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  • 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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    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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    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

    No. Identifiednaturalfrequencies,Hz Dampingratio,%

    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

    No. Identifiednaturalfrequencies,Hz Dampingratio,%

    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.

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