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Blast Induced Vibration Monitoring and Waveform Analysis.

by L.W. Armstrong

ABSTRACT

Blast induced vibrations are a by-product of mining operations. These vibrations can be

as insignificant as a rumble in the far distance or they can be extremely devastating and

can cause structural damage beyond repair. Measuring or monitoring the vibration fromany blasting operation is not only recommended but in some mining operations, in close

proximity to urban areas, it can be a requirement of the development application. How

are these vibration waveforms recorded and what are the pitfalls that can be avoided in

practice?

Blast induced vibrations are as varied as the types of mining operations in practice today.

Although the waveform from a blast is merely a combination of wave trains from each

blasthole, the ground through which the wave travels can have a modifying effect on thewaveform measured at any location. Blasting at quarry operations is normally carry out

in hard rock that is usually jointed in some fashion. Blast designs in these cases have arapid timing sequence as the final muckpile is required to stand up so that fragmented

rock does not have to be chased all over the pit floor. The vibration waveform from this

type of operation is short in duration and is usually uncomplicated (if a vibration

waveform can be described as such). Viewing this waveform can show evidence ofmisfired holes or out-of-sequence hole firings. Open cut mining operations range from

large cast blasts in coal operations to small quarry type blasts in metaliferrous mining

operations. The waveforms from such operations range from long duration shots foropen cut coal throw blasts to the typical short duration quarry type blast. Vibration levels

are typically higher from these types of blasts due to the large charge weights used in

each blasthole. Underground blasting operations produce vibration waveforms thatpredominantly have a higher frequency component due to the more competent nature of

the rock being blasted. The waveform recorded from these blasting operations range

from individual hole wave trains to waveforms similar to quarry blasts but with a much

higher frequency content.

The vibration waveform peak level may be related to the damage level for a particular

blast but, is that all that is important when vibration waveforms are interpreted? Anindication of other factors that can be extracted from the vibration waveform are given

and are discussed in detail.


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INTRODUCTION

Blasting has long been seen as a cost effective means of fragmenting rock or overburdenprior to its removal to expose the more valuable mineral below. However, blast induced

vibrations are a by-product of such blasting operations which must be properly managed

as exceedances can cause closure of mining operations.

The first step in the process of controlling blast induced vibrations is to measure the

vibrations. This is not an easy matter as many problems can occur if this task is notproperly carried out. Standards associations throughout the world have been addressing

this problem for many years and have now realized the importance that must be placed on

the correct measurement procedure. Many litigation cases have occurred over the past

decade relating to community concerns about blasting practices and if the miningoperators do not adhere to correct procedures then the vibrations that are being recorded

might be the result of something other than the blast induced vibrations. At present, one

parameter is extracted from the blast record, the vector peak particle velocity, and this

parameter is used as a pseudo-measure of what could cause structural damage.

The damage parameter extracted from the blast record is only one parameter from acomplicated event that occurs over a time frame of up to 10 seconds. The collection of

the data from the vibration sensors can reveal some interesting facts about how the

explosive has reacted with the ground. However, these facts are mostly ignored, mainly

through lack of understanding. The blast-induced vibrations are transient in nature butduring the time they are active a large amount of energy is delivered to the confining

ground. This energy can cause excessive damage (fragmentation) close to the blasthole

and perceived damage (building movement) at distances of 1to 2 km and more from theexplosive source.

Quarry operations are relatively small in terms of the number of blastholes detonated andalso the charge weight in each blasthole. Such operations use what is termed a fast blast

in that the total time of the initiation sequence of the blastholes takes between half and

one second. The initiation sequence is reasonably consistent in that a typical control row

would have 65 ms between blastholes and blastholes in each echelon would be delayedby 25 ms. This results in a muckpile that is fairly compact and rocks are not thrown very

far. Thus the waveform recorded will show a fairly tight and compact structure with a

small peak and energy dumped into a frequency band that is related to the hole initiationsequence.

Open cut coal operations and metalliferous mine operations usually have many moreblastholes in a pattern and the charge weight in each blasthole can be up to 2 to 3 tonnes.

The blasthole initiation sequence can be similar to quarry blasting operations as in the

case of small metalliferrous mines but in some open cut coal operations throw blasting iscommon practice. In throw blasting operations the control row is initiated in quick

succession and the blastholes in adjacent rows are slowed down to allow the preceding

row to move away. Of course there is an increase in energy (powder factor) to provide


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this movement. So the waveform recorded will show an increase in peak levels as well

as a change in the frequency of initiation compared to the quarry blast.

Underground blasting poses completely different problems and some unique solutions

have become available over time. The major difference between underground and

surface blasting is that a void has to be formed into which the fragmented rock is thrown.Once this void has been established, similar initiation sequences to that those used in

surface blasting practices can be adopted. Minimisation of fragmented waste material is

more important and quite specific to underground operations as the main aim is toremove the valuable ore. Underground safety is of prime importance and to this effect

smooth wall blasting techniques are regularly employed so that the integrity of the backs

is maximised. A vibration waveform recorded for a typical development heading in an

underground operation will show multiple individual peak levels as well as a change inthe frequency of initiation compared to the above ground blasting operations.

Blasting near existing structures can provide some difficult situations but blasting can be

carried out with safety and no damage to these structures. Each and every structure has anatural or resonance frequency of vibration and it is wise to have knowledge of this

frequency before any near field blasting is attempted. With this knowledge it is possibleto design blast initiation sequences that minimize the energy being dumped into the

structures resonance frequency band.

BLAST INDUCED VIBRATION DAMAGE CRITERIA.

Compliance measures are part of any responsible mining operation and limits are detailed

in standards. The limits set out in these standards are as a result of input from researchersand also industry consultation. The Australian Standard of reference for blast induced

vibrations is AS 2187.2-1993 particularly Appendix J of this standard. The limits are

shown in Table 1 and local and state authorities use these limits when mining companiesapply for development applications to begin mining operations.

Table 1. Blast induced vibration limits

Type of building or structure Peak particle velocity

mm/s

Houses and low-rise residential buildings 10

Commercial and industrial buildings or structures of

reinforced concrete or steel construction

25

These recommendations have become mandatory over the past decade as miningoperations are now occurring close to populated areas and residential buildings havebecome affected by blast induced vibrations. Over the years, work by Siskind (1986),

Dowding (1996) and others has shown that residential buildings can withstand a certain

level of vibration. In fact stresses incurred during normal temperature and weatherfluctuations can place excessive loads on some structures. It is not only the peak level

that affects structures but also the frequency of the vibration loading along with the

duration that the vibration is acting on the structure. A lot of work carried out by the


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USBM and the Swedish blasting industry in the early 1960s developed damage criteria,

which are related to the frequency and vibration levels. Again these are guidelines and a

degree of safety is built into them. A typical damage criteria chart is shown in Figure 1.

Figure 1. Damage criteria charts in common use today. Peak particle velocity is

assumed to be directly related to damage (after Technical Services, 1998).

EQUIPMENT AND MONITORING PROCEDURE

To obtain a vibration record of any blast the first step is to have equipment that willadequately detect, sample and store the blast induced vibrations. The primary sensormust be designed to detect motion and two types are readily available today,

accelerometers and geophones. No matter which type of primary sensor is chosen it isimperative that a good signal-to-noise ratio is obtained from the monitoring exercise. If,

for example, low vibration levels, less than10 mm/s, are to be monitored at a local

residence and the primary sensor can measure up to 200 mm/s then this type of primarysensor will normally be operating in the low sensitivity part of its operating range and

some errors could result. It is always best to operate in the mid-range of the primary

sensor for best accuracy.

Next is the electronics required to sample the electrical signal from the primary sensor

and store this signal for post processing either on board or through an external software

package. The primary sensor output is usually an analogue electrical signal which mustbe digitized for storage, handling and analysis. The digitizer has a resolution that can

affect the quality of the signal detected especially if the vibration level is low and the

sensor output signal is low. Typically today 8-bit resolution is available with somemanufacturers offering the option of 12-bit resolution. The only sacrifice in choosing a

12-bit resolution digitizer is the consumption of storage capacity and power, which

1 10 100 1000

10

100

PeakParticleVelocity(mm/s)

Frequency (Hz)

USBM

Swedish


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should not be a problem today with memory capacity availability. Storage capacity on

field equipment has improved dramatically over recent years. The vibration monitoring

equipment is usually only required to store one event, which is downloaded and thewaveform analyzed prior to the next blast. Sampling rate is another important feature and

one that is often overlooked. When a waveform is sampled too slowly for the frequency

of the primary event, errors can occur. This effect is best shown in Figure 2 in which thesolid line is the actual waveform and the solid squares are the sampled points. The

resulting waveform, dashed line, shows where peak values can be missed by an

inadequate sampling rate.

0 5 0 1 0 0 1 5 0

-1.0

-0 .5

0 .0

0 .5

1 .0

ParticleVelocity(m

m/s)

B la s t T im e (m s)

Figure 2. Inadequate sampling rate of a vibration waveform.

An effect described in signal processing as aliasing can, at times, cause erroneous

waveforms to be recorded. Aliasing is related to the Nyquist frequency, which is definedas half the period of the maximum frequency of the waveform. Sampling at frequenciesless than the Nyquist frequency will cause aliasing and a poor representation of the

vibration waveform will be recorded. Vibration monitoring equipment today usually

offers a choice of sampling rates up to 1 or 2 kHz. This sampling rate has been shown toadequately sample the vibration waveform frequency (< 150 Hz in most cases) and not

cause any aliasing problems.

Possibly the most important aspect that an operator can affect is the way in which the

primary sensor is coupled to the ground. The primary sensor is required to detect any

movement of the ground due to the blasting operations and only those due to the blasting

operations. If the primary sensor is improperly bonded to the ground then what is the


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primary sensor measuring? Blair (1995) has shown that the effect of poor ground

coupling can result in extraneous frequencies and amplitude peaks resulting from the

mount rocking within its confinement following a transient vibration loading. Armstrongand Sen (1999) have shown the effect of different types of primary sensor mounting

techniques recommended by vibration monitoring equipment manufacturers. The

standards give recommendations on coupling techniques to use but do not assign theimportance that this part of the monitoring procedure deserves. If this bond is not

successful then the entire monitoring exercise is wasted. Bonding the primary sensor to

the ground in soil and in rock is a different process. In soil the primary sensor isembedded in the soil as shown in Figure 3. In the case of rock it is best to find some

outcropping bedrock and secure the primary sensor to the bedrock with a rigid cement as

shown in Figure 3.

Figure 3. Recommended ground coupling procedures for the primary sensor.

WAVEFORM ANALYSIS

The explosive source for a single blasthole typically detonates in less than 10 ms

depending on the column length and this is the order of the time that the shock wave actson the surrounding ground. However, the ground takes time to react to this shock wave

(due to inertia and particle-to-particle resistance etc.) and depending on the ground

structure the reaction time of the shock wave on the ground will vary. Stratifiedoverburden type ground in effect has room to move with a resultant recorded waveform,

which is extended, and there is often evidence of wave separation in the recorded

waveform. More competent underground rock structure, where internal rock rigidity


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resistance to deformation is higher, results in a recorded waveform that is more

compressed.

Traditionally blast induced vibrations are measured as the peak particle velocity. As the

vibration wave propagates through the ground from the explosive source it causes a

displacement of neighboring particles and also of the bulk of the ground. Localizedparticles move or vibrate about some local equilibrium point under the influence of the

vibration wave. It is the velocity that these particles move about their local equilibrium

point that gives rise to this unit (velocity unit) of measure.

The main concern with blasting operations is the potential damage that can be caused.

Damage is a result of differential movement (displacement) between neighboring

particles in a structure and movement with respect to time is velocity. In the early1960s, when studies were carried out by the USBM and the Swedish blasting industry,

velocity and acceleration gauges were readily available and the use of displacement

gauges was not well received. Besides, displacement could be obtained from integration

of velocity signals.

A vibration waveform is basically a collection of wave trains from individual blastholedetonations following traversal through the ground. The primary sensor is a triaxial array

of accelerometers (or geophones) that measure the ground movement in three orthogonal

directions. The primary sensors are usually oriented with one sensor pointing towards the

blast; this sensor is called the radial or longitudinal direction. The other directions aretransverse and vertical. But for each of these directions there is a peak level as well as

a frequency component. The peak vibration level reported for a blast is the maximum of

the vector sum at each sample point of the three components.

Vector Peak Particle Velocity = Maximum{v ([radiali]2+ [transverse

i]

2+ [vertical

i]

2)}

where iis the component amplitude at each sample point.

But is this the best way to represent what the explosives detonation have applied to the

ground? For example a vibration level of 16 mm/s, which peaks once in the vibrationwaveform at a relatively high frequency (> 60 Hz) might not cause any damage to a

structure. But a vibration level of 4 mm/s occurring at several points on the vibration

waveform indicating a more even distribution of the vibration energy throughout the timeframe of the blast and occurring at a much lower frequency (< 30 Hz) might cause severe

damage to the structure. In the second case the peak vibration level might have been

below the limit set (for example 5 mm/s). One value extracted from a transient eventsuch as blast-induced vibrations does not tell the whole story.

If we look at the energy in a wave (vibrating body) simple harmonic motion can be usedto explain this approach. The total mechanical energy in a simple harmonic oscillation is

the sum of the kinetic energy and the potential energy of a point at some time (Halliday

and Resnick, 1966):


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E = K + U = m v2+ k x

2 = k A

2

where k is a force constant and A is the amplitude of oscillation.

Thus, the total mechanical energy of a particle executing simple harmonic motion is

proportional to the square of the amplitude of the motion. The energy in a particlevibrating about some equilibrium point can thus be determined even though the event is

transient in nature.

So at any point on the vibration waveform trace the energy can be determined and a

summation of these energy values over the time that the wave acts could be an indication

of the energy imparted to the ground by the vibration wave. Even vibrations at lower

levels than the peak vibration level impart some energy to the ground or structure. Theenergy imparted below the peak vibration level is neglected but it could still have an

effect on the ground or structure (frequency dependent). Inclusion of the energy in the

reporting of the vibration effect at a point could shed some light on potentially damaging

blast-induced waveforms. This could give a more meaningful indication of the effect ofthe vibration wave on the ground or structure.

The frequency of the vibration waveform is one parameter that is becoming more

relevant in todays blasting operations. The frequency content is also one parameter used

in the damage criteria approach. The vibration waveform is represented by a voltage

signal from the primary sensor in the time domain. The primary sensor is designed tooperate at frequencies below a certain resonance frequency of the sensor. This resonance

frequency of the primary sensor is usually in 30-plus kilohertz range and well within the

frequency range of blast-induced vibrations. This time domain signal may be convertedto a frequency domain signal. Over many years of signal analysis a procedure known as

fast Fourier transform, after the great 18th

century French physicist Joseph Fourier (1768-

1830), has been developed to a stage that this type of analysis can even be carried out inreal time with some data acquisition devices. Basically the time domain transient signal

is represented by a sine and cosine series and the coefficients are used to convert the time

domain signal into the frequency domain. The mathematics behind the fast Fourier

transform is quite complicated but the resultant output from the Fourier series is a plot ofamplitude (or distance) as a function of frequency. There are many ways of representing

the frequency of the waveform and the traditional approach is to determine the maximum

value and quote this frequency. Again one parameter is used to represent the entirewaveform. It has been reported (Siskind 1986, Dowding 1996 and USBM) that normal

residential structures have resonance frequencies below approximately 35 Hz. To this

extent some authorities require that blast-induced vibrations be reported as some peaklevel less than a frequency of 35 Hz.

VIBRATION PREDICTION

Blast-induced vibrations recorded at a particular location are affected by many factors.

The effect of the shock wave on the ground and the ground structure are characteristics


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that cannot be controlled. These characteristics must be understood, for the local area of

operation, if any control of the blast-induced vibrations is possible. Blasting operators

can however exert a degree of control on their practices to maintain the blast-inducedvibrations within prescribed limits. The level of blast-induced vibration is affected by

both the charge weight that is initiated at any time and the distance from the explosive

source to the monitoring location. For example, for a charge weight of 1000 kg, a highervibration level will be produced at a distance of 100 metres from the source than that at a

distance of 1000 metres from the source, all other conditions being the same. Similarly,

at a monitoring location 100 metres from the explosive source a charge weight of 1000kg will produce a higher vibration level than a charge weight of 100 kg, all other

conditions being the same. The usual approach is to use a decay power law which relates

vibration level as a function of scaled distance (distance /v charge weight). The site law,

as it is termed, is of the form:

Vector Peak Particle Velocity (mm/s) = a *(Scaled Distance)-b

where a and b are site specific coefficients and the scaled distance is measured in metres.

Blast-induced vibration prediction is not an exact science as there are many unknown

variables associated with blasting in rock and overburden material. The rock and

overburden material is usually not consistent or known and vibration wave propagationthrough this inconsistent material is difficult to predict. There are three approaches that

can be taken to predicting blast-induced vibrations.

a) Text book approach

This type of approach is usually taken when there is no information available that can be

used to predict the blast-induced vibration from blasting operations. Text books andmanufacturers handbooks have published a generic decay power law for this situation.

The standard parameters used result from many years of monitoring exercises in a lot

of different materials. This generic first approach law is suited for locations where nohistoric data is available and is usually termed a greenfield site. A generic site law is of

the form (Technical Services, 1998):

Vector Peak Particle Velocity (mm/s) = 1140 x (Scaled Distance)-1.6

b) Single hole site law approach

This type of approach to vibration prediction is more site specific and relies on singleblastholes being detonated and the vibration level at various distances being measured. If

several monitoring locations and several blastholes are detonated a reasonable amount ofdata can be collected and some confidence in the predictions can be assumed. Once

again the decay site law equation is constructed and any scatter in the data can indicate

variability in vibration transmission through the ground even on a single site. Thisapproach should be used to gather useful site information that can be used in more

powerful prediction programs.


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c) Monte Carlo approach

This type of approach to vibration prediction is possibly the most powerful as it takes into

account the effect of detonation of a sequence of blastholes. A schematic diagram is

shown in Figure 4 of the unit operations involved in this form of prediction. The basicstructure of Monte Carlo analysis (Blair, 1999) is a probability approach using the

expected variability of any input function to the event being modelled. There is variation

in all measurements and so the charge weight and the initiation sequence will both havevariability and influence the predicted output. Once a model is established and calibrated

it can then be used to predict the changes in vibration level when blast pattern conditions

are altered.

Seed model

Sin le hole scalin law MONTE

CARLO

ENGINE

Ground velocity

Blast design

Waveform broadening

Blasthole screenin

Single blasthole seed shapes

Output for all

simulated blasts

Figure 4. Schematic diagram of Monte Carlo prediction approach.

TYPICAL BLAST-INDUCED WAVEFORMS.

Blasting operations are many and varied, as are the vibration waveforms recorded fromthese operations. In the following sections some typical waveforms are shown (if there

is such a thing as a typical waveform) that have been recorded at mining operations.

These are not the only waveforms that would be experienced at these operations but someof the characteristics are explained. Viewing the waveform can help explain anyanomalies that might occur from a normal blast from which a complaint is received.


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Quarry blast waveform.

0 1 2

0 .0

0 .5

1 .0

V e c t o r S u m

ParticleVelocity(mm/s)

Blas t T ime (s )

0 1 2

-2

0

2

Ver t ica l

0 1 2

-2

0

2

T r a n s v e r s e

0 1 2

-2

0

2

R a d i a l

Figure 5. Quarry blast with low vibration level.

In this example a vibration monitor was embedded in the ground and coupled in the

recommended manner approximately 2 kilometers from the blast. The waveforms foreach orthogonal component and the vector sum are shown in Figure 5.

This was a quarry blast where the rock was fairly competent and the operation requiredthat the resultant muckpile be stood-up and not thrown very far. Quarry blasts are usually

fast and have a small number of blastholes. The initiation sequence used for this blast

was 25 milliseconds in the control row and 42 milliseconds along the echelon. This blastwas a 3 m x 3 m square pattern with 89 mm diameter holes drilled to a depth of 10

metres. There were 80 holes in the pattern with 50 kg of explosives per hole.

The design blast duration was approximately 1.25 seconds and the vibration pattern wasconsidered to be evenly distributed. This shot was quite smooth but there is some

persistent vibration after the blast finished. This can be expected at this distance (2 km)

as the higher frequencies attenuate at a higher rate than the lower frequencies giving alowering of the average frequency of the waveform. The ground conditions over this

distance can change which can cause other types of waves to be generated. The point to

note here is the higher vibration frequency of the wave at the beginning of the shotfollowed by a decrease in the vibration frequency at the end of the blast. At these

monitoring distances this change in vibration wave frequency is often recorded and if this

frequency excites any structural resonance frequencies there is the possibility ofstructural damage.


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Open cut metalliferrous waveforms.

0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

0

10

20

V e c to r S u m


Blas t T ime (s )

0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

-10

0

10

Vert ica l

0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

-10

0

10

T ra n s v e rs e

0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

-10

0

10

R a d i a l

Figure 6. Vibration waveform from a gold mine blast.

For the waveforms shown in Figure 6, the vibration monitor was glued to a rock outcrop

that was clean and dry. The monitoring location was approximately 250 meters from the

blast. The rock being blasted was reasonably competent and massive. The majority of

the bench had been excavated and the final pit wall was being cleaned up. Displayed are

the waveforms for each orthogonal component and the vector sum.

The initiation sequence used was 100 milliseconds in the control row and 42 millisecondsalong the echelon. This blast was 5 m x 5 m square pattern with 140 mm diameter holes

drilled to a depth of 16 metres and loaded with 150 kg of explosives per hole. This was a

trim blast shot with approximately 60 holes and the two rows closest to the final wallwere lightly loaded with explosives to minimize the vibration level on the final wall.

The design blast duration was approximately 1.25 seconds and the vibration pattern wasconsidered to be irregular as the vibration level showed a rising and falling pattern. From

the vibration waveform it could be inferred that the timing sequence could be modified to

provide a more even vibration loading on the final wall with the possibility of minimizingdamage to the final wall. The point of interest here is as this was an in-pit location themine could use the vibration waveforms to fine tune their blasting practices and produceminimal vibration loading on the final wall while still maintaining enough blasting

energy to fragment the rock for excavation purposes. Vibration is one way ofrepresenting structural damage and as can be seen the interpretation of the vibration

waveform can provide valuable information to help minimize any structural damage.


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Open cut cast blast waveforms.

0 2 4 6

01 0

2 0

3 0

V e c to r S u m


B l a s t T im e ( s )

0 2 4 6

-20

0

2 0

Ver t i ca l

0 2 4 6

-20

0

2 0

T ra n s v e rs e

0 2 4 6

-20

0

2 0

R a d i a l

Figure 7. Vibration waveform from an open cut coal mine blast.

For the waveforms shown in Figure 7, the vibration monitor was embedded in the ground

and coupled in the recommended manner approximately 450 meters from the blast. This

blast was a cast blast in which the overbruden material needed to be thrown in a

controlled manner as far across the strip as possible to minimize dragline handling.

Displayed are the waveforms for each orthogonal component and the vector sum.

The initiation sequence used was 9 milliseconds in the control row and 100 millisecondsbetween rows increasing to 175 milliseconds in the last couple of rows. This blast was 8

m x 11 m staggered pattern with 251 mm diameter holes drilled to a depth of 30 metres

and loaded with approximately 850 kg of explosives per hole. There were approximately800 holes in the pattern.

The blast time was approximately 3.5 seconds and the vibration pattern was considered tobe reasonably even. The overburden material was stratified and as such was composed of

layers of different density material which have different wave transmission properties.

Even though the control row was fired on short delays, because of the structure of theground, surface waves were set up in the ground as can be seen in the low frequencywave persisting up to 5 seconds after the blast. This low frequency wave is difficult to

control as it is a property of the structure of the ground. The point of interest here is that

low frequency waves can be generated by blasting in this type of ground. This lowfrequency waveform can cause residential building damage due to differential movement

of local components of buildings.


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Single blasthole waveforms

0.0 0.2 0.4 0.6 0.8 1.0-200

0

20 0

R a d i a l

Ope n c u t coa l m ineHa rd rock qua r ry

ParticleVelocity(mm/s) 0 1 2 3 4

-2

-1

0

1

2

R a d i a l

0.0 0.2 0.4 0.6 0.8 1.0-200

0

20 0

T r a n s v e r s e

0 1 2 3 4-2

-1

0

1

2

T r a n s v e r s e

0.0 0.2 0.4 0.6 0.8 1.0-200

0

20 0

Ver t ica l0 1 2 3 4

-2

-1

0

1

2

Ver t ica l

0.0 0.2 0.4 0.6 0.8 1.0-200

0

20 0

V e c t o r S u m0 1 2 3 4

-2

-1

0

1

2

V e c t o r S u m

Blas t T ime (s )

Figure 8. Vibration waveform from single blasthole detonations.

Single hole waveforms are shown in Figure 8. For the hard rock quarry the monitor was

glued onto bed rock approximately 50 metres from the blasthole. For the open cut coal

mine the monitor was embedded using the soil in the recommended procedure at adistance of approximately 2 km for the blasthole. The waveforms for each component

and the vector sum waveform for each single blasthole are shown in Figure 8.

The hard rock quarry blasthole was 89 mm diameter drilled to a depth of 13 metres and

loaded with 50 kg of explosive. The open cut coal mine hole was 152 mm diameter

drilled to a depth of 15 metres and loaded with 125 kg of explosives. In both cases onlysingle blastholes were detonated to produce the waveforms shown in Figure 8.

In both cases the blast time was approximately 10 milliseconds as only one blasthole wasdetonated. In the hard rock quarry example the blast effect on the ground has dissipated

in approximately 100 milliseconds even though the monitor was placed close to theblasthole. In the open cut coal mine example the single hole waveform has causedcomplicated waves to be formed in the stratified overburden material which are seen to

vibrate the ground for nearly 2.5 seconds after the single blasthole was detonated. The

point of interest here is the difference in the two waveforms for similar explosive

detonations (one blasthole). The ground the wave travels through will have a majorinfluence on the waveform recorded from any blast initiated.


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Open cut blasting near electric power poles.

0 1 2 3 4

0

100200 Pole Vector


Blast Time (s)

0 1 2 3 4

-200

0

200 Vertical0 1 2 3 4

-200

0

200 Along l ines0 1 2 3 4

-200

0

200 Normal to l ines0 1 2 3 4

0

10

20 Ground vibration

Figure 9. Blasting near power utilities.

A vibration monitor was embedded in the ground and coupled in the recommended

manner 20 meters from the base of the power pole in the line of the power lines. Asecond vibration monitor was glued to the top of the 20 meter high power pole and the

individual components aligned along the power lines, normal to the power lines and inthe vertical plane of the power pole. The waveforms for each component at the top of thepole and the vector sum waveforms for the ground and the top of the pole are shown in

Figure 9.

The initiation sequence used in the blast was 100 milliseconds in the control row and 9

milliseconds along the echelon. This blast was 6 m x 6 m square pattern with 200 mm

diameter holes drilled to a 10 metre depth with 130 kg of explosives per hole. There

were approximately 250 holes and the centre of the pattern was approximately 50 metresfrom this set of power poles. The set of power poles consisted of three poles connected

together and secured in a plane normal to the direction of the power lines.

The blast time was approximately 3 seconds and the vibration pattern was considered to

be lumpy. This shot was felt to stop and start indicating a poor distribution of

explosive energy or a poor timing sequence used. The point to note here is the resonancebehavior that was set up at the top of the power pole and the amplification of the

vibration level due to the structure itself. Resonance behavior can occur in any structure

if the right frequency is excited.


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Underground development heading waveform.

0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

0

3 0

6 0

V e c to r S u m


Blas t T ime (s )

0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

-6 0

-3 0

0

3 0

6 0

Vert ica l

0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

-6 0

-3 0

0

3 0

6 0

T ra n s v e rs e

0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

-6 0-3 0

0

3 0

6 0

R a d i a l

Figure 10. Vibration waveforms from an underground development heading blast.

Figure 10 shows part of the waveform from a vibration monitor glued to a solid rock

outcrop approximately 100 meters from the blast in a cuddy on the decline to the miningoperations. The rock surface was dry and smooth and the coupling to the rock was

considered to be good. The primary sensors were placed with the radial component

directed towards the blast. Accelerometers were used as the primary sensor and theresultant waveforms are an integration from the acceleration trace, consequently some

low frequency artifacts occur between the blasthole records.

A typical underground development heading initiation sequence was used with delay

numbers up to 12 (6400 milliseconds). This blast was 0.75 m x 0.75 m pattern with a

diamond burncut and 64 mm diameter holes drilled to a depth of 3.6 metres. There wereapproximately 45 holes with 10 kg of explosives per hole. The centre of the pattern was

approximately 50 meters from the main decline of the mine.

The blast time was approximately 6.5 seconds and the vibration pattern showed the

typical individual hole separation for this type of blast. Due to the harsh conditions underwhich explosives are required to perform in this type of blasting situation there is always

the possibility that some blastholes fail to detonate. A misfire occurred during this blastas seen by the absence of signal at approximately 1.5 seconds. The point to note here is

that the vibration waveforms can be used as a diagnostic tool to examine the performance

of any blast and determine any remedial action that needs to be addressed for futureblasts.


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Underground stope blast waveform.

0 1 2 3 4 5

0

1

2V e c t o r S u m


Blas t Time (s )

0 1 2 3 4 5

-2

0

2Ver t ica l

0 1 2 3 4 5

-2

0

2Transve rse

0 1 2 3 4 5

-2

0

2R a d i a l

Figure 11. Vibration waveform from an underground slot and ring blast.

Figure 11 shows the vibration waveform from a vibration monitor glued to a solid rock

outcrop approximately 500 meters from the blast. The rock surface was dry and smoothand the coupling to the rock was considered to be good. The primary sensors were

placed with the radial component directed towards the dip direction of the ore body.

For this blast an opening or slot had to be established for the fragmented material to be

thrown into before the subsequent rings in the stope were fired. A delay of 400

milliseconds was used in the slot formation period and then 25 milliseconds betweenblastholes in each ring. The slot blastholes were drilled on a 0.75 m x 1 m pattern and the

rings were drilled with a toe burden of 2.2 m. The blastholes, 102 mm in diameter, were

drilled up to 30 metres deep. There were approximately 9 sets of holes in the slot and 10

holes in each ring. The first ring was 1 metre from the slot that was formed.

This is a similar waveform to the development heading blast, discussed above, except

that the ring firings can be seen to appear at approximately 4.8 seconds. It can be seenthat approximately 4.5 seconds was allowed for the formation of the slot which provide a

void for the ring blasts to throw the fragmented material into. The point to note here is

that the vibration waveforms can be used, in conjunction with post blast inspections, as adiagnostic tool to examine the performance of the slot formation in relation to the

fragmented material formed by the ring blasting.


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CONCLUSION.

Vibration monitoring has been an integral part of responsible mining operations for abouta decade now. As the urban sprawl begins to affect most mining/quarrying operations

measures must be taken to minimize the blast-induced vibration effects on the nearby

neighbours. These measures, in particular in some quarries, have become imperative ifthe quarry is to remain in operation. It doesnt matter that the quarry has been in the

same location for 50 years the urban sprawl is here to stay and both the quarry and the

local residents can co-exist provided the mine/quarry operations are carried outresponsibly.

The vibration monitoring procedure should be viewed as a constructive part of the

mine/quarry operation and not something that just has to be done. When a diagnosticapproach is taken to the recorded waveform it can not only help to reduce blast-induced

vibrations for subsequent blasts but also provides a tool to view the effect of blasting on

the ground. If final walls are to be protected then less damage can result from properly

designed blasts as a result of analyzing the vibration waveforms from previous blasts.Smart blast designs are a key responsibility of mine/quarry operators not only in the

efficient use of sometimes costly raw materials but also in the management of some ofthe environmental outputs from the mining/quarrying operations.

REFERENCES.

Armstrong L.W. and Sen G.C., 1999, The measurement of blast induced vibrations in

soil. Explo 99, Kalgoorlie, WA, 7-11 November, pp. 99-104.

AS 2187.2-1993, Explosives Storage, transport and use. Part 2: Use of explosives.,

1993, Standards Australia, Homebush, NSW, Australia.

Blair D.P., 1989, Ground coupling of vibration detectors., CSIRO Division of

Geomechanics, Institute of Mineral Energy & Construction, External Report No.1,

September, pp. 1-45

Blair D.P.,1999, Statistical models for ground vibration and airblast., Int, J of Blasting

and Fragmentation, 3, pp 335-364

Dowding C.H., 1996, Construction vibrations., Prentice-Hall Inc., New Jersey, USA

Halliday D. and Resnick R., Physics Parts I and II., Wiley International Edition, NewYork, USA

Siskind D.E., 1986, Frequency analysis and the use of response spectra for blast vibrationassessment in mining., Proceedings of 12

th Annual Symposium on Explosives and

Blasting Research, Orlando, Florida, 4-8 February, pp.1-11.


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Technical Services, 1998, Safe and efficient blasting in surface coal mines., Orica

Explosives, April

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