A proposed mechanism of rock failure and rockburstingLe mecanisme fondamental de rupture violente de la roche

Angenommener Mechanismus von Gesteinsbruch und Gebirgsschlag

ZOU DAIHUA, Department of Mining and Mineral Process Engineering, The University of British Columbia, Vancouver,Canada

HAMISH D.S.MILLER, Department of Mining and Mineral Process Engineering, The University of British Columbia,Vancouver, Canada

ABSTRACT: Sudden rock failure in the form of rockbursting has long been a problem in underground mines. Thebasic mechanism of this phenomenon is still unresolved. This paper describes the research work conducted atthe University of British Columbia to study the basic mechanism of violent rock failure, and to identify re-liable precursive behaviour. Acoustic emissions were tested from rock specimens, the rock failure mechanismpostulated and the experimental results obtained are in agreement with measurements made in situ in a deeplevel South African mine. The shear failure mechanism proposed has been modelled and this numerical modelallows rock tests and their associated acoustic emissions to be realistically simulated.RESUME: Les affaissements violents de roche ont ~t~ ~tudi~ de fa~on a identifier un comportement precurseurfiable.· Un m~canisme de fracture de la roche a ete postul€ d'ap~s les emissions accoustiques d'echantillonsde roche. Le m~canisme de cisaillement a ete modelise et il permet une simulatlon des essais de roche et desemissions acoustiques associ~es.ZUSAMMENFASSUNG: Um eine zuverlaessige Voraussage ueber Gesteinsbruch zu erhalten wurden ·im Test Bruchge~raeusche gemessen. Der Mechanismus des Scherbruches ist ebenfalls im Modell untersucht worden und dies erlaubteine realistische Simulation von Gesteinstests und ihre begleitenden Bruchgeraeusche.INTRODUCTI ONThe phenomenon of rockbursts ha long be~n a pr?blemin underground mines. It has been assoclated wlthmining excavations throughout the world in all rocktypes and at all depths. As min~ng depth cont~nuesto increase, this problem ~s becoming more se:lous.Despite the work of many researche:s, the basIcmechanism of this phenomenon is stIll unresolved.With the development of microseismic monitoring, .warning of impending rockbursts has been greatly Im-proved. However, the low reliability of this tech-nique as a predictive tool has largely limited it toevent location. The commonly faced problem is thefailure without anomaly or an anomaly with no ac-companying failure.Generally a rock burst can be described as a sud-den release ~f strain energy stored in the rock mass,sometimes resulting in catastrophic failure and ex-tensive damage to underground openings and.miningfacilities. It is characterized by expulSIon of :ockin varying quantities from the surface of an openIng.This phenomenon usually happens instantly and with-out any visual warning. Once it happens it threatensminers' lives and gives rise to considerable opera-tional problems in the mine. .'The rock mass is highly jointed and anlstroplc.Before excavation takes place, a stress field existsin the rock mass, known as the virgin str~ss •. As ?nopening is excavat~d, the virgin st:ess fleld IS dlS-turbed, resulting In stress redistrIbution aroundthe opening. Close to the boundary of the excava-tion a zone of stress concentration is formed. Ifthe maximum stress is less than the strength of therock mass the structure will be stable. Otherwise,failure o~curs either by yielding or cracking. Atthe same time the stored strain energy which is pro-portional to the square of the stress, is released.If the energy release happens sUddenly,.t~e fai!urewill be violent •. However, until today, Ilttle ISknown about the way this energy release occurs. .

This research work is intended to study the basIcmechanism of rock failure and rockbursting. We aretrying to approach this problem from the very be-ginning, studying both the conditions under which a.burst is likely to occur, and the release of acoustIC

energy prior to rock failure.1. FAILURE MECHANISM OF ROCK MASS1.1 Failure principleAs a geological material, rock mass is generallyjointed and anistropic. The failure process of rockmass is also complex. An examination of the fail-ures of rock mass, such as of a pillar, or of theremnants of a rock specimen, shows that failureusually takes place along a surface. This surfacemakes an acute angle with the major loading direc-tion. For an isotropic material, this surface canbe determined on Mohr's circle, Figure 1[1J. However,this kind of ideal material rarely exists. The fail-ure of rock mass will occur along the weakest sur-face, which may be a major fault, joint, or any otherweakness when the shear stress on that surfacereaches the corresponding shear strength. As can beseen, the failure of jointed rock mass will be con-trolled by the shear process.

T

b)

Fig. 1 Schematic showing shear failure plane.

For intact rock, this principle of shear failuremay not apply until a failure surface is formed. Inthis case, the failure is believed to initiate fromlocal microfracturing, since even intact rock alwayscontains microcracks. When a force is applied to arock specimen, these microcracks initially close.Then the rock mass exhibits perfect elastic deform-ation under further load. As the stress reaches

1357

some level, these cracks start to develop, or micro-fracturing initiates. This results in acousticemissions that occur due to the vibration of rockparticles in the immediate vicinity of the fracturing.These vibrations are extremely small. As the loadcontinues to increase, the number of microfracturesincreases and fracturing propagates further. Atfirst the fracture propagation is stable and fractur-ing can be halted by maintaining a constant load.When the fracture propagation reaches the unstablestage, the fracture process is self-sustaining be-cause the energy required to maintain the crack pro-pagation decreases. Even if the load is held con-stant, fractures will develop. Any increase of theload will accelerate the fracture propagation. Ap-parently the seismic event rate increases due to theintensive fracturing, but the increase of acousticenergy is not significant because the vibrations ofrock particles remains low. Meanwhile, the releasedenergy from crack extension increases with the cracklength. When additional energy is available, thecrack tends to fork in the weakest direction. Theonset of forking represents a transition within theprocess of the unstable stage. Once this transitionhas taken place, successive forking will lead to co-alescence of many fractures, forming macrofractures.Thus, the acoustic energy is expected to increasedramatically. These macrofractures will join to-gether to form a surface on which the final failuretakes place. From now on the shear principle con-trols the failure process.1.2 Stick-slip in shear failure.During the shear process, once: the shear stressreaches the shear strength, slip begins. However,the behaviour of slipping varies with the loadingconditions and the surface properties. In shear ex-periments. either stable sliding or stick-slip is ob-served. Generally, the phenomenon of stick-slip isexpected to occur if the shear surfaces are verysmooth or if the normal stress is'very high. [2,3]

The stick-slip is of significance. During sticktime, shear stress and potential energy graduallybuild up. When slip occurs, the shear force dropsand the potential energy is released. If the sliptakes place suddenly, the energy will be releasedvery quickly, resulting in violent failure. Thephenomenon of stick-slip has drawn great attentionfrom seismologists and is considered as a mechanismof shallow earthquakes [4], esp~cially for thoseoccurring along geological faults. The high stressfield in the earth's crust tends to initiate relativemovement along the fault. Once the potential energyexceeds the shear strength ofithe fault, stored en-ergy is released by a sudden slip in the crust.

A natural earthquake and a rock burst are extreme-ly similar in terms of seismic events. The onlydifference between the two is 4 matter of scale.They both involve the violent release of seismicenergy. For an earthquake, the stress build-up isthe result of many decades or even centuries of move-ment in the earth's crust. For a rockburst, it isusually caused. by mining activity in a relativelyshort time. Therefore, the shear failure is signi-ficant to the study of rockbursting.1.3 Conditions of violent failure.In this research, the shear behaviour is analyzed ona numerical model. Figure 2 schematically illus-trates the model. The whole system can be describedby an equrt ibrium equation.

mx ='F(t,x) - f(u,P,x) •••••••• (1)where m is the mass of the particle, P the normalpressure, u the frictional coefficient, f the totalresistance, F the shear force, x. x and ~ the slipdistance, velocity and acceleration respectively.

x

Fig. 2 Simple shear model.

The shear force F which is a function of time andslip distance, is modelled by a spring which repre-sents the elasticity of rock mass. This spring issupported by a moving base. The moving speed Vsimulates the loading speed. The effects of slipvelocity and seismic radiation are also considered iNthis model. Scholz and Engelder (1976) observed fromexperiments that the frictional coefficient is in-versely proportional to the logarithm of slip velo-city [5]. The seismic radiation is complex. Onemethod in which radiation effects can be simulatedwithout making the model unduly complicated is toattach a semi-infinite string to the mass particlein such a way that the motion of the mass particleexcites an elastic wave which propagates along thestring. The derived force exerted by the string onthe mass is linearly proportional to the slip velo-city. Thus, the term f includes the frictional re-sistance and the seismic dissipative force,both ofwhich are functions of slip velocity.

The stick time, which is the peace time betweenadjacent slips, obviously varies with conditions.If stable sliding is considered as a special case ofstick-slip, in which the stick time is zero, a tran~sition condition between the stable sliding andstick-slip exists. In this model. stable sliding isconsidered to occur when the stick time is equal toor less than IO-~ seconds instead of zero, mainly dueto the numerical approximation and computing costs.Figure 3 shows some typical numerical results. Thisfigure shows the transition chart. AS,expected, thedriving speed, normal pressure and elasticity of therock mass have a significant effect, although theeffect from frictional coefficient is negligible.For a given material, the elastic modulus is specified.If the loading conditions of loading speed and normalpressure fall within the lower part of this chart, theshear behaviour will show stick-slip. The upper partrepresents the stable sliding. This chart can alsobe used to determine the maximum loading speed orminimum normal pressure for stick-slip for a giventype of rock mass. It should be pointed out thatthis chart only shows the basic principle. The tran-sition condition for a particular material should beobtained from experiments.

·,•l~;;

~ -,..• -,••...• -aIIC -,•· -J...

-,•

".bl. ,lldlnt

U.-,OI,·.95

• ••no.~.1 lo.d LOCI0IPn)-P.II

Fig. 3 Transition between stable sliding andstick:-slip.

From the physical conditions of the shear processand the transition chart, violence failure is ex-

1358

pected to occur during shear in the following threecases:MODE I. Violence is the stick-slip under very highnormal pressure because of the large amount of energyreleased at each slip.MODE II. Violence comes from the transition fromstick-slip to stable sliding. If a shear system whichshows stick-slip behaviour suddenly changes intostable sliding for some reason, such as sudden reduc-tion of normal pressure or quick increase of loadingspeed, extra energy is available. This energy has tobe released quickly in order to keep up with thesudden change of loading condition.MODE III. Violence occurs under sudden loading.Whether the shear behaviour is stable sliding orstick-slip, violent failure is bound to happen if ashear force much higher than the shear strength isapplied to the system suddenly, because extra poten-tial energy is always available. The failure of in-tact rock, such as a rock specimen under convention-al compressive test, belongs to the Mode III violence.If the shear stress starts from zero, which isusually the case, failure happens only when the shearstress reaches or exceeds the shear strength. Underthis condition, there are only two modes of violentfailure, namely Modes I and II.2. ACOUSTIC SIGNALS FROM ROCK SPECIMENSDuring this research, laboratory experiments on rockspecimens were carried out to study the acoustic ac-tivity prior to rock failure. They were designed toexamine the acoustic emissions pattern from similarrock types loaded under compression and shear. Thetesting results are very encouraging and agree wellwith the mechanism postulated previously. Detailsof the test ~rogram and results have been previouslypublished [6].During compressive tests, generally few acousticsignals were observed before stress had reached somelevel, say 75% or 80% of the compressive strength,.Figure 4. This stress level corresponds to the frac-ture initiation. As the fracture propagates furtherunder continuous loading, the acoustic activity be-comes more intense. In the period between the frac-ture initiation and the final failure"the acousticemission is most active. The most significant phen-omenon during this period is the fact that the eventrate increases rapidly initially and then dies downimmediately preceding the specimen failure. At thesame time, the energy release rate increases. Whenfailure is approached, the energy rate shows a peakvalue. The drop of event rate and the peak value ofthe energy may indicate the coalescence of micro-fractures. These events are in perfect agreementwith the failure principle discussed above.

In the shear tests, both sawcut surfaces and nat-ural breakage surfaces were loaded to failure. Thesurface roughness seems to have little effect onacoustic emission pattern, although the magnitude ofthe acoustic signal from the breakage surface ishigher for the sawcut surface. In general, theacoustic activity is low before the slip and remainssomeWhat unchanged as sliding continues.

The effect of' normal pressure on acoustic emis-sion seems significant. As normal pressure increases,the acoustic activity increases throughout the shearprocess, characterized by higher magnitude, but theacoustic emission pattern changes little, Figure 5.As previously described, stick-slip phenomena wereobserved under high normal pressure. Following eachslip, the'acoustic emission shows a'sharp increase.The acoustic activity remains at a low level as thestress builds up again.

47flO••...~ :sll70

~ 2380

1190

o

AI.

1100ROCK IA U1ll 21/ :5llD8

10 c.~...

1 /II/IS 12.50.1.

880

~ 1060

I440 , •• .;

0:220 •

o

01.TJI1E 18ec:1 FIRST ARRIVAL

ROCK I A ,... 21/ SSD8." '.,11920 • ~

~ .: "40 .~

~"flO .:

'" i

12/11/85 t2.~.oe14900

o

C)AXIAL LOAD '~"Nl

Fig. 4 Acoustic emission vs. axial load forspecimen #1.•••• 4.1 ke'

• n..-!··~lJI a)'.

...., •• k ••

I. I' " ",..

· ...:: ......, '"c• "'·• ••

~·2 "'••

_ •••.••..~.I kei

",. II II

·-:: .-......•.~I-• -e• .---,-e)' • • II ,. I. II

.M::.. ,..•...•.. '"•e· "..d) • • • "

II ,." I.

,h •• ,. di,,,1 18,1 In)

Fig. 5. AE vs. shear displacement for sawcutsurface #4 at various normal pressure.

1359

From testing results it is found that compressivefailure and shear failure are two different modes ofthe failure mechanism of intact rock. Failure undercompression is a matter of fracturing up to the pointwhere a failore surface is initiated. After the for-mation of this surface. the failure process obeys thelaw of shear. Unfortunately. this shear process incompressive tests occurs extremely rapidly and cannotbe easily observed. This is because the shear stresson the newly-formed fracture surface is much higherthan the corresponding shear strength. Detailedanalysis shows that this shear stress is up to threetimes higher than the strength for the specimens tes-ted under compression. It is this extra shear stressthat makes the failure viole~t. If this extra forcecan be extracted at the formation of the fracturesurface. the failure can be reduced to non-violence.On the contrary. if a large shear load is suddenlyapplied to the shear specimen. violent failure canalso be expected during shear process. This has beenproven in the experiments by releasing the normalpressure instantly when slip began. resulting inbursting. Figure 6 shows the acoustic signals fromone of these tests. The acoustic emission beforeslip has been completely shadowed by the peaking ofsignals at the instantaneous failure. Because theload is reduced to minima instantly. after shock isscarcely observed.

•••• SAliCUT BUDDEN l.OADJPIOzso

: l~•..100..

c•: '5Q

00

AJ. TIME <S.clFIRST ARRIVAL

I. SAWCUTSUDDEN LOADING246:;0

19720

14790

B J. TUE {~."t:HqR5T (\RntV(\L

Fig. 6 AE from sawcut surface #4 under suddenloading. normal pressure 1. 2.5 and4.5 ksi respectively.

3. NUMERICAL SIMULATION OF ACOUSTIC ACTIVITY3.1 Numerical modelFrom microseismic monitoring of rockbursts in situand from acoustic emission tests of rock specimensin the laboratory. a large quantity of data ofacoustic signals before rock failure is available.However. these data are only recordings of theacoustic signals. To date. the causes of these sig-nals and how they actually occur remains unknown.

In this research. a numerical model is developedto simulate the acoustic activity prior to rockfailure. This model is not based on any physicallaw of acoustic emissions or on any empirical for-mula from previous recordings. It is entirely basedon the proposed shear failure mechanisms. Whetherthe failure is in the fracturing stage or at s~ipstage. any movement of rock particles at a localarea will induce vibration among the surroundingrock particles. It is this vibration which causesacoustic signals. It is expected that .• if this model

can produce results similar to the acoustic signalsrecorded during tests. this model can serve thefollowing purposes:1. To justify the shear process as a mechanism ofi

rock failure and rockbursting.2. To verify the acoustic emission as a useful

precursive signal.3. To provide a tool to study the acoustic activity

prior to rock failure.As in the finite element method of stress-strainanalysis. the rock mass is discretized into indivi-

dual elements. Because the shear process takes placeon the contacting surfaces. the movement only occurson the failure plane. Only two variables are neededto describe an exact location in a plane. However.this model is not involved in the exact descriptionsof locations of the elements. only the behaviour ofthe elements during the movement is of interest.Therefore. only one degree of freedom is permitted.

The model consists of a series of particles. N.connected together by springs. Figure 7. The massof the material is concentrated on those particlesand the spring represents the elasticity of the rockmass. Let the mass of particle i be m. and thestiffness of spring i be ~i. If we further assumethat at the beginning all particles are at rest. bythe force equilibrium of particle i as shown inFigure 7. the equation of motion of particle will be

mixi = Fi - Fi-l - fi* ••••••••• (2)i = 1. 2 •••• N

where Fi and Fi-l are the forces in the springs infront and behind particle i. and fi* is the total re-sistance including frictional resistance and seismicforce.

x

b) •

Fig. 7 Diagram of acoustic activity model.After SUbstituting all these forces. the above

equation becomesmixi = >"j(a+xi+l-Xi) - ).t-I (a-x] - xi.•l)

- fi - Eoxi •••••••••••••••••••• (3)i=I.2 •••• n

where a is the space between two particles. fi thefrictional force. Eo the seismic coefficient. Xi.Xi and Xi the location. slip speed and accelerationof particle i. respectively.When the initial conditions are considered. equation(3) can be solved for the unknowns Xi. Xi and Xi foreach particle at any time. Because fi contains anunknown Xi in denominator. and explicit solutioncannot be found. Therefore it is desired to searchfor a numerical solution. which is obtained by theRunge-Kuta method. The tedious work of calculationis left to the computer.

A computer program was written"lWhich in additionto the above unknowns, can also calculate the kineticenergy, the work done against friction and the seis-mic energy radiated, at any moment. The event num-ber is counted by checking the change of status ofevery particle continuously. Obviously, at the be-ginning only parts of the system will move under theload. As loading continues, the number of particlesin movement will increase. If the onset of movementof all particles is considered as the final failureof the system, the energy changes and acoustic acti-vity prior to the failure can be simulated.3.2 Modelling resultsThis model generates excellent results. Some re-sults from a typical analysis are given in Figure 8.As can be seen, the event rate increases sharply asthe failure is approached and then drops to the pre-vious low level immediately preceding the failure.Meanwhile, the seismic energy remains low when theevent rate goes up, and increases dramatically priorto the failure. In the results from all runs of theprogram, the energy rate and energy ratio show asimilar behaviour, although the energy ratio showsthe anomaly more clearly.

~. "',w~ •••N,

rate• IIevent

w...~ •• illlut •...

!z ••~w ••'.L_.••.•......•....•.••.•, .......•L.o.••••••••.••••••_ •••z:'"'.._,"".•..."•......•._.....•..J.••...•.•-, ""••'-ee ..L•••••••'"'.Ioo.•• J...., •...••• _, .~.

1111

•. IMI,....in •••N-,~ "IW>-:i •••

1'" I" ••

seismic energy rate

~ .11

It.... •• ••• , .11 .115 .IN .eas .11 .en .04 .&45 .115.915 .ee .015 .e- .e-e .11

,I'.l."1111

0

>- '00'~>- "'~ .aow

••••••

••Fig. 8

seismic energy ratlo

••• , .1' .115 ,M .ees .03 ,SiS .••. Dot'5 .a, .ess .•.• ., .e- .07'5 .0'TIME '(5)

Computer results from the numericalacoustic model.All the results from this model are in very good

agreement with the exp~ri~ental resu!ts, even thoughthe model itself has no dIrect relatIon to theacoustic emission. The increase of the event ratecorresponds to fracture propagation. The d~op.ofevent rate and increase of acoustic energy IndIcatesthe formation of macrofractures. This shows that the'postulated shear failure mechanism to interpret rockfailure and the acoustic activity prior to the fail-ure is justified. It also verifies that the acousticemissions are indeed a precursive signal for rock

failure and can be used for predictive purposes.The problem is how to use this signal correctly andefficiently.

Consequently, this model provides us with a methodto study the rock failure and the related acousticactivity. It can be used to simulate the acousticemission under various conditions and to study theinfluence on the acoustic emission from the changeof geological conditions and loading conditions.This part of the research is still in progress.4. COMPARISON WITH FIELD RESULTSIn order to check the acceptability of the above re-sults from experiments and the numerical modelling,some field results of rockbursting monitoring havebeen studied. By comparison, it is found that theabove results are in agreement with measurementsmade in situ in a deep level South African mine [7].Two typical cases are presented:4.1 Case 1: Rockburst on May IS, 1983A large rockburst of magnitude 3.4 occurred on 10lWlPanel, No.3 shaft on May 15, 1983, at 03.37 hours.A concentration of microseismic events prior to theburst is apparent. In Figure 9 the number of micro-seismic events per hour originating from the panelfor the period 8th to 15th May, 1983, is plotted. Asteady increase can be seen, from approximately 60events 6 days before the burst, to almost 300 eventsonly 24 hours beforehand. A sharp drop in the rateof microseismic activity was measured immediatelybefore the burst. For this particular case, thechanges in the ratio between numbers of larger andsmaller events provided the researcher with addi-tional information to make a reliable prediction.

! I \/

, \1\,,,

/ --\ : \ " ,•• t-JlU •• '.n ..' ."

.'.' /: \ : ,\' ,, .',./ \<,

~ I--

.

. .... ..I- ~..~...

1!W.!l!!!..!!!!' l'-_.Fig. 9. Event rate and relative energy one week andthree days after the May 15 event (afterBrink)

In this example, the agreement between the fielddata and the results from experiments and the modelis apparent. In all cases, the event rate increasessharply at first and drops immediately preceding thefailure. The abrupt increase of the ratio betweenthe number of large and small events is equivalentto the increase of event energy, because this changeof the ratio is due to both the decrease of the smallevent number and the increase of the large event num-ber, with more energy being released.4.2 Case 2: Rockbursts on October 4 and 10, 1984On October 4, a 2.6 magnitude rockburst occurredduring shift time (16:31 hours) on the 110 level.Figure 10 shows the event rate, average corner fre-quency and average event energy as observed fromthat area for the time window 2200 to 0400 hoursevery night. On September 27th, influence from anexternal source made the measurement unreliable. Onthe basis of event rate alone, the rockburst would

1361

not have been anticipate on October 4th, as the eventrate parameter is very sensitive to the mining acti-vity and no blasting took place in that area the pre-vious afternoon. However, the corner frequencyshowed a steady drop for tne.precedinq 11 days and afurther drop to below 600 Hz is indicated a few hoursbefore the burst. This behaviour of the corner fre-quency gave a clear precursive indication of a pend-ing rockburst. The average event energy also con-firmed what was expected. Five days later, regularblasting started and was followed by a small burst(magnitude 1.4) at 4:39 hours on October 10th.Again, a relatively low corner frequency and a rela-tively high event energy preceded the burst. Theblasting the previous afternoon made the event rateunusually high.

I··1-r:....j··~I .•.

II ::1f'I'_-"'_~I • t ,

fII •••1tltnn24I1HJ1I1Jt)t, •It ••••••

J J 10 S • , • , 111 0 U ••t ., •• 11M

••.••• , IlItIlIWIf.•.. "' ..Fig. 10. Corner frequency, event rate and energy

over 25 days, covering 2 burstS(after Brink)In this example, similar results as given pre-

viously were recorded. The energy goes up sharplyin both bursts. The event rate shows an increaseand drop prior to the burst, except the secondburst, which was influenced by blasting.

The corner frequency may be another importantparameter to be used. It is schematically definedas fo in Figure II [8], which shows that when f < fo,the amplitude spectrum is level, and when f ~ fo' thespectrum decays. In other words, higher frequencycorresponds to lower magnitude or to smaller event.The drop of fo may indicate a greater number ofevents in the low frequency band and more energy re-leased. The increase of magnitude of individualevents will be accompanied by a decrease of eventrate because of the coalescence of microfractures.This hypothesis is supported by the empirical rela-tion between the number of events and their magni-tudes derived from years of observations of seismicevents [7].

log N = a - b M ••••••••••••••••••••••••• (4)where a and b are constants, M is the magnitude andN the number of events of magnitude ~ M. Therefore,the downshift of the corner frequency also indirect-ly indicates the drop of the event rate and thesharp increase of the energy release.

5. CONCLUSIONSIn this research, the source mechanism of rockburstsis studied and the acoustic emission prior to thebursting is analyzed. The shear failure is postu-lated as the basic mechanism'of rock failure androckbursting. Important results were obtained fromexperiments on acoustic emission from rock specimens.

log A

log Co

log f. log f

Fig. II. Schematic far-field seismic spectrum (A=amplitude spectral density, f = frequency),clarifying the concepts of low-frequencyamplitude level (~.o)' corner frequency(fo), and high-frequency amplitude decay(-f-n with n = 2 or 3). (after Bath).

A numerical model based on the shear failure mechan-isms has produced results very similar to those fromexperiments. All these results are in agreement withmeasurements made in situ in a deep mine. Thissuggests that the postulated mechanism is true andthe experimental results obtained clearly indicatethat the monitoring of acoustic emissions can beused to reliably predict rockbursts.6. ACKNOWLEDGEMENTSSpecial thanks are given to Professor J. Nadeau inthe Department of Metallurgical Engineering at theUniversity of British Columbia for the loan ofacoustic monitoring equipment. Help from Mrs •Melba Weber and Mark Stoakes during the preparationof this paper is'acknowledged.REFERENCESJaeger, J.C. & Cook, N.G.W. 1969. "Fundamentals of!Rock Mechanics". Textbook.Hoskins, E.R., Jaeger, J.C •.& Rosengren, K.J. 1968.

"A Medium-Scale direct friction experiment". Int.J. Rock Mech. Min. Sci. Vol. 5, p. 143-154.

Stesky, K.M. 1978. "Rock friction effect of con-fining pressure, temperature and pore pressure".Pageoph, Vol. 116, p. 690-703.

Brace, W.F. & Byerlee, J.D. 1966. "Stick-slip asa mechanism of earthquakes". Science 153, p. 990.

Dieterich, J.H. 1978. "Time dependent frictionand the mechanism of stick-slip". Pageoph, Vol.116, p. 790-805.

Zou, Daihua & Miller, Hamish D.S. "Acousticemissions from rock under unioxial compressivetest and direct shear test". Proc. 29th AcousticEmissions Working Group Meeting, Royal MilitaryCollege, Kingston, Ontario, June 23-26, 1986.Brink, A.V.Z. & Mountfort, P.I. 1985. "Rockburstprediction research at Western Deep Levels Ltd. -A review report of the period 1981-1984". Inter-nal report No. R, p. 21. \

Bath, M. 1984. "Rockburst Seismology". Rockburstand seismicity in mines, S. Afr. Inst. Min. & Met.Symp. Series No.6, p. 7-15.

1362