Chapter 11 Rock Support And Reinforcement

1 1

andreinforcement Rocksupport 11.1 Terminology The term supportis widcly useclto describeüe procedüresand lnatei s usedto in provethe stabilily rnd maintainthe load carryingcaprbilily otrcck nearthe boundaries of undergroundercavations.As will be shoivn in this chäpßr the primar\ objectiveofsuppor präcriceis to mobiliseand coDseNcthe inhercmslrengthoflhc rock massso that it becomesselt-snpporiins. In accordwith modernpracticepaticularly in Arstralia. adistinctionwi11benade betwecn the Lennssupport and reinlbrcement,using the detinilions introducedb\ Windsof and Thompson (1993). Support i,i tfie applic:rtionol a reäctlvelbrce to rnd includestechnique!xnd devicc\ suchrs lin1ber.llll. üe surläceofan excav.tLion drotcfete,nrcshmd sleelorconcretesetsor line.s.Reinlbrcem€nt.on the olherhand. is a meansofconscr\'ing or improving the overallrock masspropertiesfiom within the rock massb) lechniquessuchas rock bohs,cablc bolts and ground anchors. hlvas oDccthc customto describesupportas beingtcnporary or permanent.l€mporary support rvasthat supportor reinlbrcementinstrllcd to cDsuresale workins conditioD,jduriDgmining.Forcenturies.suchsuppoftconsistedof sornelonn of linr bering.lfth€ cxcä\ationwas requiredlo remain openfof an cxtendedperiod of Iin.. permänent support was installedsubsequendy. Qtrilc often. the LenporaJysupton was p{rll} or wholly renoved lo erable lhc pcmanent supportto be inslalled.A\ rhal can b. will be denonstfuledin secdonI1.2, this practioenegatcsthe advanLage obtainedby applying the principles of rock-suppoft inte.rction mechanicsanLI\o qhouldbe avoided. Modem pracriceis to describethe supportor relnfbrcementof permanenrexcavatioDsasbeingpriinaryorsecondary.Prim.lsuppoforreintbrcementisapplied during orinn1edilrtelyafterexcavation.to cnsurcsaleworkingconditionsdunng subsequenreicavation. ardlo initiatelhc p.occssoftnobilising and conservingrock 'na!. The primary supportor reinforce' sLrenglhby cortrolling bornda.y displacernenls. mcnt will form part. and may forn the lvholc. of Lhetolal supporlor reinfbrcemenr rcquired.Ant, additionalsupportor rcinforae enl appliedat a later stageis terme,i Ii was once common p.ictice to regard stopesas temporaryercavationshavins dilltrent suppof requirementsfrom the morc permanentmine installationssucbrt nnjor accessways. haülages,crushcrchambers.workshops.punping staiionsand shafis.lndeed,this distinctionn1 y still be made,parliculaflyin the mining of naft)u orebodieswherethe supporttcchniquesusedin the vicinity ofthe face may be quire diflerent from those Ned for pennancntmine irstallations.However.many large. scalel]1elalliferousminesnow use mechaDiscdstopingnethods in which indn'idurl stopesmry be rery large and may have operationallivcs mcasurcdnr yexrs rath.r than weeksormontbs.In thesecases.the supportand .einforcementtechDi.tuc\u\.d may havemuch nr comnon with lhosexsed for permanertnine irstdllatilD$ and in cjvil enginceringcoDstrucLion.

312

PRINCIPLE5 AND REINFORCEMENI 5L]PPORT

may also be classifiedax beingeirheractiveor pasSupportor reinforcement sive.Active support imposesa predeterminedload to the rock suditceat the tlme of installätion.ll cantakethelbm of rensionedrock bolts or cables.hydraulicprops.ex pandablesegmentedconcrctelinings or poweredsupportsfor longwall faces Active supportis usuallyrequiredwhenit is necessaryto suppot the gravity loadsimposed by individual rock blocks or by a loosenedzone ofrock. Passivesupport or rein forcementis not installedwith an appliedloading,but rathet devetopsits loadsÄt tbe rock massdeforms.Passivesupponmay be providedby steelarches,timbercdsets or compositepacks,or by untensionedgroutedrock bolts,reinforcingbarsor cables Untensioned,gouted rock bolts, reinfbrcing bars and cablesare often descnbedas The telm strata control is usedto describethe suppot and reinfbrcemeft rechit evokesa conceptof ihe niquesusedin coalmin;ng.Thetem is a goodonebecause control or limitation of stratadisplacemenlsratherthanoneof suppod Nevertheless, suppot in the strici senseis a najor fünction of someshalacontrol measures,nost notably of hy&au1icpropsusedinmediately behindthe läce in longwall mining. Becauserhisbook is concemedwith all typesof modernundergroundrnining, ihe tems süpportandreinforcementwiil be usedin preferenceto stratacontrol ln the presentchapter.emphasiswill be Flacedon the principlesandmajot techniquesused in goodsüpportandreinforcementpraclicefor lnining excavationshavinganextended life and for large undergroundexcavationsgenerally-Techniquesusedin pariicular will typesof mining,includingtheuseof filI andlongwallstratacontrolme.tsures, chapters. in subsequent be discussed ll.2

prin(iples Supporldnd reinforcemenl

in Figure1i.1 in whichaheadi'gis beingadvanced Considertheexanpleillustrated by conveniionaldrill ändblastmethods.The pre mining stateofstressis assumedto be hydrostaticandof nagnitudep0.Blockedsleelselsäreinstalledaftereachdrill and thedevelopment of rndialdisplacement concems blastcycle.Thefollowingdiscussion andradial support'pressure'at a point on the excavationperipheryat sectionX X as the headingprogressivelyadvancesto änd beyondX X. In this discussion,the tern süpportwill be üsedthroughoutallhoughthe processinvolvedtnay be one of supportand reilrforcemenlor reinforcemen!alone.Following customaJyusage,the equivalentnormal stressapplied!o the excavationperipher] by the suppodsystem, will be temed thesuppotpressure. ln step I , the headinghasnot yet reachedX-X andthe rock masson the periphery pi. acting prolileis in eqüilibriumwith anintemalsüpportplessure. of theproposed po. änd opposiG ro equal ln step 2, the fäcehasbeenadvancedbeyondX-X rnd the suppof pressure,A. previouslyprovidedby the rcck ins;de the excavationperiphery,hasbeenreduced to zero. However.the äpparendyunsuppoted sectionof the headingbetweenthe face and the last steel set installed,is constrainedto someextent by the pmtimity of the face-Figue 1L2 showsthe developmentwith dislancetuomthe fice of radiäl at the peripheryoi a circulartunnelin an elasiicmaterialsubjectto displacement a hldrostatic I,r sit l strtss field. In this case,the zoneof influenceof the face may

313

R O C K5 U P P O RATN D F EN F ( ' F C E M E N T

f'r il

al tt t__J ^

tl fi

lll

u ----!

eilJn Fisure l1.r (al Hypolhetical ple ol r tunnel being advancedbt rrI taceddll andbldi nethodswith blockedsteelseßbeinCinndled after (b)therad'aLsup cachnuckingcyclei por.pressur€iisplacenEnt cuNesfor the rcch m* turdthe supponsyscfr

i

Figure 11.2 Distnbulioi n€d dre Ia.e. of the üdial eh$ic displaccnent. ,i, of the&cultr bonndaryof I tunnelof radius,a, in a hydrcslatic strelsfidld, ,,0. nornalisedwith ß ste.t to thc pllne struindistlrcement,

314

l0

SLJPPORT ANI] REINFORCEMENT PRINCIPLES be definedas 2.25 radii. al rvhich disrancelioin rhe frcc. lhe radial displacementis within approxinarely5E ofthe comparableplane sträinvalue. The graphin Fignre I1.1 showsa plor of the rrdial suppon pressure!/r, required al a point to linrit the radial bonndary displacenent.öi. io the value given by thc abscissa.If the restraintprovidedby Lheface ar step 2 wcre not alailab]e, inrcmil supporlpress'tr.es givenby the ordinatesofpoints B and C would be requiredto hnit the displaccmeltsto their actuallalucs. Differenrcurvesäre shownfor rhe side walls and lbr the rool Extra srpporr pressureis requircdto limit rhe displacenlentot' the roof to a pnrtictrlu value becauseoflhe extra load irnposedb)r the action ofgravily on loosenedtuck in the rcof. By step:, Lheheadins has been muck€d out and sreel rers have been insralled close ro the face. At rhis sLage.the sets carry no load becauseno defonnarionof the rock has occnrredsiDcetheif installarion.This lssumesüat rhe rock massdoes not exlxbit time-dependentstrcss-strainbebrviour.On ihe graph in Figure 11.1.the radial displacenrntr ofpoints in the roof and in the side wall. are slillrhose given by poinrs B and c. In step 4. the headingis advancedabout one and a halftunnet diamete.sbeyond X X by a fwther cyclc ofdrilling andblasting.Thc rcsrraintoffercdby thc proximity of the face is now negligible. and there is lurther radial displäcenent of rhe rock surtaceat X Xas indicatedby rhe curvesCEG andBFHinFigure I LI. This induces load in the sleel sets which are assxned ro show linear radial srrc,ij displacemenL behaviour Thus rhc lupports typically load along a path such as DEF. known as the support reaction or ayailable support lire. The curve represenringthe behaviour of lhe rock massis known a! the gound chäracte.istic or rcquiftd support tine. Equilibrium betweentherockand the sleelsetsis reachedatpoinr E for ihe side wall and poinl F for the rcot it is imponanr to norerhar mosr of rhe redisrributedstress arising from creationof the excavationis canied by the .ock and nor by the steel

OFG Radial dhplaenenr, 6i tilu.€113 Ilnßralionof dieinn! -i. .f supp.frltiliress aDdol üc jrns of rs in*all!tun on lupporr

If steel sets had not been installedaibr the läst rwo ilages of headnrgrdünce. the radial displaceneDtsat X X wouid have incrcasedalong the dashedcunes EG änd FH. In the caseof üe side walls, equilibrium would havebeenreachedat poinr G. However.the suppofl prcssurcrcquired to limil displacementof the roof may drop to a minimum and then increaseagain as rcck becomesloosenedand has ro be held up. ln this illustr"tjvc example.the roof wo! d collapseil no strppor were providedThe .ational designof suplon and reinforcemenrsystenrsmust lake inro accouDt lhe inleractioDbetweenthe supportor reinforcing elemenrsänd the rock mass,de scribedquilitatively for this simpleeranple.lt is clearfion this analysisihat conhol of rock displacementsis the major r6le of supporrand reinlorccmenrsystems.As Figure 11.1 shows.enoughdisplaccmentmusr be allowed ro enablerhe rock rnas! strcngthto be mobilisedsulficientlyro restrid requißd suppon loadsro practicrbte levels. However excessivedisplncement,which would lead to a toosennrgof the rock mass and a reduction in its load-carrynrgcapacity.must nor bc permitied io The stifinessand the time of installarionof the supportelemenrhavean importanl inltuence on this displacementcontrol. Figure ll.3 shows a rock suplorr interac tior dragramfbr ä problen similar to that illusrratedin Figure 11.1. The ground characleristicor.equired suppon llne is given by ABCDE. The earljestpraciicable

315

ANII REINFORCEMENT ROCKSUPPORT time at which supponcan be installedis älter radial displacenentofan ämountOF support l isinnalled atFandreachesequilibriunwiththerocknrässatpointB. Thls support is too stiff for the purpose and aitracts an ercessjve share of the redistribured the süpportelementsmay failcausing catastrophictailure of load. As acons€quence. the.ock sunoundingthe excavxlion. Support2, having a lower sliffness,is installedat F .tnd reachesequilibrium with the.ock massat C. Providedthe correspondjngdisplxcementof the peripheD'ofthe exclvationis acceptäbleoperalionally.this syslemprovidesa good solution The rock mNs cäies a major portion of the redisrributedload. md üe supportelementsare not str€ssedexcessjvely.Nole thai if, asin the temporäry/pernanentsupportconcept, this suppod wcre to be removed after eqüilibdun had been reached,uncontrolled displacementand collapseof the rock miss would älmost certainlyoccur Support 3. having a much lower stiffnessthaD support2, is also installed at F bul reaches equilibrjum wilh the rock mass at D wherc the rock mass has stated to loosen.Although this n1ayprovide an acceptabletenporary solution.the situationis a dangerousone becanseany extra load imposed,for exampleby a redisrributio. of withnearby mining. will hrve 1()be carriedby the supponele'nents. stess associated ln general,support3 is loo flexible for this particulärapplication. Suppoit,l, ofthe sane iype and sriffnessas supporr2, is not installeduniil a radial displacementoftherockmassofOG hasoccuned.In this case.the supportis insralled 1oolate.excessiveconve4enceofüe excavationwilloccut andthe suppot elements beforeequilibriunis reached wil probably becomeoverstressed ln Figures11.1and 11.3.constantsupportdflnesses are assumed.In pracrice,thc stiffnessesofsupport.tnd reinforcingelements:[e usuallynonlinear. Figure I l.'l ii lüstratessomeof the cffectsthat Inay arise.Thereis ofteninitial nonlinearbehaviouf becaüseof poor or incomplele contactbetweer the rock and the support systerr Figure11,4 N)n linetusupporrEaclioncuryes.bswed1orsonesup

Radial diphcemenr. d,

316

ANALYSIS ROCKSUPPORT NTERACTION Concrete and shotcrete n1aycreep as ihey cure, as nalr grortcd rock bolis and do\tels. The suppartsystemswith the poo€st stiffnesscharacteristicsde those using intermiuent blocked steel or tinber sels. Even if well installed, tnnb€r blocking provides a very ltexible element in the syst€m. Steel sets älso suffer frolrl the disadvartage that thcy often fail by sideways buckling. Frcm theseconsiderationsof rock srpport inteücdon mechnnics,it is possible to develop a set of principles to guide süpport and reinforcementpactice. These principles are not meant to apply to the cäse of providing support fbr the self-weig|t of än individual block ofrock, but to the more generalcasein which yield ofthe rock masssunoündingthe excavationis expectedto occur

(r) Inslall the support and reinforcement close to ihe fäce soon after excavatron. (ln some cases,ir is possible.and advisable.to inslxll some reinlbrcenent befbre excavation.This caseof ple-placedreinforcementor pre reinforcementwill be discussedin seclion 1i.4.) Thcre should be good contact between the rock mass and üe supporl and rein-

(o The defornabitiiy

of the supl)on and reinforcemenl syslem should be such that i t can conform to and acconmodate the dilplacelnenls of the excavation surface. (d) Ideally. the s pport and reinforcementlystem shouldhelp preventdeterioration of the mechanicalpropertiesof üe rock mass with time due to wealhering, .epeäted loading or wear. (e) Repeated removal and repiacement of süppot änd reinforcing elements should

(t) The süpport and reinforcement system should be readily adaptable to chatrging rock masscondilionsand excavationcrosssecti(,r. The suppona.d reinforcingslslem shouldprovideminimum obstructionto the excavations and the working face. ( h ) The rock nass suffoundingthe excavalionshouldbe disturbedaslitt1easpossible during the excavation process so as to conserve ils inlereni stength. (,) For accessesand other infrafucture excavaiionsunder high strcssconditbns, support and reinforcemenl performance can be improved by "closing the ring" of shotcrete or a concretc liiing across the ltoor of the excavation.

11.3

Rock-support interaction analysis

ln sectbn 7.6. a solution was given for rhe radiusof tbe yield zone and ihe stesses within the yield zone tollned around a circular ercavrtion in massive.elasticrock subjectedto an initial hydroslaticstresslield. Ertensions of anallses of this llPe to include more rcalistic rock rnassbehaviourand to include the calcul.tlionofdis placementsal lhe excavationperiphery,can be usedio obnin numericalsolutionst) rock supportinteractionproblems. 7.6andillüsträtedinFigue7.20, In Lheadsyrnmetdcploblemconsideredinsection with lcttherockmasshavea Coulombyield criterionin whichpeakstrengthcoincides yield and lhe stress strainbehaviouris as shown in Figure 11.5.Nole that dilatancy äccompanies posl-peat deformation of the rcck mats. As belbre, the limiting states

317

ROCKSUPPORT ANDREINFORCEMENT of stress in the elastic and fracnlred rock are given by o

8

(o

bo-

(7.8)

and

(7.9)

01 = do\ The principal stesses within the fractured zone are q = lsadienll=/

' 'P' \t n( L) \ "

",,:

and

,r.=.s^_dp,l,)"'

(7.r1)

and the radius of the fraclured zone is

f 2P

Callt\d t)

L(l+ r)p l

(7.15)

Theradialstresstransmittedacrosstheelastic fracturedzoneinteface arradius. = .. Figurc 11.5 Idedised elßtic-brlrtle stres strin frodel (after Brcsn

2P co

(7.14)

In the elasticzone,the radial displacementproducedby a redücrionof the rädi.tl stressfrom p to ]?r is

(p

p'),? 2Gr

b

p),. 2G

ff no fractured zone is formed, the radial displacementat rhe peripheryof rhe excavation(r.. : d) is

"_

,c

(11.r)

Note thai ihe rädial displacement,z, is posinve outwärdsfrom rhe cent e of rhe Wilhin the ftacrued zone, for infinitesimal strain and with conpressivestrains positive,considerarions of the compatibiiig,of displacemenisgive

(r1.2) and

( rr . l ) 318

ANALYSIS ROCK SUPPORI NTERACTION

Assumethat within the fracturedzone e3: €r"

l(€r

€r.)

(1r.4)

where€r., €:r.arethestains at theelastic-plasiicboundtuyandf is anexperimentally deiermined constant asdefinedin Figure11.5.Substitution of a"=

t.-=

(p

pt) ._

(|| s)

into equation I 1 ,l and rearranging gives

4:-Jt\

(1- J)\! !!

(11.6)

FIom eqnationsi 1.2, 1i.3 and 11.6

du -J ,u -.(p - ,r') + O - l)ä: dt: The solution;o this differentialequationis

u

c,

ttt, p,lt tltl 2 C i. l) ) |

where C is ä constänl of integratio. which may be evalüäted by substituting the value ofel at r =.. given by equrtion I1.5. This leadsto the solution

u

;=

\t-p)\ fr/-lr

G(r+rlL ,

. /r.\r+rl -\;/ l

(11.7)

EquätionI 1-7cänbeusedto plota relationbetween radialdispläcenrnt,genemlly represented by ör : -,i, andsupportpressüre,Ä. at the excavationperipherywhere / = d. Theditrerences thedisplacements between experienced by therockin theroof, sidewallsandfloor canbeestimatedby assumingthat,in thefloor theresultantsupport pj , tessa pressüre pressure is theäppliedpressure, thatis equivalent to theweightof the rock in the f-racturedzone,.y(r" a). In the sidewall,the suppot prcssureis p. Dd in the roof, gravity acts on the fractued zo.e to increasethe rcsultantsupport pressure Io pi '|'ylre - aJ. Consideras an exanple, a circular runnel of radius.r : 3 m excavatedin a rock masssubjectedio a hydrostaticstresslield of : 10MPa.Thepropertiesof therock massare1 : 25 kN m 3, c = eooup4 f" =2o,ö=45", OJ = 30' andC= 2.414MPa,whichgivelheparameier valuesb = 5.828,C0= 11-657andd = 3.0An internalradial suppot pressureof pj : 0.2 MPa is applied. Fron equation7.16,the radiusof ihe fracturedzoneis calculatedas f )^

+

.^1'd-'

" L#tr l+ r r ? , I

_ . . 1 t 5m

ard theradialpressureatthe interfacebetweenthe elasticandfractwedzonesis given by equation7. 1,1asI r : 1.222MPa.Theradial displacementat ihe tunnelperiphery is thengivenby equationI1.7 as b, :

319

,, :0 228in

AND REINFORCEMENT ROCKSUPPORT for sanlle problom Table11,1 RequiEdsuplonlinc calculations l0

.1

2

1.222

05

0.2

0.t 10.,187 0228 0.612 0ll0 lj.t37 0.3t0 0.287 0.090 ( 0.087)

i , l o 4 690 r..1li poor = Pi+.r(,i pdoor= rL t(.c

d)(MPa) dXMP.)

0 0 015 0000 t0 4 ,1 t0

0020 2 2

0 . 0 2 2 0421 0.063 \ 222 1222

0 008 0 0 4 2 1.008 0 5 4 2 0.4s3

To determinethe grcund characterisricor reqniredsupportcurve. substilutcsuc cessivevaluesof pi in equation7.15 to obtain a seriesof valuesof r" which n.e then Radial düplacemenr.6, (m) substituledinto eqnation t 1.7 to oblain the correspondingvalüesof 6r = "i.The Iigürc 11.6 Calculatcdßqutred resulisso obrainedare iabulatedinTablc i Ll and plottedin Figure 11.6.The critical supponlinetnrthcsidcsallsin a sar supportprelsnrebelow which a fracluredzonelvill developis found by puttingr" = a inequationT.15whichgivespi-=1.222MP!.Inorderrorestrictradialdisplace mentsto valuesof Ei, calcularedfor sidewä]]sxpportpressuresof l,j, roof md floor pressurcsof Z +"y(re a) and ?i - 'y(/" - d) win be requi.ed. The complele solution of a rock support interacrion problem reqüires determinatjon of the support reaction or av^ilable support line in addition to the ground characteristicor requiredsuppot line consideredso far Using methodsintroduced by Daemen (1975), Hoek and Brown (1980) have presentedmethodsof calculat ing supportreactionlines for concreteor shotcretelinings, blocked steel sels änd ungroutedrock bolts orcables.Detailsofthese calculationsare givenin AppendixC. Figure I1.7 shows the result$of a set of calculationscanjed out lbr a sanple problem using the naterial model of Figur€ 11.5.A 5.33 m radius accesstunnel is driven in a fäir quality gneissat a depth of 120 m where lhe ir rr& stateof streis is hydrostaticwith p = 3.3 MPa. The propediesof the rock nass are o. : 69 MPa. n = 0 . s , r = 0 . 0 0 0 1r. = 1 . 3 8 G P 4r = 0 . 2 . J : 4 2 . , ? , = 0 1 s , : 0 a n d 1 . : 20 kNm 3. In this problem,the self weighr of the fracturedrock aroxnd lhe tunnel as shownin Figure 11.7. has rn imporiantinfluenceon radial displacements. The supportreactionor ävailablesupportline for 8I23 s|eelsetsspacedat l 5 nr centreswith good blocking was caiculatedusing thc following input data: lv: j ma, I], = 207 GPa. 0.1059 n. x : 0.2023 m, .1. = 0 0043 m'?,.I, : 2.67 r l0 : 0 = 2 5 m , E n 1 0 0 G P aa n d ö r o : o y . = 2 4 5 M P a . S : 1 . 5 m . 0 = 1 1 . 2 5 ' ,l B 0.075 m. The availablesuppo( provided by thesesteel sets is shown by line I in Figlre ll.7 which indicateslhat the naximum availäblesxppot pressureof about 0.16 MPa is quite adequateto stabilisethe tu.nel. Howevet becauserhe set spacing of 1.5 m is quite lärge comparedwith the likely block size in the fracturedrock. it will be necessaryto provide a meansof preventingunravellingofthe rock bet$een 0

005 010 0 15020 025

The importanceof conect blocking of sreelsetscan be demonstratedbychanging the block spacing and block siiftness.Line 2 in Figure 11.7 shows the available supponline calculatedwith e = 20' and ÄB = 500 MPa. The suppo capacityhas now droppedbelow acritjcal level,rnd is not adequateto stabilisethe tuDnelroot: Sinceit has alreadybeen recognisedthat some other suppot in {ddition to sleel sets will be required.the use of shotcretesuggestsitself. Line 3 in Figurc 11.7 is the availablesupportcurle for a 50 mm thick sholcretelaycr calculaßd using the following data:-g = 20.7 GPa,r,. :0.25. t" = 0 050 m. o.. : 3,1.5MPa. Because

320

RUC|\ 5I-]PPORT INTEFACION ANALYsS

I 3I 23 nal sß { t.5 n c.r6 2 8 I I !r..1 sß .i 1.5 n ent6

*irh lood bhriit enh p@r blo.ti.!

rchoEtl rck botß a 25 rr [email protected]. 3 m l,on3d..t.nidly ar 1.5 fl €rE6 idrlLd *hhi' 3 h .t le rncho'.il 6lo!3 tud.iiollt 5 ?5 'm diftr.r.3 Et boh. d 1.5 ß erk6 in$rl.d .bour ro fr

Rock strplort intcracIipr€ llj donanaltsisfora5.33nradirstunnel in iair qu.liq gneissara dcpih ot 120 n (arid Hoe! andBtutn. 1980).

50 75 lü) Radial dilplrc.mcnt, q (mh)

shotcretemay be placed close to the face soon after excavation. ü0 was taken as 25 mm. It is clear that this shotcretelayer has adequatestrengrhand stiffnessro srabilise tunnel displacements- Indeed, it may well be too sliff and develop unacceptably high compressive slresses within th6 shotcrete ring- Briltle tiacture of a shotcrele lining such as this shoüld be avoided. Wire mesh or fibre reinfbrcement could increase the tensile and shed strengtis and üe ductiljty of the shotcrete. Patem rock bolting is anotherpossiblemeansofprovidingpdnary supponfor this tunnel. Line 4 in Figure i1.? ;s the availäblesupportcurve calcularedfor a rockbolt system using the following parameters:1 = 3.0 m, d : 0.025 n, E6 = 2O1 GPa. d O : 0 . 1 4 3m M N ' , 4 r : 0 . 2 8 5M N , r " = 1 . 5m , r r : 1 . 5m a n dü o = 0 0 2 s m . It rppears that this patteflr of rock boldng provides a satisfactory solution. The strengttr of the rock massis highly mobilised. änd the rock bolts are not excessivelyloaded except in the roof where an adequatehad factor nay not exist. It would be preferable, iherefore. to increäse the density of bolting in the roof and to decreaselhal in the side walls and the floor. It will also be necessaryto use mesh or a thin layer of shotcrele. to Fevent uffavelling of blocks of rock from between the rock bolts. Line 5 i1lusüatesthe disastronseffect of delayingthe installationof the rock bolts until excessive defomation of the rock mass has occured. In this case, equilibrium of lhe rock in the roof änd the support system cannot be reached and roof collapse

321

R O C KS U P P O RATN D R E I N F O R C E M E N T

The analysispresentedsofar is a very simpleone.Il usesä simplifiedconstilutive model for the rock mass and appliesto an axisynnetric problen modified only by an empirical corection for the influenceof gravity. A range of analytical and for otherboundarycondilionsand semi-analytical solutionshavebeendeveloped the Hoek-Brown empirical rockmassstrenFhcdterion constiruiive models.inclüding (e.g.,Anagnostou flow rules and Kovari, 1993,Brown4d1.. 1983, andnon-associated 2002. Cananza-Tores and Fairhurst, 1997, 1999.Detoumey Carranza-Toffes ,t dl., useful means of reducingthe 1987, Pänel, 1995, Wang. 1996). A and Fairhürst, transfomations andscaling complexity ol lhe solutions is to adoptthe mathematical (1987). (1993) a.d Kovdi Detoumey and Fairhwst Anaenostou methodsusedby (1999). äre usuälly and Fai]hu6t The results ofthese analyses andCaffanzaTonens preseDted in dimensionless form as in the exanpleshownin Fignre11.8.ln this ground zonemdius,€ =.e/r, are reactioncurvesandthescaledplastic example,lhe shownfor a sectionfive diametersremovedfrom the face of an advancingtunnelin a rock massthat satisfiesa Hoek-Brownstength criterion andis subjectto an initial hydrostaticstresslield of magnitudecro.Solutionsare giveDfor a ser of selected pitrmeiers andlbr düeepossiblelalues of the GeologicalStrenglhIndex, GSL Although analylicalsolutionssuchasthoseoutlinedabovernaybe of väluein pre lininary studiesof arangeoI problems,mostpracticalündergroundmining problems in Chapter requiretheuseof numerical methods of thetypesdjscussed 6 for theircom haveall pletesolution.Finiteelemeni,linite difference anddistinctelemenrmethods purpose. using the finite dif The resultsof calculationscardedort beenusedfor this I1.8. Figure 1l.9 shows the fercncecodeFLAC'" areshownsuperimposed onFigure J(dLrion rl r { 2000, ü.ing n ACrD for e rore curre\cJl.uhledb) Leacf. a/. s..,und geometncallycomplexcaseof extractionor productionlevel drifts i' the Premier blockcav;ngmine,SoüthAfrica.Thegound reacrioncunesshownin Figure11.9 arefor severallocationsalonga producliondrift with respecrb üe undercurface(see Chaprer15 for an explanationof theseternls).Thesecurveswere xsed to estimate to acceptable levels. thelevelsofsuppot prcssure requi.edto limit drift closüres

11.4 Pre-reinlorcement In sone cncumslances,it is dillicult to provide adequatesuppoft or rcinforcement io rhe rock masssufficiently quickly after the excavalionhasbeennade. If suitable accessls available.it is often practicableto pre reinforcetherock massin advanceof excavation.ln othercases,extrarcinforcemenrmay beprovidedäsparl of thenornal cycle,in anricipationof higher stressesbeing irnposedon the rock ät a later stägein rhe lifb of the mine. ln mining applicarions,pre reinforcementis often provided by grouied rods or cablesthat arenot pre-tensionedandsomay bedescribedasbeingpassiverätherrhän active.Snchpre-reinforcementis effectivebecauseit allowsthe rock nass to defom in a controllednanner ändmobiliseits stength, but limits the amountof dilation and looseningthat csnoccur The efiectivenessof this lorm of reinforcemenl subsequent is critically dependenton the bondingobtainedbetweenthe reinforcing elementand the grout, andbetweenthe grout andlhe rock. The initial najor useof pre-reinforcementin undergroundmining wasin cuFandthecrownsof cut-and-fill fiI miring (Fuller.I 981).Theuseof cablesto pre-reinforce

322

ss

v : 0'(nd'.sociatennownls)

GSI

n

40 30

12 0.8

s

G lcPal

3.910. t.310.

2.2 t2

t'

vV" (b)

----

GrcundRdcrion Cwe lxtenl ofplEtic zone

: { GSI=30 tlgüre rl.8 Analysn of conve! Fn.. and the exient .l the ph*i. ^{. in atunnel1i r ro.L nas haling . Hck Brown nreigth ..ienon and ßatecr h an idtlal hydro$alc $es i.id (x) problcrndclinition turddatai l, -sround rea.tion .ufles aid nor GrlNd eie.t ofthe pla*1c z.ie ai dron SS' lof rhicc valrcs of GSL lü. co*es tu! valucs calculatcd us .! üe code FI-ACrD (arrer CaraDza' and rairhürst, I 999) lveni

E 5

z 025

1.0

1.5

]0

Nomaliserl wallßdialdisrlscnen! D/2! I-"/"1 slopesis illustraled in Figure 11-10-Ar a given slage of mining (Figure 11.10a). cablesare lnstalledto relnfbrcethe rock massover thrce or four lift! oflniDing. The citblexare inslalledon apprcximatcly2 m lquare gridst this spacingmay bereduced or inc.caseddcpcndingon the rock nass quality. Cablesare inlralled normal to the rock suface when they are usedfor general pre-reinfofcemenr. If shear on a padicular discontinuityis to be rcsisted,thecrbles shouldbe insialledat an angleof20'-,10'to lhe disconlinuni As illustraledin Figure 11.l1a. cäblesnrstalledin cur and fill srope backsm.y also be usedro providesomepre reinforcementto the hangingwäll.

323

AND RFNF')R'INlENT ROCli 5L]PPORT

7.

i

a

a 'a-.-

Iieü.c11.9 (n.urdre&1r.ncurlcs .alculxtedlnr s.v.ral Poirnsrong I produ.rnJn drilr ir thePrtrnierIUinc. 21)00i SourhalrfatalicrLerh.r{l

-

(d)

ligure ll.l0 ljs. .l iI lle relnaoren.nt

cxble 'lore! cu! r.d Jill

The pre'reinfo.ccmenlofhangmgwrlls i5 also imNrtant iD rhe now nlore widc\ uscclsublelel and longhole open stopiDgmethodsof miniDg If practicable.morc xnlfbrm colcrlge of the lrangjDgNallthan that illusträLedin Figures I I llt .rnd b .ry be obtainedb) installing fans oi cabler liom a ncx.by hangingwalld ft r\ nr thc caseshoün in Figure 1l.1lc. Furhcr eramplesof the p.c reinlbrcementof opcf sbpes 'vill be gi\'en jn Ch.tpterl,l. ln m{nr ol rhe early applicrlions of fully grotrrcdc.rbledoq'el renrfofcemenlard pre-reinfofccncnl.the flLllpotenLiälof the reinfo.cing s)stem \\'asnol rerlised Thi\ $as generallybccäuseof failtlle of thc grout-cablebond ard the consequentiDcftcc tive load tmnsfer bexveerrthe dcfonning rock mxssand lhe cable. Since thtt titnc cor,jidcrablealtention hxs bccn paid to tendon dcsign.rnd to installation.gmutin: rnrl restifg proccdures(e.g.Matthc{s er d1.,1986.Thompsoner.l.. 1987.Windsor

324

P R ER E I N F O R C E M E N T

l0

1 ..!)\"

/

\

l5

I It.ll

Pre{einforencnt at

Mine .JnJJ. (r) cut-sd

bicl €iiforcenen! O) loryhole {oping hmglngw.ll reinfore(c) lon8holcopenstopinghang Einfo@dent ftom a hang I ddlt (after Bourchid .r al,,

andThompson. 1993,Hoek€rdl., 1995,Hutchinson andDiederichs, 1996.Windsor, 1997.2001).As a result,theseproblems havenowbeenlargelyovercome P.e-reinforcemenr rnayalsobeusedro goodelTecrin permanentandinfrastructure excavations.Severalexämplesar€givenby Hoekeral ( 1995).Figüre I l . t 2 illusrrates ihe use of groutedreinJorcingbars to Fe-reinforce a &awpoint. For drawpoints that lrlay be heavily loadedand subjectto wear. their continucdstabiliiy js vjtaly

325

RI NFORCEMENT ROCKSLJPPORTAND

Iistrrc 11,12 Lse or grout.d ßinlorciDg bas rli pre renrlor.c a dmw|oirn ii , ldg. ncchanised mrne. The bm* dea, sho$n shaded, is bla{cd lasr, aier Einforcemenr hN beeninialled lrom the drxsp.lntand llon the trough dnr€ (rfter Hock and

xl

3mx3mbrowdea

' d c a l d r a s p o i nl rc . g l h l 5 m

inporrant in mary uDdergmtrnd mining operations. In panicular. failui: of the bro$ ofthe ercavationcan resuhiü conpiele loss ofcontrol oflhe stopedraw operation. Figure 1l 12 snowsa suitablemerhodot' pre-reinforcingthe brow sJeawith grouEd reintbrcing b.trsinsralledftom the drawpoint and from the trough drive befbre the brow areais blasted.

11.5

Support and reinforcement design

Frequently,supportand reinfo.ceüenl design is basedon precedentpr&tjce or on observationsmide, and experiencegained,in trial excavationsor in the early st.tges of mining in ä particulararea.Howevef.it is prcferablethät a morc rigorousdesign processbe uscdrDd thai expedenlialor presumptivedesignsbe suppodedby sone tbrn of analysß. Depcrding on the application.design cxlcuhtions may be of .r siinple limiring equilibnum type or may use more comprehensiveconrpütätioDal approaches involving rock supportinleractioncalculationsand taking accountofthe delbrmationand strengthprcpeniesoflhe supportand rcinforcementsystemand the completest.essst.rin responseof the rock mass.Different deüignapproachesma) be requied for threemain appli.alionsofsupport and reinfbrcement:

326

SUPPOR AT N DR E I N F O R C E MDEENSTI C N . . '

local supportand reinforccmeDt1()suppon indiridual biocks or loosenedzones on an excavationboundary; geneül or systematicreinfo.cementln which the objccLiveis to inobilise and conservethe inhereDtstrengihof the rock mass;and supporiand reinforcenent systemdesignedto rcsisl lhe dynamicloading asNcj rted with rock buNt conditions.

Staticdesigninalyseslbr the firsttwo applicalionswill be discussedhere.The nore complex caseof dynamic or rockburstlording wil be consideredin section15.2.3. 11.5.2 Local suppoft and reinforcement Two typcs of designanalysiswill be preseded here.The lirst type involves simple static liniting equilibriüm analyseswhich essenliallytreat the systemcomponents as rigidbodies and nse sinplified nodels of systemmechar cs. Thc secondtype äre more rigorousand comprehensiveanalyleswhich rakeinto accountthe deformation and slip or yield ofrhe suppot and reinfo.cingsysiemelementsand the mckmass. Design to suspenda roof beam in laminated rock As illustEted in Figure 11.i3 rockboltsmay be üsedto suspendapotentially unstableroofbeam in lan atedrock The anchoragenustbelocaiedoulsidethe potentillly unslablezone ffit is assuned that the weight of the .ock in the unstablezone is supporledentirely by the force developedin tbe rockboltsthen

(11.8)

I:lDll

/r\. where 7 = working load per rock bolt, 'y : unit wcighl of the rock. , : height of Lhexnstablezonc. rnd r : rockbolt spacingin both the longitudinal and tknsrerse

'-Eü.e 11,13 Roctbox designto sppo.. the reieht of a rool berm in

reinforced potenliallyu.stable

1 L

t

327

1 D

ROCK SLJPPORT ANDREINFORCEMENT ff, for exanpie, ? = l0tonne:100kN,'y =25kNm 3 änd, = 4 m, eqMtioD 11-8givesr = 1.0m. In this application, caremustbe täkento ensurethät the bolr anchorshavean adequatefactor of safety againstfailurc under lhe working 1oad.?. This design neüod is conservativein that it doesnot allow for the shearor flexußl strengthoI the strataabovethe abutments. Lang and Bischotr (1982) extendedüis elementaryanalysisio incorporatethe shearstrcngthdevelopedby the rock masson the vetical boundariesof the rock udt reinforcedby a singlerockbolt.The rock is assunedto be destressed to a depth,D, asin Figurc11.13.butvariablevefticalsiresses. o". andhorizontalstresses,lo,, are assnmed to be inducedwithinthede-stressed zone.T}?ically.l maybe takenas0.5. The sherr strengthdevelopedat any point on the perineter of the reinforcedrock unitis givenby c + ptrcr,,wherec is thecohesion andp = tan0 is üe coeflicieni of friction for the rock mass.l,ang andBischotr's analysisleadsto the result

r

= "/. * tk \'

. \ [r-e\pr-plD/R)l

1 R / L I e \ p ,u r r l n r l

( rr . 9 )

where I : rockbolt tensiori,,4 = äreaof roof canyi.g one bolt (: r' for a s x r bolt spacing), R = sheärradius ofthe reinforced rcck unit.: A/P, whereP is the sheärperin1eter(= 4r for a r x s bolt spacing).a : a facror dependingon the time of installationof the rockbolts (o : 0.5 for activesuppot, and o = l 0 tbr passive reinforcement), md a - bolt lengthwhichwill oftenbe lessthan,. theheightof the de-stessedzoneof rock. Lang and Bhchoff suggestthat, for preliminary analyses,the cohesion,., shonld be täkenas zcro.Designchartsbasedon equation11.9showthat.particularlyfor low vdluesofO. üe requnedbolttension,?, increases significantly asL/r decreases belowabouttwo, but thai no significantreductionin 7 is producedwhen,/r is increasedabovetwo. This result providessomeconoborationof Lang's empirical rule that the bolt length shouldbe ar ieasttwice the spacing.For a given serof data. equationI1.9 will give a lowerrequnedbolt iensionthanthatgivenby equation I1.8. Clefly, Lang dndBischofft theoryappliesmoredirecdyto the caseof the development of a zoneof reinforced, self-supporting rock,thanto the simplercase of thesupponof thebral gaviry loadproduced by a loosened volumeof rockor by a roof beamin laminatedrock. Designtosuppofta triangulatottettahedralblack. In Chxpter 9, theidentification of potential failure modesof triangular and reaahedrälblocks wxs discüssed.and analyseswere proposedfor the cises of symmetricand asyrnmetrictriangularroof pdsms.Theseänalysestäkeaccountof inducedelasticsiresses rnd discontinüity defomability, 15 well as allowing for the self-weiglt of the block and for suppon fo.ces. The completeanalysisof a non-regulartetrahedrdwedgeis morc complex. An otherwisecompletesoluiion fbr the tetranedraiwedgewhich doesnor allow for inducedelasticstresses is sivenby HoekändBrown(1980). The analysespresentedin Chapler9 may be incor?oratedinto the designproceproblemilhst.atedin Figure11.14to whichthe dure.Consider thelwo-dimensional analysisfor an asymmetrictriangularprism may be applied.ff it is assunedrhat rhe nomal stitrnessesof bodrdiscontinuitiesaremuch srcaterthanthe shearstifinesses.

328

TT JE5ICN 5 L ] P P O RATN D R FN F O R C E M E N

H0

:0 MN

Figür€11,14 Elampleolr da.gD

eqxation9.39 Inay be used.Subsliluting,ilo : 20 MN (corfespondnrgto aboundarv s t r e so s f 5 M P ä ) .o r : 1 0 ' , o r = 2 0 ' , d r r: ü = ' 1 0 ' i n e q u a l i o n 9 3 9 g i l e s t h e v e F ticalforce requiredlo producelimitirg cquilibnum ofthe pnsm as ,rr : 3.6'1MN per merrethickness.Sincethe weight oftlre pnsrn is W = 0.26 MN per neire th'ckness, it is concludedrhat thc prism \rill remrin stableunder thc inflnenceof the induced tf the wedge is permittcd to displacelerdcalll' so that joinl relaxation occurs' rhe liniting vertical force is gilen by eqüation9.'10,wirh valucsof i1 being deter mined fiom equation9.11. In the presentcäse.the post rclüation limitjng verticd force is & = 0.l8 MN per netre thickncss This is less than the value of W and so. without reinfofcemcnl.the block will be unstable Thc rcintbrcementforcc, R, required 1o n1aintaina given valne of facto of safety agailst prism failure, r'. is F = 1.5. then R = 0.14 illN per nretreihickness.This given by ll =tv nlF.It force could bc provided by groured dowels nadc lioln steel rope or Ieinibrcing

Figure 11.15 Design .l r rockboh .: üble s){em to prelenl slldiig ol r rLangulf prisn (aicr Hoek and

wereto bc completelv Ifthe stabilisinginltuenceofthc inducedhorizoDtalstresses rcnoved. it would be necessxfyto provide suppor for lhe total weight of the prism For a factor of sattiy of 1.5, the requjredequi!.rtentunifonn u)f suppon presnrc would be 0.08 MPa. a valuercadily aitainableusingpatternrock bolting Figure I 1.15shows a casein $,hich a rwo Llinrcnsionalwedge is liee to slide on a discontnruityAB. lf stfesscsinducedarouDdthe excavati{rnperipheryare ignorcd' teDsionedrock bolt or cable srppo mal be designedby considcing lin ting equr librium for sljding on AB. IfCoulonb's shearsheDgthla\r applieslbr AB. the tactor of safetyagain!t sliding is

.A + (Wcos ' + I cos€)lanir wsinü Isin0 wherc W = weight ofthe bk)ck, Ä = areaoflhe sliding suface, 7 = total forcc in thc bolts or cables,ü = dip of Lheslidirg sutface,0 = anglebetweenthe plunge of thc bolt or cablc and the normal to Llresliding suface, ., .l : cohesionand angleof füclion on the sliding surlace. Thus rhe totalforce requiredto nraintaina given faclor ofsafety is

lY(Äsinü cosü tänO) .Ä coslJtanO+ Fsh0

329

ROCKSUPPORT AND RENFORCEMENT

Figun 11.16 Local rinlb'.cnEnt &ft,n throughanoctiveleDgthofbolt {ati.r BrMy x.d Lo.ig.1988)

A fäclor of säfet! of 1.5ro 2.0 is generallyusednr suchcasesThe valtreof I.equired io mainlain a gileD valueof F will be minimised if 0 = I cot.l. Conprehensive analysis of local reinforcement. A comprehensiveanalysisof rcck reinforcenentmü( be basedon loadsmobilisedm rcinforcemenrelemenrsby thcif defonnarionand by relativcdisplacemenrbetween hosrrock and componenrsof the reinforcement.For local reinforcernen!.representedby a rcinforcing bar or boll fülly encaplulatedin a srong, stiffresin or groxt, a relatively lafge axiat resishnce to exlensloncan be deyehped over a relativelyshort lengthof the shankof rhe bolr. and a high resistanceto shenrcanbe developedby ar elemenrpenetraLing a slipping joinr. Analysis of local reinforcementis conducredjn terms of the blds mobiiised in the reinfbrcementelementby slip and scpüxtion at ajoini and rhedeformarionof an 'actire length' of thc element,as shown in Figure I l.16. This reflectsexpefinental observationsbyPells (1974),Bjurstrom(1974),andHaas(1981)rhar,in disconrinuous rock. reinfbrcementdcformationisconce.rratcdneaf an acrivejoinr.The conceprual nodel of lhe locai operationof the aclile length is shown in Figure ll.17a. where locat load and defonnationresponsei! siDrulatedby two spnng!. one para el to thc local axis ofthe elemeniand one peperdicular ro it. When sheafoccursar thejonrt. as shownin Figure I t.17b. the axial sp.ing renuins pmallel ro the new oriemadonof the acliveiengdr.and the shearspringir rakento remalnpcrpendicularro the originrl axial orienlation.Displacementsnormal ro thejoint Äre accompxDiedby analogous changesin ihe \pring orieftations. The loads nDbilised h ihe elcnrentby local delbrmationare.elared to thc dis placemcntsthroughtheaxial and sheüstiffnessesolrhebolt. Kr and,(. respectively. Thesecan be estimatedliom the expressions(Cerdcenet al., t911)

(11.10)

( 1 r . 11 )

330

S U P P O R T A NR DE J N F O R C E M E DN ET 5C N

drrccrion or

Frgüre 11.17 Modeh lor dial dd $ed lction of einforcenent ar a 3lippinsjoint (after Büdy dd Lods, t938).

,Lu,.

i/ .

L

'l: fr t t l |,GpLD.td).lt L"l

G! = Shearmodulusof grour,tb = Young'smodulusof bolt, dz, dr = holeandbolt diametet rcspectively F: K/(4Ebr)z K:2Es/(d2/d.1 1) and1 : secondmomentof arcaof the rcinforcing element,and .]s = Young'smodulus of gout. Themobilisedaxial andshearforcesareboth assumedto approachhniting values asymptotically.A conrinuous-yieldingmodel fbr axial pedormance(with an änalo gouse\pre$ioolor sbed)i' de,cribed by rhee\p'e..ion

^4:

rr ^ralf(ra)

( rr .r 2 )

where AF" is an increnental changein axial force dueto an inffemental axial dis placenentA a", and/(r") is atunctiondefiningtheloadpathby which F" approäches its ultinätevalue.Pfi. Theexpression for /(4) is conveninently defnedby

= p,1, F^ (P3, F^)/(Plt)z "f(i.")

(11.13)

The valüestbr the ultimate axial and shearloads that can be suslainedideally shoüld be determinedfrom appropriateiaboralory tests on the rock grout shank system.sinceit dependson factorssucbasgrcut propenies,roughnessof the groutrcck interface.adhesionbetweenthe grout and the shank.and the thicknessof the groutannulus.Ifmeaswedvaluesfor üe ultimateloadsarenot available.approximaie \atue.canbeesümared irom rheerpression. P;l! = 0.67di(obo")l

331

(ll.r4) (11.15)

R O C ( S L ] P P O RATN I ) R FN F O R ( : E ' 1 E N T

u s . l ! l ! ) i l ( [ ] r ( rJ i 1 m * e l d B c \ h ,n n L ln L l r X ) n r m l . h , , ( i L L rnr uflrrr \ !! { üß nc.JL\c nr /blocl
.nnt|s\.

tre\\

thul,d tnf

tf-11r-*tJj i.j n ".. n9

. "..n"{ i\ü.hrrrk

_

zxlm :botr\ rrdiaL lromIhis

1 9 t u 0 or )

u s . t P h e r c n \l , \ h . r ( ) r n g l c

I

Iieür€ 11.18 Tlpical eorkin: skcth ured durins pr.liminarl latour ol ! rockbllling pitrem lnr rn excrvrtionin j.iiled nck rrticr

where ob : yicld strcngthof bolr. o. = uniaxial cornpressivestrengihof the rock. rpe|r: shearslren-sthof grout or grout rock interlace. and I = bond lenglh. In equarionI L 15.i! i! assumcdthat shcaroccursal the hole boundary.If shearoccursat the bolrgrouti erflce. thc apprcprialediameterin equation11.15 is lr. Consideing equrLions11.10 11.15.iI is oblened that any incrementof relati\e displacementrt { j{rint can bc uscd 1(rdelennine incrementaland then lotal lbrces parallel and translcrsc to the aris of lhe reinfbrcementelenent. Fron the knowr orientation of rhe element relatile to the joinL, these lin"cs can bc lranslbnncd inro componentsacting rormaL and traniverse !o the joint. In this form. they can be inroduced into a suitable finile dillbrence code. such as the distinct elemenl schemedescnbedin section 6.7. which sinrulalesthe hehaviourof a jointed rock

11.5.3 Ceneral ar systematic t€jntorcemenl whereas in the caseoflocal suppot and rcinforcemen!.the objectivewa! io \up!or1 a gilen block or zone ot' rock on the excalation periphelt, here dre objectiveis to mobilisc and consene the inherent strengthof the rock mass itself. This ls otten achiclcd by crealirg a self:supportingarch of rock as shown in Figure 11.1E.Ir) thc gcncnl case.it is erpectedthat the rcck mass surroundingthe excavalion$ill liaclure or yleld. The designapproacheslhai 'nay be applied in this caseare rocksupportinteractio| calculations,the applicationof enpirical design rules, the Lrse

332

FT 5]CN A IN DR E I N F O R C F MI -F)N S!]PPOR nuDefical of rock nass clalsificattuDbasecldesignrules :rnd rnore cornpreherrsive nr these aplroachei rre used in combination. anal)ses.Quite ofren ffacrice. Rock suppan inturactian calculations. Ilese nla) bc carricd otrLu\ing the meth ods dilcussednr secrnrn1l.l and lhe calculationprocedufes\er ou! jn AppcD.LixC. Alrhough idealisätioüsof the problen have to be made.and ldne frcto s aDd rcch niques cannot bc spccincallyallowed for in the calcülation!.lse of this appr).tch of lhc rchLi\e nedls of canpemritsthc designerto developa clear understaDding jn pafticular rptlicdtion. ln nloll cases.it \rill be didale rcinfbrcementsystems a for a nümbcr oftrial desig s belbr an ne.essaryto car) oul a senesofcalculations J p p . o t f . - r\ec . i c r c , r ro i . e . c e J o r - l - e l d i - . EnpiricaldesiEn rLtles. A wide rangeofempi.icrl supportrnd renforcenert design rules have beendevelopedover the last 50 years.Thc\c rules. ühicl a.e basedon precedentpractice.generällyapplyto permanentundergr)undcrcavrLionsralherüan io rernporarymining excavarionsnL.h as stopes.They are geometricallybasedand do not äccountexplicitly foflhe stfessfitld inducedaroundthe excavationor fbr the quality of the rock mass.For these.exsons,they mxst be usedwith extfemecaution aDdonly tbf making preliminaryestimrtcswhich nusl be checkedby makjng more conrplctcassessments. Thc ünge ofempidcal designrulesavaihblc hasbeenre!iewedby Stillborg(199,1) aDdb) Rachnad !1,/. (2002) in the contcxt of thei applicationto lhe suppod xnd rcinlorcenent of productiondrifts in a block caling mine. One ol lhe most useful änd long-lived set of enrpirical dcsig rules is that developedby Lang (1961) for pa(em rockbolting ofpermanen! ercxvarionsduring the constructlonof the Snowy MouDtaiDsHvdro-electricSchemein Ausrahä. AlLhoughLang s rulesare described here as empi.ic.rl.thel' \rere establishedon the baljs oi r range of laboratory.tield and theoreticalst ies rvhich ha\'e been reviewedby Brorvn {1999.1).Lang (1961) givesthe mininnnn boh length,Z. as üe greales!ot (a) lilice lhe bolt spacing,ir (b) threetjmes the width ofcritical and potentiallyurslable rock blocks dcfincd bt the averagediscontinurtyspacmg,r; or ( c ) U . 5 Bl b r s p a n os f B < 6 n r , 0 . 2 5 ß f o r s p i n s o l S : 1 8 3 0 m . For excavationshigher lhan I 8 m, tbe lengthsof sidewallbolts lhould bc at lcast onc llfth of the wall height. The maxinnnn bolt spxcing,r. is given by ihe leastof 0.5L rnd 1.51'.When weld or chain mcsh is used,a bolt spacingof more thtn 2 .r mäkesrttachmentolthe meshdim.ult if not imposlible. Figure 11.l8 shos.sa !rclimilrr) layoul ofa rockboltingpxtlem for a horse shoe shapedercavationinjointcdrock. prcparedusingLanS \ rule!. This llgure alro illusl lcs the basison whjch l-ang'sNle, r!crc dc\,cloped,narnelythe edablishmeDtoft sell strpporlingcompressedring or a.ch .tmtrndLlrce|lcavation.lf a highly conrtfess ible featurcsuch.ts.r läuh or a chy seanrcrolsesthe conprcssionnng. it is possible that lhe requ;rcdcomprcssion$ill not be developedand that thc reinlbrcernentqill

Rock .'ars .lassificatian scirerl]es. Schemessuch as tlrcse due to Barton cr ,/. (1974)ard Bienia'vski(l971. I976) weredevelopedasnelhods ofc\limaling support

333

R()(:K SUPPORT AND RENFORCETVIENT

(RMR 4-{)

RMRe I lnQ+44 (Bieniawski, 1989)

Q=e

tr

e

( R MR - s o)

RMR^e15 logQ+so (Barton,1995)

Q -'l 0T

f i n u n- - r az Paun.

65

61.7

7 7 . 2€ 5 . 4

100

t4

97.9 1062

20

50 d

E .E .9t

e2Q

3

10 5

2

2.1

I

0.001 0.0040.01

0.04 0.1

04

1

10

4

40 100

1.5 400 1000

R o c km a s so u a lv o - F Q D x J r x J w Jn

J3

SRF

lisure 11.19 Pernrnenr supFor reqnirenents to. undelgrounde\cavations.using prcccdcDtprxctice.Baton .i .r1. andr.nrfor.m.nl cconncndxdons proposcd38 cälegorie\()1sufport basedon their tunnelling quality index, ? and thc basedon the ! s!\len ot rock mass exc{vation suppot rrtio. l,tR, whjch varies q,ith rhe use of rhe excavadonand th. cl6rincxdon(aft{ Banon,2002)

cxtent to which sonredegrceof instabiljty is acceptable.Thesetrroposalsha\e becn discussedin detail by Hoek and Brown (1980) who point out the dangersinvolled jn blindly rdopting thci.provisions.paficnlarly wherelhe natufeofthe elcavadonand the propertiesol Lhe.ock mässdiffer frcm thosein lhe casehistofiesthat were useLt nr dcvclopingthe recomnrendations. Figure ll.l9 shows the reconxnendalionstor the sulport and reinforcemeDt.f pefmanenrercavationsoffered by Grimslad and Bat1on(1991) and Bafton (2002r uhich updatethe orlginal recommendalionsof Barton dl. 11974)and {llow lbr. in "r ptulicular.subsequentadvrncesin sholcrclclcchnology. Cotnprehens)veanalysisalgeneralar systenatic reinlorcenent. In arock mass subjectto fiacture and yield. the lpadallt, ei.tensivc.lodcl of reinforcemenrjs more

334

S U P P O RATN D R E I N F O R C E M EDNET5 I C N

dFEure11.20 ModeLof spatidlly brirc Einforcenent in Dck subj{t b diftse defomation (after ltasca,

:@r). appropriatethan the local defomation rnodel.Becauselocal resistanceto shenrin the reinforcementis noi significantin this case,a one dinensionalconstituiivemodel is adequatefor describingiis axial pefornance. The 6nite differencerepresentation of reinforcement shownin FigureI 1.20involvesdivisionof rhecompletecableor tendonintoseparate segments andassignment ofequivalent masses to thenodes. Axiäl extensionof a segmentis rcpresentedby a spnngof stifhressequivalentto the axial stiffness.limiled by a plasticyield condition. Interactionbetwe€nthe tendonandthe rock is modeled by a spring-sliderunit, with the stitrnessof the springrepresenting the elastic deformability of the grout, and witb the limiting shearresistanceof rhe slider representingthe ultn äte she,I loadcapaciryof eitherthe groul annulusitself, the tendon grout contact.or the grout rock contact. The elasticplasticperformance of a tendonsegment in axialextension is shown in Figurel1.2la. For üe elasticresponse, thebarstiffness is reiatedto theproperties anddimensionsof the tendon K.: EhAlL

(l r . l 6 )

where Ä and a are the ffoss-sectionäl area änd length of the tendon segment. Ihe leld load is relaled direcdy to the yield strength of the tendon and the crosssectional äieä. Il after yield, the segment is subjected to a phase of unloading, the unloading stiffness ;s taken to be equal to the loading siiffness. (a) Axial pe.fomance ngür€ ll.2l .a a reinronenenr sogne.ri (b) shed F ommcc ofercut urulus bctwccn Edon dd boEhole süfaces (aner

E€

E*

'i3

o splaehetll ol c.bl€ ßlalls

335

AT N DR E I N F O R C E M E N T ROCK 5UPPOR The elastic perfectly plastic pedomance of thc lendon segnrenlrepresentsthe simplestpossibleconstitudvemodei. With minimal increaseir complexitv' a kmc matic hardeningmodel or acontinuousweakedngmodel analogous1()that describcd by eqnätions11.l2 and I l.l3 could be introduced The elastic-pefeclly plastic perfornance of the gout annulusis represcntedin Figufe 11.21b.As a resuh of relative sheardisplacement,, betweenthe lendon sudaccand the boreholesurface,the shearforce Ä mobilisedper unit length of cable is rclatedto the stiffness(bod, i.e.

( 1 1l 7 l Usually.rboid wouldbc rneasureddirectly in laboütory pull-out lests Alternativel). it may be calcülaledäom thc expressjon K h r , \=d 2 n G { l l t l ( ]

(l].lE)

+ 2 t/ d ] l

wherc r is the thicknessofthe grout annulus. The ultimate loxd capacityofthe grout is de{inedby equation II 15. with lcnglh .1-of unily. and rlsk given by

(11.19r whererr is approximatelyone-halfof lhe snaller of the uniaxialstrenglhsof the grout and thc rock. and 0b is a factor defining the quality of the bond belweenthe grout and the rock. (Forperfecibond quality, Ob : l ) Calculationof the loadsgencraledin reinforcementrequjresdelerminationof the relativc disolacementbeNeen the rock and Ä nodeofa reinforcenrcnlsegment.Con siderthe constantstraintriangulaEone shownir Figure 11 22a.wilh componenlsof displacement!,i, r"i. fo.example,at the comersi(i : l, 3) The naturalco ordinate\ ti, of a reinforcementnodep lying within the triangleare givenbv the relatile area\ of the trianguld areasdefinedin Figure 1l.22b Thus

1;=AilA

(l1.20)

i:l'3

whereA is theäreaof thetnrnglewith comcrs1,2, 3 al and,l atnode p are inlerpolatedlinearlv fiom the displacc The displacements meDtsat the comers,using the nalural co-ordinatcsas weigh! facrors,i e

,l : rl, &., üf = r, tl., i = 1.3

(11.211

subscnpß. is impliedonrepeated wheresummätion Figure 11,22 BNis ol nalural.o orditratcslor interpolating rucl dn (afterllasca.2003). placc!rcnts

..';'-_

336

DESI(;N RENFORCEMENT 5UPPORTAND ln the finite differencc änaiysis,incrementalforms of equation 11.21 arc used from in successive computationcyclesto calculaieincrementalnodal displacemcnts, which rhe new configuration of an element can be determhed. The axial component of relarive displäcement at a node can then be calculated fiom the absolule displacement of a node and the absolutedisplacemenrof rhe adjacenLrock. The axial fbrce is ottained fron equalion I 1. 17 and rhe active length adjacentto the node, taldng account oftie limiting condition delinedby equation I1.19. The force Fl, Fj mobilised at the grout rock interface at a node is distnbuted to the zone comers using the natural co ordinatesof the node as the weight faclor; i.e.

rl, = rrr.j

(11.22)

whereIjr areforcesassi-sned !o the zonecomer This lbrmulationofreinforcementmechanicsmay be readily incorporatedin a dy namicrcläxalion,linite differencemethodof analysis of ä defbrmablemedium,such by Cu'dall ändBoard(1988).The solutionofa asthecodecalledFLAC described cableboltsillusaatesapplica sinpleprobleminvolvinglong,grouted,untensioned tion of the methodof analysis.The problen involvesa circular hole of I m radius, stressfield.of magnitude10 MPa. excavated in a mediumsubiectto a hydrostatic For the elasto-plasticrock mass,th€ sheärand bulk elasticmoduli were4 GPaand andMohr Coulornbplaslicitywasdelinedby a cohesionof 6.7 cPa respectively, consisted 0.5MPa,ängleoffrictionof30", anddihtionangleof 15'.Reinforcement groutedinto 50 mn of a seriesof radiallyorientedsteelcables,of 15mm diameter. diameterholes.The steelhada Young'snodulusof 200 GPa,andä yield loadof I GN. Väluesassignedto tr'b..dand Sh""dwere 45 GN In ' and 94 kN m '. These propenies conespond to agroutwith a Young\modulusof2l.5 GPaanda peakbond strength(rps0 of 2 MPa. Thener {ieldproblemgeometry is asa quanerplane. Theproblenwß analysed both illusrraEdin Figüre11.23,wherethe extentof the failurezonethätdevetops Thedistribuiions of thereinforcement is alsoindicated. in theabsence andpresence de shownin Figure 11.24for aroundthe excavation of stressand displacement the caseswherethe excavationnearfield rock is both unreinforcedand reinforced. Fqüre 11,23 Problen eeometl ri J_icldzones abo[ a cncdar ex €ution (a) Nithourdd (b) with rc.tnr.enent (dfter Brady dd Lorig,

(.)

_l

__1 l

-L

337

I

ANI] FE NI-ORCEMFNT ROCI 5UPPORT

IL

Iigüre 11,24 DiGbutiotrs of (x) {r.* arl fb) displac.Deol arctrnd r ciculr extavaiion lor unre,nlnn.d üd rcinbrccd DerF6eld rock (lher

Examinatbn of the diirribütions of radial disphcement&r and raditl and tangential stressc\q and q indicäteslhe lunctioD of the radial reinforcernenlli substanliall\ reducesthe radial displacemenÜr. and generatesa hjghcr magnitndeof o. in th. l'ractu€d zone, resulting in a higher gradient in ihe or distribütion The effect i! to shill lhe plastic-lastic tr:nsiiion closer to the excavltion boundar]' Thu!. both closüe ofthe excavationand the depthofrhe zoneofyjelded rock are reduced The Llcnsityofreinforcemenl used in this dcmonsrationprublem is greaterlhan wonld be appliedjn mining practice.Howevcr,it confirmsthe modeof actionofrern' fofcencrr andthepfospcctlbr applicalionof conputalionalnethod s in reinforcernenr

11.6

Materials and techniqu€s

11.6.1 OveNiew The emphasisnr this chapterba! bccn on the principlcsoflhe süpportand reinforcc ment ofrcck tnassesand on designanalyses.Howerer,if supportand reinforcenenr is to be fully efTective.il is necessa4'thatsuitablematerialsbe usedfor r pirrlicular appticationand that thesemdterirls be instÄlledor äpplied using sarisfactorytech niques.Thc Llctailsofthese techniquesand materialstue largely beyondthe scopeof this book. Only thc central principlesllDd so e illustrative exampleswill be given here.For full practicaldetails.the readershoüldconsulttextssuchas thoseby Hoel andBullock(2001).HütchinsonandDjedeichs (199'l),Karser erdl. (1995).Hustur.rlid (1996). Melbye ind Garshol (1999), Procto. and white (1977) and Stillbor! .r d1. {1994). and the prcceedingsof specialtyconferencessuchas thoseeditedb-! Kaiser aüd Mccrearh (1992) and villaescusaer da (1999). ln this section,brief accounlt $,ill bc given of rhe essentialfeaturesofrockbolts and do$els. cablc bolrs.shotcrele wire merh and steelsets.Deldls ot' the suppottlcchniquesusedin longwallcoal and r e n l : i p . u . n r i l i l gu . l ' e ! i \ - ' i l C h t oe r . 5 1 1.6.) Rockboltsand clowels A single tensioncdrockbolt usually coDsistsof an anchorrge.a steelshank,a lace plate.atighteningnut and somctimesa deformableplatc.For shorttemr applications the bolt n1aybc leil ungroured.but fb. permanentor long tcmr lpplications and use

338

MATER]ALS AND TECtsN IQUE5

in colrosive environments, rockboks are usuaily tully grorted with cement or resin grort fbr improving both pull-oxt stenglh and colrosionresistaice. Rockbolis are oiten classifiedaccordingto the natureof their anchorages.Early rockboltanchorswereoflhe mechanicalsloland-wedgeandexpdnsionshelltypes.ll is often diflicuh to fo|m ard maintain mechanical anchorsin very hard or in soft rocks. Mechanical anchors are also susceptible to blast-induced damage. Anchors lbrmed from Portlandcementor resin are generally nrore rcliable and permanent.A third categoryof rockbolt anchorrgeis that utilised by friction (Split Set and Swellex) bolts which rely on the generationof f.iction at the rock bolt contact along their lengthsfor their äncho.ageand strcngth.As with mechanicalanchors,friction bolts depend for rheir efficacy on the sizes dnd accuracy of the drilling of the holes in which rhey are installed-They are also susceptibleto conosion. Althorgh they may be given a pre-tensionto ensurethat an anchorageis formed,ftcdon boltsa.c usually not inslalledwith the levelsof pre tension(5 20 tonnes)usedfor otherrockbolts.ln this case, the), xct a dowels rathe. than rcckbolts. Other types of dowel arc usually grouted along theA lengths on installation and develop their tension with deformation of the rock massin which they are installed.Groudng of Splil Sel bolts rnd dowels may innease their load cärrying capacity fbr longer lerm applicadons (Thonpson and Finn, 1999). Figure 11.25showsa nunber of types of rockbolt and dowcl classifiedilr]cording 1othe anchorägemethodusedbut wiLhseverullypesofshank illusträted.Figure I 1.26 shows turther details of the installation and grouting of ä rcsin anchored and grouted bolt made from threadedbar. Resin encapsulatedrockbolts are widely uscd for flre reinfbrcementof longer term openingsin meralliferousmines.(e-g.Slade al.. "r 2OO2). 11.6.3 Cable bolts Cable bolts äre long, grouted,high tensilestrengthsteelelementsusedto reinforce rock masses.They may be usedas pre or post rcirforcement and may be left un tensionedor be ple- or post-tensioned. Windsor (2001) defnes the following ienns associaredwiih cable bolring: . .

lvire - a singie,solid sectionelement. Strand a set ofhelically spunwires.

MsrrscrborßE-:

11,25 Types of tuckbolt Jowel (aiter Hadjigeorgiouand

.200r).

arrcBBzffi

@

339

--

---.ffi

|

R O C I 5 U P P O RATN D R I N F O R C E M E N T

Figure 11.26 Resin Ctuuted rcck' bolr nade lrom threadcdbr (aiter

resi. mixed wirh harde.et by roralionof bar during

. . .

Cable än anangementofwlres or stnnds. Tendon- pre-tensionedwires or strand Dowel - un-rensioncdwires or strand.

Cable bolring as delinedhere was first uled nr ündergroundmetalliferousmines in South Africä and Canadabul it was probablyin A'Ntralia that cablc boll and dowel reinforcementwas first developedas a major fonn of syslematicreirrforce ent ln cufand-fil1 mining (Clffiord. 1974, Brown, 1999b) Figure 1127 summanscsthe developmentof cablebolt configurations. windsor(200i) nolesthat the developmentof hardwde for cablebolting hasbccD matchedby improvementsin designphilosophyand meüods.Inrhis context,design includes choosing a suitabletype of cable bolr, the boll orientations.lenglhs and densities,an appropriateinstallationprocedure,and detemining whethefto usepre in conjunctionwill pre- orpost tensioning ln mining p.acLice. or post-reinforcement rhesedecisionsare inflüencedbylogistics.eqnipmentavailabjlity, precedentpractice in snnilar ctcumstancesand, in the case of installationprocedures.the levels of training of lhe work force. Instdlation practjce häs the potential to diclale the mechanicalperformanceof cäblebolring.The length and transversefiexibility ofcable bolls createa numberot difllculties in ensuing a high quality insiallalion.lnstallatidr can be influencedby a number of factors relating to the dilling of the hole. the configurationand state oftbe cxble,and the grouting and lensioningofthc cäble.A tull discussionofthese factors is outside the scope of this lext. For turther dctäils. the reader is refencd to the books by Hoek cr dt (1995) and HutchinsonaDdDiedenchs(1996), and thc papersby Windsor (1997.2001),fo. example.Figure I1.28 illusträtestwo alternatjle methodsof grouting cable bolts into upholes.Theie mcthodsmay be describedas gravity retmdedand gravity assisted,respectively.In the groxi lube n1ethod,the Lube nay be withdnwn progressivelyfrom thehole as it fi]ls with grout. This methodhas

340

MAIERIALS AND TECF]NIQUES

TYPI'

LONGITUDINAL SECTION

CROSSSECTION


Multiwire

XOq \o,t''

Birdcaged

^o o

iOo " o o

@

Single

:-----l

drG)& wrörw

@ @@

Singl€Slrand

(@1) \*/

(@l \..-_/

I Compoicnr

2 conponcd

@

,G)

High Calaoity

s Bulbed ,Antinode

Node

#& tigure 11.27 sumary of theds a numberof operationaland cost advantagesand is usedroutinely in a numberof nlopmenrofcableboltconnsunlions Australianmines(Villaescüsa, 1999). 11.6.4 Shotcrete Shotcreteis pneumaticallyappliedconcreteüsedto providcpassivcsupporlto therock surface.lt consistsof a mixture of Pordsndcement,aggregates, wateranda rangeof

341

AND RE]NFORCEMENT ROCK5L]PPORT

ff Fisüft 11.?8 Altcnative bethods ol snuting cablesnrto upboles(ailer

admixturessuchasacceleratorsor ret.rders,plasticisers,microsilicaandreinforcing libres. Gunite. which Fe-dates shotcretein its usein undergroundconstruction.is pnenmaticallyapplied moltar. Becauseit lacks the larger äggregatesizesof up to 25 mm typically usedin shotcrcte.guniteis nol ableto developthesameresistanceto defornaiion andload canying capaciryassholcrete.For at least50 years,shotüete hasbeenüsedwith outstandingsuccessin civil engineeringundergroundconstruction in a wide varieiy of ground types.It is so successfulbecauseii salisfiesmost of the reqüirementsfor üe provision of satisfactoryprimary supportor reinforcemenl hasfoundincreasing discussed in section11.2.Over the last 20 years,shotcrete morepermanenl practice. for the suppon ofthe initially minilrg usein underground (Brown of slopes and stope accesses for the suppon excavätionsbut now increasingly pan and be used as of tbe support 2001). lt may also 1999b.BrummerandSwan, (Hoek and et dl., 1995, Kaiser rock burst conditions systemin mild rcinforcernenl as ä in conjuncrion with, or used increasingly is being Tannant.2001).Shotcrete primäry headings. Brulnmer and Swan provide support of replacementfor, meshto (2001)describea caseof üe useof wet mix sreel6brc reinforcedshotcreteio provide the total drifi suppon in a sublevelcaving operationat ihe Stobie Mine. Ontario, Canada.Bolts areusedin dnfts only at intersections.

MATERIALSAN D TECHN IQL]ES

(b)

(c) fküre 11.29 Sone sDpponnech -isß developedby shotcEtc: (a) a -gle blocL (b) ! bem anchoredby bl6: (c) a r@f arh; (d) a closednns

Someof the supporrmech-"-, o*roo"O on the peripheriesof ":, "n*.rete excavations areillustratedin Figwe I 1.29.Thesuppoft functiors, modesoffailure and methodsof designof shotcreteasa componentof hardrock supportandreinforcemeni (2001).Hoek systems arediscussedby Holmgren(200i) andby Kaise.andTannant €t dl. (1995)provide a sei of detailedrecornrnendations for the useof shotcretein a Iangeof rockmasscondi.ions Iikelyto beencountered in hardrock mining. Shotcreteis preparedusingeither the dry-mix or the wet-mix process.In the drymix process,dry or slightly darnpene.d cemeDt.sandand aggregatearenixed at the batchingplant,andüen enträinedin conpressedair andtanspo{ed to thedischarge nozzle.Wateris addedtkough a dng of holesat the nozzle.Accwate watercontrol is essentialto avoidexcessivedust whentoo little wateris usedor an over wet mix whentoo muchwateris added.In the wet nix process.the requiredamountof water is addedai the batchingplänt, rnd the wet mix is pumpedto the nozzle wherethe compressedair is introduced.A comparisonof the dry and wet mix processesis givenin Table11.2.Until the lastdecadedry-mixmethodwasmorewidelyused, mainly becaüsethe equipnentrequiredis lighter andlessexpensive.andbecanserhe dry materialcänbe conyeyedoverlongerdistances,anirnporrantadvanrage in mining applications.However,wet-mix methodshaveimporiantadväntages for underground mining applicationsin termsof reduceddustlevels,lower skili requirementsandrhe needfor iessequipmentat the applicationsiie. They havenow becomeüe indusrry ständffd(Brown,1999b,Spearing, 200i). Shotcretemix designis a dilticult andcornplexprocessinvolving a cefain amou.t of tlial anderror.Themix designmustsatisfythefollowing criteria(HoekandBrown, ro80l (a) Shootabiliry- the rnix nust be ableto be placedoverheadwith minimum re,

343

ROCKSUPPORTANDRENFORCEMENT Table 11.2 Conpuison or wet md drt-nn shotcrelingpNesses (alier Spedins. 2001).

Los €bound.typicallyaboul5%lo l0% Moderateto hish pl&enentac, belw@r

HiBi rebound.usD.llt morethd 259. Low to modemreplacementFte, up b 6 nrrr

Low rdsport distdc. up to 200 d Moderateto high placedquality

(b) Early strength the mix mustbe saongenoughto providesupportto theground at agesof a few hours. (c) Long term saength- the mix must achievea specified28 day sFengthwith the dosageof acceleratorneededto achieveihe requiredshootabilityandea y stlength. (d) Düabiliry - adequatelong tem resistanceto theenvironmentmustbe achieved(e) Economy- low-costmarerialsmustbe used.andtherenusr be minimumlosses dueio rebound. ofdry componentsby weight: A typical ba-sicmix containsthe following percentages cemenf coarseaggregate Iine aggregateor sand accelerator

15J09o 3O4O9o 4V5O9o 2 51a

Thewater: cementratio for &y-mix shotüerelies in th€range0.3-0.5 andis adjusted by the operatorto suit local conditions.For wet-mix shotcret€,the water : cemenl ratiois generally between 0.4 and0.5. The eflicacyof the shotcretingFocessdependsto a largeextenton the skill of ihe operator.The nozzle shouldbe kept as rcarly peryendicularto the rock surfaceas possibleandai a constantdisranceof about I m lion it- A permanentshotcreteLhing is usually between50 mm and 500 rDmrhick. the larger thicknessesbeing placed in a nu4ber of layers.The addition of 2G-50Inm long and 0.25 {.8 lnm diameter deformedsteel fibres, or plastic fihes, has beenfound to improve the toughness, shock resistance,dwabiLity,and shearand flexural strengthsof shotcrete,nnd to reducethefomation of sbrinlagecracks.Fibre-reinforcedshotcretewill acceptldger d€formationsbelorecrackingoccursthanwill uffeinforced shotüete;aftercracking hasoccüred. the rcinforcedshotc€temainiainsits integrity andsomeload'carying capabiliry However.fibrc reinforcedshotcreteis moreexpensiveandmoredifücnh to apply thanunreiDforcedshotcrete. 11.6.5 Wire mesh Chain link or welded ste€l meshis usedto restrainsmatl piecesof rock between boltsor dowels,andto reinforceshotcrete.For the iatter application.weldedmeshis prefenedto chain'link meshbecauseof the difficully of applyingshotcretesatisfactonly throughthe smalleropeningsin chain ünk mesh.For undergrounduse,weld nesh typicaly has4.2 Inm diameterwiresspacedat 100nm centres.In somemining

344

11,3

MATER A L 5A N D T E C H NQ U E S

FquE l1.3lr Reinfored haülageal .d€plh ol 1540n folowing a sismic 6$t ofnasnitude 4.0.Severly dan ttd rocL is well containedby mesh d rcpe lacing (aier Onlep!, 1983).

districtssuchäs thosein WesternAustraliaandOntario,Canadä,mining regulations and codesof Factice now rcquire somefolm of surfacesupport,usually mesh,to be üsedin all personnelentry excavations.In WestemAustralia,the codeof practice appliesio atl headingsüat arebigherthan3-5 m andrequtueslhat sudacesupportbe instaled downto at least3.5 m fron the floor (MinesOccupalionalSafetyandHealth Ad|sor) Bodro.laoq,. In under$oundmetalliferousmining, rock blocks or fragmentsof ftactued rock are often held in placeby a pattemof hoist rope lacing install€dbelweenrockbolts or anchorpoints. Rope lacing mäy be usedro sdffen meshin thosecÄresin which restaint to loosenedrcck. Otlepp (1983. the meshis unableto provideadequate with of theuseof meshandlacingin conjünction 1997)givesa nümberof examples rockbolis ändgrouiedcäblesandsteelrodsto stabilisetünnelsin the deep-levelgold minesof SouthAirica. FigureI L30 showstheappüranceof anintensivelyreinfbrced haulageat a depthof 1540m following a seismicevenlof mngnihrde4.0 which had its sourceon a fault intersectingthe haulagenearthe locationof the photogmph.The haulagewasreinfo.cedwith 2.5 m longgrouiedsteelropetendonsand7.5 m long prestressed rock anchorswhich providedan overall supportcapacityof 320 kNmi. The 3.2 mm diameterby 6s mrn squaregalvanisedmeshwas backedby 16 mm diamererscraperrope. Acrossthe intersectionwiih the fault. the severelydamaged rock waswell containedby themeshandlacingeventhoughseveralofthe pres! essed anchorshad failed.

345

ROCK5UPPORTAND RENFORCEMENT

{b}

gronnd characredsricor

E $ a

vffi

Radisl displ4oenl

Figlre 11.31ToussairtHeintz nam yieldingadh: (a)cio$ section; lb) chml joinri (.) allemadlejoinri 11.6.6 Steel sets (d) dh .onngurationbelbredd atier Steel archesor steel sets are used where high loäd carrying capacityelementsare yicldinst (c) idealised load radial required to support tunnels or roadways. A wide range of rolled sleel sections are

availablefor this applicarion.Where the rock is welljointed, or becomesfractured after the excavationii made, the spacesbetweenthe sets may be Iilled with stcel mesh.steelor limber lagging. or sleelplales.Proclor dnd White (1977) provide the most detailedaccounl availableof the materialsand techniquesusedin providing Sleel sets provide srppon rä1herthnn reinforcement.They cannot be preloaded againstthe rock face and, äs pointed out in section I1.3. their emcacy largely de pends on the quality of the blocking provided to transmit loads from lhe rock to the steelset. Steelarchesare widely used to suppo roadwäysin coal mines where they are oflen required lo sustain quite large defomations. These defomations may be acconmodatedby using yielding archescontai.ing elementsdesignedto slip at predeiermnedloads (Figirre 11.31).or by pemitting the splayedlegs of the arche\ 1()punch into the floor Where more rigid supportsare requiredas, fbr exanple. in circüh tränspodationtunnels,circular steelor concreteseisare used.

346