Penney_The Use of Geotechnical Instrumentation to Optimise an Engineered Mine Design at Beaconsfield Gold Mine

The Use of Geotechnical Instrumentation to Optimise anEngineered Mine Design at Beaconsfield Gold Mine, TasmaniaA R Penney1, P B Hills2 and R J Walton3

ABSTRACTGeotechnical instrumentation has been implemented as a key step formonitoring and optimising an engineered mine design for managingseismicity at the Beaconsfield Gold Mine. Measurements ofdisplacement, stress change, ground control element load and seismicresponse of the rock mass allow for back analysis and calibration ofcritical parameters for feedback into the engineering design loop.

Instrument clusters are installed in strategic locations prior to thecommencement of stoping in order to determine the nature of theresponse of the rock mass to stoping. A selection of instruments includinginstrumented cable bolts and rod extensometers, resistance wireextensometers, hollow inclusion stress cells, vibrating wire stress metersand resistance wire extensometers are used to gather data through a datalogging system. The rate at which data is recorded is adjusted to providemore information during periods of anticipated rapid change such asaround stope firings or during periods of high seismic activity, and lessinformation during periods of anticipated quiescence.

The twofold purpose of the instrumentation is to calibrate thenumerical modelling output which forms the basis of the engineered minedesign, and to provide an alert mechanism where the behaviour of therock mass is not what was anticipated, or where the impact of thatbehaviour may compromise the integrity of the engineered ground controlsystem.

The paper describes the location of instrument clusters with respect tostoping, the type of instruments employed and the method of dataacquisition. It provides examples of results obtained and illustrates theway in which that information is used to optimise the mine design andmanage seismicity.

BACKGROUNDThe Beaconsfield Gold Mine in Northern Tasmania began toexhibit mining induced seismicity during sill driving of theorebody at a depth of 760 m below surface in 2002. Theincreasing occurrence and degree of seismicity over thefollowing four years saw a number of strategies introduced andwork plans implemented for the purpose of seismic management.Increasingly, the focus of these strategies and plans was directedat reducing the risk to personnel, equipment and the miningoperation itself, which was posed by the increasingly seismicallyactive environment.

Geotechnical instrumentation was adopted as a managementtool from the onset of seismicity, although principally, it wasdirected at measuring and monitoring the seismicity itself andmeasuring the in situ stress field. Stress change monitoring wasintroduced when in situ stress measurements were completed in2003 and 2006, but principally this was done to monitor far-fieldchanges over time rather than focusing on local changes and theimmediate impact on the day-to-day mining operation.

A complete review of all aspects of the mining operation atBeaconsfield followed a well publicised seismically inducedrockfall accident on Anzac Day 2006. At the behest ofWorkplace Standards Tasmania, a comprehensive peer reviewedmine design and safety management process, a Case to ManageUnderground Safety (or Case for Safety) was implemented. Inessence, from a purely mining perspective, the Case for Safetyinvolved geotechnical design from first principles and adoptionof a mining method suitable for the safe operation of the mine inthe prevailing geotechnical environment. In particular, the Casefor Safety led to the development of a remote stoping method inthe west zone of the Beaconsfield Gold Mine to excludepersonnel from areas of greatest seismic risk. Enhanced use ofgeotechnical instrumentation was introduced to the operation toverify assumptions made and monitor progress.

GEOLOGY AND SETTINGThe Beaconsfield Gold Mine is centred on the Tasmania Reef, asteeply-dipping tabular quartz-carbonate vein hosted by asequence of siliciclastic sediments. The sediments, which diptowards the east, comprise the Salisbury Hill Formation ofconglomerates and sandstones overlain by the Eaglehawk GullyFormation of sandstones, siltstones and limestones withoccasional pebble bands. All siliciclastic rocks have beenmetamorphosed to quartzites although they retain much of theirsedimentary character. The Tasmania Reef is essentially planarover a strike length of 350 - 400 m, with an average width of2.5 m and occupies a shear which cuts across stratigraphyorthogonally. The west zone of the Tasmania Reef is hosted bythe Salisbury Hill Formation, and it is the brittle nature of theconglomerate horizons within that formation in particular, whichare prone to mining induced seismicity. The geology of the mineis discussed by Hills et al (2001).

GEOTECHNICAL DESIGNThe basis of first principles geotechnical design adopted for theBeaconsfield Gold mine is generally that outlined in theCanadian Rockburst Handbook (Kaiser, McCreath and Tannant,1996). The Case for Safety studies (Pfitzner, 2006; King,Thomas and Scott, 2007; Scott and Reeves, 2007; Sidea, Scottand Reeves, 2007) progressively developed the design processthrough the increasingly geotechnically complex environmentsof decline development, ore driving, stoping in the generallyaseismic east zone and finally, stoping in the west zone. Theprocess was summarised by Reeves (2008) and peer reviewed inapplication by Kaiser (2008). Scott, Penney and Fuller (2008)examine the application of this process to stoping in the westzone.

REMOTE STOPING METHODA key outcome of the Case for Safety process was the realisationthat seismicity associated with stoping in the west zone of theBeaconsfield Gold Mine would continue and that the magnitudeof those seismic events was likely to equal that which occurredon Anzac Day 2006. Further, it was realised that no supportsystem could be designed to adequately ensure the safety ofpersonnel working in the western ore drives, should they be

Narrow Vein Mining Conference Ballarat, Vic, 14 - 15 October 2008 165

1. MAusIMM, Geotechnical Geologist, Allstate Explorations NL,PO Box 58, Beaconsfield Tas 7270.Email: [email protected]

2. FAusIMM, Technical Services Manager, Allstate Explorations NL,PO Box 58, Beaconsfield Tas 7270.Email: [email protected]

3. Principal Consultant, Coffey Mining Pty Ltd, 1/21 Howleys Road,Notting Hill Vic 3168. Email: [email protected]

present when such seismic events occurred. Consequently, theWestern Case for Safety (Scott and Reeves, 2007) was premisedon the development of a remote stoping method which totallyexcluded the presence of personnel from ore drives in the westzone where stoping has commenced. The development andapplication of the remote stoping method devised, is discussedseparately by Hills et al (2008). With this change, there was arequirement that geotechnical assumptions and other decisionsbehind this move are adequately assessed. These assumptionswere to be continually checked and modified as data is collected,analysed and further observations are made.

DESCRIPTION OF PROJECT ANDINSTRUMENTATION REQUIREMENTS

The need to evaluate the installed ground control system and therock mass around the excavations with a degree of confidenceand reliability as the mining front advances continues to be achallenge facing all mining operations. Areas of ground controlfailure and the failure to understand the rock mass behaviour canresult in development and production delays, equipment damage,reserve loss, or injuries. Instrumentation including microseismicsystems, stress/strain meters, instrumented support elements andextensometers are critical components in understanding thisbehaviour, and measuring the effectiveness of installed groundcontrol systems. This in turn allows for optimisation of theengineering design process through back analysis and ongoingcalibration of engineered designs and has the potential to leaddirectly to improved operating costs as well as safer operatingconditions (Bawden et al, 2007). This role for instrumentation isincumbent in the Case for Safety process adopted at theBeaconsfield Gold Mine.

Microseismic monitoring and the surrounding issue of seismicmanagement was discussed in some detail by Hills and Penney(2008), and are not reiterated. This paper describes the use ofclusters of instruments to monitor the behaviour of the rock massand the installed ground support in response to stoping. Itfocuses on the site selection and analysis of the westerninstrumentation installed in the 940 Stoping Block footwalldrives as outlined in the Western Case for Safety.

Description of instruments and loggers usedAll instruments have been selected due to their relative robustnature, ease of use and general acceptance across the miningindustry. The instruments utilised in these clusters are: CSIRO Hollow Inclusion Stress Measurement Cell

(Worotnicki and Walton, 1976), selected for its capability tomeasure stress change in three dimensions;

SMART cable bolts (Hyett et al, 1997), specificallydesigned to match the bungee (yielding) cable bolts designedfor use at the mine following the Anzac Day 2006 rockfall(Scott, Penney and Fuller, 2008) with node points at 0.4 m,0.8 m, 2.0 m, 4.5 m, 6.7 m and 7.1 m (Figure 1)4;

multi-point borehole extensometer (MPBX) (Windsor andWorotnicki, 1986), selected to compliment the SMARTcable with node points at 1 m intervals over a 6 m length;

1 m and 2 m resistance wire extensometers (RWE) (Windsorand Worotnicki, 1986), selected for their high precision andaccuracy for the measurement of pillar deformation betweenthe footwall drive and the stope; and

uniaxial vibrating wire stress meters (Dutta, 1985), selectedto provide data on stress increases perpendicular across thedrive backs may also be used but are not essential.

Data acquisition from the instrumentation in these clusters isachieved using dataTaker DT85 and DT515 data loggers. Datais transferred from the loggers to either a USB memory stick, ordirect to a laptop computer via USB connection.

Layout of instrumentation and criteria for siteselectionA typical instrument cluster for the footwall drives is shown inFigure 2. The majority of the instrument installations, andtherefore observations, are made from the drive backs. This is thecritical area identified from geotechnical mapping, numericalmodelling and empirical methods for static and dynamic failure.

Clusters are located in strategic locations in footwall drivesalong the strike of the orebody. In a standard cluster, an HI Cellis installed at a depth above the backs of around 3 m to measurestress change and provide a quantitative assessment of observedstress-induced damage. SMART cables and MPBXs are installedinto the centre of the drive backs within a metre of the HI Celland at a separation from each other of not more than 1.5 m. Thecluster allows the impact of measured stress change to becorrelated against measured deformation of the rock mass andthe corresponding response of the support element. RWEs areinstalled into the wall between the FW Drive and reef at an

166 Ballarat, Vic, 14 - 15 October 2008 Narrow Vein Mining Conference

A R PENNEY, P B HILLS and R J WALTON

4. The SMART cable design modified for use at Beaconsfieldincorporates a 1.5 m length of Garford bulbed strand at the toeanchor position. Three Garford nodes occur in this section of thecable, and the measurement head takes the place of the barrel andwedge swage block on the standard Beaconsfield bungee cable. A 4.5m long section of plain strand encased in pvc tubing provides thedebonded capacity to allow for deformation (particularly due todynamic load), and a 1.3 m section of plain strand at the collar toprovides for surface anchorage. Self aligning barrel and wedgeassemblies and domed plates are employed as surface fixtures.

FIG 1 - SMART bungee cable bolt configuration.

incline of approximately 30. At this orientation, the RWE isperpendicular to the reef footwall, and provides insight into thebehaviour of the minimum 6 m pillar between the reef and drive.A HQ diamond drill observation hole is drilled to a depth of 7 min the backs to allow visual observation by borehole camera ofany developing fractures or deformation, and is located within1.0 m of the MPBX. The observation hole also allows forvisual observations of the depth of fracture caused by eitherstress redistribution or pre-existing discontinuities (Figure 2).Occasionally, a cluster is augmented by the inclusion of an HICell installed approximately 9 m into the wall of the footwalldrive away from the orebody.

Generally one cluster is installed in each geotechnical domain.This has the benefit of allowing for determination of all changesduring stope block extraction. Instrument locations are alsoselected following the criteria outlined by ISRM suggestedmethods (ISRM Commission on Standardization of Laboratoryand Field Tests, 1978): instruments installed in an area where expected hazard zones

exist (hazards from stress, geotechnical domain, seismicity, etc); instruments installed in areas to provide the best possible

coverage of all stages of extraction of the stoping block; instruments installed in positions that can be re-accessed to

make any repairs to damaged cables (when it occurs), andthat allow visual inspections to be made to validate visual/measured responses and calibrate all other damage mapping;

loggers located in a secure area and not placed in zoneswhere access exclusions are expected to occur; and

instruments installed into a sound rock mass, and not in faultzones, or where dynamic water conditions exist.

The location of instrument clusters in the 940 and 980 stopingblocks are relative to the stoping panels in longitudinalprojection in Figure 3 (note all clusters are located in thefootwall drives 6 m behind the stope projections).

Procedure for data collectionData acquisition is undertaken by the dataTaker data loggers,which are programmed to read and store all information obtainedfrom the instruments. The data loggers and power supplies are

housed in steel enclosures. For some of the installed instruments,steel terminal boxes are used to house intermediate connectionsof the instruments to the data loggers.

Normally, the data loggers are set to six hour or 12 hour scanrates. The data loggers are set to take readings at five minuteintervals a minimum of 30 minutes before stope firings. This fiveminute scan rate is maintained for the length of the re-entryperiod that is imposed to the level (generally 24 hour exclusion).At the end of this exclusion period, all data is interpreted beforeentry to the level is granted. The instrument results are comparedto the Omori analysis results from the seismic data for the sameperiod to ensure all key trends are not exceeding the requiredlimits. Any time-dependent changes (ie slow stress redistribution,ongoing load increase, displacement from rock mass creep, etc)can be assessed before any persons enter these areas.

Once preliminary assessments are completed and entry to thelevel is granted, loggers are reset to 12 hour scan rates. This scanrate can be changed to six hour scan rates if the stope area iswithin known high seismic hazard areas. Loggers will remain insix hour scan rates until the seismicity returns to acceptable orbackground levels.

The data loggers acquire all the data in millivolts and onlydisplay the last recorded values, therefore it is not possible todetermine whether any changes have been recorded by theinstruments at the data logger itself. All recorded data must beconverted into the appropriate units to allow analysis to beundertaken. Conversion factors from the millivolt record to theappropriate units for the various instruments installed in theclusters are shown in Table 1.

OBSERVED AND MEASURED BEHAVIOUR(DATA ANALYSIS)

The 930 West instrument cluster in the 940 West Stope Block hasbeen selected for discussion in this section, and its behaviour istracked through firing of the first 11 stoping panels since itsinstallation in December 2007. All other instrument clusters inthe 940 West Stope Block exhibit similar responses and trends tothe 930 West instrument cluster, and are not discussed further.The first ten stopes were fired in the 940 West Stope Block.


THE USE OF GEOTECHNICAL INSTRUMENTATION TO OPTIMISE AN ENGINEERED MINE DESIGN AT BEACONSFIELD GOLD MINE

FIG 2 - Layout of instruments in 930 West illustrating pre- and post-stoping damage profile.

Stope 3A was in the 940 East Stope Block was fired almostconcurrently with Stope 3. Stope 11 was the first stope fired inthe 980 West Stope Block in April 2008 and was accompaniedby a ML 1.9 seismic event. The location of all instrument clustersin the 940 and 980 West Stoping Blocks and of the 11 stopesfired during the analysis period discussed is illustrated inFigure 3.

As was the practice throughout the 11 stope firings, each stopewas extracted in a single firing using electronic detonators. Littlechange was observed on any of the instruments during theextraction of stope panels one to four. This was primarily due tothe stope panels being 15 m west of the 930 instrumentationcluster. Stope 5 was the closest stope firing to the 930 Westinstrument cluster, located in the immediate hanging wall of theinstrument cluster, and the first significant change recorded bythe instruments was observed in response to that firing.Following the extraction of Stope 5, minor stress changescontinue to occur throughout the extraction of the remainder ofthe block but no large step changes were recorded.

The most significant change measured was that of the localstress field as measured by the HI Cell. The use of the HI Cells inthis application does not allow determination of principle stressdirections. Orthogonal stress components max int min refer tothe maximum, intermediate and minimum changes incompressive stress over the measurement period. In the case ofthe 930W HI Cell, this amounted to a drop in int and min by3 MPa and 10 MPa respectively, and a rotation of the stress fieldof approximately 50 with associated changes in dip (Figures 4a,4b and 4c). These changes were in line with analysis ofnumerical modelling results undertaken for the design of thedrive using Map3D. This is the key outcome and requirement ofthe analysis. With further iterative analysis, these observed stresschanges could be resolved into changes to the principal virginstress components, but this has not been done.

It was also with the firing of Stope 5 that the first real changeswere observed on the MPBX and SMART cable (Figures 5and 6). Displacements were recorded between nodes at 3 m to4 m and 4 m to 5 m on the MPBX with the greatest displacement



Conversion factor Units Notes

CSIRO HI cell (4 mV) / Vinput GF) M (micro strain) Vinput = 1.99 for system usedGF = 2.097 for cell and cable type usedResistance wireextensometer

(RWE1n - RWEDatum)0.8 mm (displacement) RWEDatum is the first record in mVRWE1n is any subsequent record in mV

SMART MPBX (N1n - N1Datum) - (N2n - N2Datum) 0.0415 mm (displacement)N1Datum is the first record at node 1 in mV

N1n is any subsequent record at node 1 in mVN2Datum is the first record at node 2 in mV

N2n is any subsequent record at node 2 in mV

SMART cable (N1n - N1Datum) - (N2n - N2Datum) 2.55 (0.212/D) Tonne (load)

N1Datum is the first record at node 1 in mVN1n is any subsequent record at node 1 in mV

N2Datum is the first record at node 2 in mVN2n is any subsequent record at node 2 in mVD is the length of cable between node 1 and 2

Vibrating wire (R1n - RDatum) G 0.00689 MPa (pressure)G = 1 for E of rock

RDatum is the first recordR1n is any subsequent record

TABLE 1Data conversion factors from mV (recorded) to engineering units for instruments in use at Beaconsfield Gold Mine.

FIG 3 - Longitudinal projection of the 940 and 980 West Stoping Blocks illustrating the location of instrument clusters and thestoping extraction sequence.



Stope

1

Stope

2

Stope

3St

ope

3a

Stop

e4

Stope

5

Stope

6

Stope

7

Stope

8

Stope

9

Stop

e10

Stope

11

-16.00

-12.00

-8.00

-4.00

0.00

4.00

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Date

Stress

change

(MPa)

maxintminStope Firing

Decrease in compressive stress

Increase in compressive stress

A

Stope

11

Stope

10

Stope

9

Stope

8

Stope

7

Stope

6

Stop

e5

Stope

4

Stop

e3a

Stope

3

Stope

2

Stop

e1

0

45

90

135

180

225

270

315

360

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Date

Bearing

ofstress

change

(degre

e)


B

Stope

11

Stope

10

Stope

9

Stope

8

Stope

7

Stope

6

Stope

5

Stope

4

Stop

e3a

Stop

e3

Stope

2

Stope

1

0

10

20

30

40

50

60

70

80

90

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Date

Dip

ofstress

change

(degr e

e)


C

FIG 4 - Stress component magnitude change (A), bearing change (B) and dip change (C) measured by the 930 West HI Cell duringthe extraction of Stopes 1 to 11.

occurring between the 4 m and 5 m node points. Load on theSMART cable bolt was recorded between the nodes at 2 m to4.5 m and 4.5 m to 6.0 m, with the majority of load beingimposed between nodes at 2 m and 4.5 m.

Average displacement rates for the MPBX following Stope 5extraction were: ~0.5 mm/firing displacement between nodes at 4 m and 5 m,

and ~0.15 mm/firing displacement between nodes at 3 m and 4 m.

Average loading rates for the SMART cable followingStope 5 extraction were: ~400 kg/firing on cable for length between 2.0 m to 4.5 m, and ~100 kg/firing on cable for length between 4.5 m to 6.0 m.

The displacement and load changes occurred in the section ofground coinciding with the debonded section of the SMARTcable. This was expected as the bungee bolts do not form activereinforcement in this zone. Rather, they provide more passivereinforcement, designed to act as energy absorption anchors

when the rock mass is subjected to high ground motions. Thedebonded section of cable is from 1.0 m to 5.5 m into the rockmass and is in the region where the load and displacementoccurred. While the use of alternative reinforcing elements suchas Garford bulbed cables may help to reduce displacements, theresulting stiffer ground control would tend to fail when subjectedto high ground motions imposed from a large seismic event dueto its low energy absorption capacity. The design implications ofthe bungee cable are discussed further by Scott, Penney andFuller (2008).

A key consideration in the results from the MPBX and SMARTcable are the ongoing step change increases of displacement andload at each subsequent firing with little change of the stressfield. This was interpreted to be slow rock mass degradation dueto reduced clamping stresses acting on the rock mass, allowing areduction in confinement. The rock mass was able to relax anddisplace due to the use of debonded bungee cables.

The results from the RWE started showing some signs ofmovement following the firing of Stope 2 and reached themaximum change following the extraction of Stope 5. There



Stope

11

Stope

10

Stope

9

Stope

8

Stope

7St

ope

6

Stope

5

Stope

4

Stope

3aSt

ope

3

Stope

2

Stope

1

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

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Date

Change

inLoad

(t) 0.4m and 0.8m

0.8m and 2.0m2.0m and 4.5m4.5m and 6.0m6.0m and 6.4m6.4m and 7.1mStope Firing

CableDa

mage

CableDa

mage

FIG 6 - Reinforcement load change measured by the 930 West SMART cable during the extraction of Stopes 1 to 11.

Stope

11

Stope

10

Stope

9

Stope

8

Stope

6

Stope

5

Stope

4

Stope

3aSt

ope

3

Stope

2

Stope

1

Stope

7

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

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Date

Dis

pla

cem

ent(m

m) 6.0m and 5.0m

5.0m and 4.0m4.0m and 3.0m3.0m and 2.0m2.0m and 1.0m1.0m and headStope Firing

Cable Damage

FIG 5 - Rock mass movement measured by the 930 West MPBX during the extraction of Stopes 1 to 11.

was only minor creep in the instrument records beyond thatpoint (Figure 7). Unfortunately, a significant amount of datawas lost due to cable damage, limiting the analysis of theseinstruments. In a gross sense, most displacement occurred onthe 2 m RWE which was installed at a depth range of 1 m to 3m into the nominal 6 m wide pillar between the footwall driveand ore drive. Observations in the area did not reveal anydamage or movement in the footwall drive, so it is mostplausible that the rock mass was relaxing towards the openstope. The fact that displacements stopped soon after extractionand subsequent backfilling supports that interpretation. Thebackfill placement will provide a level of confinementminimising any further movement. As an aside to theobservations of RWE, raw strain change data gathered from theHI Cell showed an immediate response to the backfilling ofStopes 2 and 5 (Figure 8). That change has not been analysed interms of stress change.

There has been little observed change to the rock mass aroundthe footwall drives. Damage induced from stope firing, hasoccurred around the blasthole collars in the wall. Up to 1 m ofdamage was observed around some rings with all support in thearea being totally destroyed and requiring reinstatement.However the damage has been reduced in subsequent firings dueto the introduction of improved stemming techniques.

ALERT MECHANISMS FROM ANALYSISCommunication of the occurrence of stope firings and theexpected rock mass behaviour following those firings(ie seismicity, expected areas where rock mass change is likelyupon re-entry, etc) is the most effective alert mechanism formanaging risk associated with stoping activity at theBeaconsfield Gold Mine. Advance notification to all sitepersonnel and key stakeholders a minimum of one day before a



Stope

11

Stope

10

Stope

9

Stope

8

Stope

7St

ope

6

Stope

5

Stope

4

Stope

3aSt

ope

3

Stope

2

Stope

1

Back

fill

Back

fill

-800

-600

-400

-200

0

200

400

600

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Date

Mic

ro-s

train

Change

(mV)

REF

A90

A45

B45

B135

B90

C0

C90

C45

E90

F90

StopeFiring

FIG 8 - Raw strain change data recorded by the 930 West HI Cell during the extraction of Stopes 1 to 11 and illustrating the response tobackfilling of Stopes 2 and 5.

Stope

1

Stope

2

Stope

3St

ope

3a

Stope

4

Stope

5

Stope

6St

ope

7

Stope

8

Stope

9

Stope

10

Stope

11

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

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Date

Dis

pla

cem

ent(m

m)

RWE 1m

RWE 2m

StopeFiring

FIG 7 - Rock mass movement measured by the 930 West resistance wire extensometers during the extraction of Stopes 1 to 11.

planned stope firing is desirable. There is an understanding by allpersonnel that what is predicted or expected is not guaranteedand significant departures from this can and will occur. However,there is also an understanding within the workforce, that where adeparture from the predicted or expected (beneficial orotherwise) occurs, feedback will be provided once all data hasbeen processed and analysed.

Following each stope firing in the west zone of the mine, thereis a minimum 24 hour exclusion period for personnel access toany footwall drive within that stope block. That period is basedon Omori analysis of seismicity from previous stope blasts.Seismic flaring is also considered when analysing the seismicdata. The basis for this flaring analysis is: Moderate hazard: three times the 30 day average. Entry with

caution for specific required jobs only. High hazard: five times the 30 day average. Entry will be

excluded until the seismic flaring rate reduces and causes canbe fully assessed following the seismic management systemsoutlined in Hills and Penney (2008).

With the additional benefit of the installed instrumentation,additional rock mass and time dependant changes areincorporated into the re-entry analysis. Where rock mass changesrecorded by the instrumentation exceed site guidelines, theheading re-entry restriction will be extended. Typically, therestriction will remain in force until the measured changes fallbelow the guidelines.

In the event data records indicate a cable has been damaged,then a geotechnical inspection is carried out before repairs aremade to the cable. Once communications have been restored, thedata is analysed and appropriate actions effected. Typically,damage to the instrumentation cables will result in a loss ofdetail, but the quantum change recoded by the instrument is notcompromised.

Ultimately, it is expected that real time alert mechanisms willbe established, reporting seismic hazard zones and observedchanges from instrumentation in excess of predetermined levels.

CONCLUSIONSThe change in mining method in the west zone of theBeaconsfield Mine resulting from the Case for Safety reviewprocess requires ongoing monitoring to ensure those areasrequired for personnel access remain acceptably safe andaccessible for their entire designed life. Results from the firststope block extracted under this new method (the 940 WestBlock) indicated that low levels of rock mass deformation hadtaken place, and that load capacities of the installed groundcontrol was still within acceptable limits of the design criteria.Monitoring will continue to occur to ensure that the integrity ofthe excavations is maintained. As mining progresses deeper,continued analysis of instrumentation data may allow amodification of ground control designs in line with observedrock mass behaviour.

Time-dependent changes have already been identified andongoing analysis of this cause and effect relationship is takingplace. Ultimately, this will result in a more robust re-entryprotocol and increased understanding of the rock mass behaviouras mining progresses. It will also permit better ground controldesigns to be implemented.

Results to date confirm that the current ground control designsare working well within the design tolerances. Once fullextraction of the 940 West Block is complete, a detailed backanalysis will be undertaken to determine the nature of theresponse which the rock mass exhibited to stoping. This in turnwill be fed back into the design procedures to determine thereliability of the engineered ground control design in future. Theprocess will be iterative.

ACKNOWLEDGEMENTSThe authors acknowledge the management of Beaconsfield GoldNL for permission to publish this paper, and the assistance ofTop Rock Technologies and Coffey Mining in the design,installation and commissioning of these instrument clusters.

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geomechanical instrumentation in cost control underground mining,in Challenges in Deep and High Stress Mining (eds: Y Potvin,J Hadjigeorgiou and T Stacey), pp 317-325 (Australian Centre forGeomechanics: Perth).

Dutta, P K, 1985. Some recent developments in vibrating wire rockmechanics instrumentation, in Proceedings 26th US Symposium onRock Mechanics, pp 1043-1053 (South Dakota Schools of Mines andTechnology: Rapid City).

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Penney_The Use of Geotechnical Instrumentation to Optimise an Engineered Mine Design at Beaconsfield Gold Mine