Assessment of rock slope stabi'lity using theRock Mass Rating (RMR) system

By C M ORR1, Member

ABSTRACT

The Rock Mass Rating (RMR) and Rock Mass Strength (RMS)classification systems for jointed rock masses are briefly reviewed, withparticular reference to their use in slope stability studies. A correlation isprovided between the results obtained from the two classifications and anequation is presented tentatively defining the RMR value for long-termstable slope angles. The validity of the equation is discussed in generalterms in the context of slope stability problems commonly encountered inWestern Australian open pit gold mines.

KEYWORDS: open pits, rock mass classifications, slope angles, slopestability, swelling clays.

INTRODUCTION

The Rock Mass Rating (RMR) system, also known as theGeomechanics Classification, was developed in 1973 as a means ofassessing permanent rock support requirements for undergroundexcavations (Bieniawski, 1973). Initially applied to civilengineering projects (tunnels and underground caverns),modifications to the original classification resulted in it being usedfor mining applications, rippability studies, dam foundations andslope stability (Bieniawski, 1988).

A similar rock mass classification, based on the RMR Systemconcept and known as the Rock Mass Strength (RMS) system, wasdeveloped by geomorphologists and used to correlate 'rockstrength' with stable slope angles of natural rock outcrops (Selby,1980; Moon and Selby, 1983). This classification, althoughapparently less well-known than its engineering contemporary, hasobvious applications to rock slope stability studies associated withmining and civil engineering projects.

The purpose of this paper is fourfold, namely to

1. briefly summarise the RMR and RMS systems and their use inslope stability studies,

2. correlate results from the RMR and RMS systems,

3. provide an equation that tentatively defines the relationshipbetween stable slope angles in jointed rock and RMR systemvalues, and

4. discuss, in general terms, the validity of using the RMR versusslope angle relationship in the context of slope stabilityproblems commonly encountered in Western Australian openpit gold mines.

THE ROCK MASS RATING (RMR) SYSTEM

A comprehensive description of the RMR system and itsapplication to engineering projects has recently been published byBieniawski (1988).

The RMR system classifies jointed rock masses using thefollowing six parameters.

1. uniaxial compressive strength ofrock material

2. rock quality designation (RQD)

3. spacing of discontinuities

4. condition of discontinuities

5. groundwater conditions

6. orientation of discontinuities.

Ratings are allotted to each of the above parameters, dependingon their actual measured values. The first five ratings are summedto yield a basic rock mass rating. Adjustments are subsequentlymade to the basic rating for the influence of discontinuityorientations to give a final (adjusted) rock mass rating (RMR)value. This ranges from 0 to 100 with high RMR values indicatingbetter rock mass conditions. Five rock mass classes aredistinguished on the basis of the final rock mass ratings (Table 1).

TABLE 1Rock mass classes determined/rom total ratings

(after Bieniawski (1988).

RMR Value Class Description

<20 V Very poor rock21 - 40 IV Poor rock41 -60 III Fair rock61 - 80 II Good rock>80 I Very good rock

Output from the classification is in the form of average stand-uptime for unsupported tunnel roof spans and cohesion and frictionangles for the rock mass.

Slope stability applications

TABLE 2Pit wall angles versus RMR class (after Laubscher, 1975).

The first published application of the RMR system to slope stabilitywas by Laubscher (1975) who used adjusted rock mass classes toprovide an experience-based guide to slope angles applicable toopen pit mining. Laubscher's proposed relationship is given inTable 2.

1.

2.

3.

Principal, George, Orr and Associates; Associate, James AskewAssociates (pty) Ltd (Australia).Chris Orr was born in Zimbabwe and graduated from the Universityof Natal, South Africa in 1973 with a MSc degree in Geology. Heworked with the Council for Scientific and Industrial Research, theGeological Survey and as consulting engineering geologist in SouthAfrica, before emigrating to Australia in 1986. He is theauthor/co-author of 14 publications dealing with various aspects ofengineering geology, slope stability and rock mechanics.Original manuscript received September 1991.Revised manuscript received March 1992.

Adjusted Class 1Pit Wall Angle (0) 75

2

65355

445

535

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Steffen (1976) used the average values of rock mass cohesionand friction angle, derived from the RMR system, to determine thestability of 35 slopes (of which 20 had failed) with respect tocircular failure. Results were largely inconclusive, although somestatistical trend was found between factors of safety (FOS) andincidences of failure. FOS values of up to 1.2 existed for failedslopes while some apparently stable slopes exhibited FOS values of0.7. Despite this, the general RMR classification approach wasdescribed as being useful as a preliminary investigative tool forslope stability studies.

Hall (1985) described a graphical correlation between RMRvalues and slope angles in jointed rock masses. It was used toestimate stable slope angles for railway cuttings in South Africa.The correlation was provided for slope heights of less than 20 m,excavated within rock masses of RMR ~ 20 ie, poor quality orbetter. A recommended design line was provided, the equation forwhich has subsequently been calculated as

Slope angle = 0.65 RMR + 25 .... (1).

for slope height < 20 m, and RMR ~ 20.

More recent applications of the RMR system have involvedforecasting typical stability problems and determining potentialslope support measures (Romana, 1985 and 1988). The originalRMR system was modified, taking into account the influence ofexcavation methods on slope stability and providing a more

detailed description of joint favourability with respect to potentialinstability. A similar approach was described by Singh, Elmherigand Sunu (1986) in assessing the slope stability of two granitequarries.

Robertson (1988), using a modified RMR system. showed thatwhen the RMR exceeds 40, slope stability is determined by theorientation of and strength along discontinuities. Where the ratingis less than 30, slope failure may occur through the rock mass atany joint orientation.

Figure 1 summarises the existing RMR versus slope anglerelationships proposed by various authors.

THE ROCK MASS STRENGTH (RMS) CLASSIFICATION

A Rock Mass Strength (RMS) classification was developed bySelby in 1980 and used by geomorphologists to explain therelationship between 'rock mass strength' and the long-term stableslope angles of natural rock outcrops (Selby, 1980; Selby, 1982;Moon and Selby, 1983; Selby, 1987).

The RMS classification uses similar (but not identical) inputparameters to the RMR system.

1. strength of intact rock2. state of rock weathering

DESCRIPTION®

VERY BAD:SOIL-LIKEFAILURES

BAD: PLANAROR BIG WEDGE

FAILURES

PARTIALLYSTABLE: SOME STABLE: SOME

~~::cJEtJ,:~/TC:ls BLOCK FAILURES

VERY GOOD:COMPLETELY

STABLE

1---­__J

~LAUBSCHER{1975}

2 HALL (1985)3 ROMANA{1988}

S~O.65 RMR + 2570° {RMR~20: SLOPES oc20m HIGH}

60°enu.i 50°...JCJZcCw 40° (f)£L0 ------...Jen 30°

20°

10°

80°

90°---,---------------------------/

RMR 10 20 30 40 50 60 70 80 90 100

ROCKCLASS

CLASS V: VERYPOOR ROCK

CLASS IV:POOR ROCK

CLASS Ill:FAIR ROCK

CLASS 11:GOOD ROCK

CLASS I: VERYGOOD ROCK

FIG 1 - Summary of existing RMR versus slope angle relationships.

26 No21992 The AusIMM Proceedings

3. joint spacing

4. joint width (aperture)

5. orientations of joints with respect to the slope6. joint continuity

7. outflow of groundwater.

Each of the parameters are assigned ratings which are thensummed to provide the rock mass strength (RMS).

Correlations are provided between RMS values and natural slopeangles. These were measured from long slope profiles. of up toseveral hundred metres in height, developed on a variety ofigneous, sedimentary and metamorphic rock outcrops in Antarctica,New Zealand, South Africa and Namibia. Rock outcrops werejointed. unbuttressed and lacked major continuous defects whichwere critical for slope stability.

A straight line equation (with a correlation co-efficient of 0.88).linking RMS and slope angle (S). was proposed for the Antarcticand New Zealand examples (Selby. 1980):

RMS = 49 + 0.42S ...... (2)

Selby (1980) claims that by using equation (2), the inclination ofa slope can be estimated from the RMS value with a standard errorof ±5.1°.

South African and Namibian examples (Moon and Selby. 1983)were presented in a graphical form which also showed a straightline correlation between RMS and slope angle (S). An equation forthis correlation was not presented in the original paper butexamination of the graph shows it to be identical to equation (2).

APPLICABILITY OF THE RMR SYSTEM AND THE RMSCLASSIFICATIO TO SLOPE STA8ILITY STUDIES

Although the RMR System is arguably the most well known andapplied method of classifying rock masses for engineeringpurposes, it has a number of shortcomings. particularly when it isapplied outside the area of tunnel support design, for which it wasoriginally intended. These include its lack of sensitivity to changesin the classification parameters and its predilection for the central(fair rock) class. (Kirsten. 1988); overemphasis of the intact rockstrength rating given to the classification. (Kirsten. 1988; Hall,1985); and the lack of emphasis given to rock joint shear strengthsand groundwater conditions, considering their importance to rockslope stability (Hall, 1985).

Two additional criticisms that may be added are

1. the lack of correlation between RMR values. slope angle andslope height. The latter two variables are linked and are offundamental importance in any attempt to correlate rockquality with stable slope angles. and

2. the influence of time upon the stability of rock slopes isignored. Vanarsdale. Costello and Marcelletti (1989) show, forexample, that a progressive decrease in pit wall angles over atime span of ten to 30 years occurred in coal-bearing strata inthe USA. Other examples undoubtedly exist but are not wellreported in the literature.

Although a number of the above criticisms may also beapplicable to the RMS classification, the published data (Selby.1980) relates rock mass strength to slopes of varying heightsexposed over periods of geological time (ie. slopes which may beregarded, in the engineering sense, as being in a state of limitingequilibrium). The RMS classification is therefore extremelyvaluable in predicting long-term stable slope angles. It can also beused to provide a tentative correlation with the more widely known.engineering-oriented, RMR classification and hence a relationshipbetween RMR values and long-term stable slope angles for jointedrock.

Correlation between RMS and RMR classifications

RMS data published by Selby (1980) have been re-analysed anddescribed in the form of the RMR system (using the 1988 version

ASSESSMENT OF ROCK SLOPE STABILITY USING THE RMR SYSTEM

100

90.....Cl)Cl)

80Ol~

:i 70III~IIIC eoQl

iD RMR 6 2.2 RMS-130....50 ' 6 0.88

Cl~.....

40<a:Cl)

30Cl)

<::lE:le: 20()0

10a:

0+-,--,--.------r---,----+-,--,--.-----1

o 10 20 30 40 50 eo 70 80 90 100

ROCK MASS STRENGTH (Selby. 1980)

FIG 2 - RMR versus RMS correlation.

of the latter by Bieniawski (1988». Some scaller occurs (Figure 2)but the general relationship for 15 data sets was found to be

RMR = 2.2 RMS - 130 ......(3)

with a correlation co-efficient of 0.88.

Prediction of stable slope angles from RMR values

Figure 3 shows a compilation of RMR values (derived usingequation (3» and slope angle data obtained from the informationpublished by Selby (1980) and Moon and Selby (1983). Slopeheights have been arbitrarily subdivided into the range: less than 10m. 10 - 20 m. 20 - 40 m and greater than 40 m. RMR versus slopeangle relationships for nine failed slopes less than 20 m high.extracted from Hall (1985). are also shown.

The graph shows that

1. the general RMR versus slope angle relationship is non-linear.

2. no data are available for slopes with an RMR of less than 20 orgreater than 77.

3. a lower-bound equation filled to the data gives a relationshipbetween slope angle (S) and RMR value of

S = 35 In (RMR) - 71 .....(4)

This relationship is proposed as the limit of long-term stabilitythat can realistically be expected for slopes up to 50 m highand exhibiting RMR values of between 20 and 77 (ie, poor togood rock). It must be stressed that the data are onlyapplicable to southern African. New Zealand and Antarcticexamples. A separate study in Australia is needed to verify therelationship under local conditions and to establish therelationship between RMR and stable slope angles forshort-time periods, ie open pit operational life spans asopposed to geological time. and

4. the limited data and their scatter preludes meaningful analysis.at this stage, for relationships between slope angle and slopeheight for rock masses of similar quality.

Although equation (4) gives the impression that steep stableslopes can be formed in jointed rock masses of mediocre quality, itmust be stressed that RMR values in excess of 40 cannot be

The AuslMM Proceedings No21992 27

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ROCK QUALITY (Blenlawskl, 1988)

GOODFAIRPOORVERY POOR

90 A_ _ ? LIMIT OF LONG-TERM

80 STABILITY?0

70A

0 60 SLOPE ANGLE =35In(RMR}-71w for RMR 20-80-JC'

50z4(

w40 A FAILED SLOPES <20m HIGH (Hall, 1985)D.

0 0 .. tOm HIGH }-J 10-20 HIGH NA TURAL SLOPESet)30 I li. m (Selby, 1980;

I x 20-40m HIGH Moon cl Selby, 1983)

20? • 40-50m HIGH

10

10 20 30 40 50 60 70 80 90 100

ROCK MASS RATING (RMR)

FIG 3 - RMR versus slope angle relationship.

achieved when very unfavourable Jomtmg (from a stabilityviewpoint) occurs in even the highest quality rock mass. Thissituation arises as a consequence of a rating adjustment of -60 beingapplicable for 'very unfavourable' joint orientations (Bieniawski,1988). Very unfavourable joint orientations must be analysed asspecific cases.

Excavation methods also significantly influence the stability ofartificially formed slopes (Romana, 1988) and this aspect shouldalways be considered when assessing realistic wall angles.

EXPERIENCE FROM OPEN PITS IN WESTERNAUSTRALIA

Common slope stability problems encountered in WesternAustralian open pit gold mines have been described by Rosengrenand Swindells (1988), Swindells (1990) and Orr, Swindells andWindsor (1991). Based on data in these publications and theauthor's experience, the applicability of using the RMR versusslope angle relationship (equation (4» for estimating stable pitslope angles is discussed below.

In very general terms, the geology exposed in typical WA goldmine pits may be subdivided into three broad categories.

I. a surface lateritised (caprock) layer, commonly extending todepths of 2 to 5 m and occasionally 8 m

2. an intermediate layer of weathered rock (saprolite), extendingto depths of up to 80 m

3. a lowermost layer of fresh (unweathered) rock

The uppermostlateritised layers exhibit RMR values in the range45 - 70 (fair to good rock). Stable slope angles of 65° to 75° arecommonly achieved over the life of a typical open pit (one to fiveyears). Failures within this layer are rare and only normally occuras a result of undercutting, following slope failures within theunderlying weathered rock.

The intermediate weathered rock layer, commonly referred to as'oxidised rock' or 'saprolite', generally exhibits structurallycontrolled instability in the form of planar, wedge or topplingfailures along relict joints. The high degree of rock weathering andclosely jointed nature of the material, often combined with adversejoint orientations, invariably give rise to very poor to poor rockconditions (RMR ~ 20 to 40). Stable overall slope angles above45° are seldom achieved in pit walls with lives greater then one totwo years, although steeper slopes are commonly cut for miningoperations. Increased pore water pressures, following heavyrainfall or abandonment of pit dewatering procedures, are attributedto as being the cause of most failures.

Instances also occur where wall instability is exacerbated by thepresence of swelling clay minerals, developed by weathering and/oralteration processes of the original tuff, basalt or amphibolite rock.These clays, which occur within the rock material and also as ajoint infill, swell and shrink with varying moisture regimes, givingrise to loosening of joint-bounded blocks of rock within the slopes.This loosening is often the precursor to progressive (ravelling type)slope failures. An example of this ravelling process, recorded froma 35 m high slope excavated within an apparently fresh doleritecontaining swelling clays, is described by Orr (1979). Slopesexcavated in rocks containing swelling clays should be regarded

--28 N021992 The AuslMM Proceedings

with caution, and their stability, both in the long- and short-tenn,cannot adequately be predicted by classification methods.

Depending on the inclination of rock jointing and its effect onwall stability, fresh rock commonly exhibits RMR values ofbetween 40 and 60. A moderate to steeply dipping, pronouncedrock foliation, is common. Most slope instabilities occur as planarfailures in the footwall slope (where foliation is undercut) ortoppling failures in the hangingwall. Stable slopes with overall wallangles of 55° to 60° are generally achieved. Reinforcement, in theform of cable bolting, has often been used to increase stability ofsteeper slopes.

The above general examples are in broad agreement with theRMR versus slope angle relationship given in equation (4), anddemonstrate its usefulness in providing a first approximation ofstable slope angles for slopes up to 50 m high.

CONCLUSIONS

Using published data, a tentative correlation has been madebetween the results of the RMR system (a relatively simple andwell known rock classification used primarily in undergroundengineering practice) and the RMS classification (a similarclassification used for geomorphological studies).

The relationship between stable long-tenn slope angles and RMRvalues, derived from the original RMS classification database, isnon-linear. The limited data available exhibit a degree of scatterthat precludes it being used at this stage for meaningful analysis ofthe influence of slope height on the stability of various quality rockmasses. This situation should improve as more data becomeavailable.

Some typical general examples of the applicability of the RMRsystem to slope stability assessments carried out in WesternAustralian open pit gold mines demonstrate that the overall conceptis realistic. However, considerably more field examples arerequired to improve current knowledge on the effect of time onslope stability and the relationship between slope height, slopeangle and rock quality. It is hoped that additional study and thecollection and correlation of additional data by other workers willimprove the correlation described in this paper.

It is considered that the RMR versus slope angle relationshipdescribed here is useful in providing a first approximation ofstable long-tenn slope angles that can be excavated within jointedrock masses for slopes up to 50 m high. In common with mostclassification approaches however, it should not be used solely fordetailed design purposes.

ACKNOWLEDGEMENTS

The interest shown and advice provided by Messrs C Windsor,Senior Rock Mechanics Engineer, Rock Mechanics ResearchCentre, CSIRO, Perth and M Lee, Principal Geotechnical Engineer,James Askew Associates Pty Ltd, Melbourne is acknowledged.

ASSESSMENT OF ROCK SLOPE STABILITY USING THE RMR SYSTEM

REFERENCES

Bieniawski, Z. T., 1973. Engineering classification of jointed rock masses.Trans. S. Afr.lnst. Civ. Engrs., 15:335-344.

Bieniawski, Z. T., 1988. The Rock Mass Rating (RMR) system(Geomechanics Classification) in engineering practice, in RockClassification Systems for Engineering Purposes, (Ed. L Kirkaldie), pp17-34, Publication SPT 984 (ASTM: Philadelphia).

Hall, B. E., 1985. Preliminary estimation of slope angles, in Symposium onRock Mass Characteristics. pp 120-121 (South African National Groupon Rock Mechanics: Johannesburg).

Kirsten, H., 1988. Discussion to 'Rock Mass Rating (RMR) system(Geomechanics classification) in engieering practice' by Z T Bieniawski,in Rock Classification Systems for Engineering Purposes, (Ed. LKirkaldie). p 32, Publication SPT 984, ( ASTM: Philadelphia).

Laubscher, D. H., 1975. Class distinction in rock masses. Coal, Gold andBase Minerals ofSouthern Africa, pp 37-50.

Moon, B. P. and Selby, M. J., 1983. Rock Mass Strength and scarp forms inSouthern Africa. GeograflSika Annaler. Ser.A, 65:135-145.

Orr, C. M., 1979. Rapid weathering dolerites. Trans. S. Afr. Inst. Civ.Engrs., Vo121, No 7:161-167.

Orr, C. M., Swindells, C. E and Windsor, C. R., 1991. Open pit topplingfailures experience versus analysis. Proceedings of 7th InternationalConference on Computer Methods and Advances in Geomechanics (Eds.Beer, Boorker and Carter), pp 505-510 (Balkema: Rotterdam).

Robertson, A. M., 1988. Estimating weak rock strength, in AlME - SMEAnnual Meeting, Phoenix, Az, 1988, preprint No 88-145 (AIME-SME:

Romana, M., 1985. New adjustment ratings for application of Bieniawskiclassification to slopes. Int. Symp. on the Role of Rock Mechanics,Zacatecas, Mexico, pp 49-53.

Romana, M., 1988. Practice of SMR classification for slope appraisal.?roe. 5thlnt. Symp. on Landslides, Lausanne, pp 1227-1231.

Rosengren, K. J. and Swindells, C. E, 1988. Slope stability in open pit goldmines. Confon R&D for the Minerals Industry, pp 122-127. (WA Schoolof Mines: Kalgoorlie).

Selby, M. J., 1980. A rock mass strength classification for geomorphicpurposes: with tests from Antarctica and New Zealand. Z. Geomorph. NF, Vol24,No 1:31-51.

Selby, M. J., 1982. Controls on the stability and inclinations of hillslopesformed on hard rock. Earth Surface Processes and Lanciforms,7:449-467.

Selby, M. J., 1987. Rock slopes in Slope Stability, (Eds M G Anderson andK S Richards), pp 475-504 (John Wiley: New York).

Singh, R. N., Elmherif, A. M. and Sunu, M. Z., 1986. Application of rockmass characterisation to the stability assessment and blast design in hardrock surface mining excavations in 27th US Symposium on RockMechanics, pp 471-478 (University of Alabama).

Steffen, O. K. H., 1976. Research and development needs in data collectingfor rock engineering. Proc. Symp. on Exploration for Rock Engineering,Johannesburg, pp 93-104 (A A Balkema: The Netherlands).

Swindells, C. E, 1990. Geotechnical studies for open pit mines: WestAustralian operating experience. Proc. Mine Geologists Cont, Mt Isa, pp167-170 (The Australasian Institute of Mining and Metallurgy:Melbourne).

Vanarsdale, R., Costello, P. and Marcelletti, N., 1990. Denudation ofHighwalls near Manchester, Kentucky. Eng. Geol., 26:112-123.

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