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