1990 Laubscher Geomechanics Classification System

8/4/2019 1990 Laubscher Geomechanics Classification System

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J. S. A tr. Inst. M in . M etal/., vol. 90, no. 10.Qc t. 1990. pp. 257-273.

A geo~echanics classification system for therating of rock mass in m ine designby D .H . LAUBSCHER *

SYNOPSISThe m ining rock-m ass rating (M AM A) classification system was introduced in 1974 as a developm ent of the CSIAg eom ec ha nics c la ss ific atio n s ys tem to c ater fo r d iv ers e m in in g s itu atio ns. T he fu nd am enta l d iffe re nce w as th e re co gni-tio n th at i n s it u rock-mass ratings (AMA) had to be adjusted according to the mining environment so that the finalratings (M AM A) could be used for m ine design. T he adjustm ent param eters are weathering, m ining-induced stresses,joint orientation, and blasting effects.It is also possible to use the ratings (AMA) in the determ ination of empirical rock-mass strength (AMS) and thenin the application of the adjustm ents to arrive at a design rock-mass strength (DAM S). This classification systemis versatile, and the rock-m ass rating (A MA ), the m ining rock-m ass rating (M AM A), and the design rock-m ass strength(DAMS) provide good guidelines for the purposes of mine design. However, in some cases a more detailed in-

vestigation may be required, in which case greater attention is paid to specific parameters of the system.Narrow and weak geological features that are continuous within and beyond the stope or pillar m ust be identifieda nd ra te d s ep ara te ly .The paper describes the procedure required to arrive at the ratings, and presents practical examples of theapplication of the system to mine design,S AM EV A T TIN G

Die m ynrotsm assa-aanslag-klassifikasiestelsel (MAM A) is in 1974 ingevoer as 'n ontwikkeling van die W NNAs e g eome gan ik ak la ssifik as ie stels el o m v ir u itee nlo pe nd e m yn bo uto es tan de v oo rs ie nin g te m aa k, D ie fu nd am ente leverskil w as die erkenning van die feit dat in s itu -ro tsma ss a-a an sla e (AMA) v olg en s d ie m yn bo u-omgew in g a an ge su iw ermoes word om die finale aanslae (MAMA) vir mynontwerp te kan gebruik, D ie aansuiweringsparameters is ver-wering, m ynbouge'induseerde spannings, naatorientasie en die gevolge van skietwerk,Dit is ook moontlik om die aanslae (AM A) by die bepaling van empiriese rotsmassasterkte (AMS) te gebruik,en dan by die toepassing van aansuiwerings om 'n ontwe rp ro tsmassast er kt e (DAMS) t e k ry , H ie rd ie k la ss if ik as ie ste lselis v ee lsy dig e n die ro tsma ss a-a an slag (AMA ), d ie m yn ro tsma ss a-a an sla g (M AMA), e n d ie on tw erp ro tsm as sa ste rk te(DAM S) verskaf goeie riglyne vir die doeleindes van m ynontwerp, Daar kan egter in som mige gevalle 'n uitvoerigerondersoek nodig wees waarin daar meer aandag aan spesifieke parameters van die stelsel geskenk word.Smal en swak geologiese aspekte wat deurlopend is in en verby die afbouplek of pilaar, moet ge'identifiseer enafsonderlik aangeslaan w ord,D ie referaat beskryf die prosedure wat nodig is om die aanslae te kry en gee praktiese voorbeelde van die toepassingv an d ie ste ls el o p m yn bo u-o ntw erp ,

INTRODUCTIONThe classification system know n as the m ining rock-m ass rating (M RMR) system was introduced in 1974 asa d evelopm en t of th e CSIR g eom ech anics classific atio nsystem l,2. The development is based on the concept of

in situ and adjusted ratings, the param eters and valuesbeing related to com plex m ining situations. Since thatt ime , th er e have been modif ic at ions and imp rovemen ts3 -S ,a nd th e system h as b een used su ccessfu lly in m ining pro-jects in C anad a, C hile, th e P hilip pin es, S ri L an ka, S ou thA frica, the USA, and Zim babwe.T his p ap er c on so lid ate s th e work p re se nte d in p re vio uspaper s and des cri be s t he bas ic p ri nc ip le s, d at a-coll ec tionprocedure, calculation of ratings (RMR ), adjustm ents(MRMR ), d esign rock-m ass stre ngth (DRMS), an d prac-tical application of the system s.An impo rta nt d ev elo pmen t o f th is c la ssific atio n mak esit suitable for use in the assessm ent of rock surfaces, asw ell as borehole cores.T ay lo r4 r ev iewed the c la ss if ic ati on s ys tems developedby W ickham, Barton, Bieniawski, and Laubscher and

. A ssociate C onsultant, Steffen, Robertson & K irsten Inc., 16th Floor,2 0 A nde rso n S tree t, Jo ha nn esb urg , 2 00 1.@ T he South A frican Institute of M ining and M etallurgy, 1990. SAIS SN 0038-223X /3.00 + 0.00. P aper received 3rd A pril, 1989.

concluded thatThus, the four system s chosen as being the m ost advanced classifica-tions are based on relevant param eters. E ach technique undoubtedlyyields meaningful results, but only Laubscher's geomechanicsclassification and the 'Q ' system of B arton offer suitable guidelinesfor the assessment of the main parameters; namely, the joint attri-butes. For general m ining usage and where the application of ac la ssific atio n v arie s w id ely , L aub sc he r's g eo me cha nic s c la ssific atio nhas the added advantage of allow ing further adjustm ents to the ratingfor different situations. This, coupled with the fact that the tech-nique has been in use for six years, gives no reason for changingto another system which offers no substantial improvement.The figure below shows a 98 per cent correlationbetween the RMR of the MRMR system and the NO Isystem based on the classification by Taylor4 of thirtysites ranging from very poor to very good. Thus, if NOIdata are available, this information can be used in thepract ical applicat ions.

PRINCIPLESA classification system m ust be straightforw ard andhave a strong practical bias so that it can form part oft he norma l geo logic al and rock-mechan ic s i nvest ig ationsto be used for mine design and communication. H ighlysop histicated techn iq ues are tim e-consuming , and m ost

JO UANA L O F T HE SOU TH A FA IC AN IN STIT UT E O F M IN IN G AND MET ALLU AG Y OCTOBEA 1 99 0 25 7


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80,....

60 .RMR/MRMR 40

200

10090807060

;!. 50:.:u0 40a::~ 30w

~2 0100

0,01 1,0 10,0 100,0 1000,0,1Q System

m ines cannot afford the large staff required to providecomp lex data of d oub tful b enefit to the p lan nin g and pro-duct ion departmen ts .T he approach adopted involves the assignm ent to therock mass of an in situ rating based on m easurablegeological param eters. Each geological param eter isw eighted according to its im portance, and is assigned am axim um rating so that the total of all the param etersis 1 00 . T his w eigh tin g w as review ed at reg ular intervalsin the developm ent of the system and is now acceptedas being as accurate as possible. The range of 0 to 100is used to co ver all variation s in jointed roc k m asses fromv ery po or to very go od. T he classification is divid ed intofive classes with ratings of 20 per class, and w ith A andB sub-divis ions.A colour schem e is used to denote the classes on planand section: class 1 blue, class 2 green, class 3 yellow,class 4 brow n, and class 5 red. C lass designations are forgeneral use, and the ratings should be used for designpurposes.T he ratings are, in effect, the relative strengths of therock m asses. T he acc ura cy o f the classification dep end son the sam pling of the area being investigated. The ter-minology prel iminary, in termediate, an d final sh ould bea pp lie d to a sse ssmen ts to in dic ate th e sta te o f d rillin g a nddevelopm ent. It is essential that classification data arem ade available at an early stage so that the correct deci-sions are m ade on m ining m ethod, layout, and supportrequirements.In the assessm ent of how the rock m ass will behavein a m ining environm ent, the rock-m ass ratings (RMR )are adjusted for w eathering, m ining-induced stresses,joint orientation, and blasting effects. The adjustedratings are called the mining rock-mass ratings orMRMR.It is also possible to use the ratings to determine anem pirical rock-m ass strength (RM S) in m egapascals(MPa). T he in s itu ro ck -mass stre ng th (RMS) is a dju ste das above to give a design rock-m ass strength (DRM S).T his figure is extrem ely useful w hen related to th e stressen vironme nt, and h as b een used fo r m ath em atical m odel-ling.The c la ss ifi ca tio n s yst em is v er sa til e, a nd t he ro ck -massra tin g (RMR), th e m in in g ro ck -mass ra tin g (MRMR), a ndthe design rock-mass strength (DRM S) provide goodguidelines for m ine design purposes. H ow ever, in som ecases w here a m ore detailed investigation is required,e xamp le s o f th ese situ atio ns a re d esc rib ed in whic h sp ec i-fic param eters of the system are used.25 8 OCTOBER 1 99 0 JO URNA L O F TH E SOU TH A FR IC AN IN ST ITU TE O F M IN IN G AND META LL URGY

S in ce a ve ra ge v alu es c an b e m isle ad in g a nd th e w eake stzo nes m ay de term ine the resp onse o f the w hole rock m ass,these zones m ust be rated on their ow n. N arrow and w eakg eo lo gic al fe atu re s th at a re c on tin uo us w ith in a nd b ey on dth e sto pe o r p illa r must b e id en tifie d a nd ra te d se pa ra te ly .GEOLOGICAL PARAMETERS, SAMPLING , AND RATINGSThe geological param eters that m ust be assessed include

the intact rock strength (IRS), joint/fracture spacing, andjoint condition/water. Before the classification is done,the core or rock surface is examined and divided intozones of similar characteristics to which the ratings arethen applied. These parameters and their respectiveratings are shown in Table I.

Intact R ock Strength (IR S)The IRS is th e uncon fin ed uniaxi al comp re ss iv e s treng thof the rock betw een fractures and joints. It is im portantto n ote that the cores se lecte d fo r te stw ork a re inv ariablythe strongest pieces of that rock and do not necessarilyreflect th e av era ge values; in fact, o n a larg e copp er m ine ,only unblem ished core w as tested. The IR S of a definedzone can be affected by the presence of w eak and strongintact rock, which can occur in bedded deposits anddeposits of varying m ineralization. A n average value isassigned to the zone on the basis that the weaker rockw ill have a greater influence on the average value. Therelationship is non-linear, and the values can be read offan em pirical chart (Fig. 1).SELEC T C UR VE U SING WEA K ROCKIRS AS% OF STRONG ROCK IRS

10% 20% 30% 40". 50"10 60% 70% 80"1. 9O'Y.

10 20AVERAGE IRS AS % OF

STRONG ROCK IRSF ig.1 -D eterm ination of average IA S w he re the rock m ass con-tains weak and strong zonesExample:Strong rock IAS = 1 00 MPaW eak rock IAS = 20 M PaW eak rock IAS x 100 = 20%Strong rock IASW eak rock IAS = 45%A ve ra ge IA S = 37% of 100 MPa = 37 M Pa


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/ B 0 I8 r-/ " '/1 IY ~~ I IIA ~~III.~ ~/ I I/ I60/'.;,~ I I/ I

/ / 7/ I I5 / / / I'I I I/ / I II,4 ,A 1/ I I#' / 1 I1.. / J I II/ / I I2 II I II I/ I II,/0

The rating range is from 0 to 20 to cater for specim enstrengths of 0 to greater than 185 M Pa. The upper limitof 18 5MPa h as be en se lecte d beca use IR S v alu es g reaterth an th is have little b earing on the streng th o f jointed ro ckmasses.Spacing of Fractures and Joints (RQD+JS or FF)Sp acin g is th e m easurem ent of all the d iscon tin uities

and partings, and does not include cem ented features.Cemented features affect the IRS and as such must beincluded in that determ ination. A joint is an obviousfeature that is continuous if its length is greater than thewidth of the excavation or if it abuts against anotherjoint, Le. joints define blocks of rock. Fractures andp arting s do not n ecessarily hav e co ntinu ity . A max imumof three joint sets is used on the basis that three joint setsw ill define a rock block; any other joints w ill merelym odify the shape of the block.Two te ch niq ue s h av e b ee n d ev elo pe d fo r th e a sse ssmen to f th is p arame te r:. the more detailed technique is to measure the rockquality desi~nation (RQD) and joint spacing (JS)separately, the maximum ratings being 15 and 25respectively;. th e o ther techn iqu e is to m easure all th e d iscon tin uitiesand to re cord th ese as the fractu re freq uency p er m etre(FF/m ) with a m axim um rating of 40, Le. the 15 and25 from above are added.Designation of Rock Q uality (RQD )The RQD determ ination is a core-reco very tech niquein which only cores w ith a length of more than 100 m mare r ecord ed :

RQ D, 0,10= Total lengths of core> 100 mm x 100.Length of runOnly cores of at least BXM size (42 mm) should be used.It is also essential that the drilling is of a high standard.T he orientation of th e fractures w ith respect to th e coreis impo rta nt fo r, if a BXM bore ho le is d rille d p erp en dic u-lar to fractures spaced at 90 mm, the RQD is 0 per cent.If the bore hole is drilled at an inclination of 40 degrees,th e spacing betw een the sam e fractures is 13 7mm; on thisbasis, the RQD is 100 per cent. As this is obviously in-c orre ct, it is e sse ntia l th at th e c ylin de r o f th e c ore s (so un dcores) should exceed 100 mm in length. A t the quoted 40degree intersection, the core cylinder would be only91 mm and the RQD 0 per cent. The length of core usedfo r th e calculatio n is m easu re d from fracture to fra cturealong the axis of the core.In the determ ination of the R QD of rock surfaces, thesam pling line m ust be likened to a borehole core and thefo llowin g p oin ts o bse rv ed :. experience in the determ ination of the RQD of coreis necessary;. do not be m isled by blasting fractures;. weaker bedding planes do not necessarily break whencored,. assess the opposite w all w here a joint form s the side-wall,. shear zones greater than 1m must be classified sepa-rately.

Joint Spacing (JS)A maximum of a three-joint set is assumed, Le. thenum ber required to define a rock block. W here there arefou r o r more joint sets, the th ree closest-spaced join ts areused. The original chart for the determ ination of the JSrating has been replaced by that proposed by Taylor4.From the chart in Fig. 2 it is possible to read off the ratingfor one-, tw o-, and three-joint sets.I,

0,IDN\5 0,Z~eta: 0,w-'mu;~ 0,-':!~~ 0,I-ZW:::;: 0l-)::J.,.0


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I 2 3 4 5Class A B A B A B A B A BRating loo-81 80-61 60-41 40-21 20-0Description V er y G oo d Good Fair Poor V ery P oo rColour Blue Green Yellow Brown Re d

F ra ctu re f re qu en cy , F F/mJoint RatingIR S-M Pa rating RQD ra tin g spacing A ve ra ge p er07 0 07 0 m metre I set 2 set 3 set

> 185 20 97-loo 15 025 0, 1 40 40 40165-185 18 84-96 14 See F ig. 2 0,15 40 40 40145-164 16 71-83 12 0,20 40 40 38125-144 14 56-70 10 0,25 40 38 36105-124 12 44-55 8 0,30 38 36 3485-104 10 31-43 6 0,50 36 34 3165-84 8 17-30 4 0,80 34 31 2845-64 6 4-16 2 1,00 31 28 2635-44 5 0- 3 0 1,50 29 26 2425-34 4 2,00 26 24 2112-24 3 3,00 24 21 185-11 2 5,00 21 18 151-4 I 7,00 18 15 1210,00 15 12 1015,00 12 10 7

20,00 10 7 530,00 7 5 240,00 5 2 0

ALLOW FOR CORE RECOVERY

TABLE IGEOLOGICAL PARAMETERS AND RATINGS

1. M eaning of the ratings

D istinguish betw een the A and B sub-classes by colouring the A sub-class full and cross-hatch the B .2 . P aram eters an d ratin gs

TABLE I (co ntinue d op posite ) T A BLE IIIBOREHOLE LOG SHEET

S am plin g p ro ce du re

B oreh ole N o:Z on in g o f b or eh oleI nte rv al le ng th (A )Total sound core (B)

BRQD, 070- x lOOALowangle0-29

Date:

TABLE IIFACTORS TO G IVE AVERAGE FRACTURE FREQUENCY

Factor Jointspacing

NumberM ean sp acin g (A )T rue d is tance = A x sin (0,26)NumberM ean spacing (B )T rue d is tance = B x sin (0,71)NumberM ean spacing (C )T rue d is tance = C x sin (0,97)

= Sum of individual FF/m (inverse of spacing)2F in al R atin g

a. One set of three sets on a line, or one set onlyb. Two sets of three sets on a line or two sets onlyc. All of the sets on a line or borehole cored. Two sets on one line and one on anothere. T hree sets on three lines at right-angles

1, 01, 52, 02, 43, 0

Moderateangle30-59Highangle60-90Average f requency

IR SRQDJoin t s pa cin gJoi nt cond it ionTotalRemarksS in v al ue s 0-29 = 0,26 30-59 = 0,71Signature:

60-90 = 0,97

260 OCTOBER 1990 JOURNA L OF TH E SOUTH AFRICA N INSTITU TE O F M INING AND METALLURGY


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Accumulative 07 0adjustm ent of possible rating of 40Adjustment, 070

Mod. p re ss ur e H igh p re ssu re25-125 >125

Parameter Description Dry Moist U rn UrnA Multi wavy directional lOO lOO 95 90

Uni 95 90 85 80Large-scale Curved 85 80 75 70j oi nt expres si on Slight undulation 80 75 70 65

Straight 75 70 65 60B Rough s tepped/i rregu lar 95 90 85 80

Smooth s tepped 90 85 80 75Small -scal e join t Slickensided stepped 85 80 75 70expression Rough undu la ti ng 80 75 70 65200 mm x Smoot h undu la ti ng 75 70 65 6020 0 m m Slickensided undulating 70 65 60 55

R ou gh p la na r 65 60 55 50Smooth planar 60 55 50 45Polished 55 50 45 40

C] o in t w all a lte ra tio n w ea ke r th an w all r oc k a nd o nlyif it is w eaker than the filling 75 70 65 60D Non-softening Coarse 90 85 80 75

a nd s he ar ed Medium 85 80 75 70material Fine 80 75 70 65] o in t f il li ng S of t s he ar ed Coarse 70 65 60 55mate ria l, e .g . Medium 60 55 50 45talc Fine 50 45 40 35

Gouge t hi cknes s< amplitu de o f ir re gu la ritie s 45 40 35 30Gouge t hi cknes s> amplitu de o f ir re gu la ritie s 30 20 IS 10

T ABLE I (continued from opposite page) 3 . A ss es sm en t o f jo in t c on ditio n

Exam ple: A straight joint w ith a sm ooth surface and m edium sheared talc under dry conditions givesA = 70070 , B = 65070, D = 60070 ; to ta l ad ju stmen t = 70 x 65 x 60 = 27070, and the rating is40 x 27070 = 11 .The rock m ass rating (RM R) is the sum of the individual ratings.

fo llowin g situ atio ns a pp ly :. if all the features are presen t in th e sidew alls, establishw hether they intersect a horizontal line;. if they all do not intersect the horizontal line, m easure

on a vertical line as well;. if a set is parallel to the sidewall, measure these ona line in the hanging at right-angles to the sidewall.This conflicting situation of different sam pling pro-cedures can be resolved if the sum of the m easurem entsis divided by a factor to arrive at the average frequency.These factors are shown in Table 11, which can be ap-preciated if com pared w ith the sam pling of the sides ofa cube on different lines on intersection.The need for accurate sam pling cannot be too highlystressed. Often detailed scan-line surveys are done onsidew alls that do not intersect all the features, and thenthis biased inform ation is analysed in detail.W here boreholes do not intersect all the features at 45degrees, a sam pling bias will occur unless provision ism ade for the angle of intersection, as in the log sheet ofT able Ill.

The' average fracture frequency per m etre (FF Im ) isused in Table I to determine the rating. The inverse ofthis number giv es the average fracture spacing. T he datafrom A and B can be used only if the joint spacing forall the sets is approxim ately the sam e.F ig . 3 sh ow s th e re la tio nsh ip b etw ee n FF/m and ra tin gsafter the different sampling techniques for core andunderground expo su res have been adjusted to an av eragespacing.B ec au se th e FF Im in clu de s b oth c on tin uo usGoin ts) a ndd is cont inuous (f ra ctu re s) f ea tu re s, th e con tinuity must b eestimated to give the joint spacing and rock block size(Fig. 4). T hus, the FF/m w ill give the rock-m ass rating,but this has to be adjusted by the factors given in TableIV .Cor e Recover yAs the FF/m does not recognize core recovery, theFF/m must be increased ifthere is a core loss, w hich w illoccur in the w eaker sections of the core. T he adjustm entis done by dividing the FF Im by the core recovery andm ultiplying the quotient by 100.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY O CT OB ER 1990 26 1


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v/7 I/[7///V///v///

/ V

RatingJ oi nt s pa ci ng Ratingm RQD J5 Combined FF/m

0,025 o I I 10,05 o 1,5 1,5 50,10 8 3 11 100,20 12 5 17 150,50 14 10 24 201,00 15 13 28 262,00 15 19 34 313,00 15 21 36 334,00 15 23 38 365,00 15 25 40 38

J

.JS

~3 000a:.2!SC)ZUc;t~ 20I-~021 50ZI-era: 10

!J

0 60 60 J 30 20 /!J /0 l' 5

40

35

30

25

20

/5

/0

5

3 2 /.0 0.85 0$ 02 0./5 0/3A V E R A G E F R A C T U R E F R E Q U E N C Y P E R M E T R E ( F F / m )

C O R E R E C O VE R YFig. 3-Ratings for fracture frequency per metre

T A BLE IVFACTORS BY WHICH JOINT FREQUENCIESARE MULTIPLIED

C on tin uou s featu res070 Factor1009080706050

1,00,90,80,70,60,5

Comparison of the Two TechniquesT he advantage of the FF /m technique is that it is m oresensitiv e th an th e RQD for a w id e rang e of join t spacin gs,because the latter m easures only core less than 100 mmand rapidly changes to 100 per cent. Exam ples of this areshow n in T able V , w hich assum es that there is a percent-age of core greater than 100 mm at joint intersections.The fracture-frequency technique was first used inC hile in 1985 and then in C anada in 1986. In Z im babw e,the FF/m technique was used in conjunction with theRQD and JS technique and was found to be just asaccurate.

Joint Condition and W aterJoint condition is an assessm ent of the frictional pro-perties of the joints (not fractures) and is based on ex-p re ss ion, s ur fa ce p rope rtie s, a lte ra ti on zones , fil li ng , andwater. Originally the effect of w ater was catered for in

T ABLE VCOMPARISON OF TECHNIQUES

a se pa ra te se ctio n: h ow ev er, it w as d ec id ed th at th e a sse ss-m ent of joint condition allow ing for w ater inflow w ouldhave greater sensitivity3. A total rating of 40 is nowa ssig ne d to th is se ctio n. T he p ro ce du re fo r th e d ete rm in a-tion of joint condition is show n in T able I, w hich dividesth e jo in t-a sse ssmen t se ctio n in to su b-se ctio ns A , B , C , D .Su b-section A caters for th e la rg e-scale ex pression o fthe feature, such as across a drift or in a pit face. Ba ss es se s th e small- sc ale exp re ss ion and is b as ed on the p ro -file s sh ow n in Fig. 5. S ection C is app lied o nly w hen thereis a distinct difference betw een the hardness of the hostrock and that of th e joint w all. S ection D cov ers th e va ria -tion s in jo in t filling .As the conditions of the different joint sets are not

26 2 OCTOBER 1990 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY


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~I"\." " "" "'" " "" 1"-. "" "I''\ I'I" '\ '"'" '"

"'",

JOINT SPACINGCm) o,F 0,.' Of r 0,5 1,0I IBLOCK SIZE ,~ ) . 0 , 0 01 q0 8 al2 1,0I I I IISOLAT1 ::D DRAW 6mZONE D IAMETER I Bm

A B C

4O p

. a : : w

op~.....S 5,0Z.....Z00:::0lJ...0W 1,0Cl )::>J0


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Potential w eathering and adjustm ents. %Degree ofweathering Y2y I y 2y 3y 4 + yFresh 10 0 100 100 100 100Slight 88 90 92 94 96Moderate 82 84 86 88 90High 70 72 74 76 78Complete 54 56 58 60 62Residual soil 30 32 34 36 38

I - --.......ROUGH STEPPED / IRREGULAR

][ --...SM OOTH STEPPED

m

PERCENTADJUSTMENT95

90

SL lC KENSID ED ST EPPED85

N: -ROUGH / IRREGULAR UNDULATING

x. -SM OO TH U ND ULA TING

3Z I

........ 80

- -- F ig . 5 -J oin t ro ug hn es sprofiles75SLlC KEN SID ED U ND ULA TIN G 7.0

1ZIIROUGH / IRREGULAR PLANAR 65

SMOOTH PLANAR 60

IXPO LISH ED PLAN AR 55

tion and the rate of m ining.T he three param eters that are affected by w eatheringare the IRS, RQD or FF fm , and joint condition. TheRQD percen tag e can be d ecre ased by an in crease in frac -tures. The IRS can decrease significantly as chem icalchanges take place; in fact, there is the situation withkim berlites, where solid hard rock becomes sand in ash ort tim e. T he jo int co nd ition is affec ted b y alterationof the wallrock and the joint filling. W eathering databased on the exam ination of borehole cores can be con-se rv ative owing to the large surfa ce area o f c ore relativeto th e volum e-un derg ro un d ex po sures are more reliable.T able V I show s the adjustm ent percentages related todegree of w eathering after a period of exposure of half,one, two, three, and four-plus years.26 4

TABLE VIADJUSTMENTS FOR WEATHERING

J oin t O rie nta tio nThe size, shape, and orientation of an excavation af-OCTOBER 1990 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY


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No. of joints No. of faces inclined away from the verticaldefining theblock 701170 751170 801170 851170 901170

3 3 24 4 3 25 5 4 3 2 I6 6 5 4 3 2,1

Adjust- Adjust- Adjust-Average Plunge ment Plunge ment Plunge mentrating degree 1170 degree 1170 degree 1170

0- 5 10-30 85 30-40 75 >4 0 705-10 10-20 90 20-40 80 >4 0 7010-15 20-30 90 30-50 80 >50 75

15-20 30-40 90 40-60 85 >60 8020-30 30-50 90 >5 0 8530-40 40-60 90 >5 0 90

fects the behaviour of the rock m ass. T he attitude of thejoints, and whether or not the bases of blocks are ex-posed, have a significant bearing on the stability of theexcavation, and the ratings m ust be adju sted according-ly. The magnitude of the adjustment depends on theattitude of the joints w ith respect to the vertical axis ofthe block. A s gravity is the m ost significant force to beconsidered, the instability of the block depends on thenum ber of joints that dip away from the vertical axis.The required adjustm ents are show n in Table V II.T A BLE VII

PE RCE NT AG E A DJU STM EN TS F OR JO IN T O RIE NT ATIO N

T he orientation of joints has a bearing o n the stabilityof open stopes and the cavability of undercut rockmasses.T he a dju stm en ts fo r th e o rie ntatio n o f sh ea r z on es w ithrespect to development are as follows: 0-15 = 7611/0,15-45 = 84% ,45-75 = 92%.Advance of the ends in the direction of dip of struc-tu ra l fe atu re s is p re fe ra ble to d ev elo pment a ga in st th e d ip .A n adjustm ent of 90 per cent should be m ade to previousadjustm ents w hen the advance is against the dip of a seto f closely spaced joints. T his is because it is easier to sup -port rock blocks that have the prom inent joints dippingw ith the adv an ce.The adjustm ent for shear-zone orientation does notapply to 'jointed rock'. T he m axim um rating is thereforejoint orientation m ultiplied by direction of advance,w hich is 70% x 90% = 63%.T he e ffe ct o f jo in t o rie nta tio n a nd co nd itio n o n stab ili-ty is clearly displayed in bridge arches m ade from high-fric tio n ro ck b lo ck s.Joint-orientatio n A djustm ent for P illars and S idew aU sA mod ifie d o rie nta tio n a dju stm en t a pp lies to th e d es ig no f p illa rs o r s to pe sid ew alls . A dju stm en ts a re m ad e whe rejoints define an unstable w edge w ith its base on the side-w all. The instability is determ ined by the plunge of thein te rs ec tio n o f th e lower jo in ts, a s w ell a s b y th e c on ditio nof the joints that define the sides of the wedge (TableVIII).

Min ing- induced S tr es se sM inin g- in du ced s tr es se s r es ult f rom th e r ed is trib ut io no f fie ld (re gio na l) s tre sse s th at is c au se d b y th e g eome tryand orientation of the excavations. The m agnitude andra tio o f th e fie ld s tre sse s s ho uld b e k nown . T he re dis trib u-tion of the stresses can be obtained from m odelling orfrom p ublished stress-redistribution diagram s7.8. T heredistributed stresses that are of interest are m axim um ,m in imum , a nd d iffe ren ce s.

T A BLE VIIIPERCENTAGE ADJUSTM ENTS FOR THE PLUNGE OF THE

INTERSECTION OF JOINTS ON THE BASE OF BLOCKS

Maxim um StressThe max imum p rin cip al stre ss c an c au se s pa llin g o f th ew all p aralle l to its o rie nta tio n, th e c ru sh in g o f p illa rs , an dth e d efo rm atio n a nd p la stic flow o f s oft zo ne s. T he d efo r-m ation of soft intercalates leads to the failure of hardz on es a t re la tiv ely low stre ss le ve ls . A c ompre ss iv e stre ssat a large angle to joints increases the stability of th e rockm ass and inhibits caving . In this case, the adjustm ent canbe up to 120 per cent, Le.. im proving the strength of therock m ass.M inim um StressT he m in im um principal stress plays a significant rolein the stabilities of the sides and back of large excava-tio ns, the sides of stapes, and the m ajor an d m inor apexesthat protect extraction horizons. T he rem oval of a highhorizontal stress on a large stope sidew all w ill result inrelaxation of the ground tow ards the opening.Stress DifferencesA large difference betw een m axim um and m inim umstresses has a significant effect on jointed rock m asses,re su ltin g in s he arin g a lo ng th e jo in ts . T he e ffe ct in cre as esas the joint density increases (since m ore joints w ill beu nf av ou ra bly o rie nta te d) a nd a ls o a s th e jo in t-c onditio nratings decrease. The adjustm ent can be as low as 60 percent.Fac to rs in th e A ss es smen t o f M in ing-in du ced S tre ssThe follow ing factors should be considered in theassessment of mining-induced stresses:. d rift-in duced s tre sse s;. in teractio n o f clo se ly sp ace d d rifts;. location of drifts or tunnels close to large stopes;. abutm ent stresses, particularly w ith respect to thedirection of advance and orientation of the fieldstresses (an undercut advancing tow ards m axim ums tre ss e nsu re s g oo d c av in g b ut c re ate s h ig h a bu tm en ts tr es se s, a nd v ice versa );. uplift;. point loads from caved ground caused by poor frag-mentation;. removal of restraint to sidew alls and apexes;. in cre ase s in s iz e o f m in in g a re a ca us in g c ha ng es in th egeometry;. massiv e wedge failu res;. influence of m ajor structures not exposed in the ex-cavation but creating the probability of high toestresses or failures in the back of the stope;

JOURNA L O F THE SOUTH A FR IC AN IN ST ITU TE O F M IN ING AND META LLURGY OCTOBER 1990 265


10/17

. presence of intrusives that m ay retain high stress orshed stress into surrounding, m ore com petent rock.The total adjustment is from 60 to 120 per cent. Toarrive at the adjustm ent percentage, one m ust assess theeffect of the stresses on the basic param eters and use thetotal.

B last ing Ef fec tsB lasting creates new fractures and loosens the rockm ass, cau sing movem en t o n jo in ts, so tha t th e follow ingadjustm en ts sho uld b e app lied :TechniqueBoringSmoo th -wa ll b la sti ngGood c on ve ntio na l b la stin gPoo r b la st in g

Adjustment, %10 0979480 .

The 100 p er cent adjustm ent for boring is based on nodam age to the w alls; however, recent experience withro adh eader tun nellin g show s th at stre ss deterioratio noccu rs a short distance from the fac e. T his ph enomen onis being investigated since good blasting m ay create ab et te r wal l condit io n.It should be noted that poor blasting has its greatesteffect on narrow pillars and closely spaced drifts ow ingto the lim ited amount of unaffected rock.

Summary o f A dju stm entsA djustm en ts must reco gn ize th e life o f the ex cava tio nand the tim e-dependent behaviour of the rock m ass:Parameter Possible adjustment, %Weathering 30-100Orientation 63-100Induced stresses 60-120Blasting 80-100.A lthough the per centage s a re empi ri ca l, t he adju stmentprinciple has proved sound and, as such, it forces thedesigner to allow for these im portant factors.

STRENGTH OF THE ROCK MASSThe rock -m ass streng th (RMS) is d erived from th e IR Sand the RM R5. The strength of the rock m ass cannot behigher than the corrected average IRS of that zone. TheIRS has been obtained from the testing of small speci-

m ens, but testw ork done on large specim ens show s thattheir strengths are 80 per cent of those of small speci-m ens4. As the rock m ass is a 'large' specimen, the IRSmust be reduced to 80 per cent of its value. Thus, thestrength of the rock m ass would be IRS x 800/0 if it hadno joints! The effect of the joints and its frictional pro-perties is to reduce the strength of the rock m ass.T he follow ing procedu re is ado pted in th e calcula tio nof RM S:. the IRS rating(B) is subtracted from the totalrating (A ) and, therefo re, the balan ce, L e. RQD , jointspacing, and condition are a function of the rem ain-ing possible rating of 80;. the IRS(C) is reduced to 80 per cent of its value,

RMS = (A -B) C 8080 x x 100'26 6 OCTOBER 1990 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

e.g. if the total rating w as 60 with an IRS of 100 M Paand a rating of 10, thenRMS = 100MPa x (60 - 10) x 80 50MPa .80 100 =

DESIGN S TR ENGTH OF THE ROCK MASSThe desig n roc k-m ass streng th (DRMS) is the stren gth

of th e u nco nfined ro ck m ass in a sp ecific m in ing e nviro n-m ent. A m ining operation exposes the rock surface, andth e c on ce rn is w ith th e sta bility o f th e z on e th at su rro un dsthe excavation. The extent of this zone depends on thesize of the excavation and, except w ith m ass failure, in-stability propagates from the rock surface. The size ofthe rock block will generally define the first zone of in-stability. Adjustm ents, w hich relate to that m ining en-vironment, are applied to the RMS to give the DRMS.As the DRM S is in m egapascals, it can be related to themin ing- induced s tr es se s. The re fo re , t he adju stment s u sedare those for w eathering, orientation, and blasting. Forexam ple, ifweathering = 85% , orientation = 75%, blasting =90070,total = 57% , and RMS = 50 , the adjus tmen t

= 57% and the DRMS = 50 x 57% = 29 MPa.T herefore , th e rock m ass ha s an un con fin ed compressivestrength of .29 MPa, which can be related to the totalstresses.

PRESENTA TION OF DA TAThe rating data for the rock mass should be plottedon plans and sections as class or sub-class zones. If theA and B sub-divisions are used, the A zones can becoloured full and the B cross-hatched. These plans andsections now provide the basic data for m ine design. T helayouts are plotted w ith the adjusted ratings (MRMR),which w ill highlight potential problem areas or, if thelayout has been agreed, the support requirem ents w ill bebased on the M RM R or DRM S. In the case of the DRM S,the values can be contoured.

P ract ical Appl ica tionsThe rock mass can now be described in ratings or inm egapascals; in other words, these num bers define thestrength of the m aterial in which the m ining operation

is go ing to take place. E xc avation stability o r instab ilityhas been related to these num bers. O n the m ines in w hichthe system has been in operation, its introduction waswelcom ed by all departm ents from those dealing withg eolo gy to th ose invo lve d in p ro du ctio n.W ithin the scope of this paper, the practical applica-tions are described in broad term s to ind icate th e b enefitsachieved from the use of this system.CommunicationC ommun ication betw een variou s d epartm ents has im -p ro ve d sin ce th e in tro du ctio n o f th e c la ssific atio n sy stembecause num bers are used instead of vague descriptive

terms. It is well known that the term inology used tod esc rib e a p artic ula r ro ck mass b y p erso nn el e xp erie nc edin the m ining of good ground is not the sam e as that usedby personnel experienced in the m ining of poor ground.


11/17

a ab b a ab b b b b c

c c c d dd e f f c+ l

flp h + flp h+fll h+fllh + flp flp

Suppo rt P rin cip le sT he RMR is taken into con sid era tio n in d esig ning su p-port even though the adjusted ratings (MRMR) are used.The reason is that a class 3A adjusted to SA has rein-forcem ent p ote ntial, w hereas an in s itu class SA has noreinforcing po ten ti al .Su pp ort is req uired to m ainta in th e integrity of the ro ckmass and to increase the DRMS so that the rock masscan support itself in the given stress environm ent. Theinstallation must be timed so that the rock mass is notallow ed to fail and should therefore be early rather thanlate. A support system should be designed and agreedb efo re th e d eve lopment stag e so tha t th ere is inte ractio nb etw ee n th e c omponents o f th e in itia l a nd th e fin al sta ge s.To control deform ation and to preserve the integrity ofthe ro ck m ass, the in itial sup port sh ou ld b e installed con -currently w ith the advance. T he final support caters fort he min ing- induced s tr es se s.A n inte grated su pp ort system con sists o f compo ne ntsth at a re in te ra ctiv e, a nd th e su cc ess o f th e sy stem d ep en dso n th e c orre ct in sta lla tio n a nd th e u se o f th e rig ht mate ria l.E xperience has show n that sim ple system s correctly in-sta lle d a re more sa tisfa cto ry th an c omplic ate d te ch niq ue sin w hich the cha nces of erro r a re high er. T he sup erv iso rystaff m ust understand and contribute to the design, andth e d esig n sta ff must re co gn iz e th e c ap ab ilitie s o f th e c on -s tr uc tio n c rews and any logi stic al p roblems. The con st ru c-tion crew s should have an understanding of the supportp rinciples and th e co nseq uen ces of po or installatio n.Layout of Support Guide for Tunnels Using MRMRT able IX show s how the support techniques, in alpha-betical symbols, increase in support pressure as theMRMR decreases. Both the RMR and the MRMR aresh own a s su b-c la sse s.

TABLE IXSUPPORT- PRESSURE FOR DECREASING M RM R

RM RMRMR lA lB 2A 2B 3A 3B 5AA 4B- Roc k r ein fo rc emen t-p la stic d efo rma tio n- +

lAlB2A2B3A3B4A4B5A5B

* The codes for the various support techniques are given in Table X.

A djusted ratings m ust be used in the determ ination ofsu pp ort re qu irements. In sp ec ia liz ed c ase s, su ch a s d raw-p oin t tu nn els, th e a ttritio n e ffe cts o f th e d rawn c av ed ro ckand sec ond ary blasting must b e reco gn ized , in w hich casethe tunnel support shown in Table IX would be sup-plem ented by a m assive lining.T he sup po rt tech niqu es shown in T able X are examples

o f a prog ressive in crease in su pp ort p ressu res and a re no ta c omplete spectrum o f tech niq ues. Where w eatherin g islikely to be a problem, the rock should be sealed onexposure.T ABLE X

SUPPORT TECHNIQUESRock r ei nf or cemen t

a L ocal bolting at joint intersectionsb Bolts at I m spacingc b and straps and mesh if rock is finely jointedd b and m esh/steel-fibre reinforced shotcrete bolts as lateralrestraint

d and straps in contact w ith or shotcreted ine and cable bolts as reinforcing and lateral restraintf and pinningSpillingGrouting

efgh

Rig id l in in gj Timber

]k Rigid steel sets L d f fI M assive concrete ow e orma IOnm k and concreten Structurally reinforced concrete

Y ie ld in g l in in g, r epai r t ec hn ique , high deformation0 Yielding steel archesp Yielding steel arches set in concrete or shotcrete

Fill q FillS pa llin g c on tr ol

r Bolts and rope-laced meshRoc k r ep la ce me nts Rock replaced by stronger material

Developm ent avoided if possible

5BLayout of Support Guide Using the DRM ST he support guide for tunnels using the D RMS and thesupport techniques of Table IX are shown in Figs. 6and 7.

S tabil ity and Cavabil ityThe relationship between the ratings adjusted forstability or instability (MRMR) and the size of excava-tion is show n in Fig. 8. The exam ples of different situa-tion s w ere tak en from op erations at the fo llow in g m in es:. Freda, Oaths, King, Renco, and Shabanie M ines inZimbabwe. Andina, Mantos Blancos, and Salvador M ines inChile. Bell and Fox M ines in Canada. Henderson M ine in the USA.T he d iag ram re fers to the stab ility o f th e rock a rch , w hichis depicted in three em pirical zones:. a stable zone requiring support only for key blocks

or brows, i.e. skin effects. a tr an si tio n zone requ iri ng s ub stanti al p en etr ativ e s up -port and/or pillars, or provision to be m ade for dilu-tion owing to failure of the intradosal zone,OCTOBER 1990 267O URN AL O F TH E SO UTH AFRIC AN IN STITUTE O F M ININ G AN D M ETALLUR GY


12/17

INCREASING ", /PLA~ T IC DEFORM JlON /' / k// /INCREASINI ROCK RE IN ORCINGPOT NTlAL ~/ c /'" /1 I'"-' / // / // / 11 //

I ./,' / ~- /m /,/ b / . + 0 /,/ /k / // / /../ / d / // / " .. 0/ /", ' / e // ./ /// 1/,/ a /( / /h/,' ]I / // / J/' / d1,..-/ /

/ // ,/ e / f + 0 // / // f // / "I/

a / / // m / /b // , // / , + 0 Vd/ / ,/ / // . / // / // '," 1V ".I'/ ,/ / 1 < S TABLE// /q/ ' iI'.I / IT = SUPPORTKEY BLOCKS -/ // / Y. ill =SUPPORTEFFECTIVE/ /,.I' /" N=FAI LURE CONTROLLED -// ~=COLLAPSEICAVINGJ//.//

30

r< )bIb 400a.~Cl)Cl )W0:::f-Cl )f-ZW 60~Z00:::>ZW(9ZZ

80 70 40DESIGN ROCK MASS STRENGTH MPo

00 50

10

eo

50

70

80

90

/00

//0

/20. a caving or subsidence zone in w hich caving is pro-pagated provided space is available or subsidenceorcurs.T he size o f the excavation is defined by the 'hydrau licradius' or stability index, w hich is the plan area dividedby the perim eter. O nly the plan area is used for excava-tions w here the dip of the stope or cave back is less than45 degrees. W here the dip is greater than 45 degrees, thearea and orientatio n of the b ack w ith respect to the m ajorstress direction m ust be assessed.For the sam e area, the stability index (SI) w ill varydependin g on the relationship betw een the m axim um andthe m inimum spans. For example, 50 m X 50 m has thesame area as 500 m x 5 m, but the SI of the first is 12,5268 O CTO BER 1990 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

30 20 /0

F ig . 6 --Suppor t r equ ir ementsfo r m ax imum st re ss

w hereas the SI -of the second is only 2,5. The large 50 mX 50 m stope is less stable than a 500 m X 5 m tunnel,and this is well illustrated by the difference in the SI.I nd es tru ctib le p illa rs (r eg io na l) r ed uc e th e spans so th atthe SI is applied to individual stopes. Sm all pillars, asin a post-pillar operation, apply a restraint to the hang-ingw all, w hich results in a positive adjustm ent and, assu ch , a h ig her ratin g, s o th at th e o ve ra ll sto pe d im en sio nsc an b e in cre as ed w ith in th e d ic ta te s o f re gio nal sta bility .In a room -and-pillar m ine, the pillars are designed toensure r eg io na l s ta bi lity .The stability or cavability of a rock m ass is determ in-ed by the extent and orientation of the w eaker zones.There is a distinction betw een m assive and bedded


13/17

,," ;.~/' " k./ C ",' """" ,,"./ "' ,,""k ./"." b " /'.... ,, "/" " g ,,/'.... "/ r". " d m/,"" ",,-r" "". /" /' d /',...../ a /' . ./ /'[ . + 0~,.... ", , ' /' //' f ",,' ./'/' /'./ b /' /'" h /' . 0" / //" ",,' /'//' ./ """ nr /' /""./ b

"/' ".I"

/ /' /". / d / '"" "1.../ 0/. // N /f " /;",." /'""/' ""/'

,"" ' V./'//"/'

"" 1- ST AB LEII = SU PPO RT IZW(!)~z~

60

70

80

90

/00

I/O

/2

de posits in that the bed ding cou ld be a do minan t featu re.Stability of O pen StopesL arge, open stopes are generally m ined in com petentground, the size of the stope being related to the criteriafor regional stability (Fig. 8). T he stability of the stopehangingwall has to be assessed in term s of whether thepersonnel are to work in the stope or not.If personnel are to work in the stope, the back mustbe stable im mediately after m ining. In order to achieveth is sta bility , p ote ntia l ro ck fa lls n ee d to b e id en tifie d a nddealt w ith. If the environm ent and m ining rate perm it it,or if skilled personnel are available, the support can bedesigned for the local situation. H ow ever, if the m ining

D E S IG N R O C K M A S S S T R E N G T H MPa60 50 40 30 /00 0

rate is high, or the identification of potential falls is dif-ficu lt, a blanke t-su ppo rt design is req uired .In the case of open stopes where the activity is fromsu b-levels o utside th e stop e, the local instability affe ctsthe am ount of dilution before a stable arch has form ed.In the worst situation, the intradosal zone can have aheight that is 25 per cent of the span.B y u se of a combination of join t-con dition ra tin gs an djo in t- or ienta tio n dat a, a condi ti on /o rient ati on perc en tagec an b e d erived. T hese percentages are sh ow n in T ab le X I.T hese percentages can be used to define areas requir-ing support as follow s:

600/0- 70% Highly unstable, collapse with blast,r equi res p resupportJOURNA L O F THE SOU TH A FR IC AN IN STIT UT E O F M IN ING AND MET ALLURGY OCTOBER 1 99 0 26 9


14/17

~=--~80

STABLE(L OC AL S UP PO RT )

70

en

0SUBSIDENCE

CAVING

20

/0

0/0 20 30 40 50 60

ST AB IL IT Y IN DE X = H YD RA UL IC R AD IU S = PE~~~tTEREXAMPLES0 STABLEA TRANSITiONAL. CAVING

F ig . a -S tabilit y/i ns tabili ty d iagr am7011,10- 80% U nstable, tim e-dependent falls, m ayrequ ir e p resupportRelatively stable, requires support orsc alin g, e ve n lig ht b la stin gStable.80% - 90%90% - 100%In the case of bedded deposits, thinly bedded andm assively bedded zones m ust be rated as distinct units.B ed separation occurs in the thinly bedded zones, w hilethe m assive zones contribute to the stability.

CavabilityThe joint patterns bear directly on the cavability andfragmentation of the rock mass, and can be used inassessm ents of whether a cave-m ining m ethod can beem ployed. It is im perative that the hangingw all zone forat least the height of the orebody should be classified.D iagram s like that show n in Fig. 7 are used to define theundercut area for different rock m asses.FragmentationC aving results in prim ary fragm entation, w hich is the

27 0 O CTO BER 1990 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

p artic le siz e d ev elo pe d in th e fa ilu re z on e o f a n a dv an cin gcave . P rim ary fra gm entation is d ete rm in ed by th e stress-es in the cave back and by the strength(condition) andorientation of the joints w ith respect to those stresses.The size of the potential rock blocks is based on theadjusted FF/m in Fig. 4.Secondary fragmentation is the breaking up of theprim ary rock block in the draw colum n. For com minu-tion to occur, the stresses generated must exceed thestrength of the rock block, w hich is unlikely if the blockis moving an d cush io ned b y finer m aterial or so fter rock s.T his is e vid ent in heteroge neo us orebo dies that con tainclasses 3, 4, and 5. In these cases, class 4 and 5 zonesfragment readily, but class 3 zones arrive at the draw-po in t as large b lo cks e ven tho ug h th ey co ntain joints w ithh ig h jo in t-c on ditio n ra tin gs.

Extent of Cave and Failure ZonesThe result of a block cave is the formation of a zoneof cav ed m ate rial that ha s d ifferen tial rates o f movemen tw ith in b oun da ries d efine d b y the cav e an gle . B eyo nd thecave boundary, a failure zone is developed with frac-tures(cracks) and lim ited m ovem ent. A s show n in T ableXII, the strength of the rock m ass, the am ount of draw-down, and the major structures dictate the angle of thecave and the extent of the failure zone.

M ining Method as Related to MRM RTable XIII show s how the M RM R varies with m iningmethod.

P il la r Desi gnP illa rs a re d esig ne d to e nsu re re gio na l sta bility o r lo ca lsupport in stopes and along drifts, or to yield under ameasure of control. In all cases, the strength of thematerial and the variations in strength must be knownboth for the pillar and for the roof and floor. The shapeof the pillar w ith respect to structure, blasting, andstresses is significant, and is catered for by the adjust-m ent p ro cedu re. F or ex ample , fo r a w id th -to-heigh t ratioof less than 4,5: 1, the following formula uses SI andDRMS8:

W,SP illa r s tre ng th Ps - k F,7'where

k = D RM S in M Pa W = 4 x - Pill ar a rea (SI)P il la r per imete r'H = height.

Initial D esign of Pit SlopesTable XIV can be used in the design of the initial pitslopes. If the rock mass is homogeneous, the anglessho wn are c omparative ly accu rate. H ow ev er, in a h etero-ge neo us roc k m ass, th e c lassifica tio n data of th e signifi-cant feature m ust be used. For exam ple, a shear zone dip-ping into the pit with a rating of 15 w ould dom inate evenif the rest of the rock mass had a rating of 50.OVERVIEW OF THE SYSTEM

Table XV gives an overview of the MRMR system .


15/17

Dip fr om v ertic al. D ip towa rd s v er tic al.Conditionrating 0-40 40-60 60-80 80-90 90-80 80-60 60-40 40-00-10 60 65 70 75 75 80 90 9011-15 65 70 75 80 80 85 90 10 016-20 70 75 80 85 90 95 10021-25 75 80 85 90 95 10026-30 80 85 90 95 lOO31-40 85 90 95 100

MRMRI MRMR2 MRMR3 MRMR4 MRMR51. C ave A ngleD epth, m Unres Res Unres Res Unres Res Unres Res Unres ReslOO 70-90 85-95 60-70 75-85 50-60 65-75 40-50 55-65 30-40 45-5550 0 70-80 80-90 60-70 70-80 50-60 60-70 40-50 50-60 30-40 ~'()-502. Extent of

Failure ZoneD epth, m Surf. U /G Surf VlG Surf VlG Surf U /G Surf V lGlOO IOm IOm 20m 20m 30m 30m 50m 50 m 75 m lOOm500 IOm 20m 20m 30m 30m 50m 50 m lOOm 75 m 200m

TABLE XIPERCENTAGE ADJUSTMENTSFOR DEGREE OF DIP

.Angl es f rom hor iz on ta l.TABLE XII

THE ANGLE OF CAVE AND THE FAILURE ZONE

Unres = No l at er al r es tr ai ntRe s = L at er al re st ra in t

CONCLUSIONSThe RMR/MRMR classification system h as b een in u sesince 1974, during w hich period it has been refined andapplied as a planning tool to num erous m ining opera-tions.It is a com prehensive and versatile system that hasw idesp rea d a cceptance by m inin g p erson nel.The need for accurate sam pling cannot be too highlystressed.T here is room fo r further im pro vem ents by th e ap plica-tion of practical experience to the em pirical taL les andcharts.The DRM S system has not had the same exposure buthas proved to be a useful back-up tool in difficult plan-n ing situation s, a nd has been u sed successfully in m athe-matica l model ling .The adjustment concept is very important in that itfo rc es th e e ng in ee r to re co gn iz e th e p ro blems a sso cia te dw ith the environm ent with w hich he is dealing.

ACKNOWLEDGEMENTST he contributions of H .W . T aylor, T .G . H eslop, A .D .

Surf = At sur faceU/G = Underground

Wilson, N.W . Bell, T. Carew, and A. Guest to thed evelopment and app li ca ti on o f th is c la ss if ic at io n sy stemare acknowledged .REFERENCES

1. B IFN Ii\ W SK I, L T. Engineering classification of jointed rock m asses.Trans. S. Afr. Instn Civ. Engrs, vo\. 15. 1973.

2. Li\lIBSCHER, D.H. Class distinction in rock masses. C oal, G old,B ase M in era ls S. Afr., vo\. 23. Aug. 1975.

3. Li\lIBSCHER, D.H. Geomechanics classification of jointed rockm asses-m in ing ap plication s. Trans. Instn Min. Metall. (Sect. Aj,vol. 86. 1977.

4. Ti\YLOR, H.W. A geomechanics classification applied to miningproblems in the Shabanie and King mines, Zimbabwe. M .Phi!.Thesis, Univ. of Rhodesia, Apr. 1980.5. Li\IJBSCHER, D.H. Design aspects and effectiveness of supportsystem s in different m ining conditions. Trans. Instn M in. M etall.(Sect. Aj, vo\. 93. Apr. 1984.

6 . EN GIN EER S I NT ERNi\T IO Ni\L (N e. C av in g m in e ro ck m ass classifica-tion and support estimation-a manua\. U.S. Bureau of Mines, con-tract J0100103, Jan. 1984.

7. HOEK, E., and BROWN, E.T. Underground excavations in rock.London, Institution of Mining and Metallurgy, 1980.

8. STi\CEY, T.R., and Pi\GE, CH. Practical handbook for under-ground rock m echanics. Trans Tech Publications, 1986.

JOURNA L O F TH E SOU TH A FR IC AN IN ST ITU TE O F M IN ING AND META LLURGY OCTOBER 1990 27 1


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IR S0-20RQD0-15JS0-25 orFF/m0-40JC0-40Majorstructures.

TABLE XVOVERV IEWOF THE MRMR SYSTEM

Weathering30-100070

r

Orientation63-100%

I

0 RMS100 RMRIPresentation

I. .CommunIcatIOnI.Basicdesign

Induced s tr es ses60-120%

IdjUstment ~ DRMS30-120% MRMRID etaile d d es ig n:

support, stability, cavability, sequence,d rift o rien tatio n, area o f in flu en ce, p illars,excavation geometry, initial design of pitslopes

Blasting80-100%

I

Technology developmentInnovation in South African equipm ent*

T he synergy of South A frican research and m anufac-tu rin g h as re su lte d in se ve ra l h ig hly in no va tiv e meta llu r-g ica l dev ice s in recen t y ears.C arb on -c on ce ntra tio n Mete rThe latest of these, the ultrasound-based carbon-concentration meter, was developed by Mintek witho riginal spo nso rsh ip by the C hamber of M ine s R esearchOrganization, and is now being manufactured andm arke ted w orldwide b y D ebex E lectron ics in Joh ann es-burg.D esigned to achieve precise on-line m easurem ent ofcritical carbon-concentration levels during the gold-recovery process, the novel instrum ent m akes possibles ignif ic an t imp rovemen ts and cos t s av ings in me ta ll ur gi ca lgold-recovery plants, and has excited international in-

terest as a new and valuable m etallurgical tool.D uring the carbon-in-pulp gold-recovery process,carbon granules are added to the gold slurry, whichfollows the initial cyanide-leaching stage. The gold-cyanide com plex w ithin the slurry is deposited onto thecarbon granules, and the carbon-concentration level isth ere fo re c ritic ally importa nt fo r o ptimum gold re co ve ry .B efore the developm ent of the new instrum ent, therew as n o w ay of con tinu ou sly and ac curate ly assessin g thislev el throu gh th e six to e igh t abso rp tion stage s in vo lv ed,since ca rb on -in-pu lp is p umped from on e tan k to a notherin counter flow to the flow of pulp or slurry.G iinte r Sommer, th e D irector of th e M easu rem en t andCon tro l D iv isio n a t M in te k, sa ys th at th e c arb on -c on ce n-tratio n m eter is an in ternatio nal first and w as de velope d. Released by Group Public Affairs, De Beers Industrial Diamonds,p.~. Box 916, Johannesburg 2000. The Debm eter system. On the left, cutaway views of the de-a era to r ta nk a nd u ltra son ic tra nsd uce r arra y system. T he slurrypresentation system is on the right of the draw ingJOURNA L O F TH E SOU TH A FR IC AN IN STIT UT E O F M IN ING AND META LLURGY OCTOBER 1 99 0 27 3

1990 Laubscher Geomechanics Classification System

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