identification of grouting discontinuity of rock bolts as an efficient way to control safety...

8/22/2019 Identification of Grouting Discontinuity of Rock Bolts as an Efficient Way to Control Safety Conditions in Mine Road

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JournalofMiningWorldExpress(MWE)Volume2Issue1,January2013 www.mwejournal.org

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IdentificationofGroutingDiscontinuityofRockBoltsasanEfficientWaytoControlSafetyConditionsinMineRoadwaysA.Staniek

CentralMiningInstitute,POL

[email protected]

AbstractA rock bolt which is grouted underground may not be

properlyinstalledwiththeresultbeingadiscontinuityofthe

resin layer. Such discontinuitymay also occur inworking

conditionsdueto typicalrockbehaviouranddisplacement.Itmaybeveryhazardous tomine safety. In thispaperan

outline of amethod for nondestructive identification of a

discontinuity of a resin layer surrounding rock bolt is

presented.Themethodusesmodalanalysisproceduresand

is based on an impact excitation where a response

transducerispositionedatavisiblepartofarockbolt.Asan

installed rockbolt acts as anoscillator,different lengthsof

discontinuityofresinlayerchangeitsmodalparameters.By

proper extraction of these parameters, from which a

resonant frequency is seen asmost valuable, the intended

identificationispossible.Theresultsofidentificationofresin

discontinuitiesarediscussed. Experimentswereperformedinanexperimentalcoalmineaswellas incoalandcopper

mines. Also the influence of explosions on installed rock

boltsispresented.

KeyWordsMine;Rockbolt;SupportSystem;Method;ModalAnalysis

Introduction

The function of the rock bolt support system is to

anchorandreinforcetherockzoneinthenearfieldof

anundergroundopeningtodeeperrockstrata,Li[1].Predominantly for that purpose steel rockbolts are

used,Tadollini[2].Therockboltconsistsofasteelbar

grouted in an oversize hole. A portable installation

machine is used to spin thebolt into the hole filled

with fast setting epoxy resin cartridges. After

hardeningoftheresinaplateandnutisdrivenupthe

bolt.Althoughrobustresincartridgesareavailable in

themarket, inmining practice the rockbolt is quite

frequentlynotfullyencapsulatedasaconsequenceof

rock divergence, resin deterioration, escape of grout

into crevices, rock strata movement or impropergrouting.

Inorder toverify theproposedmethod researchwas

performed in an experimental calmine aswell as in

existingcoalandcoppermines.Theinvestigatedcases

of discontinuity of resin layermay be divided into

caseswhere:

discontinuitywasknownbeforethetest, discontinuitywasunknownbeforethetest, discontinuitywas knownbefore the test only to

thestaffofaminedepartment.

Below the example results of undertaken study are

presented.

Description of the Method

The methodology is based on the assumption that

boundary conditionshave tobe such that the insitubolt behaves like a cantilever. Different lengths of

grouting discontinuity form different boundary

conditions and change the modal parameters of an

installedrockbolt.Excitedmodestransfertotheouter

part of the rockbolt,which enablesmeasurement of

global characteristics. The method, as Staniek [3]

described, uses modal analysis procedures and is

based on impact excitation. It consists of twomain

phases:experimental,which entails themeasurement

ofmodalcharacteristicsofan investigatedobject;and

theoreticalinvolvingthereferencetothefiniteelementmodeldatabases.Toobtainsuchdatabasestheoretical

modal analysis is performed on the FEmodel of a

tested structure and for different border conditions

related todifferent casesofdiscontinuity.Themodel

includesover1000theoreticalcaseswherethechange

indiscontinuitywasmadewitharesolutionof5cm.

Themodel isscalable todifferentrockboltdiameters

and rock bolt lengths. The parameters of rock and

grout i.e. density, Poissons ratio and Youngs

moduluswereestimated experimentallyaccording to

standardmethods,Ulusay [4]andEN1936 [5].Tobeused as a reference, the theoreticalmodel demands


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www.mwejournal.org JournalofMiningWorldExpress(MWE)Volume2Issue1,January2013

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refinementinrelationtoverificationoftypeofborder

conditionsandcontactsassumedinFEmodel,Dossing

[6], Ewins [7],Maia [8] andUhl [9]. Thatwas done

taking into account 28different cases grouted in the

experimental coal mine with preset, known

discontinuities in the resin layer. To enable in situinvestigations a portable measurement system was

designed and built. As a programming tool the

LabVIEW environment was used, Bishop [10].

Samplingratewaschosenas16kHz,whichseemedto

besufficientconcerningresolutionofFRFplotsaswell

as frequency range. Simultaneously, the import of

recorded data and derivation of modal parameters

wereperformedutilizingCADAX[11]modalanalysis

software.Themeasurement setupand itsapplication

concernedwith acquisition and recording of data in

realmining

conditions

isshown

in

Figures

1,2a

and

2b. After removing the plate and nut, a response

transducer (accelerometer) was fixed onto a visible

partofa rockbolt,approximately2cm from itsend.

After transversevibration excitation of the rockbolt,

signals from both the force transducer (which is

located on the impact hammerhead) and the

accelerometerwererecorded.Toavoidnodpointsand

improve the results through averaging ofmeasured

FRFcurvestheexcitationwasrepeated5timesandat

4 to 7 points selected at equally spaced intervals of

approximately2cm

each.

The

accelerometer

was

mountedusingametalbeltandfrompracticalpointof

view itwasquiteeasytoclampitontotheendofthe

rock bolt. Other methods of mounting were also

investigated, using wax and magnet. For wax

attachment no significant difference was observed.

Magnet isnot recommended since itdoesnotensure

absolute repeatability and positioning especially in

impact testingwith the result that coherence isquite

poor,Bruel&Kjaer[12].

FIG.1EXPERIMENTAL SETUP: (1)ROCKBOLT,TYPICALLENGTH1.5m2.5m; (2)ACCELEROMETER; (3) IMPACT HAMMER; (4) PORTABLE

MEASUREMENT SYSTEM; (5)WORKSTATION FORMODAL ANALYSIS; (6)

SURFACEOFUPPERROOFSECTION;(7)GROUTLAYER L ISAGROUTED

LENGTH

FIG. 2 a: PORTABLE UNIT FOR ACQUISITION AND RECORDING OF

FREQUENCYRESPONSEFUNCTION,INSITUMEASUREMENT

FIG.2b:EXCITINGAROCKBOLTWITHANIMPACTHAMMER,RESPONSE

OFTHEIMPACTMEASUREDWITHPIEZOELECTRICACCELEROMETER

Results of Undertaken Study

Research Work in the Experimental Coal MineBARBARAGIGIn this section the results of validity check are

discussed. Figures 3a, 3b and 4a, 4b below present

exampleresultsofresearchworkofa totalof28rock

boltswithdifferent lengthsofgroutingdiscontinuity

installedinanexperimentalcoalmine.Forthesecases

theprocessofgroutingwascontrolledat thestageof

installationi.e.knowndiscontinuities.

Examplesofidentifiednaturalfrequenciesforboththe

experimental cases and their theoretical counterpartsare presented in Tables 1 and 2. The degree of


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refinementmaybe seenby comparing these natural

frequencies. For the casepresented inFigure 4b and

Table2thesefrequenciesare306.9Hzand1872.5Hz,

which are characteristic of the outerpartof the rock

bolt togetherwith 585,2Hz and 924,4Hz,which in

turnarecharacteristicof thehidden,notgroutedendpart of the rock bolt. Although in the theoretical

analysis more natural frequencies were calculated,

onlysomeofthemwereobservedintheexperiments.

The explanation for this is that energy which is

transported from a vibrating structure to adjacent

structures constitutes a loss of the structures

vibrationalenergy.Forastructuralcomponent that is

in intimate contact with others, energy transfer to

adjacent components can contribute considerable

damping andgenerallymakes itdifficult tomeasure

the components energydissipation itself,Remington

[13].Attenuationofvibrationmayevenapproachtens

of decibels per meter as far as transmission of

vibrating signal along the rock bolt is concerned

(higher damping for lower frequencies), Beard [14].

Suchattenuationmakesamplitudeofvibratingsignal

for modes propagating through grout region

comparable with noise level and immeasurable; it

seemedtobethecasewiththefirsttwomodes inthe

example above. The other modes were informative

enough for the relevant case to enable grouting

continuity identification. It must be mentioned,

however,thatincertaincasestherearetoofewnatural

frequencies for the evaluation tobeperformed.Care

should be taken while mode matching and in

problematic situations FRAC (Frequency ResponseAssuranceCriterion) have tobe calculated to obtain

correlation and adjustment of experimental and

theoretical models. For excitation at point j and

responseatpointkFRACjkisdefinedas(1):

L

i

L

i

ijkAijkX

L

i

ijkAijkX

jk

HH

HH

FRAC

1 1

22

1

2

(1)

Where:

XHjk frequency response function for

experimentalmodel,

AHjk frequency response function for analytical

model,

i=1,Ldiscretepointsforafrequencyrange

In mode matching of experimental and theoretical

models visual comparison of mode shapes for the

outerpartofarockboltmightbeusefulandoughtto

beperformedineverycase.

FIG.3a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORAROCKBOLTGROUTEDONOFITS

LENGTH,STARTINGFROMAREAR,HIDDENEND.AXISXFREQUENCY,AXISYINERTANCE


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FIG.3B:EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESINLAYERFORAGROUTEDROCKBOLT(ROCKBOLTLENGTHEQUALS2m)

TABLE1:COMPARISONOFIDENTIFIEDNATURALFREQUENCIESFORAROCKBOLTGROUTEDAT OFITSLENGTH,STARTINGFROMAREAR,HIDDENEND

No.Frequencies,Hz

FEM Experiment1 89.3 90.5

2 250.8 247.8

3 492.8 475.8

4 816.5 780.9

5 1225.3 1147.4

6 1725.6 1596.7

7 2317.3 2118.5

8 2992.3 2702.6

FIG.4a:

FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORAROCKBOLTGROUTEDONOFITS

LENGTH,STARTINGFROMACEILINGSURFACE.AXISXFREQUENCY,AXISYINERTANCE


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FIG. 4b: EXPERIMENTALLY EVALUATED DISCONTINUITY OF A RESIN

LAYERFORAGROUTEDROCKBOLT(ROCKBOLTLENGTHEQUALS2m)

TABLE 2 COMPARISON OF IDENTIFIED NATURAL FREQUENCIES FOR A

ROCKBOLTGROUTEDON OFITSLENGTH,STARTINGFROMACEILING

SURFACE, FREQUENCIESMARKEDWITH *)ARECHARACTERISTICOFNOT

GROUTED,REARPARTOFAROCKBOLT

No.Frequencies,Hz

FEM Experiment1 98.9

2 277.1

3 307.9 306.9

4*) 544.3 585.2

5*) 903.2 924.4

6 1356.7

7 1815.1 1872.5

8

1909,1

Another trial to check the usefulness of themethod

wasperformedoncaseswhereadiscontinuityof the

resin layerwas locatedsomewherebetween the front

andendpartoftherockbolt.Tocheckthevalidityof

results, theprocessofgroutingwasdivided into two

stages: first the endpart of a rockbolt and then the

frontoneweregroutedboth withtheknownlengths.

Figures5aand6ashows themeasuredFRFfunctions

for such cases, and Figures 5b and 6b are graphical

presentations of the determined discontinuities. The

natural frequenciesranddamping ratiosr for thatcasesarelistedinTables3and4.

Thedampingratiorisdefinedby(2):

cr

r

rc

c (2)

where:

crdampingcorrespondingtomoder

ccrcriticaldampingcorrespondingtomoder

FIG.5a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,LACKOFRESIN

LAYERINTHEINNERPARTOFAROCKBOLT.AXISXFREQUENCY,AXISYINERTANCE


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FIG.5b: EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESINLAYERFORAGROUTEDROCKBOLT(ROCKBOLTLENGTHEQUALS2m)

TABLE3:IDENTIFIEDNATURALFREQUENCIESFORACASE,WHERETHEMIDDLEPARTOFAROCKBOLTISNOTGROUTED,FREQUENCIESMARKEDWITH*)

ARECHARACTERISTICOFNOTGROUTED,INNERPARTOFAROCKBOLT

No. Identifiednaturalfrequencies,Hz Dampingratio,%

1 327.8 1.1

2*) 653.6 1.3

3*) 978.3 1.0

4*)

1634.9

0.6

5 1817.3 2.7

FIG.6a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,LACKOFRESIN

LAYERINTHEINNERPARTOFAROCKBOLT.AXISXFREQUENCY,AXISYINERTANCE


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FIG.6b:EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESIN

LAYERFORAGROUTEDROCKBOLT(ROCKBOLTLENGTHEQUALS2m)

TABLE4:IDENTIFIEDNATURALFREQUENCIESFORACASE,WHERETHE

MIDDLEPART

OF

A

ROCK

BOLT

IS

NOT

GROUTED,FREQUENCIESMARKED

WITH*)ARECHARACTERISTICOFNOTGROUTED,INNERPARTOFAROCK

BOLT


1 286.7 1.6

2*) 571.0 1.5

3*) 859.5 0.4

4*) 1408.6 0.7

5 1493.5 3.5

6 1699.4 0.9

On thewhole, the resultswerequite satisfactory. In

situ measurements showed that calculated damping

mightvarytoacertaindegreefromsampletosample,

thusreducing itsutility. Itcanbe incurredasaresult

ofaconflictbetween theassumeddampingbehavior

andthat

which

actually

occurs

in

reality,

also

the

presence of small debris in the bore hole may

influencethedamping.

ResearchWorkintheCoalMineJANKOWICEInaccordancewithotherinvestigationsonthesubject,

asKidybinski [15]described,researchontheeffectof

seismicityon continuityof the resin layerofgrouted

rockboltwasalsoundertaken.The investigated rock

boltswere located in a coalmine roadway near the

regionofablastsite.Thetestswereperformedbefore

and after explosions. Centers of explosions were

located atadistanceof5 to 10meters from the rock

bolts, shown on Figure 7. The loads of dynamite

chargewere as follows:partA: 30kg,partB:66kg,

partC:60kg,partD:60kg.Asanassumptionallthe

rockboltsweretobegroutedonthewholelengths.

PartA:Archsupports

PartB:Rockboltsupports

PartC:Archsupportsreinforcedwithrockboltsandsteelnet

PartD:Rockboltsupportsreinforcedwithenergyabsorbingboltsandsteelnet

FIG.7:ASKETCHOFLOCATIONSOFCENTERSOFEXPLOSIONSINANINVESTIGATEDOPENING


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Figures8aand9ashowstheexamplemeasuredFRFfunctionsbeforeexplosions,Figures8band9bsubsequently

themeasuredFRFfunctionsafterexplosionsandFigures8cand9caregraphicalpresentationsofthedetermined

discontinuitiesandeventualchanges.ThenaturalfrequenciesforthatcasesarelistedinTables5and6.

FIG.8a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,BEFOREEXPLOSION.

AXISXFREQUENCY,AXISYINERTANCE.

FIG.8b:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,AFTEREXPLOSION.

AXISXFREQUENCY,AXISYINERTANCE


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FIG.8c:EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESINLAYERFORAGROUTEDROCKBOLTBEFOREANDAFTEREXPLOSION(ROCKBOLT

LENGTHEQUALS2.3m).

TABLE5IDENTIFIEDNATURALFREQUENCIESFORACASE,BEFOREANDAFTEREXPLOSION,FREQUENCIESMARKEDWITH*)ARECHARACTERISTICOFNOT

GROUTED,REARPARTOFAROCKBOLT

Lp. Identifiednaturalfrequenciesbeforeexplosion,Hz

Identifiednaturalfrequenciesafterexplosion,Hz

1 134.7 133.2

2*) 269.2 269.9

3

658.5

647.5

4 794.1 785

5*) 928.4 1011.4

Thecaseanalysedabovewasnotfullygroutedbutnodistinctchangeswereobserved.

FIG.9a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,BEFOREEXPLOSION.

AXISXFREQUENCY,AXISYINERTANCE


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FIG.9b:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,AFTEREXPLOSION.

AXISXFREQUENCY,AXISYINERTANCE.

FIG.9c:EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESINLAYERFORAGROUTEDROCKBOLTBEFOREANDAFTEREXPLOSION(ROCKBOLT

LENGTHEQUALS1.8m).

TABLE6IDENTIFIEDNATURALFREQUENCIESFORACASE,BEFOREANDAFTEREXPLOSION

Lp.Identifiednatural

frequenciesbeforeexplosion,Hz

Identifiednaturalfrequenciesafterexplosion,Hz

1 247.8 232.1

2 1406.9 1319.6


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For theabovecasegroutnear theceilingwasslightly

crumbled with the result of longer length of not

groutedouterend.

The test results increased knowledge about the

predictedbehaviorofarockboltsupportsystemwhen

anexplosiontakesplaceinthenearvicinity.Totally18caseswereanalyzed.In10casestherewerenodistinct

changes in resin layer continuity ( 5cm). In 4 cases

considerabledeformationof rockboltswas recorded,

whichmadethedeterminationofaresin layer length

impossible. In one case, the increase of stiffness of a

rock bolt was observed, probably as a result of

collapsing rocks, and the 3 remaining cases proved

difficulttointerpret.

ResearchWorkintheCopperMineLUBINAnother test to check the usefulness of themethod

was a researchwork in a coppermine,where rock

boltsintheroofstratahaddiscontinuitiesknownonly

tothe

staff

of

amine

department.

After

evaluating

the

discontinuitiesutilizing themethod, the resultswere

confrontedwith real, preset discontinuities. Twenty

rock bolts were investigated in that experimental

study.Theresultswerequitesatisfactoryandproved

the accuracyof themethod as illustrated inTables7

and8andFigures10a,10b,11aand11b.

FIG.10a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FORANANALYZEDCASE,METHOD

VERIFICATIONTRIAL.AXISXFREQUENCY,AXISYINERTANCE.

FIG.10b:EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESINLAYEROFAGROUTEDROCKBOLTWITHAVISIBLELACKOFRESINLAYERATTHEEND

PARTOFAROCKBOLTBEINGDETERMINEDASTHEMOSTDANGEROUSCASE,METHODVERIFICATIONTRIAL(ROCKBOLTLENGTHEQUALS1,8m).


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TABLE7IDENTIFIEDNATURALFREQUENCIESFORANUNKNOWNCASE,METHODVERIFICATIONTRIAL,FREQUENCIESMARKEDWITH*)ARE

CHARACTERISTICOFNOTGROUTED,REARPARTOFAROCKBOLT


1

145.2

4.2

2*) 314.4 2.7

3*) 568.1 2.9

4 902.9 2.9

5*) 1290.6 0.6

FIG.11a:FRFCHARACTERISTICSOFDIFFERENTPOINTSOFEXCITATION(MARKEDWITHDIFFERENTCOLORS)FOR ANANALYZEDCASE,METHOD

VERIFICATIONTRIAL.AXISXFREQUENCY,AXISYINERTANCE.

FIG.11b:EXPERIMENTALLYEVALUATEDDISCONTINUITYOFARESINLAYEROFAGROUTEDROCKBOLT,METHODVERIFICATIONTRIAL(ROCKBOLT

LENGTHEQUALS1,8m).


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TABLE8IDENTIFIEDNATURALFREQUENCIESFORANUNKNOWNCASE,METHODVERIFICATIONTRIAL

No. Identifiednaturalfrequencies,Hz Dampingratio,%1

43.8 13.4

2105.3 3.7

3205.2 2.4

4337.6 1.9

5500.2 0.8

6693.7 0.7

7919.7 1.1

81178.9 0.4

91468.6 0.6

101772.7 0.7

Conclusions

The developedmethod enables the identification of

the length of discontinuity of grout in a rock bolt

installation. Itusesmodalanalysisproceduresand is

based on impact excitation and on reference of

experimentalmodalmodeltothetheoreticalone.The

advantagesofthemethodinclude:

a. reliabilityofgroutingdiscontinuityassessmentonacontinuousbasisfollowinginstallation,

b. anondestructivecharacterofthemethod,c. the lack of necessity to install any additionalequipmentintoaroofsection.

Dynamicparametersof the tested structure (installed

rockbolt)aredeterminedbyitsboundary conditions,

which are directly related to the length of grouting

discontinuity. The dynamic measurement results

support theassumption thatboundaryconditionsare

such thatthe insituboltbehaves likeacantilever.Of

coursenot inallcases istheassumption fulfilled, i.e.

the rockbolt isbent or cracked. It canbe observedwhen the FRF function does not include

distinguishableresonancesandisquitedifferentfrom

typicalcharacteristics(bytypicalwemeanobtainedby

taking into account a very large number of

investigated cases). To overcome this problem and

determine that the rockbolt isnotbentordestroyed

by rock stratamovement, ultrasonic guidedwaves

ought tobeused to check that there isno failure in

inserted rock bolt (which should be straight and

without any damage). That constitutes a limitation,

but themain purpose of themethod is to check thestate of the grouting and not the state of rockbolt

alone.Nevertheless, theapparatuswasmodifiedand

equippedwithanultrasonictransducer.

Additionalwork is necessary to overcome problems

with nonlinear behaviour of the analysed object,

which resulted in a shift of relevant natural

frequenciesfortheoreticalvs.experimentalmodels.

REFERENCES[1]

Li

Charlie

C.:

Principles

of

rock

bolting

in

high

stress

rockmasses,Mining& Environment,Vol. 2/1, pp.

133143,CentralMiningInstitute,Poland,2010.

[2] Tadollini S., C.: Current roofbolting applications,technologies and theories in USmines,Mining &

Environment, Vol. 2/1: 305314, Central Mining

Institute,Poland,(2010).

[3] StaniekA., IdentificationofDiscontinuities inResinLayer of Grouted Rock Bolts, Experimental

Techniques,Vol.36,No.2,USA,2012.

[4] TheComplete ISRM (International Society forRockMechanics) Suggested Methods for Rock

Characterization,TestingandMonitoring:19742006.

Editors: R. Ulusay, J.A. Hudson, Ankara, Turkey,

2007.

[5] EuropeanStandardEN1936:2006Naturalstonetestmethods Determination of real density and

apparentdensity,andoftotalandopenporosity.

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[9] UhlT.:ComputerAidedIdentificationofMechanicalModels,WNT,Poland,1997.

[10] BishopR.H.,LearningwithLabVIEW,PrenticeHall,NewJersey,2001.

[11] CADAXUserManual,LMS,Lueven,2000.[12] Mechanical Vibration and Shock Measurements,

Bruel&Kjaerhandbook:122129,Naerum,1982.[13] RemingtonP.J.:EncyclopediaofAcoustics,Vol.2,Ch.

63,71,pp.715734,843855,JohnWiley&Sons,New

York,1997.

[14] BeardM.D.,LoweM.J.S.:Nondestructive testingofrock bolts using ultrasonic waves. International

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identification of grouting discontinuity of rock bolts as an efficient way to control safety...

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