hatched rectangle) can be achieved by making two
mining steps leading to equal stress increments, i.e. the optimum mining sequence would be
the one which satisfies the condition (Figure 6).

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Fig. 3: Relationship between ERR and number of seismic events
Fig. 4: Definition of mining-induced energy components
Mine support: Rock support in burst-prone ground requires a good understanding of rock
mass behavior under high stress conditions and the behavior of the rock support system. Rock
support in burst-prone ground differs from conventional rock support where controlling
gravity-induced rockfalls and managing shallow zones of loose rock are the main concern. In
addition to these design issues, rock support in burst-prone ground needs to resist dynamic
loading and large rock bulking due to violent rock failure. Uniform and quick placement of
support and use of rapid yielding hydraulic support is more effective in dynamic loading
situation.

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Fig. 5: Energy components when mining in sequences
Fig. 6: Reduction of energy released when mining in steps

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Rate of extraction: Higher rate of extraction of ore from a comparatively small face and
immediate placement of supports resulting in better ground control.
Mine induced stresses: 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.
Stope sequencing: Strict sequence of stoping with adequate leg and lead when pallel loads in
close vicinity.
Stoping near faulted zone: Stoping away from the zone of weakness such as faults, dykes
etc., should be done to avoid risk of seismic activity along large geological structures. Slip on
such structures has been observed to cause substantial damage in large parts of a mine.
Shape of opening: By avoiding sharp corners that cause or enhance stress concentration.
High density mine fill: Use of high density fill rockburst is minimized.
De-stressing of abutment: Additional fracture/ de-stressing could be induced in the rock
ahead of the face to extend the fracture zone by
a. 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.
b. 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.
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.

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In situ stresses: The in situ stresses are 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. Current technology cannot predict when rockburst will occur, and the best we can
achieve today is to identify areas of high rockburst potential using numerical models and/or
experience. Microseismic monitoring systems have become an integral part of most hard rock
deep mines in an effort to characterize mining induced seismicity for a quantitative
evaluation of the seismic hazard. These systems have a considerable impact on the mitigation
of the seismic risk by minimizing the exposure of personnel and equipment to seismic
hazards remains.
The origin of microseismic activity in rock is related to process of displacement or
deformation or fracturing in rock which are accompanied by sudden release of stored strain
energy that generates an elastic stress wave/ seismic wave which rediates from the point of
origin within the rock to the boundary. Using a suitable sensor, it is detected remotely as a
microseismic signal/ discrete microseismic event. The seismic signals contain information
about source characteristics, such as the origin time, the location and the fracturing
mechanism. Microseismic monitoring is to observe the seismic waveforms originated from
fractures and to obtain the source parameters from an inversion of the recoded waveforms.
The amplitude and frequency of seismic waves radiated from such a source depend mainly on
strength and state of stress of the rock, the size of the source of seismic radiation, and on the
magnitude and the rate at which the rock is deformed during the fracturing process. The
microseismic monitoring system mainly comprising of seismic sensors, signal conditioning
unit, data acquisition and software for data processing, visualisation and interpretation. The
systems are designed to operate automatically and continuously. It allows real time display of
microseismic activities and location of strong events induced by mining.
Geophones are used for sensing microseismic signals generated in rock. The selection of
geophone is made depending on its required range of frequency and amplitude of ground
motion which can be reliably converted to electric signal. The required numbers of
geophones are installed in rock through boreholes from surface at different depths to make a
three dimensional network configuration. The geophone network configuration is decided
depending on stratigraphy around the mine working. The geophone signals are transmitted to
data acquisition system. The data acquisition system records amplitude and time of ground
motion at sensors distributed throughout the volume of interest assembles data at a central
point for processing. Generally the data are digitised, as close to the sensor as possible, since
in this form it is possible to protect against corruption. The acquired data is communicated to
a central site through cable or radio. Triggering, validation and data reduction is used to
select and pre-process data corresponding to valid seismograms for storage and later
transmission.

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The central site provides sufficient resources for performing seismological processing,
archival storage of event and assisting in interpretation by visualization techniques. In recent
years hardware and software advances in microseismic monitoring systems have allowed this
geophygical technique to provide practical geomechanical measurements at operating mines.
Many organizations including CSIRO (Australia), ESG Solution (Canada), Vulcan (Maptek
Pty Ltd- Australia) and ISS International (South Africa) have developed sophisticated
microseismic systems for monitoring rockburst in mines.
Fig.7: Seismic Net Work of underground hard mine
ACKNOWLEDGEMENT
This lecture material has been compiled from the author’s works and from various references
with due acknowledgements to all the concerned.

Lecture on rock bursts

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