HSEHealth & Safety

Executive

Novel mobile and portable methods for detecting rock failure

Prepared by Rock Mechanics Technology Ltd and the University of Exeter for the

Health and Safety Executive 2004

RESEARCH REPORT 248

HSEHealth & Safety

Executive

Novel mobile and portable methods for detecting rock failure

David Bigby BSc. (Hons.), PhD., MIMMM CEng Alan Bloor BSc. (Hons.), PhD.

Rock Mechanics Technology Ltd Bretby Business Park, Ashby Road

Burton-on-Trent, Staffordshire, DE15 0QD

Chris Chester BSc. (Hons.), MSc., MCSM Camborne School of Mines, University of Exeter

Redruth, Cornwall, TR15 3SE

At the outset of the Project, there was no portable non-contacting technique available for determining the condition of a mine tunnel roof. Such a technique would be of great benefit to Health and Safety in the mining industry as it could allow unsafe roof to be identified without the investigator having to be positioned within the hazardous area. A suitable technique could be of particular use in the cut-out area of a tunnel excavation, prior to support placement, and could facilitate dynamic decision making on the timing of support placement.

The Project investigated a number of physical phenomena, such as acoustic response, ultrasound emissions, electromagnetic emissions and thermal imaging which were considered to have a potential to provide warning of failing roof rock, to indicate when initial or additional support placement might be required. A practical instrument utilising one of these phenomena would have to be capable of machine mounting or remote operation. Initial tests using these techniques had already produced encouraging results.

The research indicated that the detection of emissions, ultrasonic, acoustic or electromagnetic, as an indication of microfracturing and the imminent failure of a rock mass, has a number of inherent problems relating to its use in a mining environment, in particular problems of filtering background noise.

The Project went on to examine how more conventional means of rock failure detection could be modified to become remote methods. This involved considering the different responses of a rock mass, which had failed, but not detached completely from the surrounding mass, to exterior stimuli such as vibration or a thermal gradient.

It was concluded that the most promising principles for development of a mobile or portable, non-intrusive instrument for detecting failing and/or failed rock in a working mining environment are; measurement of induced vibration using a laser vibrometer or similar device and detection of an induced thermal gradient using a thermal imaging system. However both principles require further research and development before a practical tool can be developed.

This report and the work it describes was jointly funded by the Health and Safety Executive (HSE), the European Coal and Steel Community (ECSC) and several mine operators. Its. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

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First published 2004

ISBN 0 7176 2866 3

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ii

CONTENTS

EXECUTIVE SUMMARY 1

1. INTRODUCTION 51.1 Background to Project 5

1.2 Aims 5 1.3 Objectives 6

2. LITERATURE REVIEW 72.1 Electromagnetic Emissions 72.2 Thermal Response 92.3 Ultrasonic / Acoustic Emissions 13

2.4 Induced Vibration 162.5 Laser Displacement Measurements 18

3. LABORATORY WORK 23 3.1 Introduction 23

3.2 Electromagnetic Emissions 233.3 Ultrasonic Emissions 243.4 Thermal Response 253.5 Laser Vibrometer 27

4. FIELD TRIALS 31 4.1 Introduction 31

4.2 Thermal Response Trials 314.3 Laser Vibrometer Trials 33

5. CONCLUSIONS 355.1 Ultrasonic / Acoustic Emissions 355.2 Electromagnetic Emissions 355.3 Thermal Response 355.4 Laser Vibrometer 365.5 Overall Conclusions 37

6. RECOMMENDATIONS 39

7. REFERENCES 41

8. FIGURES 43

iii

LIST OF TABLES

Table 1 Results of 2-D slab modelling calculations, after Kononov 2000. 12 Table 2 Potential ‘hard’ applications of the AEM with assigned confidence 18

levels, after Piper et al, 2002.

iv

LIST OF FIGURES

Figure 1 Examples of Data Collected From Underground Experiments, 44 (a) Shows a Clear Electromagnetic Anomaly Prior to a Seismic Event, (b) Shows No Clear Anomaly Prior, During or After a Seismic Event, Possibly Due to the Level of Background EMR

Figure 2 Temperature Profile In A Simulated Slab (1-d Fin Model) After 45 Kononov, 2000

Figure 3 Plotted b-values from the Underground Research Laboratory, 46 Canada (a) During a Seismically Quiet Period, and (b) a Seismically Active Period

Figure 4 Acoustic Energy Meter. 47 Figure 5 (a) Polytec Portable Digital Vibrometer (PDV) 48

(b) Schematic Representation Of A Michelson-Morley Interferometer. (c) Schematic Drawing Of A Heterodyne Interferometer, With Bragg Cell.

Figure 6 (a) Schematic Of Instrumentation Configuration 49 (b) Photograph Of Instrumentation And Rock Press.

Figure 7 Screen Dumps Showing the Measurements of Background Noise 50 Seen Inside the Faraday Shield

Figure 8 Reduction in Background Noise Achieved Using the RC Filters. 50 Figure 9 (a) Schematic Representation Of The Instrumentation 51

Configuration, After Modification (b) Photograph Of Instrumentation And Rock Press.

Figure 10 (a) Modifications Made to Thermal Test Blocks, Position of Four 52 Thermistors Embedded Into Slab, (b) Enlarged Image of Embedded Thermistor

Figure 11 Thermal Response of Loose and Solid Test Blocks to a Period of 53 Heating Followed by a Period of Cooling.

Figure 12 Laboratory Thermal Response Trials, Using Liquid CO2, to Lower 54

the Temperature of Simulated Failed and Intact Rock Mass. Figure 13 Computer Simulation of the Thermal Response of Loose and Solid 55

Rock Mass to Cooling After an Initial High Temperature (Natural), Figure 14 Computer Simulation of the Thermal Response of Loose and Solid Rock 55

Mass Heated Then Subsequently Allowed to Cool (Enhancement).

Figure 15 Vibrometer Trace From Simulated Loose and Solid Rock Mass, 56 Figure 16 Vibrometer Trace Parameter Plotted Against Acoustic Energy 56

Meter (AEM) Value Figure 17 Surface Temperature Profiles of a Pillar at a Dimension Stone 57

Mine, Near Bath. (a) Shows the Surface Temperature Immediately After a Heating Period of Four Minutes. (b), the Surface Temperature Difference After Four Minutes of Cooling 58 (c), the Surface Temperature Difference After 12 Minutes of Cooling.

Figure 18 Acoustic Energy Meter Value Plot For The Same Pillar. Failed 58 Rock Mass Represented By The High AEM Values.

Figure 19 (a), Approximate Roadway Profile (Not to Scale), 59

v

(b), Temperature Variation Across Roof Profile. Figure 20 Vibrometer Trace Parameter Value Plotted Against AEM Value, 60

From the Dimension Stone Mine Figure 21 Vibrometer Trace for Failed and Intact Rock From the Dimension 60

Stone Mine. Figure 22 Vibrometer Traces for One Target Excited From Different 61

Locations at Derbyshire Limestone Mine. Figure 23 Algorithm Value Versus AEM Values For Different Separations 62

From Limestone Stone Mine. Figure 24 Algorithm Value Versus AEM Values For Different Separations 62

From Dimension Stone Mine.

vi

EXECUTIVE SUMMARY

This 3 year HSE research project, 4273/R33.097, entitled “Novel Mobile and Portable Methods

for Detecting Rock Failure” was co-funded by a European Coal and Steel Community funded

RTD Project, 7220-PR092, entitled “Advanced Geotechnical Instrumentation for Detecting

Rock Failure and Monitoring Support Loads”. The HSE Project commenced on 1st April 2001

and was completed on 31 March 2004. The HSE Project was undertaken by Rock Mechanics

Technology Ltd of Burton on Trent, Staffs. The ECSC Project was undertaken by 5 partners

drawn from 4 Western European coal mining countries (France, Germany, Spain and the UK).

Some of the work reported here was undertaken in collaboration with Camborne School of

Mines, Exeter University, under an Associate Contract to the ECSC Project.

At the outset of the Project, there was portable non-contacting technique available for

determining the condition of a mine tunnel roof. Such a technique would be of great benefit to

Health and Safety in the mining industry as it could allow unsafe roof to be identified without

the investigator having to be positioned within the hazardous area. A suitable technique could

be of particular use in the cut-out area of a tunnel excavation, prior to support placement and

could facilitate dynamic decision making on the timing of support placement.

There were a number of physical phenomena, such as acoustic response, ultrasound emissions,

electromagnetic emissions and thermal imaging that were considered to have a potential to

provide warning of failing roof rock, to indicate when initial or additional support placement

might be required. A practical instrument utilising one of these phenomena would have to be

capable of machine mounting or remote operation. Initial tests using these techniques had

already produced encouraging results.

The research indicated that the detection of emissions, ultrasonic, acoustic or electromagnetic,

as an indication of microfracturing and the imminent failure of a rock mass, has a number of

inherent problems relating to its use in a mining environment. Emission strength is a key

problem. When background noise is also considered, the problem of detection becomes even

more difficult. In a working underground environment there are a number of sources of

acoustic, ultrasonic and electromagnetic emissions, which create significant background noise.

However, the detection of such emissions, especially acoustic and to some extent

electromagnetic, as an indication of failing rock mass, is important for research purposes.

Under laboratory conditions, where variables such as background noise and failure rate can be

controlled, such emissions can provide a great deal of information about the failure process.

Due to these inherent problems, the researchers went on to examine how more conventional

means of rock failure detection could be modified to become remote methods. This involved

considering the different responses of a rock mass, which had failed, but not detached

completely from the surrounding mass, to exterior stimuli such as vibration or a thermal

gradient.

Technology and experience already gained during the development of the Acoustic Energy

Meter, a contact device capable of quantifying the integrity of a surface, were applied. Research

was conducted into the transient vibrational characteristics of failed rock, and how this could be

stimulated and measured remotely. Using a laser vibrometer, measurements of transient

vibration were taken from both intact and failed rock mass in the laboratory and underground.

Results showed that, with sufficient vibration, distinguishing characteristics could be seen

between the two rock mass classes. Mathematical algorithms applied to the results, could

1

quantify the integrity of the surface under investigation and give an indication of its potential to

cause harm.

Further work on induced vibration is recommended in the following areas: -

x� Safe and mine worthy techniques for the delivery of sufficient energy to a surface under

investigation should be the main focus due to its critical role in the whole concept.

Without sufficient vibration, measurements are inaccurate and unreliable, hence the

requirement for extra research.

x� Increasing the return signal strength is also important. Without a good return signal the

measurements are inaccurate and unreliable. During the field trials, reflective tape was

placed on the target surface; this however falls short of a wholly remote method. It is

recommended that further work should examine the potential of the paintball marker,

initially used unsuccessfully for the delivery of vibrational energy, as a method of

placing reflective paint onto the target surface.

The thermal response of intact and loose rock mass to two different temperature environments,

either naturally occurring or enhanced by man in the underground environment was also

researched. Initial laboratory work demonstrated that by heating a surface, simulated loose rock

mass could be distinguished from solid rock mass by its different thermal response. Field trials,

however, failed to provide similar positive results, with only minor success in identifying loose

rock mass seen at one location in an evaporate mine. It is the authors’ opinion that this remains

a viable concept and requires further research, and the following further work is recommended:

x� In the authors’ opinion the concept is most suited to deep excavations where virgin rock

temperatures are high, hence research should be focused here, rather than shallow

excavations where more energy is required to develop a thermal gradient. Work should

be conducted into ways in which normal mining practices can be modified in order to

enhance a temperature difference to detectable levels. The following are some

suggestions of how a temperature difference could be enhanced between intact and

loose rock mass: -

�� Increasing ventilated airflow in a development for a few hours prior to a survey.

�� Use of an auxiliary cooling fan to lower ventilated air temperature in a

development prior to a survey.

�� Stop ventilating a development for an over night period, causing the rock mass to

reach an equilibrium closer to virgin rock temperature. After the over night

period, restart the ventilation, whilst surveying the area with thermal imaging

equipment.

�� Spraying hot rock with cooled water. This could be particularly successful in

deep, hot mining environments where cooling water is often already available for

other purposes.

In summary, it was concluded that the most promising principles for development of a mobile or

portable, non-intrusive instrument for detecting failing and/or failed rock in a working coal

mining environment are; measurement of induced vibration using a laser vibrometer or similar

device and detection of an induced thermal gradient using a thermal imaging system. Both

principles require further research and development before a practical tool can be developed.

The areas requiring most attention are;

x� a remote means to impart sufficient vibrational energy into the rock,

2

x� a means of improving the return signal strength to a laser vibrometer in a mining

environment,

x� a means of producing a sufficient thermal gradient; this is most likely to be achieved in

a deep mining environment

3

4

1 INTRODUCTION

1.1 BACKGROUND TO PROJECT

This 3 year HSE research project, 4273/R33.097, entitled “Novel Mobile and Portable Methods

for Detecting Rock Failure” was co-funded by a European Coal and Steel Community funded

RTD Project, 7220-PR092, entitled “Advanced Geotechnical Instrumentation for Detecting

Rock Failure and Monitoring Support Loads”. The HSE Project commenced on 1st April 2001

and was completed on 31 March 2004. The HSE Project was undertaken by Rock Mechanics

Technology Ltd of Burton on Trent, Staffs. The ECSC Project was undertaken by 5 partners

drawn from 4 Western European coal mining countries (France, Germany, Spain and the UK).

Some of the work reported here was undertaken in collaboration with Camborne School of

Mines, Exeter University, under an Associate Contract to the ECSC Project.

The ability to detect/predict the onset of rock mass failure has been the goal for a considerable

number of geotechnical researchers over the past decades, with earthquake prediction being the

main driving force behind the research. Such research has produced a number of novel ways in

which to detect rock failure, all with different degrees of success, and with none being 100%

accurate. In recent years the same methods have been developed for use in the mining

environment, with the aim of predicting / detecting mining induced, rock mass failure.

Seismic techniques first developed for earthquake prediction, have seen the most use in the

mining environment, with a number of deep, hard rock mines using the technique as a method

of identifying seismically “quiet” periods. These periods are often seen as fore runners to a

rockburst or bump, a dangerous hazard in deep seated mines.

In mining terms the ability accurately to detect failed or failing rock mass would have two

significant advantages; firstly safety. Areas where rock mass failure has occurred, or is

currently occurring, could be identified and the correct action taken before an accident, or

fatality occurs. The second advantage would come from increased production, a result of less

“down time” caused by falls of ground and other rock mass failure problems, and the accidents

and fatalities which accompany them.

Devices are currently available which can give an indication of the integrity of the rock mass.

They are however often crude and rely heavily on the operator’s experience and judgement.

These available devices also require direct contact with the surface under investigation, a

situation, which puts the operator at increased risk. An example of such a device is the scaling

or sounding bar, used to sound an excavation in search of loose rock, often after a round of

explosives has been fired. Such a technique is subjective as it is not able to quantify the

integrity of a surface, leaving the decision as to the level of risk to the operator. A device

capable of detecting failed or failing rock mass and quantifying its integrity from a remote

location, would remove such risks, making the underground environment a safer more

productive place.

1.2 AIMS

The primary aim of the work undertaken under this Report was to develop a portable or semi-

portable means of detecting areas of rock mass failure in a mine, by using measurements of

emissions and/or responses to certain stimuli that failed or failing rock mass might display. The

technique(s) developed, would be capable of detecting failed rock mass from a remote location,

and not require entry into an area under investigation by the operator.

5

A secondary aim was the design and manufacture of a product capable of fulfilling the primary

aim, for economic, commercial application in the mining and extractive industries. This had to

be considered when researching and developing concepts and techniques as it imposes a number

of constraints on any potential device. Consideration was also be given to the following when

assessing possible devices and their operation.

x� Intrinsic safetyx� Mine worthiness, (robust, capable of handling extreme environments)x� Useable during normal mining activities x� Portabilityx� Potential for use as a survey tool and / or as a continuous monitoring device x� Short survey time.

1.3 OBJECTIVES

The objectives were as follows:

x� Complete an extensive literature survey into research undertaken by other authors in the

field of rock mass failure prediction / detection. In particular, examine work focused

towards the mining and extractive industries and the problem of rock mass failure found

in underground environments

x� Using the knowledge gained from the literature review and from the experience held by

the Partners, identify and acquire sensory devices and instrumentation, which showed

potential of fulfilling the primary aim.

x� Undertake laboratory trials of the selected prototype devices on simulated rock mass

conditions, and make appropriate modifications to the devices and operating procedures

if required. The laboratory experiments must simulate underground conditions as

closely as possible.

x� After assessing the results of the laboratory experiments, undertake field trials of the

device(s) which demonstrated the most potential for fulfilling the stated aims. It may be

necessary to use laboratory facilities to further develop the operating procedures for the

devices under investigation before their trial in an underground environment.

6

2 LITERATURE REVIEW

A great deal of research has been undertaken into how rock mass characteristics change during

the failure process, with most research looking at the emissions and responses produced during

and after failure. In recent times this research has advanced to another level, with the progress

in computer technology and sensory devices enabling vast quantities of data to be collected

during the failure process. From such data a better understanding of what occurs during the

failure process has been achieved with much of what has been learnt used in the construction of

increasingly complex computer modelling software. Sensory devices have also improved, not

only in their range but also their sensitivity and size, enabling easy placement around test

samples, and accurate measurements from around their circumferences.

This Chapter provides an overview of work undertaken by other researchers in this field in

recent times, with emphasis on work directed towards the mining industry and the

environmental conditions therein.

2.1 ELECTROMAGNETIC EMISSIONS

Also known as electromagnetic radiation (EMR), the first observation of electromagnetic

emissions from fractured material under stress was made in 1933 (Urusovskaja, 1969). Since

then a number of authors have investigated the emissions from different materials under

laboratory conditions and have witnessed high counts of EMR readings prior to failure. The

exact origin of electromagnetic emission produced during the fracture process is not truly

understood, with a number of different theories being proposed. The most accepted theory

among them states that the fracturing of atomic bonds during the failure process is the likely

cause of the emissions. The theory is substantiated by the mirroring of electromagnetic

emissions by acoustic emissions, whose origin has long been known to be micro fracturing

during the failure process.

The frequency range over which emissions have been witnessed from failing rock mass is

between 1 kHz and 10 MHz with wavelengths from 30m to +300 km. Cress et al (1987) found

that maximum EMR occurred over the frequency range 0.5 - 1 MHz, during their studies using

rock samples and a loading frame to induce failure. Earlier research by Hanson et al (1981) had

detected EMR during the catastrophic failure of quartz rich rocks. A more important finding

from Hanson et al’s research was that fracture size is directly proportional to the amplitude of

the emissions. This finding prompted Hanson to state that “the most important application for

electromagnetic emissions observation is the monitoring of unstable rock faces in mines”,

(Hanson, 1981). He went on to say that a portable detection system, which would not require

direct contact to the rock surface like geophones, could be produced for use in mining

situations. The device would simply detect increases in the number or amplitude of EMR

events and warn of imminent rock failure.

While studying solar activity using an array of world-wide radio receivers for cosmic radio

noise, Warwick et al., 1982, witnessed large, strange, fluctuating signals, which lasted 20

minutes on the 16th May 1960. A terrestrial source for the emissions was deemed the most

probable origin due to the emissions being some magnitudes larger than normal background

solar emissions. Five days later on the 21st May 1960 a series of earthquakes struck along the

Chilean fault zone, devastating large areas of Chile. Warwick realised the emissions he had

witnessed five days earlier, could be the result of stress-induced micro-fracturing of quartz

bearing rocks along the Chilean fault prior to mass failure and the subsequent earthquake.

Although the direct evidence obtained by Warwick falls somewhat short of identifying the

7

Chilean fault as the source of the emissions, it was possible to calculate that the source would be

of similar dimensions to the fault and lie some distance from the antennae.

Other researchers have looked into EMR as a precursor to earthquakes and other seismic

activity. A controversial group often referred to as VAN (Panayiotis Varotsos et al), have

claimed success in measuring EMR prior to earthquake activity in Greece. However various

other authors have presented papers criticising the VAN group’s methods, Stravakakis (1998)

and Geller (1997) have both argued against it. The arguments against the VAN method include

claims that no actual EMR has been recorded at the time of an earthquake and neither has

seismic activity been recorded at the time when the EMR precursors are claimed to be

measured. Also many EMR measurements were only detected at one VAN measuring station

and not at others in Greece. Finally they claim evidence that the source of much of the EMR

was digital radio-telecommunications transmitters and other industrial sources rather than the

proposed failing rocks.

More recently, Rabinovitch et al, 2000, conducted a series of tests on chalk. They failed a

number of samples while simultaneously recording electromagnetic emissions. Sophisticated

equipment and techniques were employed to block / filter out background electromagnetic

radiation so that only emissions from the failing sample were recorded. From the tests,

Rabinovitch, like Hanson in 1981, noted that the amplitude of the EMR emissions increased as

the fracture length grew, a feature they put down to the severing of atomic bonds as the fractures

increased in length. This theory has since become the most accepted for the cause of EMR.

Assuming a constant crack velocity (Vcr), Rabinovitch also stated that the time from the origin

of the pulse to its maximum amplitude envelope (Tl) is proportional to the crack length l.

Therefore as the crack length increases the time between the emission’s origin and the

maximum of its envelope increases proportionally. This is represented in the following

equation:

' l T [2.1]

v cr

A further observation was made that the frequency of the EMR emissions (Z) was related to the

fracture width (b) and could be represented by the following equation:

SvelY [2.2] b

Where vel is the Rayleigh wave speed.

The combination of the two relationships above means the fracture area can be calculated using

equation [2.3], providing the velocities are known:

T

Y

'

S vv elcr S

1 [2.3]

Where S = l x b = the fracture area.

The equations can be used for both the determination of the exact fracture area or comparison of

different fracture areas. The advantage of this mathematical approach is that it is less affected

8

by external background radiation, which may overprint the actual EMR produced by the failing

sample. The use of the equation above in a device used to measure EMR in a mining

environment would allow an interpretation of the extent of fracturing that has occurred to be

made. Once a limit of fracture area has been reached and failure is imminent, the EMR

detection device could sound a warning to avert disaster.

Research has also been conducted in mining environments for the detection of electromagnetic

radiation caused by rock failure due to mining activity. Such underground experiments allow

for a more realistic testing environment, for which a device would be used. Also background

levels of EMR in mines are drastically reduced allowing measurements of fracture induced

EMR to be better observed.

(The following is from a private communication with the Safety in Mines Research Advisory

Committee SIMRAC)

One such experimentation was conducted in an actively failing coal mine in South Africa. Both

EMR and seismic activity were recorded using a radio receiver and antenna, a pocket radio and

a geophone all connected to a multi-seismometer and ruggedised laptop for data recording. The

frequency over which the antenna and radio receiver were set, changed periodically to increase

the overall range in which measurements were taken. Only EMR and / or seismic activity above

a set background levels was recorded by the seismometer and all background nose was

disregarded.

At a frequency of 4.92 MHz the EMR was observed most clearly and occurred just prior to 80%

of confirmed seismic events during the period that frequency was used. Figures 1 (a) and (b)

illustrate the type of data recorded from the experiments. Figure 1(a) clearly shows an EMR

emission prior to a seismic event. Figure 1(b) however shows a clear seismic event of similar

magnitude to that in figure 1(a) but no discernible EMR emission. The level of seismic and

electromagnetic background noise encountered in a mining environments can also be clearly

seen in figure 1(a) & (b), it is believed that this noise could be masking any small EMR

emissions prior to seismic events.

2.2 THERMAL RESPONSE.

The use of thermal imaging and remote temperature measuring devices in industry has evolved

over the last few decades, with a growing number of applications, helped by the development of

more sophisticated equipment. The mining industry is one of the industries which have applied

the technology in a number of its key areas, however it is only in recent years that researchers

have looked at the possibilities of using remote temperature measurement technology to detect

areas of failed rock mass.

Two different methods of using thermal response in mining situations to detect failed rock mass

have been developed in recent years. The one that has seen most interest involves looking at

temperature differences between solid and failed rock mass. The other method has seen less

mining related research, with authors concentrating on using the method for the prediction of

seismic activity. The method involves using satellite based thermal imagery to locate areas of

increased stress, highlighted by higher temperatures, caused by increased friction. Both

applications will be discussed further in the following sections.

2.2.1 Loose rock detection As with many new ideas, this phenomenon was discovered by accident during work by the

United States Bureau of Mines (USBM) on roof beam failure in a limestone mine in 1958. The

original experiment involved the widening of a room till failure was induced; Merrill and

9

Morgan (1958). However due to cost and safety implications it was decided not to mine until

failure occurred, but to use compressed air forced into the separation between the lowest roof

beam and the rock above to induce failure. It was during this phase of the work when the roof

beam had detached from the overlying rock that a difference in temperature was noticed

between loose (beam) and solid rock mass. The temperature gradient was sufficient for it to be

detected by feel alone using the palm of the hand.

The cause of the noticeable difference is that two different temperature environments act on the

rock mass within an excavation. Firstly, there is the virgin rock temperature at which the rock

lies before excavation. Secondly, there is the ventilated air temperature, which can be

significantly lower than virgin rock temperature. If rock mass failure occurs an air gap or

separation may form, creating a thermal barrier. Heat from surrounding rock, which lies at

higher virgin rock temperatures, would be restricted from conducting into the cooler failed rock

mass, which lie at a temperature between virgin rock temperature and ventilated air temperature.

If the flow of heat by conduction is restricted sufficiently, a noticeable difference should be seen

between failed and solid rock mass. At the time of Merrill and Morgan’s (1958) discovery the

technology available for remote thermal imagery / temperature measurement was unable to

detect the small changes in temperature between failed and solid rock mass.

In 1970 Merrill and Stateham revisited the idea, with the advantage of more sensitive

instrumentation capable of detecting temperature differences of 0.2 oC. During the experiments,

a single point measurement device, call an infrared pyrometer, and a thermal imager, which

displayed a visual indication of an objects temperature, were used.

Trials using the equipment were conducted in a number of localities in order to witness the

variety of air to rock temperature conditions found in mines. In some of these localities the air

temperature and flow was controlled in an effort to determine the time taken for failed or loose

rock mass to adjust to changes in temperature compared to solid rock mass. Although few

figures and details relating to the conditions to which the excavations were subjected are given,

Merrill and Stateham (1970) state that the rocks studied took, on average, between 15 and 30

minutes to develop a detectable temperature difference.

An example of the trials conducted involved the stopping of all ventilation and the sealing of a

mine for an overnight period, allowing the excavation to form a natural equilibrium.

Measurements taken the following morning with the pyrometer and imager showed no

discernable difference in temperature between loose and solid rock mass. The ventilation

system was then switched on. After 15 to 30 minutes of normal ventilation conditions a

difference between 1 and 7oC, depending on the ventilated air temperature that day, was

witnessed between loose and solid rock mass. Merrill and Stateham also discovered that freshly

blasted faces produced temperature differences of 1 to 1.5 oC between intact and loose rock,

compared to the 0.2 to 0.3oC difference shown by pillars only 30ft away.

Since Merrill and Morgan (1958) first discovered the concept and then Merrill and Stateham

(1970) follow up, little work has been done on the subject, with the most significant work

undertaken in the last decade. Yu et al (1990) conducted a more systematic approach to

examining the concept, by using a 350 kg lump of rhyolite which contained loose “flakes” of

rock, Yu et al were able to repeat experiments without encountering any significant changes.

Yu et al also enclosed the test block in a wooden box in order to reduce the effects of the

surrounding environment. The block was initially heated using thermocouple wires attached to

the solid rock and the tip of a loose flake. Once a predetermined temperature was reached, air

was blown across the block to simulate ventilated air, the temperature and velocity of which was

varied. Again an infrared pyrometer and thermal imager with sensitivities of 0.2 oC were used

to obtain visual and numerical data for analysis of trends.

10

Yu et al’s (1990) initial experiments provided the expected conclusions, that loose rock cools

faster than solid rock mass and creates a discernable temperature difference between the two

rock mass conditions. Field trials, conducted also by Yu et al at Kidd Creek Mine, again

confirmed that a temperature difference was observable between areas of loose and solid rock

mass. However from the trial, Yu concluded that the age of an excavation has a negative effect

on the observable temperature difference, with older excavations demonstrating few detectable

temperature differences. To answer this problem Yu et al developed a number of techniques to

improve or enhance the temperature difference. The following enhancement methods were

used: -

x� Hot diesel exhaust from an LHD machine blowing against the roof for 20- 30 minutes.

x� Airflow rate acceleration using a local fan.

x� Radiant heat from an IR source.

x� Evaporation of water from the rock surface.

The hot air from the LHD exhaust provided the best results according to Yu et al, with a more

significant temperature difference seen due to this method than any of the others. Yu et al took

the research a step further by mounting a thermal imaging scanner onto a mechanical scaler, the

intention being to aid the operator during the scaling of a roof. To enhance the conditions the

diesel exhaust of the scaler was directed towards the roof being inspected.

The effect of using the equipment was to cut the time taken for the roof inspection, with a 30m

section of tunnel taking 30.4 minutes using the IR equipment. This compared with 64.4 minutes

taken to inspect the same area without the IR equipment. However Yu et al also state that the

checking of areas scanned using the IR scanner, using a scaling bar demonstrated that a

significant amount of loose rock had been missed by the IR method, a problem that may be

alleviated using other temperature enhancement methods.

Kononov (2000) conducted the most recent work on the subject with his work for Safety in

Mines Research Advisory Committee (SIMRAC), South Africa. The research focused more on

the theoretical aspects, with estimations on heat transfer between tunnel surface and airflow

being the main aim. Calculations on the conduction of rock were performed using a 1-D “fin”

type model, designed to simulate a slab of rock, the aim being to see how heat dissipates along

the slab (“fin”). The affect of ventilated air on the temperature of the slab was also taken into

account, by including convection calculations in the simulation. The results, figure 2, from the

calculations clearly reiterate what Yu et al (1990) had observed that better results are found in

newly excavated areas. Also, with time the surface temperature of both solid and failed rock

mass become closer to that of the ventilated air temperature.

Kononov (2000) also produced more sophisticated 2-D models using more realistic geometry

and global assumptions in the calculations. Four variations of the calculations were conducted

with different air velocities being the main variable, slab dimension being the other. The results

of the calculations are shown in Table 1.

The results highlight the effect that ventilated air velocity has on the temperature difference

between loose and solid rock mass, with slower velocities producing the best conditions for

loose rock detection. The 2-D results again illustrated the effect of time on the temperature

difference, with a marked contrast between the year old slab and the 1 month old slab. One new

conclusion drawn from the modelling relates to the effect the size of the slab has on the results

with a thinner slab showing a larger temperature difference compared to a thicker slab with the

same variables.

11

Table 1. Results of 2-D slab modelling calculations, after Kononov 2000

Air velocity Slab dimensions Calculated temperature Time since simulated

(m/sec) (w.l.t) difference (oC) excavation

(m)

0.5 0.8 x 1 x 0.4 3.47 1 year

1 0.8 x 1 x 0.4 3.26 1 year

5 0.8 x 1 x 0.4 1.75 1 year

0.5 0.8 x 1 x 0.3 3.72 1 year

5 0.8 x 1 x 0.4 2.48 1 month

Like Merrill and Stateham (1970), and Yu et al (1990), Kononov conducted field trials to

corroborate the results of the modelling. Two South African gold mines were used as test

locations, with an IR radiometer (thermometer) for remote temperature measurements, Kononov

also used a device to measure air velocity and temperature.

The results of the field trials again reiterated the findings of Merrill and Stateham (1970) and Yu

et al (1990) that a significant temperature difference between rock and ventilated air is the most

important factor for the concept to work. Kononov stated that this is the reason why such a

concept isn’t viable in the United States, where the temperature gradient is not significant. The

deeper South African gold mines where conditions are more favourable should, in Kononov’s

view, prove more suitable. Other recommendations drawn from the research include the

development of a mine worthy, intrinsically safe IR radiometer that is capable of simultaneous

measurements of air and rock temperature and air velocity.

2.2.2 Stress induced thermal radiation This section discusses the application of infrared thermal imagery in the detection of areas of

rock mass subjected to abnormal stress loads. In their paper on infrared radiation features of

coal and rocks under loading, Wu and Wang (1998) discuss how abnormal rises in surface

temperature could be detected by the thermal imager of NOVA climate satellites in 1990. The

abnormalities, according to two other Chinese scientists, Qiang and Ning (paper unavailable),

witnessed several days before and after strong earthquakes had occurred, were caused by the

changing stress fields in the rock, releasing infrared radiation.

The two scientists have since gone on to predict 68 earthquakes based on the thermal infrared

anomaly theory, 50% of which Wu and Wang (1998) state gave “good” results. Although the

research in this area is predominantly focused towards earthquake prediction, some authors have

realised its potential in a mining situation. Again Wu and Wang (1998) present further evidence

for stress induced thermal radiation, but within a mining environment. They state that mining

engineers have experienced abnormal high temperatures in some mines running up to gas bursts

and other major changes in the stress field. An example is given of a coal mine in Britain,

where the temperature of outburst coal was as high as 60oC.

The mechanism behind the infrared radiation discharged by highly stressed rocks is not

completely understood. Wu and Wang (1998) suggest both inner structure reconfiguration of

the rock material and associated physical and chemical phenomena during loading as the causes

of the temperature change. These “phenomena” according to Brady and Rowell (Nature 321),

include inner fracturing, ionisation, energy accumulation, energy consumption, electrical

resistance variation etc., the result of which is the conversion of mechanical energy, which is

causing the failure, into thermal energy and electromagnetic radiation, as discussed above.

12

Wu and Wang undertook their own experiments with a series of tests on coal and sandstone

samples loaded to failure in a loading frame, whilst recording the changes in infrared radiation.

From these simple experiments, Wu and Wang (1998) discovered that a forewarning of failure

occurs in the form of infrared radiation (IR) change. However this change in IR is not uniform,

with three separate categories of IR change witnessed by Wu and Wang, as follows: -

Type I: Forewarning by low temperature

Type II: Forewarning by high temperature

Type III: Forewarning by continuous high temperature.

(After Wu and Wang, 1998)

A further finding from the research was that during loading of a “fat” coal sample the surface

temperature increased significantly more than the internal temperature, whilst the opposite was

found when loading a blind coal sample.

In 2000, Wu et al (2000), produced a paper on the new subject of Remote Sensing Rock

Mechanics (RSRM), in which they laid out the background to the new subject and an overview

of research that had gone on previously. The paper also contains an interesting framework

which they believe the RSRM studies and applications should follow. Over an 8 year period

Wu and his team have developed this subject and conducted a large quantity of research in the

new area. To detect, measure, record and analyse any electromagnetic radiation being emitted

by rock while it is subjected to stress the group has used a variety of sensory equipment. In this

paper Wu et al sum up their research with the following significant conclusions: -

1. Both infrared radiation temperature and infrared spectral radiation temperature

increase with load.

2. Infrared radiation temperature increases with the increase of rock strength.

3. Infrared radiation temperature increases with the depth inside the sample

surface.

2.3 ULTRASONIC / ACOUSTIC EMISSIONS

An ultrasonic emission is defined as physical vibrations in matter occurring at frequencies

above 20 kHz, which is approximately the limit of human hearing. Many prolific users of

ultrasonic sounds are found in nature, with the bat being one of the most well known users of

ultrasonics as a system for nocturnal navigation. Dolphins also use ultrasonics in the

underwater environment for navigation, the detection of food and for a basic communication.

Ultrasonics has been adapted for a variety of uses in the human environment, including

medicine (echocardiography), engineering (ultrasonic fracture detection, NDT) and military

(submarine sonar ping). Ultrasonics has also been developed for use in seismology (or

microseismology) where it is known under the term acoustic emission. A more detailed

description of an acoustic emission is a transient elastic wave emitted from small-scale cracks

(microcracks) that form due to changes in stress (Hazzard, 1999). Lockner (1993) states that the

frequency range of these emissions is 100 kHz to 2000 kHz.

In order to avoid confusion for the reader the following definitions will be used when discussing

this subject: - Acoustic Emission (AE) will be used to describe emissions (frequency 100 kHz -

2000 kHz) detected by devices (e.g. piezoelectric receivers) attached directly to the surface of

the rock sample. The term Ultrasonics Emission (frequencies > 20 kHz), on the other hand will

be used to describe emissions detected with devices that have no contact with, or surfaces

adjoining the rock from which the emissions originated. For such devices to work, the emission

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must travel through the rock material, across the air - rock boundary, through the air and still be

detectable by a device.

Whilst researching the background to ultrasonic emissions and acoustic emissions it became

clear that little or no research had been done on ultrasonic emissions as a precursor to rock mass

failure. However research into acoustic emissions, as a precursor to rock mass failure is wide

and extensive, with detailed observation being made on all scales. The reason for this

imbalance is described by Obert (1941) in his paper for the United States Bureau of Mines,

where the theory of AE is first put forward. It was during some seismic investigations in a mine

that Obert (1941) noticed that the normally “quiet” mine had become “noisy” (seismic noise).

The noise continued to increase for about 15 minutes until it terminated with a rockburst, which

occurred within 50 feet of the point of observation. The observation prompted further research

into what Obert (1941) refers to as sub-audible noise for use in the prediction of rock failure

(rockbursts).

In order to detect the sub-audible noise Obert bypassed the idea of using a non-contact device

and opted for a device that had contact with the rock surface. The reason for the decision is

mainly due to the large signal impedance at the rock to atmosphere boundary. When a

microfracture forms, an acoustic emission is released, radiating outwards from its origin.

Sudden changes in density of the medium through which the AE is travelling, will cause loss of

energy, refraction and reflection of the emission. In the case of acoustic emissions, they must

travel across the rock to atmosphere boundary, a massive density change, which creates a huge

loss of emission energy, between 80-90%. Such a large loss of energy lessens the chance of

detection using a non-contact device, located some distance from the emission origin.

The absorbing characteristics of the atmosphere, through which an ultrasonic emission would

travel from source to detector, would also hinder the performance of such a technique. An

analogy of the problem can be given by a thunderstorm. At the centre of the storm the

thunderclap is sharp, short and loud, but at some distance the atmosphere absorbs the high

frequencies of the thunderclap. The result being a low drawn out rumble caused by sound

reflecting off the surrounding landscape. For a non-contact device to work it would need to be

sensitive enough to detect the weakened emissions and positioned close enough to capture them

also. Due to these reasons, Obert, and those who have followed, have opted for contact devices

to detect the emissions from failing rock mass.

Obert conducted his experiments using sensitive vibration microphones, referred to as

geophones, placed 100 feet apart at the end of drill holes in an active part of a mine. The early

geophones were connected to an amplifier, a recording device and a pair of earphones, some

distance from the active area. These initial trials highlighted the problem of background noise

created by the everyday mining activities of drilling, blasting, ore shoot movement etc. Obert

(1941) states that the background noise could be identified by the different amplitude,

frequencies and wavelengths, however at the time no equipment was available to filter it out.

In 1942 Obert and Duvall (1942) returned to the same mine with more sophisticated equipment

purposely built for the job of recording acoustic emissions under normal mining conditions.

Mining induced noise was recorded and the resulting waveforms were analysed. Combinations

of filters were then employed to remove the mining induced noise and leave just AE.

Unfortunately Obert and Duvall discovered that much of the mining induced noise has a

frequency which overlaps with that of acoustic emissions causing them to be obscured. Rock

drilling was found to be particularly effective, not surprising as drilling is the controlled

fracturing of rock mass.

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In conclusion Obert and Duvall (1942) state that the equipment then available was unable to

filter out mining noise, in effect restricting the use of such a method to periods when mining

was not active. Also, more importantly, a rockburst often followed a period of increased

acoustic activity, which could be deemed a precursor. It was however impossible to determine

at what point, after what length of time or after how many AEs would a rockburst occur,

seconds, minutes, hours or even days.

Since the discovery and early trials by Obert (1941) and Obert and Duvall (1942) the use of

acoustic emissions as a precursor to rock mass failure has advanced, with most trials in this area

now conducted on laboratory scale. By conducting trials on a lab scale researchers have more

control of the variables, enabling experiments to be repeated with the same parameters. It has

also long been known that the results from laboratory trials can be scaled up. Even with the

advantages of laboratory based experiments and the development of sophisticated detection,

recording and analysis equipment, the ability accurately to predict rock mass failure using

acoustic emission techniques still eludes researchers. Progress has been made to an extent, with

acoustic emission techniques having some success at predicting the onset of rockbursts. The

following are two techniques used in the mining and research industry to both predict rockbursts

to some degree and monitor failure patterns.

2.3.1 Gutenberg - Richter (b-value) relationship. Also known as the frequency magnitude relationship, this well known relationship was first

discovered through the research of earthquake fore and aftershocks and relates the frequency of

earthquake occurrence to the earthquake magnitudes. The relationship can be shown using the

following equation: -

log N a � bM

Where

N = Number of earthquakes greater than magnitude M

a & b = Constants (the b value can however vary from one earthquake

region to another but is usually close to one, (Lockner, 1993)).

The relationship was first used in earthquake prediction when Scholz et al (1973) proposed that

a significant change in b-value might be used as an indication of impending large earthquakes.

Due to the close relationship between earthquakes and acoustic emissions the relationship has

been applied to AE analysis. In 1987 Hirata (1987) discovered a power law frequency-

magnitude relation for acoustic emission events, which corresponds to the Gutenberg-Richter

relationship. Laboratory AE experiments have since shown a decrease in b-value with increased

stress in both uniaxial and biaxial directions during deformation of intact samples, Meredith et

al, (1990).

Due to these findings the relationship has been employed in several South African deep hard

rock mines. Using geophones placed throughout the mine, acoustic emissions are monitored.

The results are then plotted on a log frequency-magnitude graph and interpreted. With a build

up of stress, which could lead to a rockburst, a change in b-value should occur causing a visible

change in the gradient of the plotted data. The b-value is often displayed at the shaft top for

miners to take note of an impending rockburst, and high risk areas are closed to all personnel

until after the seismic event occurs, or the b-value returns to normal. Figures 3 (a) and 3 (b)

show b-value plots produced from data collected by Young and Collins, (1997) at the

underground research laboratory (URL), Canada, which is discussed in the following section.

Figure 3 (b) illustrates the b-value plot for data taken during a period of acoustic activity prior to

larger seismic activity, while figure 3 (a) illustrates the b-value plot for a more stable period.

15

A clear difference can be seen between the two illustrations with a much steeper gradient

formed during the period of increased acoustic activity prior to a larger seismic event, than the

seismically quiet period. Another difference lies in the magnitude of seismic events with the

seismically active period having seismic events up to a magnitude of 2.3 compared to the

maximum magnitude of 1.6 seen during the seismically quiet period. Although these

illustrations look like an ideal method for predicting seismic activity and hence rock mass

failure, they are in fact idealised examples, with a lot of b-value plots showing little to no

difference in slope. Another problem arises from the time frames involved; b-value plots may

give an indication of the onset of increased seismic activity, but no indication of when an event

may take place can be gleaned from the plots.

2.3.2 Acoustic monitoring The monitoring and analysis of acoustic emissions is another way that researchers are using

acoustic emissions to study rock mass failure with the hope of some day predicting it. A major

research area, which has invested heavily in acoustic monitoring, is the search for safe

repositories for spent nuclear fuel. The research on the repositories covers everything from the

most appropriate geological setting to the type of storage containers, in which the spent fuel

rods would be kept. The problem caused by fluid ingress and groundwater contamination is one

of the major areas of research. The repositories must have low fluid flow rates to avoid

groundwater contamination in the event of a fuel rod storage container failure.

The simple answer to the problem would seem to be to build the repository in unfractured

crystalline rock i.e. deep seated igneous plutonic rock. It is however important to take into

account the effect mining has upon the stress field and in turn the structure of the surrounding

rock. Acoustic monitoring is used to study these effects as experimental repositories are

excavated in a number of localities throughout the world. One example, mentioned previously,

is the Underground Research Laboratory (URL), Canada, where acoustic monitoring techniques

have been used and developed during extensive experimentation. Young and Collins (1997)

used acoustic emissions and microseismics (MS) to assess the effect of a shaft extension. Four

piezoelectric geophones were placed into four inclined boreholes located around the planned

extension of the mineshaft. Acoustic and microseismic events were monitored and recorded

during the quiet periods following each blast to assess the impact of the excavation. By using

four sensors carefully placed and located in the shaft wall it was possible for Young and Collins

(1997), to locate the source position of each event. Each event can be plotted relative to the

excavations, and areas of serious concern can be identified early. Event magnitude can be

shown also.

Although the method is restricted to quiet periods, when no mining is taking place, it is

invaluable as a method of observing failure in rock mass, and could lead to a greater

understanding of the process. Such a system would, however, prove difficult to use in an active

mine with its use restricted by background noise and its inability to predict failure accurately.

2.4 INDUCED VIBRATION

An alternative to monitoring vibrations occurring naturally in the rock is to induce a vibration

and then monitor how the rock responds to that vibration. Seismic exploration techniques apply

this principle, using explosives or other seismic sources to impart energy to the rock, and rely

upon reflection and refraction of seismic waves from interfaces within the rock and differences

in transmission velocities between beds to build up a tomographic image of the solid rock.

On a much smaller scale, a contact instrument, known as the Acoustic Energy Meter (AEM),

(Figure 4) was developed by Rock Mechanics Technology (Altounyan and Minney, 2000). The

instrument was first designed to locate voids behind concrete tunnel linings. It has later been

16

developed and tested on a variety of failed rock mass conditions, and has proved successful in

identifying surface “looseness” in almost all. The device uses a geophone placed on the surface

under investigation to measure the transient vibrations caused by a hammer blow to the same

surface. By processing the geophone signal a value, quantifying the integrity of the surface is

displayed on an LCD screen. A traffic light style indication is also shown, with a green light

illuminated to symbolise solid rock mass. The instrument effectively provides a more objective

version of the “sounding bar” technique that has been used by miners for hundreds of years to

detect loose rock. It overcomes the reliance of the older technique on experience and good

hearing, an sense which has often been impaired in experienced miners working in noisy

conditions. The algorithm applied to the geophone signal to determine a single parameter for

display effectively calculates the time that the struck surface continues to vibrate or “ring”.

The AEM has been positively assessed for application in small mines in the UK (Hurt,

MacAndrew, Bigby 2000), South African coal mines (Altounyan, Clifford and MacAndrew,

1999), shotcrete lined tunnels (Cartwright, Clifford, Armanen and Vuori, 2001), and South

African gold and platinum mines (Piper, Bron, van Rooyan, Goldbach and Clifford, 2002) and it

is currently being assessed for application in Australian coal mines. The conclusions of the

assessment for South African gold and platinum mines, which was undertaken independently of

the manufacturers during the period of this Project, are summarised below.

The work was divided into four areas, i) Theoretical evaluation, ii), AEM calibration under

controlled laboratory conditions, iii) AEM surface field trials, and iv) AEM underground trials

in gold and platinum mines.

The theoretical analysis showed that such “seismic” techniques have the potential to detect

changes in hanging wall integrity in gold and platinum mines and that the AEM has application

potential in gold and platinum mines provided that the correct frequencies are introduced into

the rock mass and that the instrument’s specification is such that the required frequencies are

measured to correctly interpret the reflected seismic energy. Although the AEM had been

successfully applied in coal mines, where the nature of the hazards is much simpler than the

complex fracturing expected in gold and platinum, its performance was difficult to predict in the

gold and platinum environments. Thus the degree of success in the various potential

applications had to be established in a detailed field evaluation programme.

As a result of the work it was concluded that the applications for the AEM fell into two main

categories; ‘hard’ and ‘soft’. The hard applications are where the meter is used as a scientific

tool (see Table 2). The soft applications are where it is used as part of a documentation and

management control system rather than placing all the emphasis on the readings themselves. For

example, it was shown that the AEM has the potential to identify the leading edge of dome

structures. In the application of the AEM against the hanging wall the obvious presence of a

dome is likely to be recognised by the operator. Therefore a management control system can be

put in place which will ensure that the operator checks all critical parts of the hanging wall and

provides necessary proof of the work in the form of a completed survey sheet. Furthermore, the

AEM could be used as a training tool to assist mine personnel in confirming the presence of

loose blocks which could be identified visually in the future.

The accuracy of the AEM was determined using a calibration table. The influence of the

following parameters was analysed: i) hammer weight, ii) distance of hammer from AEM, iii)

intensity of hammer blow, iv) hammer side. The most consistent results were obtained when the

intensity of hammer blow was constant, the distance between hammer and AEM was 200mm,

the ball-point side of the hammer was used and a 0.75kg hammer was used. Except for the rock

volume and distance between the hammer and the AEM, all the other parameters are a function

of the person conducting the survey and the hammer used. The influence of these factors could

17

be eliminated by introducing a constant impact device. An in-situ calibration of the AEM

should be conducted at each site before conducting a survey.

Surface trials, which were undertaken on unweathered andesite rock showed that the AEM

detects slightly open discontinuities up to 0.8m inside the rockwall, but does not detect tight

discontinuities beyond 0.3m into the rockwall.

Table 2. Potential ‘hard’ applications of the AEM with assigned confidence levels

Potential application Confidence level

Detection of loose rock High

Leading edge of dome structures High

Indicate de-bonding of shotcrete support from rock surface High

Monitor deterioration of hanging wall over time Medium to high

Determine opening on planes of weakness within 1st metre of rock Medium to high

surface

Indicate ‘hot spots’ within a stope panel as a function of ground Medium to high

conditions

Indicate high damage geotechnical areas relative to other Medium to high (if large number

geotechnical areas in seismically active areas of readings taken)

Indicate effectiveness of pre-conditioning Medium to low

Determine opening on planes of weakness beyond 1m into the rock Very low

surface

The AEM results from the gold mines showed that the AEM could detect slightly open

discontinuities up to 0.5m into the hanging wall at Tau Lekoa, 0.65m into the hanging wall at

Mponeng and approximately 0.7m into the hanging wall at South Deep. The AEM results from

the platinum mines showed that the AEM could detect open discontinuities up to approximately

0.7m depth at Bleksop and Eastern Platinum, 0.9m depth at Frank Shaft and 0.3m depth at

Waterval. A comparison between the AEM and Infrared Thermography was conducted but

showed no correlation between the two techniques.

Table 2 gives a summary of the potential ‘hard’ applications of the AEM in hard rock mines and

the confidence level to which it can be applied. The confidence levels are high, medium, low

and very low and are based on using a 0.75kg hammer. Due to the variability of the hammer

blow, some of the applications have a lower confidence level.

From the above, it would seem that the principles employed by the Acoustic Energy Meter are

highly suitable for development into a remote reading device for detecting failed rock, if it were

possible to impart energy into the rock and measure the resulting vibration without direct

contact with the rock in the potentially hazardous zone.

2.5 LASER DISPLACEMENT MEASUREMENTS

The use of lasers as a remote method for the accurate measurement of distance has increased

dramatically recently, with many new applications being discovered. Most laser products, such

as the laser speckle pattern interferometer, designed to measure the displacement field from

objects with rough surfaces, are lab based only. However other products, such as laser range

finders and laser doppler vibrometers, have been developed into workshop and field instruments

capable of making accurate distance measurements. These two devices were identified during

the literature review stage as showing some potential for detecting rock mass failure. They were

identified when assessing conventional methods of detecting rock mass failure, such as the

sounding bar and Acoustic Energy Meter, and how they might be adapted to become remote

methods. It is unknown if either of the devices have been used previously for such an

18

application, as no published information was discovered. The following section discusses each

device in turn, describing their specification, some of their applications and how they could

replace conventional methods

2.5.1 Laser vibrometerLaser Vibrometers measure the vibration of a surface remotely as the component of surface

velocity, by measuring the Doppler effect of a reflected laser speckle pattern. Accurate

measurements of velocities above 500 mm/sec, equal to 100mm of displacement can be

obtained using modern, small, portable devices. Figure 5(a) shows a portable laser vibrometer

produced by Polytec of Germany.

Laser Vibrometers or Laser Doppler Vibrometers are based on the principal of the detection of

the Doppler shift of coherent laser light, that is scattered from a small area of the target object.

The object scatters or reflects light from the laser beam and the Doppler frequency shift is used

to measure the component of velocity, which lies along the axis of the laser beam.

The basic principle of a laser (light amplification by stimulated emission of radiation), is the

induced emission of photons. These emitted photons possess identical properties and thus

produce the coherent light of the same wavelength, which is necessary for this application. The

laser used for Laser Doppler Vibrometers including the PDV 100 by Polytec, is a helium neon

(He-Ne) laser, which produces a visible red laser beam (O = 0.6328 Pm).

Due to the laser light’s very high frequency, a direct demodulation of the light is not possible

and an optical interferometer is therefore employed. This mixes the scattered light coherently

with a reference beam. The photo detector measures the intensity of the mixed light, whose

(beat) frequency is equal to the difference in frequency between the reference and the

measurement beam. The Michelson-Morley, see figure 5(b), is a typical interferometer used in

LDV. A laser beam is divided at a beam splitter into a measurement beam and a reference

beam, which propagates in the arms of the interferometer.

Due to the sinusoidal nature of the photo-detector signal, the direction of the vibration is

ambiguous. Two methods have been developed to introduce a directional sensitivity to LDV.

x� Introduction of an optical frequency shift into one arm of the interferometer to obtain a

virtual velocity offset.

x� Adding polarization components and an additional photo receiver in such a way, that at

the interferometer output a second homodyne signal occurs being in quadrature to the

primary photodetector output.

The most common method used to solve the problem of directional ambiguity is solution 1

listed above. This method involves the incorporation of an acoustic optic modulator (Bragg

cell) into one arm of the interferometer. The Bragg cell is driven at frequencies of 40 MHz or

higher and generates a carrier signal. The signed object velocity determines sign and amount of

frequency deviation with respect to the centre frequency. This type of interferometer is called a

heterodyne interferometer, see figure 5(c), and has a number of significant advantages. As only

high frequency AC signals are transmitted there is no disturbance from hum and noise,

introduced from all types of power supplies. The high efficiency achievable by Bragg cells,

produce less losses than the polarizers needed for solution 2 listed above. As almost all

vibrometers use the Bragg cell method including the vibrometer used in this trial, it is not

necessary to give details of the other solution to the problem of directional ambiguity.

19

As mentioned above Polytec laser vibrometers use heterodyne interferometers with an acousto

optic modulator in one arm of the interferometer. This generates a frequency modulated carrier

signal in the RF region, whose centre frequency is identical to that of the acousto optical

modulator drive signal. The directional sensitive Doppler information is thus contained in the

RF carrier. The signed object velocity determines sign and amount of frequency deviation with

respect to the centre frequency. The Doppler frequency is proportional to the surface velocity

and the phase change with respect to the phase of the reference signal is proportional to the

displacement of the object.

The complicated physics used to decode this RF carrier signal and its reference beam are

beyond the scope of this description into the workings of a laser vibrometer. It is however

worth noting that the output signal from the decoder can be both analogue or digital and give

results as velocity and/or displacement

The remote capabilities and accurate measurement achievable with such a device mean it has

been adopted by a number of industries for a variety of uses. Industries, which have adopted the

laser vibrometer technology, use its unique ability to measure vibration remotely, often without

the need for any surface preparation. By examining the vibration of a rotating gearbox, for

example, manufacturers can check for faults with any of its components. Neural networks can

also be developed to recognise the different vibrations caused by specific faults, within the

gearbox. Using the vibrometer and neural network technology together, manufacturers have a

fast and effective method of conducting quality control on their products as they leave the

assembly areas. Researchers also use the technology to study a wide range of subjects from the

beating of an insect’s wing, to the workings of the inner ear.

The existence of laser vibrometer technology was discovered during research into the Acoustic

Energy Meter (AEM) and how this device / technology could be developed into a remote

device.

It is not known by the authors whether an instrument such as the laser vibrometer has been

previously used in such a way. Neither of the two main manufactures of laser vibrometer,

Polytec and B&K, had heard of previous work in this field. The concept of using a laser

vibrometer to identify areas of failed rock mass by their vibrational characteristics, would

appear to be “blue sky” in it’s thinking and deserves further research.

2.5.2 Laser range finderLaser range finders use the travel time of a reflected laser beam to measure the distance between

the reflecting object and the range finder, accurately. Large distance can be measured

depending on the laser power and the reflecting surfaces. Range finders used in “total station”

for surveying purposes can measure large distances with the help of a reflecting mirror on the

sighting poll. The accuracy of the devices can be a high as 0.02 mm, again depending on the

strength of the reflecting surface.

The laser range finder was identified whilst investigating conventional strata displacement

measurement equipment and how the technology could be developed into a novel remote

device. Equipment such as the tell-tale extensometer and Magnesonic extensometer measure

the movement in both roof and rib strata to give an indication and forewarning of failure in the

rock mass, which surrounds the instrument. With the use of a laser range finder it was

suggested that the dimensions of an excavation could be monitored remotely, with any changes

in rock mass integrity being detected due to the change in excavation shape.

Although such an instrument is capable of measuring the dimensions of an excavation and

detecting any significant changes, a number of problems arise with such a technique. The lack

20

of a stable reference point, to which the laser range finder could be placed, is the main problem.

Without a stable point of reference it would be impossible to tell what movement was being

measured, either the surface under investigation or that of the range finder. The use of

gyroscopes was investigated as a possible means of eliminating the problem of an unstable

reference point. However the “random walk” factor, which gyroscopes suffer unless up dated

from a stable reference, meant a range finder fitted with such a device would not solve the

problem. It was decided that no further research should be conducted into the area and that

other areas such as the laser vibrometer offered more potential for success.

21

22

3 LABORATORY WORK

3.1 INTRODUCTION

Using the knowledge gained from the literature survey of work completed by other authors in

this field, and from the experience of the Partners, four areas were pinpointed for further

research and trial. Electromagnetic and ultrasonic emission detection, thermal response of

failed rock mass and the transient vibrational characteristic differences between intact and failed

rock mass were the areas selected for further investigation. Laboratory trials were used to test

each of the concepts, as to their capabilities of fulfilling the primary aim of this research. This

chapter discusses the aims, experimental procedure, results and conclusions of the laboratory

trials conducted into each concept.

3.2 ELECTROMAGNETIC EMISSIONS

3.2.1 Aims Conduct experimentation under laboratory conditions, to identify whether electromagnetic

emissions are detectable during the controlled failure of rock samples common to coal mine

roofs.

3.2.2 Experimental procedure Using two different sandstone samples, both similar to those found in many coal mine roofs, a

series of trial experiments was conducted to observe for electromagnetic emissions during the

failure of the samples. A calibrated, 1000 kN, servo controlled rock press was used to induce

failure of the rock samples, while a high gain antenna with in built amplifier was used to

detected the emissions. The antenna with a frequency range of 1 MHz to 2 GHz used an in built

amplifier allowing only EMR to be amplified. A digital oscilloscope, a Fluke Scopemeter, was

used to capture and store the amplified emissions.

Hollington sandstone, quarried at Hollington, Staffordshire, for use as a building stone, was one

of the sandstones used in the trials. This a coarse grained, flesh pink coloured, fairly

homogeneous sandstone, with well rounded, well sorted, poorly cemented grains, which

crumble easily and could be described as weak (UCS 20 MPa and Young’s modulus 3.97 GPa).

Horton sandstone, quarried at Horton, Staffordshire was the other sandstone used in the trials.

Again quarried for dimension stone, it is medium grained, light camel brown coloured, with

some small patches of hematisation, well consolidated, fairly homogeneous and could be

considered hard sandstone (UCS 67 MPa and Young’s modulus 12.52 GPa). Both samples

were prepared to ISRM guidelines, their dimensions being 42.3 mm (average) diameter and 105

- 126 mm in length.

Figure 6 (a & b) show a schematic representation of the experimental set up and a photo of the

load frame, antenna and oscilloscope. Once the equipment and the sample to be tested were in

position, a measurement of the background noise was taken; the automatic trigger on the

oscilloscopes was then set just above that level. The samples were then subjected to a slow rate

of loading (1 kN/sec) whilst being monitored for electromagnetic emissions, as they gradually

approached failure.

The initial trials proved unsuccessful with no electromagnetic emissions detected during or after

the failure process. Due to these initial findings, further alterations were made to the testing

procedure. These involved the building of a Faraday shield around the loading frame, using

aluminium foil attached to the safety shielding of the loading frame. A Resistance (5k:)-

23

Capacitor (1000 micro Farad) (RC) filter was also constructed with the aim of reducing the

background noise further, so as to reveal underlying emissions. The filter was connected

between the oscilloscope and the antenna in its own aluminium casing, before the experiments

were conducted a second time.

3.2.3 Results and discussion During the second phase of the laboratory experiments into electromagnetic emissions, it

became clear that the Faraday shield was ineffective. The level of background noise recorded,

see figure 7, within the Faraday shield itself, although reduced, was still too high, indicating the

ineffectiveness of the shield. The captured radiation displayed a clear frequency, which could

be the result of FM radio broadcasting and which may also contain an RDS signature. The RC

filter did achieve a small reduction in the level of noise. This can be seen by comparing figures

7 and 8. Again, however, the reduction was not sufficient, with no emissions observable above

even the reduced radiation level.

The reason for the ineffectiveness of the Faraday shield became clear following investigation of

the loading frame construction. It was found that the frame, a Dartec 1000 kN press, contains

an earth wire in the centre of one of the four legs. The earth wire, which is also connected to the

mains earth, acts as a large aerial, drawing in electromagnetic radiation and interfering with the

function of the Faraday shield.

3.3 ULTRASONIC EMISSIONS

3.3.1 Aims To examine whether ultrasonic emissions created by the micro fracturing of sandstone during

the failure process can be detected, and used as a precursor of rock mass failure.

3.3.2 Experimental procedure The experiments were conducted along similar lines to those of the electromagnetic radiation

experiments. The Hollington and Horton sandstones were used as samples again and the

loading frame used to provide a means of inducing failure. The sensory device was a sub-

audible or ultrasonic sound amplifier, with variable volume gain and a frequency range of 20 to

130 kHz. After later modifications it was installed in an aluminium enclosure and the inbuilt

microphone was placed onto a 60 cm length of coaxial cable. The oscilloscope was again used

as a capture and storage device for any emissions detected by the ultrasonic amplifier.

During the experiments the microphone was initially situated within 0.15m of the sandstone

sample as it was loaded to failure. Due to problems with background ultrasonic noise however,

the microphone, after the modifications described above, was attached to the sample itself for

the later experiments. Figure 9 shows both a schematic representation and a photograph of the

experimental set up.

As with the electromagnetic radiation experiments, the oscilloscope was set with a trigger level

just above the background ultrasonic noise level. The frequency of the ultrasonic amplifier was

set at 90 kHz, a frequency where background ultrasonic radiation was at its least. The sample

was then loaded at a rate of 1 kN/sec, whilst being monitored for ultrasonic emissions, until total

failure occurred.

3.3.3 Results and discussion As with the electromagnetic emission experiments, background noise, this time ultrasonic,

served to mask or obscure all but the strongest emissions during the rock failure process. The

emissions, which were detected by the amplifier and captured by the oscilloscope, occurred at

the point of sample failure, which is often explosive. No emissions were detectable above the

24

level of background noise during any other part of the loading process. The origin of the

emissions, which were captured, i.e. at the point of sample failure, cannot be identified as being

the result of failing rock mass. At the point of failure a number of ultrasonic sources of noise

are present. Not only the failing rock sample, but also the loading mechanism tripping out and

the loading platens striking the loading frame base are all potential sources at that moment in

time. This made drawing any conclusions as to the source of the emissions very difficult.

The observed background noise was produced predominantly by the loading mechanism of the

rock press and proved impossible to filter out. This is due to the noise occurring across the

whole ultrasonic range, for which the ultrasonic amplifier was designed, including the frequency

at which most rock failure noise occurred. Even later modifications, described above in section

3.3.2, to the ultrasonic amplifier, which were designed to reduce the effect of background noise,

were insufficient. Again background noise continued to obscure any precursory emissions. The

probable source of the background noise, witnessed after the alterations were made, was the

loading mechanism, transmitting ultrasonics through the loading platens to the sample and the

attached microphone.

3.4 THERMAL RESPONSE

3.4.1 Aims Investigate under laboratory conditions whether a temperature difference between loose and

solid rock mass can be detected and or enhanced by means of applying a heating or cooling

element, in order to use it as a method of distinguishing the two rock mass conditions.

3.4.2 Experimental procedure In order to conduct these experiments a set of test blocks, designed to simulate failed (loose)

and unfailed (solid) rock mass, were built. This was achieved using heavy duty domestic

paving slabs, cemented together to form a solid mass, with the upper most slab being left loose

on one block in order to simulate a loose or failed section of rock mass.

The next stage involved the heating of the test blocks using a 1.5 kW infrared heater suspended

directly above the thermal test blocks at a distance of 1.5m. A Raytek MX 2 G, series infrared

thermometer was used to remotely measure the surface temperature at the centre of the two

slabs during both the heating and cooling processes, when the IR source was removed. The

device had the following specification: -

x� -30 -900 oC measurement range, x� Laser sighting circle, defines measurement area, x� K-type thermocouple for emissivity calibration, and air temperature measurements,x� LCD display, with current temperature display and graphical representation of last 10

readings,x� 0.1oC resolution,x� Accuracy of ±1 oC or ± 0.75% of reading, andx� Measurement spot size of 29mm at a distance of 1.5 m.

The two test blocks were heated until the surface temperature of the two blocks had first

developed a thermal gradient, and then had stabilised at a higher temperature. At this point the

IR source was removed and the blocks allowed to cool. Readings were taken every minute and

recorded for later entry into a spreadsheet program.

The test blocks were modified for a further series of tests. The blocks were rebuilt with

thermistors placed within the surface of the four uppermost slabs (see figure 10), including the

25

top surface, which was then skimmed with a layer of the cement adhesive. A Campbell data

logger and multiplexer were used to interrogate all the thermistors during the heating and

cooling process. The logger stored the data for later download onto a PC, for analysis. The

modified test blocks were designed to give a better understanding of heat transfer through the

entire block instead of just the surface temperature as examined in the initial experiments.

A further change was made to the experimental procedure in the later trials. The test blocks,

now with embedded thermistors, were also subjected to cooling using liquid CO2, at a

temperature of -80 degrees. As for the initial trials, the blocks were subjected to different

periods of cooling (15, 30 and 45 sec), over which time their internal and surface temperatures

were observed using both the in built thermistors and the infrared thermometer. The CO2 was

delivered from a 25 kg dip cylinder via an insulated hose and nozzle which was directed at the

surface of the test blocks and moved slowly across their whole area.

Measurements were taken before, during and after the cooling period in order to observe at

which point, if at all, during the whole process, a temperature difference between the two blocks

appeared most notable. Periods of time to which the blocks were subjected to the liquid CO2,

were kept deliberately short. In an underground environment it would be unwise to release

large amounts of CO2, and so the laboratory trials were designed to follow the same restrictions.

3.4.3 Results and discussionThe initial experiments proved successful, with an observable temperature difference seen

between the loose and solid simulated test blocks, see figure 11. The surface temperature

difference between the two blocks became clear almost immediately, although a small initial

difference in temperature of 0.1 oC was present. The maximum thermal gradient was seen after

only 4 minutes of heating, at which point a 0.5 oC difference in surface temperature was

observed. After a further 4 minutes the gradient had gradually reduced until no detectable

difference was seen. At this point the heat source was removed, allowing observations to be

made during the cooling process. A period of 3 minutes elapsed before a temperature difference

began to reappear, with the largest gradient, of 0.2 oC, occurring 15 minutes into the experiment

i.e. after 7 minutes of cooling. A thermal difference of 0.1 oC continued for a further 5 minutes,

when measurements were ceased.

The second phase of the laboratory experiment proved less successful than the previous

laboratory trials. Using CO2 to lower the temperature of the test blocks, it was envisaged that a

difference in surface temperature would appear during cooling and or re-heating by the ambient

conditions of the surface. Figure 12 shows the resulting average surface temperature for the

loose and solid test blocks before, during and after being subjected to 30 sec of cooling by the

liquid CO2. The temperature at 0.04m depth is also shown for both the simulated failed and

intact rock mass blocks. Similar results were observed after the 15 and 45 sec cooling periods.

At no point during or after the cooling periods, can a significant difference in surface

temperature be seen, between that of the simulated failed rock mass and that of the simulated

solid rock mass. Small differences in the temperature do appear, see figure 12, but they do not

occur for any significant length of time or to any significant amount. Figure 12 also

demonstrates how only the immediate surface is affected by the liquid CO2, with no change in

average temperature seen at a depth of 0.04m. The simulated failed rock mass block has the

uppermost slab loose, forming an air gap at a depth of 0.04m into the block. It is this air gap,

which, in theory, should cause the surface temperature to return to equilibrium at a different rate

than the surface temperature of the solid test block. However figure 12 demonstrates that the

cooling effect did not penetrate that deep, hence the effects of the air gap were unseen. A longer

period of cooling may return better results; this however would not be possible with liquid CO2

due to the underground constraints of its use as discussed above.

26

The trials conducted here involved relatively simple simulations of failed rock mass, and the

results may not accurately mirror what would be seen during underground trials of such a

technique. The results from these initial trials should, therefore, not be taken as evidence

against the use of such a technique, but as useful data for designing further underground trials.

3.4.4 Computer simulationsAs part of the thermal response experiments, 2D computer models where constructed using

basic 2D thermodynamics and heat transfer equations, with Microsoft Excel used as the

modelling program. Both a thermal enhancement technique, similar to that used in the

laboratory experiments, and a naturally occurring thermal gradient were modelled. Figure 13, a

plot of data produced from the Excel spreadsheet model, demonstrates the effect of heating two

rock surfaces, one solid the other loose, for two hours and then allowing them to cool. A clear

temperature difference can be seen during and after the heating process between the solid and

loose surfaces, with the loose surface heating faster and cooling more slowly than the solid rock.

Figure 14, is a plot of data also produced from the Excel spreadsheet model, but this time

demonstrating a naturally occurring temperature gradient. Again two surfaces are modelled,

one loose and the other solid; they are initially given a high temperature, similar to virgin rock

temperature in deep mines. The model then simulates excavation, causing the surfaces under

investigation to come into contact with the cooler ventilated air. The model is left to run for a

sufficient length of time to simulate the gradual cooling effect of the ventilation air. From

figure 14 a clear difference in temperature can be seen between the loose and intact rock after

only a short period of time.

3.5 LASER VIBROMETER

3.5.1 AimsTo investigate the potential of laser Doppler vibrometery as a method of identifying failed rock

mass remotely, by way of its different transient vibration characteristics.

3.5.2 Experimental procedureThe laboratory and field experiments with the laser vibrometer were conducted during two

periods of 1 week and a later period of 1 month, due to limited availability of the instrument.

The laser Doppler vibrometer, a PDV 100, see figure 5 (a), used in the trials is produced by

Polytec of Germany and is designed to be a portable unit. The unit uses the frequency Doppler

shift of a reflected laser beam from an object under investigation, and by processing the return

signal it is possible to measure the vibration of the object remotely. The PDV 100 is one of the

more basic units produced by Polytec, and has the following features: -

x� 3 velocity range settings (±20mm/s, ±100mm/s, ±500mm/s).

x� Frequency range 0 to 22 kHz.

x� Working distance 0.2m to >30m.

x� 3 digital low pass filter ranges, 1 kHz, 5 kHz, 22 kHz.

x� Analogue high pass filter, 100 kHz.

x� 5 hours working time using Li-ion batteries.

x� Output is through a BNC socket, which can be connected to an oscilloscope for capture

and storage.

The laboratory based experiments, using the laser vibrometer, were initially designed to test if such a device was capable of detecting the vibrations, created by a light hammer blow to a rockmass. This was tested using a large concrete mass, which simulated solid rock mass conditions.

27

Once it was established that a strong, clear signal could be produced from striking a simulated

solid rock mass, it was clear that a vibration could be measured from less solid / looser

materials. When this important characteristic had been confirmed, the next task was to develop

and trial a testing procedure, which could identify loose rock remotely , on surface before taking

it underground for field trials.

A laboratory test site was designed and built using concrete paving slabs attached to a solid wall

with different amounts of cementitious grout applied to the slabs. By covering only a third of a

slab with adhesive and attaching it to the wall, it was possible to simulate a failed rock mass.

An intermediate slab, half covered, and a solid slab, which had a full covering of adhesive, were

also produced. Using two different slab types, it was also possible to examine the effect of

surface texture and colour on the return signal strength of the laser vibrometer, an essential

element for a clear, accurate measurement.

In order to measure the degree of looseness simulated by the slabs, an Acoustic Energy Meter

(AEM), see figure 4, capable of quantifying the integrity of a surface, was used. The instrument

measures the attenuation of a seismic impulse created by a hammer blow to the area under

investigation. The seismic signal is measured using a geophone placed on the surface and then

processed in real time. A value representative of the degree of looseness is displayed, along

with a “traffic light” style indication with green representing solid rock mass. The values

obtained using the AEM were used to assess the effectiveness of the vibrometer.

A paintball “gun”, which fires paint filled gelatine spheres at a velocity of 100 m/sec was used

to provide a means of imparting vibrational energy to the surface under investigation from a

remote location. The “gun” could also be used to place retro-reflective paint, remotely, onto a

surface under investigation in order to improve the return signal strength of the vibrometer. The

experiments consisted of firing a paintball at one of the slabs, while targeting the laser

vibrometer at the centre of the same slab. The transient vibration measurement was then

captured using the trigger capabilities of a digital oscilloscope. The laser and paintball marker

were positioned 5 metres from the target surface, and the shot was delivered at an angle

perpendicular to the target surface.

3.5.3 Results and discussionFigures 15 shows transient vibration traces for both the loose and solid slab, after been struck by

a paintball from a distance of 5m. Figure 15 clearly illustrates a difference in vibrational

characteristics between the loose slab and the solid. The initial amplitude of the vibration of the

loose slab is much larger and it continues vibrating long after the solid slab has stopped

noticeably vibrating.

Numerous signal processing methods were investigated to derive a single parameter from the

LDV traces which could be related to the looseness of the samples tested. Eventually, a simple

algorithm, similar to that used for signal processing by the AEM, was derived and found to be

appropriate. Figure 16 shows the resulting values plotted against the AEM value.

The values produced using this algorithm and plotted in figure 16, show a distinct correlation

with the Acoustic Energy Meter data, values increasing as the integrity of the surface under

investigation decreases. For the purposes of the laboratory and later field trials, the algorithm

was applied to the signal by post processing the digital waveform captured from the LDV via a

Microsoft Excel spreadsheet.

The derived algorithm was as follows:

28

§¨ ¨V 2 �V n

·V x¸

¹¸

© V max

Where :- V = Velocity

Vn = Background Noise (max from first 30 measurements)

Vmax = Velocity max

Ri = Rock integrity

Th = Threshold value

In order to obtain a single quantitive value, IF logic was applied to the data, with the resulting

values being added together to give the single value required. The IF logic equation, applied

was as follows

R = 1, IF Vx � Th

R = 0, IF Vx < Th

Ri = SUM R

The equation can also be written in Microsoft Excel format, as follows:

IF (Vx < Th = 0, Vx > Th = 1) SUM = Ri

29

30

4 FIELD TRIALS

4.1 INTRODUCTION

After completion of the respective laboratory trials, each concept that had been experimented

upon was examined in detail as to its future potential. From these examinations and

considerations of practicality, it was decided that both the thermal response and transient

vibration response of failed rock mass, should be pursued to the field trial stage. Field

experimentation to test the two concepts under more realistic conditions was designed for

different underground locations, where failed rock mass conditions could be accessed under a

safe operating environment. A Bath dimension stone mine, a Derbyshire limestone mine, an

evaporate mine and a hard rock test mine in Cornwall, were the locations used during the field

trials.

4.2 THERMAL RESPONSE TRIALS

4.2.1 AimsTo conduct field based experimentation into the thermal response characteristics of failed and

unfailed rock masses, with the aim of identifying a difference in temperature between the two

rock classes. Also to examine the potential for the creation and/or enhancement of a pre-

existing thermal gradient for each of the two rock mass classes.

4.2.2 LocationTwo locations were identified as suitable for field based experiments of thermal response, a

dimension stone mine in the Bath area, and an evaporate mine in the north of England. Both

localities demonstrated situations where a remote technique for the detection of failed rock mass

would be invaluable as an everyday survey tool to check on support performance. At the

dimension stone mine a practical technique would also provide a safety tool to check the

condition of old, unsupported developments.

The dimension stone mine at Bath provided an ideal locality to conduct trials using a thermal

enhancement technique to create a discernable difference in temperature between loose and

solid rock mass. The mine lies at a shallow depth of 20 - 30 m, and has a constant low

temperature throughout the year, hence the requirement to create an artificial temperature

gradient. A large support pillar in an old development was surveyed using the Acoustic Energy

Meter (AEM) discussed in section 2.4 (also see figure 4). One of the faces contained a large

loose flake, with areas of solid rock mass present on either side, ideal for a thermal response

trial.

The evaporate mine, due to its depth and high virgin rock temperature, provided a model

location to examine the theory of a naturally occurring temperature gradient between solid and

loose rock mass. It was hoped that interaction of the hot virgin rock temperature and the cooler

ventilated air, would create a discernable temperature gradient between the loose and solid rock

mass. The mine was suffering failure problems in the roof of their main longer term

developments, a situation where a remote device capable of surveying the roof at speed to detect

areas of failure would be ideal.

4.2.3 Experimental procedure The first field trials on the thermal response theory, were conducted at the dimension stone mine

near Bath. A form of thermal enhancement was employed; with the aim of identifying a loose

flake on a pillar face by a discernable difference in temperature from the surrounding solid rock

31

mass. Using an infrared heater, placed 2m away, the pillar face was heated for a period of 4

minutes, after which time temperature measurements were taken on a grid pattern across the

pillar face. An infrared thermometer, with a sensitivity of 0.2 oC, was used to measure the

temperature remotely at 3 minute intervals.

A second trial conducted at the deep evaporate mine, required no enhancement techniques as a

steep temperature gradient already existed between the hot virgin rock and the cooler ventilated

air. A number of localities were visited throughout the mine, where failure had occurred both

recently and at sometime in the past and in both side wall and roof. Again the infrared

thermometer was used to measure the rock surface temperature remotely. Where failure was

visible in the roof or sidewalls the thermometer was used to scan across the area, while the

operator looked for any discernable temperature difference. The thermometer was also used to

measure air temperature using a thermocouple probe attachment.

4.2.4 Results and discussionFigures 17 (a, b, & c) illustrate the temperature, at different time frames, of the pillar at the

dimension stone mine as it cooled after the initial heating period of 4 minutes. Figure 18 shows

the AEM values taken at the same positions on the pillar as the temperature measurements. High

values illustrate areas of loose rock mass. As the graphs demonstrate, no correlation can be seen

between the pillar surface temperature and the AEM values, which define the areas of loose

rock mass. The heater footprint is the only clear thermal feature in the surface temperature

plots.

Due to problems with the ventilation it was impossible to extend the heating period to more than

4 minutes, which prevented further results from being gathered. Another problem lay with the

infrared thermometer used, which caused the “blurring” of results. It took 2.5 minutes to take

all the readings required to produce each surface temperature plot. This led to a significant

offset in time between the first and last measurements of each scan. The effect is a blurring of

the results over a 2.5 minute period. The use of a thermal imaging camera would eliminate this

problem and would allow measurements to be taken throughout the heating and cooling period.

The field trials conducted at the deep evaporate mine, provided a variety of situations under

which the theory of a naturally occurring temperature gradient between loose and solid rock

mass could be assessed. In general, the roof temperature was found to be constant to within +/-

0.1 oC over its width, as were the sides and floor. The air temperature, as measured by the

thermocouple probe, was typically 2 oC below that of the rock.

However these conditions proved ineffective in producing a detectable temperature difference

between loose and solid rock mass. In only one area, a visible and recent shear failure in the

roof, was a detectable change in temperature from loose to solid rock mass observed. The shear

failure had created a wedge shape across half the width of the development, where also a small

portion of the wedge near the sidewall had fallen. This created a partially detached area with an

air-gap between it and the intact roof above. Figure 19 (a) shows a schematic cross section of

the structure observed.

A scan across the width of the development at this location produced a small but noticeable

temperature change, with a drop in temperature of 0.4 oC between the partially detached area

and the adjacent intact roof. The scan across the roof showed an increase of 0.2 oC to the

opposite sidewall, see figure 19 (b).

Although the temperature gradient is small and at the lower end of the instrument’s capabilities,

this result appears to confirm the theory behind the thermal response concept. Figure 19 (b)

clearly illustrates the change in temperature between the detached area and the adjacent intact

32

roof. However it must be noted that this change could also be due to the two surfaces having a

slightly different emissivity, which would also result in a small change in the temperature

reading.

4.3 LASER VIBROMETER TRIALS

4.3.1 AimsTo examine the potential of a laser vibrometer as a tool to detect failed rock mass by measuring

the transient vibration of a surface caused by the impact of a paintball.

4.3.2 LocationThree separate locations were used during the course of these field experiments. The dimension

stone mine near Bath, used during the thermal response trials, became the focus of most of the

work. The mine again provided a number of ideal test sites, which were safe enough to conduct

the trials but also contained areas of failed rock mass in both roof and wall on which

measurements could be taken. Measurements were taken at four different localities throughout

the mine, all of which had areas of rock failure adjacent to solid intact rock mass, again defined

using the Acoustic Energy Meter. Three of the localities were in the roof, whilst the fourth was

part of a large support pillar.

A hard rock test mine in Cornwall provided another location for experiments to take place. The

regular jointing pattern of the granite created suitable test situations, where blocks of granite

were loose but keyed in by their shape, preventing them from falling. A large block, loose but

still keyed into the sidewall provided an ideal test site.

The third location was a limestone mine in Derbyshire, where joint interaction created areas of

loose material adjacent to solid rock mass. A low level roof also created easy access for

alternative excitation methods, other than the paintball marker, to be examined.

4.3.3 Experimental procedureAs with the laboratory trials the initial procedure involved the firing of a paintball at a surface

whilst the laser vibrometer was focused on the same surface. The shot was fired to impact

within 0.15m of the laser spot of the vibrometer. A digital oscilloscope connected to the

vibrometer captured any transient vibration trace detectable above background signal noise

(electrical noise). Captured traces were then downloaded to a PC for later processing and

analysis. Again, an Acoustic Energy Meter was used to measure the integrity of the surfaces

under investigation and provide a value of looseness, with which the vibrometer measurements

could be compared.

The procedure was modified after the series of experiments at the dimension stone mine and the

hard rock test mine in Cornwall, as discussed in the following section. The modified procedure

involved the striking of previously installed roof bolts with a 2 kg hammer at varying distances

from the area targeted by the laser vibrometer. A second nut was placed on the end of the bolt

both to increase the area on which to strike with the hammer and also to protect the bolt from

damage. This new technique allowed more energy to be transferred to the target surface but still

from a remote location. The bolt struck by the hammer transmitted vibrational energy to the

surrounding rock mass more effectively than directly striking the rock mass.

4.3.4 Results and discussionFigure 20 was produced from the traces generated during the initial field trials at the Bath

dimension stone mine. The same simple algorithm for processing the traces was used as during

the initial laboratory trials. Over 30 measurements were obtained during these trials at the

33

dimension stone mine and the Cornish hard rock mine. Figure 21 shows an example of the

traces from areas of solid and loose rock at the dimension stone mine.

The results from the initial set of tests, at both the dimension stone mine and the hard rock test

mine in Cornwall, were unclear, with traces showing little of the characteristics shown in the

initial laboratory trials. The reason for the unclear results was identified as laying with the

paintball marker used to provide a remote source of vibration. It appears that the energy

transferred into the rock mass by the impacting paintball was insufficient to cause a significant

vibration. Although the impact of a paintball had been calculated to be equal to a 1 kg mass

falling 1m (around 10 joules), other effects had not been considered. Atmospheric friction,

impact angle and the lateral dissipation of energy on impact all served to reduce the impact

energy delivered to the surface by a paintball.

A change in experimental procedure, described above in section 4.3.3, was made, and further

trials were conducted at the limestone mine in Derbyshire, and again, at the Bath dimension

stone mine. Over 50 measurements were obtained during this second series of experiments

using the alternative procedure of imparting energy into the rock by striking a roof bolt head

with a hammer.

Figure 22 (a & b) shows a sample of some of the traces produced using the alternative

experimental procedure at the limestone mine in Derbyshire. Four different traces are plotted

showing measurements taken at the same position by the laser vibrometer, with the vibrational

energy source (hammer blow) at four different distances from the target. The graph shows a

good representation of the type of traces obtained during this part of the trials.

By applying the mathematical algorithm to the traces, a single value was obtained, which was

plotted against the AEM values taken from the same target surfaces, see figures 23 and 24. The

two graphs are plots of the algorithm derived parameter values against the acoustic energy meter

values from both the Derbyshire limestone mine and the Bath dimension stone mine and also

show the distance between the energy source and the LDV target point. Figure 23 shows the

values obtained at the Derbyshire limestone mine with Figure 24 showing the values obtained

from the Bath dimension stone mine.

Figure 22 shows the effect that distance between hammer impact and the target surface had on

the vibration measurements. The traces produced when the impact was close to the target

surface are, clear, well formed and distinctly different for loose and solid rock mass. As the

distance increases, the traces become weak and unclear, and difficult to analyse.

Figures 23 and 24 also highlight the problem of insufficient vibrational energy. For the

Derbyshire limestone mine (Figure 23), when the separation distance is large, (4.4m) the

algorithm values obtained show little in the form of a trend when compared to the AEM value,

with only a slight rise in value from solid to loose rock mass. When the separation distance is

smaller (1.56m) a definite trend can be seen with algorithm values increasing in a similar trend

to those of the AEM. The figures also illustrate that the technique used to obtain and process

the data, produces better results when the rock mass being examined is of intermediate to

extremely loose condition, see figure 23, as at the Derbyshire limestone mine. The transition

from solid to intermediate rock mass conditions as at the Bath stone mine, see figure 24,

delivered more random algorithm values with no correlation between LDV and AEM

measurements. This again is likely to be due to the insufficient vibrational energy.

34

5 CONCLUSIONS

5.1 ULTRASONIC / ACOUSTIC EMISSIONS

The work undertaken in the laboratory, discussed in section 3.3, on ultrasonic emissions and the

findings of the literature study into the principles of the theory provided strong evidence against

such a device being a practical solution to the problem of detecting failing and/or failed rock.

Problems with signal strength restrict the distance over which a device could work and

background noise serves to obscure all but the largest of emissions. If such a device was

capable of overcoming these hurdles one final problem still remains. Science has not yet found

a reliable way of using precursory emissions either acoustic or ultrasonic successfully to predict

rock mass failure.

In conclusion, the measurement of ultrasonic emissions as a portable means of predicting local

rock mass failure in a mine does not appear practical and does not warrant further research at

this time. The use of acoustic emissions as a measure of rock mass failure is a more reliable and

proven technique.

5.2 ELECTROMAGNETIC EMISSIONS

Other authors in this field have successfully demonstrated how electromagnetic emissions can

be detected during the failure of rock mass. However the initial laboratory trials conducted into

this phenomenon demonstrated the problem of background radiation and its masking effect on

failure induced emissions.

Two different conclusions can be drawn from the initial laboratory trials; firstly the failure

mechanism of the sandstone sample may not have caused the fracture of atomic bonds and the

subsequent release of electromagnetic emissions. During the failure of the samples it was noted

that the Hollington sandstone tended to crumble, rather than fail dramatically, which could

result in no electromagnetic emissions being released. The failure of the Horton sample was

however more dramatic, yet again no electromagnetic emissions were detected during its failure.

It is therefore probable that the electromagnetic emissions released during the failure of the two

sandstone samples lay below the level of background noise.

As with ultrasonic emissions, if a device was capable of detecting electromagnetic emissions in

a working underground environment, a method of processing the emissions to give accurate

identification of failure would have to be found. This problem has not yet been resolved after

many years of research; with some scientists now believing such a method is unachievable, as

failure is inherently chaotic. From the work done under this Project it would be unfair to

dismiss the concept as unusable in a practical mining situation but it does not appear to be the

most likely technique to produce a successful and viable device.

5.3 THERMAL RESPONSE

Work completed under this project, both in the laboratory and field, and by other authors, into

the thermal response of failed rock mass and its detection has highlighted the problems involved

with such a concept. Computer simulations of the concept, conducted during this project and by

other authors, show the theory behind the concept to be sound.

The laboratory trials of the concept demonstrated that a simulated failed rock mass could be

distinguished by its difference in surface temperature when its surface is heated with an infrared

35

heat source for a period of only 20 minutes. The difference in temperature was seen twice,

during the heating period and also during the cooling period, when the IR source was removed.

When a cooling source, liquid carbon dioxide, was applied to the same simulated rock mass

conditions, no discernable difference in temperature was observed.

Field based experimentation, into the concept, like the lab based work, proved inconclusive with

only one area of failed rock mass detected using the concept. However only two sites were

available for the field trials, neither of which were ideal. The field and laboratory trials were

important in highlighting the problems with the concept, and in the development of a better

understanding of what is required for such a concept to be practically applied in the field.

5.4 LASER VIBROMETER

The results of the initial laboratory trials conducted on the simulated test slabs and the

preliminary work conducted on the large concrete mass, were able to demonstrate the potential

of the laser vibrometer. The results from these trials proved that, as with the AEM, loose or

failed rock mass can be identified from intact rock mass from its different vibrational

characteristics. The traces produced from the vibrometer were also successfully processed

using an algorithm similar to that used by the AEM and a single representative value was

returned.

Due to these clear conclusions it was recommended that further field trials, under realistic

conditions, should be undertaken. Under these conditions the experimental procedure was

unable to reproduce the same quality of results as were seen during the laboratory phase of the

trials. A lack of vibrational energy resulted in unclear traces and irregular mathematical

parameter values from the processing algorithms which were applied. An alteration in the way

the target surface was excited worked up to a distance of 1.5 - 2 m from the target surface but

only if no discontinuities lay between the roof bolt being struck and the target surface.

From these laboratory and field experiments it is concluded that failed rock mass demonstrates a

noticeable difference in transient vibration characteristics compared to that of solid rock mass.

Using a laser vibrometer it is possible to measure the transient vibrations caused by an impact to

a surface under investigation and identify these differences using mathematical algorithms

applied to the signal traces.

However, for reliable measurements to be made, two things must be achieved. Firstly sufficient

vibrational energy must be applied to the surface area under investigation, from a remote

position. The methods used during these experiments to excite the surface under investigation

were not effective in delivering sufficient vibrational energy from a distance of more than 2m.

Secondly a strong return signal from the surface under investigation is also required for reliable

measurements. To overcome the problem during these trials, retro-reflective tape was used to

obtain a good return signal from the target surfaces, which improved the accuracy of the

measurement and the reliability of the reading. The placement of such tape onto the surface

under investigation does not follow the brief, that the device should be used remotely from the

surface under investigation.

If a testing procedure can be developed, which fulfils the two requirements described above, it

is the authors’ opinion that such a concept would be capable of detecting and quantifying areas

of failed rock mass. The system would be completely remote, allowing operators to survey an

area suspected of being unsafe without having to enter it and put themselves at risk.

36

5.5 OVERALL CONCLUSIONS

The detection of emissions, ultrasonic, acoustic or electromagnetic as an indication of

microfracturing and the imminent failure of rock mass has a number of inherent problems

relating to its use in a mining environment. Emission strength is a key problem, when

attempting to detect such emissions. When background noise is also considered, the problem of

detection becomes even more difficult. In a working underground environment there are a

number of sources of acoustic, ultrasonic and electromagnetic emissions, which create

significant background noise. Such noise then serves to obscure the emissions released by the

failing rock mass, making detection almost impossible in a working underground environment.

However, the detection of such emissions, especially acoustic and to some extent

electromagnetic, as an indication of failing rock mass, is important for research purposes.

Under laboratory conditions, where variables such as background noise and failure rate can be

controlled, such emissions can provide a great deal of information about the failure process.

Even though it has not yet proved possible to use the emissions to give accurate and reliable

predictions of rock mass failure, a better understanding of the processes of failure has been

gained. This knowledge has since been used in the design of safer mines and excavations, to

improve their performance under high stress conditions, and reduce the risk of violent rock

mass failure.

Due to these inherent problems, the researchers went on to examine how more conventional

means of rock failure detection could be modified to become remote methods. This involved

considering the different responses of rock mass, which had failed, but not detached completely

from the surrounding mass. Technology and experience already gained during the development

of the acoustic energy meter, a contact device capable of quantifying the integrity of a surface,

were applied. Research was conducted into the transient vibrational characteristics of failed

rock mass, and how this could be stimulated and measured remotely. Using a laser vibrometer,

measurements of transient vibration were taken from both intact and failed rock mass. Results

showed that with sufficient vibration, distinguishing characteristics could be seen between the

two rock mass classes. Mathematical algorithms applied to the results, could quantify the

integrity of the surfaces under investigation and give an indication of its potential to cause harm.

Although the ability of the vibrometer to measure the different transient vibrations of rock mass

was proven, a safe, mine worthy, technique, by which, sufficient energy could be delivered to a

surface under investigation, must still be found. A method of improving the return signal of the

laser vibrometer, from surfaces under investigation, is another area, which must be resolved,

before such an approach would be suitable for further development.

The thermal response of intact and loose rock mass to two different temperature environments,

either naturally occurring or enhanced by man in the underground environment was also

researched. Initial laboratory work demonstrated that by heating a surface, simulated loose rock

mass could be distinguished from solid rock mass by its different thermal response. Field trials

however failed to provide similar positive results, with only minor success in identifying loose

rock mass seen at one location in an evaporate mine.

37

38

6 RECOMMENDATIONS

From the work conducted under this Project it is recommended that the following areas deserve

further research into their potential as novel mobile and portable methods for detecting rock

failure.

(i) The initial laboratory and field trials into the use of remote transient vibration

measurements and their use to identify failed rock mass, showed positive results. Further

work is recommended into the following areas: -

Safe and mine worthy techniques for the delivery of sufficient energy to a surface under

investigation should be the main focus due to its critical role in the whole concept.

Without sufficient vibration, measurements are inaccurate and unreliable, hence the

requirement of extra research.

Increasing the return signal strength by means of reflective material placed onto the target

surface by remote means. Without a good return signal the measurements are again

inaccurate and unreliable, hence the need for reflective material on the target surface to

ensure a strong return signal. During the field trials, reflective tape was placed on the

target surface; this however falls short of a wholly remote method. It is recommended

that further work should examine the potential of the paintball marker initially used

unsuccessfully for the delivery of vibrational energy, as a method of placing reflective

paint onto the target surface.

Calculations on the impact energy required to excite loose rock mass of different

dimensions and the displacements likely to be seen, would help in the analysis of the data.

Such work would help in defining the limitations of the concept and give an

understanding of what is possible with the current procedure.

(ii) The work conducted into the thermal response of failed rock mass identified the problems

relating to such a concept. Although previous researchers in this subject were able to

demonstrate how failed rock mass can be identified by a temperature gradient between it

and surrounding intact rock mass, it proved difficult to repeat the same findings outside of

the laboratory during this research. It is the authors’ opinion that this remains a viable

concept and requires further research, and the following further work is recommended: -

In the authors’ opinion the concept is most suited to deep excavations where virgin rock

temperatures are high, hence research should be focused here, rather than shallow

excavations where more energy is required to develop a thermal gradient. Work should

be conducted into ways in which normal mining practices can be modified in order to

enhance a temperature difference to detectable levels. The following are some

suggestions how a temperature difference could be enhanced between intact and loose

rock mass: -

x� Increasing ventilated airflow in a development for a few hours prior to a survey.

x� Use of an auxiliary cooling fan to lower ventilated air temperature in a

development prior to a survey.

x� Stop ventilating a development for an over night period, causing the rock mass to

reach an equilibrium closer to virgin rock temperature. After the over night

39

period, restart the ventilation, whilst surveying the area with thermal imaging

equipment.

�� Spraying hot rock with cooled water. This could be particularly successful in

deep, hot mining environments where cooling water is often already available for

other purposes.

40

7 REFERENCES

Altounyan P.F.R. and Minney D. 2000, Field Experience of Measuring the Acoustic Energyfrom a Hammer Blow to Coal Mine Roof and its relationship to Roof Stability. Proc. 19th

Conference on Ground Control in Mining, Morgantown USA. 8-10 Aug 2000. pp 12- 18. Altounyan P.F.R., Clifford B. and MacAndrew K.M. 1999. Assessing and evaluating acoustic techniques for testing roof conditions in coal mines. Final Report SIMRAC Project COL 610.Brady, B.T., Rowell, G.A. 1986. Laboratory investigation of the electrodynamics of rock fracture. Nature 1980;321:488-492.Cartwright P., Clifford B., Ärmänen E. and Vuori A. 2001. Application of the Acoustic Energy Meter for assessment of tunnel lining condition. Proc. ISRM Regional SymposiumEurock 2001. Epso, Finland, 4-7 June 2001, pp333-338Cress, G.O., Brady, B.T. and Rowell, G.A. 1987. Source of electromagnetic radiation from fracture of rock samples in the laboratory. Geophys Res Lett 1987;14:331-4.Geller, R.J., 1997. VAN cannot predict earthquake – nor can anyone else. International centre for disaster-mitigation engineering (INCEDE) newsletter, vol5-4 Hanson, D.R. and Rowell, G.A. 1982. Electromagnetic radiation from rock failure. USBM RI8594, 21p:,27cm, US Department of the Interior. Hazzard, J.F. 1999. Numerical modelling of acoustic emissions and dynamic rock behaviour.PhD thesis, Keele University, Staffordshire, UK, 1999.Hirata, T. 1987. Omori’s power law aftershock sequences of microfracturing in rock fracturing experiment. J. Geophys. Res. 92. Pp 6215-6221.Kononov, V.A. 2000. Pre-feasibility investigation of infrared thermography for theidentification of loose hangingwall and impending falls of ground. SIMRAC final reportGAP706. 26p.Lockner, D. 1993. The role of acoustic emission in the study of rock fracture. Int. J. Rock.Mech. Min. Sci. & Geomech. Abstr. Vol.30(7) 1993, pp883-899.Meredith, P.G., Main, I.G., and Jones, C. 1990. Temporal variations in seismicity duringquasi-static and dynamic rock failure. Tectonophysics 175, pp249-268.Merrill, R.H. and Morgan, T.A. 1958. Method of determining the strength of mine roof. U.S.Bureau of mines, Report of Investigation R.I. 5406, 22p.Merrill, R.H. and Stateham, R.M. 1970. Loose rock can be detected by infrared devices.Mining engineering, November 1970, pp59-62.

Nitsan, V. 1977. Electromagnetic emissions accompanying fracture of quartz-bearing rocks.

Geophys Res Lett, 4(8):333-6.Obert, L. 1941. Use of subaudible noise for prediction of rock bursts. U.S. Bureau of mines,Report of Investigation R.I. 3555, 1941.Obert, L. and Duvall, W. 1942. Use of subaudible noise for the prediction of rock bursts, part

II. U.S. Bureau of mines, Report of Investigation R.I 3654, 1942.

Piper P.S., Bron K.B., van Rooyan H., Goldbach O.D. and Clifford B., 2002. The

application of acoustic techniques for identifying rock-related hazards in gold and platinum

mine. Final Report SIMRAC Project GAP 822.

Rabinovitch, A., Frid, V., Bahat, D. and Goldbaum, J. 2000. Fracture area calculation from

electromagnetic radiation and its use in chalk failure analysis. Int J Rock Mech Min Sci

2000;37:1149-1154.

Stravrakakis, G. 1998. Claims of success in using geoelectrial precursors to predict earthquakes are criticised and defended. Physics today, January 1998.Urusovskaja, A.A. 1969. Electric effects associated with plastic deformation of ionic crystals.Sov Phys-Usp 1969;11:631-43.

41

Warwick, J.W., Stoker, C. and Meyer, T.R. 1982. Radio emissions associated with rockfracture: possible applications to the Great Chilean earthquake of May 22, 1960. J Geophys Res 1982;87(b4):2851-6.Wu, L., Cui, C., Geng., Wang, J. 2000. Remote sensing rock mechanics (RSRM) andassociated experimental studies. Int. J. Rock Mech. Min. Sci. Vol. 37(6) pp879-888.Wu, L. and Wang, J. 1998. Infrared radiation features of coal and rocks under loading. Int. J.Rock Mech. Min. Sci. Vol.35(7), 1998, pp 969-976.Yu, T.R., Henning, J.G. and Croxall, J.E. 1990. Loose rock detection with infraredthermography. CIM bulletin, May 1990, pp 46-52.Young, R.P., Collins, D.S. 1997. Acoustic emission/microseismicity research at theunderground research laboratory, Canada (1987-1997). Report# RP037AECL, University Keele,1997, p139.

42

8 FIGURES

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10

20

30

40

50

60

70

80

90

100

Alg

ori

thm 0.73m

1.73m

2.71m

5.54m

20 40 60 80 100

AEM value

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62

0

Printed and published by the Health and Safety ExecutiveC30 1/98

Printed and published by the Health and Safety Executive C1.10 06/04

ISBN 0-7176-2866-3

RR 248

78071 7 628667£15.00 9