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Rock Mass Classification is Only Part of the Answer to AssessRaise Boring

R Bertuzzi1 and J Wallis2

ABSTRACT

The stability of a raise bore is often assessed using the rock massclassification system published by McCracken and Stacey as amodification of the Q system. Often the assessment is done using a

spreadsheet to manipulate geotechnical log(s) of boreholes. This paperdiscusses the inherent difficulties with this approach. Recent work byothers to provide guidelines to the classification system is discussed.A recent example of the design and raise bore performance at Macraes

Gold Mine, New Zealand is presented. A key issue is that a visualassessment of the drill core and/or core photographs is necessary toground-truth the classification.

INTRODUCTION

This paper follows the design of a raise bore using the methoddescribed by McCracken and Stacey (1989), the raise bore rockquality classification system, QR. The system essentially attemptsto compare the likely ground conditions at a proposed site withthe experience gained at previous raise boring sites. For thispaper the proposed raise bore is a 350 m deep, 4 m diameterventilation shaft at Oceana Golds Frasers mine, Macraes Flat,New Zealand. It is assumed that the reader is familiar with theQR system so that it suffices only to reproduce its relationshipwith the maximum diameter of the raise bore, spanmax:

span RSR QRmax.= 2 0 4

where:

QRQD

Jn

Jr

Ja

Jw

SRF

Adjust Adjust

R

wall orienta

=

tion weatheringAdjust

RSR = 1.3 typically quoted value for medium- to long-termservice life for a raise bore.

A review of QR recently completed by Peck (2000) correctlynotes that the system, despite being 18 years old, is not yetsupported by a specific raise bore database but relies on the Qdatabase, which is primarily derived from civil tunnellingprojects. The system also does not address the concept ofstand-up time. Perhaps this paper can contribute to a specific

raise bore database.

GEOLOGICAL SETTING

The Macraes Flat area is within the chlorite zone of theextensively deformed Otago-Haast Schist Belt (Figure 1). Theschist is strongly foliated and depending on origin is either lightgrey, quartz rich and laminated (psammite) or dark grey to green,micaceous and finely laminated (pelite) (Angus, 1992). In theregion around the mine, schistosity dips at between 15 and 30towards the east. All directions quoted are relative to Macraesmine grid, which is 45 west of true north and approximately67.5 west of magnetic north.

The structural geology of the area is dominated by two mainorthogonal fault sets, striking to the north and to the east, and aminor set of westerly dipping structures referred to as rampshears (Figures 2 and 3). The main ore-bearing structure is theapproximately 100 m thick Hyde-Macraes Shear Zone (HMSZ).

Tenth Underground Operators Conference Launceston, TAS, 14 - 16 April 2008 191

1. MAusIMM, Principal, Pells Sullivan Meynink Pty Ltd, G3, 56 DelhiRoad, North Ryde NSW 2113. Email: [email protected]

2. Underground Geotechnical Engineer, Oceana Gold, PO Box 84,Palmerston, East Otago, New Zealand.Email: [email protected]

Macraes

FIG 1 - Regional geological map of the Otago Schist. The Hyde-Macraes Shear Zone (HMSZ) is truncated by the Footwall Fault (FWF or

simply FF). Also shown is the Rise and Shine Shear Zone (RSSZ), which is truncated by the Thomsons Gorge Fault (TGF).


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It is one of the north striking structures and is defined by therelatively continuous Hanging Wall Shear (HWS) and FootwallFault (FF). Tectonic displacement along the FF has been inferredto be hundreds of metres, with the strain associated with thisdisplacement probably concentrated within the HMSZ pelite,which could absorb strain more readily than the coarsely grainedpsammite.

The raise is to be bored through the psammite-rich rock massthat overlies the HMSZ (see Figure 4).

DATA

The specific data for the raise bore comprised the log and corephotographs of one borehole drilled at the proposed raise borelocation. This was complemented by the authors knowledge ofthe mining and rock mass conditions at Macraes since open pitmining commenced in 1991. Perhaps similar to other minesthe knowledge of conditions relevant to any undergrounddevelopment varies. There is a large database and extensiveknowledge of the lithology and rock mass structure; lessinformation in relation to groundwater, particularly for

underground mining; and limited knowledge on in stress.

192 Launceston, TAS, 14 - 16 April 2008 Tenth Underground Operators Conference

R BERTUZZI and J WALLIS

FIG 3 - Typical stereonet showing the main structures at Macraes within the Hyde-Macraes Shear Zone (HMSZ). Typically three sets occur

in the rock mass above the HMSZ; the schistosity, the steeply dipping east-west trending faults/joints and the easterly dipping faults.

FIG 2 - Typical cross-section through the Hyde-Macraes Shear Zone (HMSZ) at Macraes.

FIG 4 - Interbedded psammite and pelite through which the raise is

to be bored. Joint spacing is 60 - 200 mm, with surfaces rough to

slickensided. Minor narrow (


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QR

The calculation of QR from a large amount of data can berelatively straightforward using spreadsheets. The borehole log isoften entered directly into a spreadsheet-suitable format so thechallenges are to manipulate that data consistently and tointerpret what the data means geologically.

Although the Q system is widely published, it is the authorsexperience that it is often not rigorously applied. The process of

assigning values to the parameters used in calculating QR isdescribed below for the Frasers raise bore; however, themethodology is easily transferable to other sites.

RQD

The RQD is taken directly from the borehole log.

Joint set number, Jn

It is often difficult to determine the number of joint sets fromborehole core. In the case, the mapping of the open pit slopesindicates that there are three joint sets in the psammite-rich rockhigh above the HWS, possibly increasing to three joint sets +random joints to four joint sets in the rock mass approaching the

HWS. This means that Jn = 9; possibly 12 and 15 nearer theHWS (Figure 3).

Joint roughness, Jr and joint alteration, Ja

Conditional statements were incorporated into the spreadsheet toassign values for Jr and Ja.

Joint water reduction factor, Jw

The experience at Macraes is that groundwater flowspreferentially along large-scale structures. Hence, it is assumedthat large inflows with considerable outwash of infill will occuralong major faults (Jw = 0.33), medium inflows along otherfaults (Jw = 0.66) and minor inflows elsewhere (Jw = 1.0).

Stress reduction factor, SRF

The stress reduction factor (SRF) is dependent upon the ratio ofintact rock strength, c, to the major principal stress, 1, andwhether there is a fault that could influence the excavation. Theresults of in situ stress testing carried out by Oceana in 2005indicate that 1 is horizontal and approximately 1.5 times the

vertical stress. Field estimates of the intact rock strength, c,based on index testing are made on the logging sheets.McCracken and Stacey (1989) recommended using Kirstensapproach to estimate SRF from the c/1 ratio (Figure 5). Peck(2000) follows the same approach to produce an estimate forAustralian rock mass conditions.

QR adjustments

The following three adjustments are cumulative:

wall = 2.5QR, where QR > 1 and = QR where QR 1; orientation:

to assess the face stability = 0.85QR (one flat dippingjoint set), and

to assess the wall stability = 0.75QR (two steep dippingjoint sets);

weathering, if the rock mass was logged as unweathered

(UW), then 0.9QR else 0.75QR.

Results

The results of the assessment are presented in Figure 6 as a series

of histograms for the six parameters that form the Q-value. Thehistograms are aligned so that:


ROCK MASS CLASSIFICATION IS ONLY PART OF THE ANSWER TO ASSESS RAISE BORING

0

20

40

60

80

100

120

140

10 30 50 70 90

RQD

0

50

100

150

200

250

0.5 1 1.5 2 3 4

Jr

0

50

100

150

200

250

300

350

0.75123456810121320

Ja

050

100

150

200

250

300

350

400

0 .0 5 0. 1 0 .2 0 .3 3 0 .5 0 .6 6 1

Jw

0

50

100

150

200

250

300

350

400

0.5123469121520

Jn

from log

0

50

100

150

200

250

300

350

0.5122.557.510152050100

SRF

FIG 6 - Frequency distribution of the factors making up the Q value. Histograms plotting to the right side represent best qualities in each

case. It can be seen that Jn (number of joint sets) and Jr (joint roughness) contribute to lower Q values.

FIG 5 - Stress reduction factor (SRF) with respect to in situstress

and intact rock strength (Kirsten, 1986).


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the best quality plots to the right side; and the variability in the three ratios RQD/Jn, Jr/Ja and Jw/SRF

can be visualised.

The data shows most of the parameters to be fair to goodquality but the number of joint sets (Jn) and joint roughness (Jr)to be low-quality values. This is in keeping with the generalimpressions of the schist exposed at Macraes.

Strip-logs presented in Figure 7a show the variation of theratios RQD/Jn and Jr/Ja with depth. Difficult raise boringconditions are expected when these ratios result in a rating ofpoor or less as defined by McCracken and Stacey (1989). These

zones are highlighted by the horizontal band of pink crosshatching, which are shown at the depths: 0 - 5 m, 25 - 30 m,110 - 112 m, 155 - 160 m, 205 - 207 m, 211 - 212 m (first passwas 195 - 213 m refer to text for details), 251 - 254 m, 276 -288 m (first pass was 275 - 290 m) 325 - 347 m, 372 - 375 m.Not surprisingly, the expected zones of difficult conditionscoincide with substantial faults or shear zones in the core.

Strip logs showing the variability of QR with depth for the wall

and for the face of the proposed raise bore are given in Figure 7b.The minimum required QR values for 3.6 m (QR = 2.3) and 6 m(QR = 8) diameter raises are also plotted. The geotechnical risk isdeemed acceptable if the QR value exceeds the minimum



Fair Good

HWS, 361 mdepth

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30

RQD/Jn

faults & shearzones

Fair Good Very good

0

50

100

150

200

250

300

350

0.5 1 1.5 2 2.5 3 3.5 4

Jr/Ja

Dep

th(m)

Verypoor

Poor

400Verypoor

Poor

400

FIG 7a - Interpreted log of borehole DDH4811 showing RQD/Jn, faults and shear zones, and Jr/Ja.


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requirement by at least one class. So for a 3.6 m diameter raise,the actual QR should exceed four, classifying as fair quality rock

mass. Apart from the zones of poor RQD/Jn and Jr/Ja identifiedabove, this is achieved. However, for a 6 m diameter raise, theactual QR should exceed ten and classify the rock as goodquality. As can be seen in Figure 7, good quality rock mass is notconsistently achieved.

McCracken and Stacey (1989) further state that a five per centprobability of failure is acceptable for a raise bored ventilationshaft that has a nominal service life of ten years. Figure 8 showsthe distribution of Qwall. The area under the graph up to therequired minimum QR value for a given raise bore diameter givesan estimate of the probability of failure. Using this method, theprobability of failure of the 3.6 m diameter raise bore isapproximately ten per cent. This increases to about 30 per centfor a 6 m diameter raise bore. It is interesting that there appearsto be an inflection in the cumulative percentage curve at the

point coinciding with a diameter of 4 m.

GROUND TRUTH

The spreadsheet calculated QR using the above parameters on aper metre basis. Peck and Lee (2007) recommend that rollingaverage techniques be used to average rock quality over 3 mincrements to calculate lower bound QR values, suggesting thatzones less than 3 m wide do not significantly impact the stabilityof raise bore walls. However, QR and its parent Q, was designedto classify the rock mass and not necessarily arbitrary lengths ofcore. The advantage of using the spreadsheet is that largeamounts of data can objectively be assessed; the disadvantage isthat it may falsely impress the accuracy of the classificationsystem.

To overcome this disadvantage an alternative to the 3 m rollingaverage is suggested by the authors. The QR results should becompared with the actual rock mass, or as shown in this paper,the core photographs, for example Figure 9. By comparing the

QR strip logs with the core photographs it is possible to confirm



3.6m 6m

Very poor Poor Fair Good

Very

good

0

50

100

150

200

250

300

350

400

0.01 0.1 1 10 100

Qwall

3.6m 6m

Very poor Poor Fair GoodVerygood

0

50

100

150

200

250

300

350

400

0.01 0.1 1 10 100

Qface

FIG 7b - Interpreted log of borehole DDH4811 showing Qwall and Qface.


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potentially unstable zones. At Macraes this validation suggestedthat while there are several small zones where conditions areexpected to be poor based on the QR system, only two zones arelikely to provide issues for a raise bore: 276 - 288 m and 325 -347 m.

Figure 10 plots the lower bound QR values for the two zoneson the chart published by Peck and Lee (2007) summarising

some experience from Australia and Papua New Guinea. Thischart suggests that the risk of instability is an issue for these twozones, particularly for 325 - 347 m. The raise bore is not to be

stopped in these two zones and ground pretreatment is alsoconsidered for the 325 - 347 m section.

KINEMATIC

In addition to QR, a kinematic analysis is undertaken herein toassess the likely blocks formed by a raise bore. This analysisbetter considers the impact of defect orientation than a

classification system. An excavation in the schist at Macraes islikely to encounter loosened blocks of rock bounded by some, ifnot all, of the following defects (Figure 3):

steeply dipping faults/joints that dip either towards the southor towards the north; foliation parallel shears, that dip 15 - 20 towards the east; faults/shears that dip at 45 - 60 towards the east, typically

close to HWS; and

closer to the HWS, joints/faults that dip at 45 - 60 towardsthe west.

An estimate of the likely block sizes created by the

intersection of a nominally 4 m diameter raise bore and theabove defects has been made using the program Unwedge fromRocScience. The results, an example of which is presented in

Figure 11, suggest that:

Small loosened rocks will be common, particularly from thenorthern and southern walls.



FIG 9 - Core between 279 and 288 m depth.

3.6m 6m

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

0.1 1 10 100

Qwall

Frequency

Area under the curve up to Q=2.3

(3.6m raise) = 11%and up to Q=8 (6m raise)=34%

Inflection point?

FIG 8 - Distribution of Qwall. The cumulative percentage is shown in orange (thick dashed curve) with the area under the curve up to the

minimum required QR value corresponding to a 3.6 m raise and 6 m raise. The shape of the cumulative percentage curve implies an

inflection point where a 4 m diameter rise would plot. The apparent inflection point is not a unique design tool in itself but it does identify the

point at which a rapid increase in risk is evident.


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Blocks of rock 300 to 400 kg will form on the western wallsand be marginally stable (a factor of safety = 1.5 was chosento reflect the simple models able to be analysed withUnwedge). It is considered that some of these blocks will fall.

GROUNDWATER

Oceana conducted two rising head tests within the borehole. Thefirst test was from the collar to 220 m depth and suggested a very

low permeability, in the order of 10-8 m/s. The second test was

carried out over the full length of the borehole 0 - 377 m, but alarge inflow rate of between two and 2.5 litres per second fromthe fault(s) between 250 and 377 m, presumably the faulting at325 - 347 m depth and in particular the core loss zone from 326 -329 m, prevented detailed readings. The results of these two testsindicate that:

the general permeability of the schist rock mass is very low; however, high inflows are expected from the faulted zone

around 325 - 330 m depth.



FIG 11a - Side, top and perspective views of a 4 m shaft and the likely rock block sizes formed by the joints steeply dipping towards

the south, the foliation shears and the westerly dipping faults/joints.

FIG 10 - The lower bound QR values for the proposed 3.6 m diameter raise bore plotted on the Australian experience (Peck and Lee,2007). The 22 m section between 325 and 347 m depth plots in the unstable region. The 12 m section between 276 and 288 m plots

in a transition zone.


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CONCLUSIONS

Loosened blocks of rock are expected to occur during the raiseboring of the ventilation shaft. The majority of these blocks willbe small and most likely indistinguishable from the cuttings.Larger blocks up to several hundred kilograms are inferred to be

marginally stable within the wall of the raise and hence it isprobable that some will fail. The expectation is that these willfail within or immediately adjacent to those poor quality zonesassociated with major faults. There are approximately nine suchzones, most are less than a few metres long with the exception oftwo zones approximately ten and 20 m in length, 276 - 288 mand 325 - 347 m.

The raise bore should not be stopped or lowered when cuttingthough these zones. Ground pretreatment is planned for the lowerzone. It is expected that these zones will need to be shotcretedafter the excavation is completed.

At this stage, from a risk perspective, the maximum likelyraise bore diameter is 4 m.

High groundwater inflows are expected from the zone offaulting between 325 - 347 m, particularly from the core losszone of 326 - 329 m. The inflow needs to be controlled prior toraise boring. A cone of pressure grouting and a ring of drainholesaround this zone are likely to be required. This work would bebest done from the underground workings.

REFERENCES

Angus, P V M, 1992. The structural evolution of the Hyde-Macraes ShearZone at Round Hill, Otago, New Zealand, in Proceedings 26th Annual Conference (The Australasian Institute of Mining andMetallurgy: New Zealand Branch).

Kirsten, H A D, 1983. The combined Q-NATM system The design andspecification of primary tunnel support, South African Tunnelling,6(1).

McCracken, A and Stacey, T, 1989. Geotechnical risk assessment forlarge diameter raise-bored shafts, in Proceedings Shaft EngineeringConference, pp 309-316 (The Institution of Mining and Metallurgy:London).

Peck, W, 2000. Stability of raisebored shafts The limitations of theMcCracken and Stacey raisebore risk assessment method [online].Available from: .

Peck, W A and Lee, M F, 2007. Application of the Q-system toAustralian underground metal mines, presented to Eastern AustralianGround Control Workshop.



FIG 11b - Side, top and perspective views of a 4 m shaft and the likely rock block sizes formed by the joints steeply dipping towards

the north, the foliation shears and the westerly dipping faults/joints.
http://www.amcconsultants.com.au/2006_dec.asp?ID=1#1http://www.amcconsultants.com.au/2006_dec.asp?ID=1#1http://www.amcconsultants.com.au/2006_dec.asp?ID=1#1http://www.amcconsultants.com.au/2006_dec.asp?ID=1#1

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