Rock mass characterisation using the IQ' and RMR systemsCaracterisation des massifs rocheux utilisant les systernes 'a' et MRMR pour la conception

du depot souterrain des matenaux radioactifs au RUCharakterisierung der Felsmasse unter Verwendung der Q- und RMR-Systeme des UKRadioactive Waste Repository Design (Entwurf eines Endlagers fOrradioaktive Abfalle

in GroBbritannien)

COLIN G. RAWLINGS, WS.Atkins Consultants Limited, Woodcote Grove, Epsom, Surrey, UK (Formerly: NorwegianGeotechnical Institute, Oslo, Norway)

NICK BARTON, Norwegian Geotechnical Institute, Ullevaal Hageby, Oslo, NorwayALAN SMALLWOOD, W S.Atkins Consultants Limited, Woodcote Grove, Epsom, Surrey, UK

NICK DAVIES, UK Nirex Limited, Harwell, UK

ABSTRACT: Both the Q and RMR rock mass classification systems have been used to log a series of volcanic rocks and to subdividethem into (structural and) geotechnical zones ranging in thickness from 1m to 20m. In general the core was logged in units of 3mlengths (a core box). Some 640 observations of Q-values with the corresponding Rock Mass Ratings have been obtained enabling anew bilinear relationship between the Q-system, Barton (1974) and Rock Mass Rating method, Bieniawski, (1973).

RESUME: Les deux systemes de classification Q et RMR ont ete utilises pour enregistrer une serie de roches volcaniques et pour lessubdiviser en des zones (structurelles et) ge6techniques variant en epaisseur entre 1m et 20m. En general la carone a ete enregistreeen des unites de 3m de longueur (une boite de carotte), Quelques 650 observations des valuers Q, ainsi que les Rock Mass Ratingscorrespondants, ont ete obtenues, ce qui permet d'identifier un nouveau rapport bi-lineaire entre Ie systeme Q (Barton, 1974) et lamethode de Rock Mass Rating (Bieniawski, 1973).

ZUSAMMENFASSUNG: Sowohl das Q-als auch das RMR-System zur Klassifizerung von Felsmasse wurden dazu verwandt, eineSequenz von Vulkangestein zu erfassen und in (strukturelle und) geotechnische Zonen mit einer Dicke von I-20m einzuteilen. In derRegel wurden die Bohrproben in Einheiten von 3m Lange erfaflt (Bohrprobenkasten). Dadurch konnten an die 640 Erhebungen vonQ-Werten mit den entsprechenden Rock-Mass-Ratings durchgefiihrt werden, die es ermoglichten, einen neuen bilinearen Zusammenhangzwischen dem Q-system von Barton (1974) und der Rock-Mass-Rating-Methode von Bieniawski (1973) aufzuzeigen.

I. INTRODUCTION

The use of rock mass classification systems for designing supporthas gained increasing acceptance over the last 20 years. Sincetheir development in the early 70's ( e.g. the Q-system, Bartonet aI., 1974, and the Rock Mass Rating, Bieniawski,I?13,) thesesystems have been modified and updated e.g. Grn~s~ an~Barton, (1993), Barton and Grimstad, (1994); and Ble~law~kl(1974,1975,1976,1979,1989), to include more data, case ~Istor.lesand further refinement. The main rock mass classiticationsystems are the Rock Quality Designation (RQD), the RockStructure Rating (RSR), the Rock Mass Rating (RMR) and the Q-system.

The Rock Quality Designation (RQD) Index, Deere (1967) ~asintroduced almost 30 years ago at a. time when rock qU~h~information was usually only available from geo.lo.glst sdescriptions and the per cent core r~very. Although Originallydeveloped for predicting tunnelhng conditions a~d sup~rtrequirements, its application was ext:ended to CC?rrelationWith ~nsitu rock mechanical properties and, m the.I970 s to form a basicelement of other classification systems (i.e. the RMR and Q-system). . ped rnaiThe Rock Structure Rating (RSR) was develo mainly todetermine steel arch tunnel support under contract research by theU.S. Bureau of Mines, Wickham et al. (1972). The RSRcorrelation with support (steel ribs, shotcrete, and ~ock bolts)utilizes the "rib-ratio" which relates the actual support installationto a theoretical support spacing. The RSR system is based ~nTerzaghi's (1946) rock classification and support system and IS

a further development of it.The Q-system, Barton et al. (1974) .w~ d~veloped

independently of Wickham et al. (1972) and Bieniawski (1973)but it builds extensively on the RQD method of Deere et ~1.(1967) introducing five additional parameters in order to modifythe RQD value.

The Rock Mass Rating (RMR) method, also ~o~n as. ~eGeomechanics Classification, was developed by. Ble.mawskl m1973 and subsequently modified as. more ~e histories becameavailable and to conform With international standards andprocedures (Bieniawski 1974,1975,1976, 19?9, 1989). The RMRsystem also includes five parameters in additon to RQD.

Of the various rock mass classification systems the Q-systemand Rock Mass Rating (RMR) method are the ones which seemto be most widely used internationally. These two rock massclassification systems are now described in detail.

2. Q-SYSTEM

The Q-system of rock mass classification was developed inNorway in 1974 from the analyses of212 tunnel and cavern casehistories from Scandinavia and elsewhere. It has since beenupdated (Grimstad and Barton, 1993) being now based on afurther 1050 recent case histories where the rock support installedhas been correlated with observed Q-values. The Q-system isb~~ on a numerical assessment of the rock mass quality usingSIX different parameters: I. RQD (Rock Quality Designation); 2.Number of joint sets (J, = joint set number); 3. Roughness of themost unfavourable joint or discontinuity set (J,=joint roughnessnumber);4. Degree of alteration or filling along the weakest(m~t .critical ~or the type of excavati~m proposed) joint set(J,=Jo.mt alteration number); 5. ~8:ter inflow (J.=joint waterreduction number); 6. Stress condition (SRF=stress reductionfactor). .

These six parameters are grouped into three quotients to giveoverall rock mass quality:

X JT.. x .Tv.

J.. SRI"

(see the most recent updates of the Q-system given by Grimstadand Barton (1993), and Barton and Grimstad (1994) for thevalu~ assigned to these six pararnetrs). In the Q-system the rockquahty can range from Q=O.ool to Q= 1000 on a logarithmicr~k mass quality scale being subdivided into 7 classes (A to G)With recommended support based on the Norwegian Method ofTunnelling (Figure 1). This elaborate support method with itscommentary almost amounts to an expert system for groundsupport selection. The very detailed treatment of joint roughnessand. ~lteration, are the strongest features of the Q-system. Inaddition, the numeric value of Q ranges from 0.001(exceptionally poor - Class G) to 1000 (exceptionally good -

29

ROCK MASS CLASSIFICATION

e.s

h"'~!

" £I, ..8i~I

i;u.

,. II

O.OlM0.01 OJ)I 0.1 U t •• 10 0&0

Rock muI quality Q. ~ II 1;- II ~;F

"" '000

REINFORCEMENJ' CAnGORlES,\lU_2) Spot bo' •••••••3) Sya&ematie bo1tina. B4)S_lic: bo•••••

(and unreinfon:ed IbotCI'ete.~lO em), B(+5)

5) Fibre tCUlforcecllhotcrete and bo11inl. 5-9 em, Str+B6) Fibre reinrorctd shocereteand bollinl. 9-12 em. Sf,+B7) Fibre reinforced lhotcrete and bollin •• 12-15 em, Sf,+ B') Fibre reinforced shotcrete > 15 em,

reinfot'ccd ribl of sbotcme aDd bo1tinl. srr.RRS+ B9) Cast concrete linina, CCA

Figure 1. Simplified diagram for design of rock support based onthe Q-system, Grimstad and Barton, (1993).

Class A) which encompasses the whole spectrum of rock massqualities from heavily squeezing ground up to sound unjointedrock. Of the six parameters in the Q-system, J. (Joint AlterationNumber) and SRF (Stress Reduction Factor) are probably the twomost subjective. A correct assessment of the Joint AlterationNumber (J.) is probably the most subjective aspect of the Q-system and requires proper training in the use of the system.

Grimstad and Barton (1993) have modified some of the ratingsfor the Stress Reduction Factor. This was done to bring rockburst cases into the correct bolting and fibre reinforced shoteretelocation on the chart. The SRF rating in rock affected by highstress is difficult to estimate by visual observations. If possibleSRF should be estimated by the ratio between unconfinedcompressive strength (CT,) and the major principal stress (CT,), orby the ratio between the estimated tangential stress (CT9) and thecompressive strength (CT.).

Grimstad and Barton (1993) note that for squeezing rock furthercase records are required before provision of specific SRF valuescan be made but provisionally recommend SRF values in therange 5 to 10 when CTBICTc ratios are in the range from 1 to 5 whileSRF values as high as 10 to 20 will be applicable when the CTBICTcratio exceeds 5. Singh (1993) has confirmed that squeezing mayoccur when the overburden H (metres) exceeds 350 Q'13.He alsoproposed that the compressive strength of the rock mass can beexpressed as 7"YQ'13 where "y is the rock density in gm/cc.

The original (1974) Q-system recommends predominantly meshreinfored shoterete and rock bolts or dowels as permanentsupport. In the recent update, Grimstad and Barton (1993) showhow wet mix, steel fibre reinforced, sprayed concrete (Stfr)together with rock bolts has become the main components ofpermanent rock support in underground openings in Norwaysince the early 1980s. In addition, in extremely poor rock massquality a concept using rebar steel reinforced sprayed concreteribs in addition to S(fr) and rock bolts has been developed whichhas been actually replacing cast concrete linings as permenentsupport over the last few years.

3. ROCK MASS RATING

The Rock Mass Rating (RMR) method was developed byBieniawski in 1972-1973. It is based on relatively large openingsusing both steel ribs, rock bolts, and shoterete. Originally 49case histories were investigated in 1973, followed by 62 coalmining case histories that were added by 1987. By 1989 theRMR method of predicting stand up times was based on 351 casehistories. The RMR method is based on six parameters used toclassify the rock mass: I. Uniaxial compressive strength of rockmaterial CT,; 2 RQD; 3. Spacing of discontinuities; 4.Conditionof discontinuities; 5. Groundwater conditions; 6. Orientation ofdiscontinuities.

In the RMR method ratings are assigned to each of these

parameters i.e (RMR) = [1 +[2]+ [3]+[4]+[5]+[6](see the most recent update, Bieniawski (1989), for the ratingsassigned to each of these six parametrs). The rock mass quality(RMR) can range from <20 (very poor rock) to 81-100 (verygood rock) and is subdivided into 5 classes (I to V). To applythe Geomechanics Classification the rock mass is divided into anumber of structural regions so that certain features are more orless uniform within each region. A (conserative) relationshipbetween the stand-up time and span for various rock massclasses, accordi.ngto the RMR method, is given in Figure 2. TheRock Mass Rating Method has undergone several changes sinceits introduction in 1973 (1974 -reduction of classificationparameters from 8 to 6; 1975-adjustmentof ratings and reductionof recommended support requirements; 1976 -modification ofclass boundaries to even multiples of 20; 1979-adoption of ISRM(1978) rock mass descriptions and 1989 -detailed subdivision ofjoint condition). In order to avoid confusion care should be

ld lyt lOy't •••• lmo30

20

108

Er:~II)

'8a:

10-

-----4-.-.- ..

10 1 102 103 104

Stand·up Time, hr.

Figure 2. Relationship between the stand-up time and span forvarious rock mass classes, according to the GeomechanicsClassification (RMR method).

taken to use only the latest version of the system (currently thatpublished in 1989).

The RMR method has the advantage that its six parameters arerelatively easy to estimate and the RMR value is formed simplyon the basis of adding the ratings for the various parameters.However this limits the range of materials over which the systemcan be applied. Tallon (1982) found that the RMR methodexhibits a predeliction for the central class, i.e, fair rock. Kirsten(1987) notes that this problem can be avoided by not hesitatingto assign values for the parameters across their full range. TheRMR system considers RQD and joint spacing separately, bothof which are measures of block size which is thereforeoveremphasised (receiving up to 50% of the total rating) at theexpense of other parameters which may have a greater influenceon engineering properties. In addition, Bieniawski (1989)recommends that where less than three joint sets are present thejoint spacing ratings should be increased by 30% which puts evenmore emphasis on block size. The most recent update, Chart E -Guidelines for Classification of Discontinuity Conditions(1989).gives a more detailed explanation on how to treat the conditionof joints, i.e, discontinuity persistence, separation, roughness,infilling and weathering are included. The output of the RMRsystem tends to be rather conservative which can lead tooverdesign of support. Although originally applied to tunnellingin rock, subsequent applications and extensions feature the use ofthe RMR Method in hard rock mining, coal mining, rock slopestability, foundations in rock, rock boreability and others.

4. CORRELATIONS BETWEEN RMR AND Q INDEX

Various authors have put forward correlations between the Q-system and Rock Mass Rating. These include:Bieniawski (1976) Rutledge & Preston (1978)RMR = 9.OlnQ + 44 RMR = 5.91nQ + 43

Moreno (1980)RMR = 5.41nQ + 55

Cameron-Clarke and Budavari (1981)RMR = 4.61nQ + 56 ± 19 (drill

30

Abad et al. (1984)RMR = lO.5lnQ + 42

core)RMR = 5.01nQ + 61 ± 27 (insituresults)

Kaiser et al. (1986)RMR = 8.71nQ + 38 ± 8

Trunk and Homisch (1990)RMR = 9.lInQ + 45 ± 6

These correllations have been plotted in Figure 3. It should benoted that these correlations have been formulated during the

90 Bianiawski

80RuUadge

& Preston

70Moreno

Csmeron·Clsl1<a

a: 60& Budavari

~ Abed alai.

a: 50 Keiser at &I.

Tn.nk&

40 Honisch

30

28.01 0.1 1 10 100 1,000Q-value

Figure 3. Published correllations between Q and RMR.

various stages of the developmentlr:nodificatio~ ?f ~e RMRmethod and this may partly explain the variation m thesecorrelations. Another factor is that the ratio afa, and the relatedSRF in the Q-system make direct comparison of the two numbersQ and RMR difficult when significant SRF values are inoperation in the Q-system. In order to account f?r the effect ofthe SRF factor on the final Q-value and Its consequentcomparison with Q an "unfactored" SRF factor of 1 .has ~nused to produce an "unfactored "Q"""",-, for comparl~on WIthRMR. In this paper both Q and Q"""""",!are corr~lated WIth RMRto investigate the Q and RMR correlatIOns obtained.

5. ROCK MECHANICAL DATA COLLECTION

A recent comparison has been .made of. Q and RMR. The Qand RMR logging was carried out m three non-pw:all~1drillholes, in unit lengths of about 3m .wlth~nstructural/geotechnical zones in the rock mass ranging mthickness from 1m to 20m. Some 30m to 40m of core wasobserved at anyone time to enable assessments to be pr~rlymade. Approximately 1700I? ~f rock. core was logged within onegeological formation compnsmg various volcanic rocks.

6. CORRELATIONS OBTAINED

A comparison of the Q-system an~ th~ Rock M!lSs ~tingmethod data obtained gave the followmg hnear relatlo~hlps:

RMR = 6.5lnQ + 48.6 for unfactored Q values (i.e. SRFvalue of 1 used) .RMR = 6.lInQ + 53.4 for Q values With relevant SRFfactors

As expected the addition of the SRF factor has most effect atthe lower end of the Q scale leading to many more valu~ ofQ < 1. In addition, there is the poorest fit to the hnearrelationships in this range where Q < 1. .

In order to obtain a better fit of the correlations between Q andRMR bilinear relationships were applied and the results were:for Q~ 1 RMR = 1O.31nQ + 49.3 for unfactored Q values

(i.e. SRF value of 1 was used)for Q> 1 RMR = 6.21nQ + 49.2 for unfactored Q values

(i.e. SRF value of 1 was used)for Q~0.65 RMR =6.6InO + 53.0 for 0 values with relevant

SRF valuesfor Q>0.65 RMR = 5.71nQ + 54.1 for Q values with relevant

SRF factorsThe bilinear relationship for the unfactored Q values fitted very

well with the data and this relationship has been plotted in Figure4.

90Bi-linear

80Rutledge

70 & Preston

Morenoex: 60~ Abad etal.ex: 50

Cameron-Clarke& Budvari

40Kaiser et al.

30Linear

200.01 0.1 1 10 100 1,000

Q-valueFigure 4. Plot of linear and bilinear relationship of unfactoredQ against RMR together with published correllations.

7. CONCLUSIONS

A brief review of the Q-system and Rock Mass Rating methodhas been made. Both rock mass classification approaches haveadvantages and disadvantages and consequently in many rockmass classification exercises it is recommended that both systemsare used. The published (linear) correlations between Q valuesand Rock Mass Ratings have been reviewed and there appears tobe a wide range of published correlations. Based on recent rockmass classification data a bilinear relationship seems to be mostappropriate and the concept of using unfactored Q values hasenabled improved correlations. As noted by Kaiser et al. (1986)correlations between Q and RMR can depend on the choice ofdependent variable and the method used to obtain a relationship.This needs to be investigated further. The authors wouldwelcome the publication of more data which will add to improvedcorrelations between the two rock mass classification systems invarious geological formations and at various sites.

REFERENCES

Barton, N., Lien, R., and Lunde, J. 1974. Engineering classification of rockmasses for the design of tunnel support. Rock Mechanics, Vol.6, No 4,pp 189-236.

Barton, N., and Grimstad, E. 1994. The Q-sysletD following twenty years ofapplication in NMT support selection. In: 43rd Geomechanics Colloquy,Salzburg, Austria.

Bieniawslri, Z.T. 1973. Engineering classification of jointed rock masses.Transactions of the South African Institution of Civil Engineering, Vol 12,pp 335-344.

Bieniawslri, Z.T. 1974. Estimating the strength of rock materials. J.of SouthAfrican Institute Mining and Metallurgy, Vol. 74, No.8, pp. 312-320.

Bieniawslri, Z.T. 1975. Case studies: Prediction of rock mass behaviour by theGeomechanics classification. In: Proceedings 2nd. Australia-New Zealandconference on geomechanics. Brisbane, Australia, pp. 36-41.

Bieniawslri, Z.T. 1976. Rock mass classifications in rock engineering. In:Exploration for Rock En~ineering, Z.T.Bieniawslri ed., A.A.Balkema,Johannesburg, South Africa, pp. 97-106.

Bieniawski, Z.T. 1979. The geomechanics classification in rock engineeringapplications. In: Proceedings of the 4th International Congress in RockMechanics, ISRM, Montteux, Balkema, Rotterdam, Vol.2,pp 41-48.

Bieniawski, Z.T. 1989. Engineering Rock Mass Classification -A CompleteManualfor Engineers and Geologists in Mining, Civil and PetroleumEngineering, Wiley-Interscience Publication.

Bieniawski, Z.T. 1994. Personal communication.Grimstad, E. and Barton, N. 1993. Updating of the Q-syStelU for NMT. In:Proceedings of the international symposium on sprayed concrete - modem useof wet mix sprayed concrete for underground support, Fagernes 1993.

31