Measuring In situ Rock Stress by means of the Compact Conical-ended Borehole Over-coring (CCBO) Technique
by
Katsuhiko Sugawara* and Yuzo Obara*
•Department of Architecture and Civil Engineering (Received 15 October 1998)
Abstract
Compact conical-ended borehole overcoring (CCBO) is proposed for in situ rock stress measurement applicable to the faulted and highly jointed rock formations. Apparatus and operating procedure are described in detail. The observation equation on the stress tensor to be used in practice is presented, along with the possible ways for representing and interpreting the results. Thus, it is stated how the three dimensional state of in situ rock stress is determined from the strains on the conical end surface of a single borehole. The error in stress and stresses in an isotropic or transversely isotropic rock mass are obtained. Additionally, the reliability and the applicability of the present technique is successfully verified from the case studies.
1. Introduction
In situ rock stress is of prime importance for construction of such rock structures, as underground openings and tunnels, because the mechanical behavior of rock masses surrounding underground openings is dominated by rock stress, and also the stability of rock structures strongly depends upon the state of rock stress. For the rock stress measurement, various methods and devices have been developed and applied to a wide range of rock types. For example, the suggested method of the International Society for Rock Mechanics prepared by Kim and Franklin covers flatjacks, hydraulic fracturing and USBM and CSIRO overcoring [1].
It is desirable that the stress tensor can be determined from measurements in a single borehole, and that the necessary stress relieving is completed by the compact overcoring of smaller diameter. The compact conical-ended borehole overcoring (CCBO) technique proposed in the present paper is a development of the hemispherical-ended cell proposed by Sugawara et al. [2-7]. The stress tensor can be determined from the strains on the conical end surface of a single borehole, and the error in stress can be also determined. Calculation of the rock stresses from the rock strains can be conducted for isotropic and transversely isotropic rock. The necessary stress relieving is readily completed by the compact overcoring of the same diameter as the installation borehole. Continuous strain monitoring system is possible as well as the compact overcoring. In general, the cell utilizes sixteen elemental strains on the conical end surface of the borehole.
In the present paper, the theory and practice of the CCBO technique are clarified. The apparatus and operating procedure for measuring in situ rock stresses are described in detail, together with the data recording and reduction. Guidance is given on the number of strain gauges required for a specified accuracy. Possible ways for presenting and interpreting the results are explained. The proposed measurement is supported by case studies.
— 19 —
Pilotborehole
PlotterCompuIer DigiIalsIrain
meIer
FigurelAschematicviewoftheCCBOstressmeasurement；１：drillmga76mm- diameterborehole；２：creatingaconicalboreholesocket；３：boreholesocket cleaning；４：gluingthestramcellintotheplace；５：compactovercoring．
2．ApparatusandOvercormgProcedure
AschematicdiagramoftheCCBOstressmeasurementisshowninFigurel，andtheCCBO straincellandillustrativephotographsofthecomponentｓａｒｅｓｈｏｗｎｉｎＦｉｇｕｒｅ２Ｔｈｅｆｏｌｌｏｗｉｎｇ equipmentandsuppliesareneededtoconducttheCCBOtests：
１１１１１１１１１１ｊａｂｃｄｅｆｇｈ・ｌｊｋ
Rockdrillcapableofdrillinga76mm-diameterborehole；
Diamondbitforcreatingconicalboreholesocket；
Forward-facingboreholecameratomspectqualityofboreholesocket；
Boreholesocketcleaningmaterials；
Thel6or24elementconicalstraincell；
Straincellinsertiondevice（whichincludesorientationalcapability)；
Electricalconnectionfromthestraincellthroughtherod／waterswivel；
76mm-diameterdiamondovercoringbitwiththin-walled（3mm）barrel；
Displacementtransducertomonitortheadvanceofovercoring；
Digitalstrajnmetertoprocessandrecordthestraincelldata；
Computerandsoftwaretocalculatestressesfromstrains．
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Figure2TheCCBOstraincellandillustrativephotographsofthecomponents；（a）ｔｏｐ ｖｉｅｗｏｆｔｈｅ２４ｅｌｅｍｅｎｔｃｏｎｉｃａｌｓｔraincell；（b）sideviewofthe24element conicalstraincell；（c）borzcrowndiamondbit；（｡）impregnateddiamondbit；
（e）topviewofforward-facingboreholecamera；（f）forward-facingborehole camerasystem；（９）straincellinsertiondeviceandindicatoroforientation；
（h）compactdiamondovercoringbitwiththin-walledbaｴTel．
－２１－
Rockdrillcapableofdrillinga76mm-diameterborehole（i､e・NXborehole）isatfirstrequi1℃｡，
andtheinitialboreholeisdrilleduptotherequireddistance（themaximumissofar40m)．At thebottomoftheborehole，theconicalsocketiscreatedusingthespecifiedbolzand impregnatedｄｉａｍｏｎｄｂｉｔｓａｎｄｂｙａｐｐｌｙｉｎｇｔｈｅｓａｍedrillingequipment･Oncompletionofthe boreholesocketforming，aboreholecameraisusedtoinspectthequalityofthesocket・The socketshouldappearuniformandisotropic・Thereshouldbevisuallynoopencracksinthe socketandnorunnmgwater・Usmgthesoftclothandacetone，thesurfaceoftheborehole socketiscleanedtoremovethedustparticlｅｓｐｒｉｏｒｔｏｔｈｅｇｌｕｉｎｇｏｆｔｈｅｓｔｒａｉｎｃelLOn completionofthecleanmg，thesocketisagainmspectedbytheboreholｅcamera・Iftherestill existdustparticlesevidently，cleaningShouldbedoneagain，untilthesocketbecomesvisually clean・Priortoinstallationofthestraincell，alltheelectricalcircuitsandstraingauge resistancearechecked、Thenthestraincenisattachedtotheinsertiondevicemakingsurethat thecableisthreadedthroughtheinsertiondevice・Theglueisthendistributedoverthｅｈｅａｄｏｆ ｔｈｅｓｔｒａｉｎｃｅｌＬＴｈｅｓｔｒａｉｎｃenistheninsertedmtheboreholeandpushedtocontacttothe socket・Theinsertiondevicecanberotatedsothatthecellisplacedintheboreholesocketata specificorientation，usingtheinclmometerintheinsertiondevice・
Oncetheorientationhasbeendecided，themsertiondeviceispushedhardagainsttheborehole socketandheldｆｉｒｍｆｏｒ３０ｍｍｓｕｎｔｉｌｔｈｅｇｌｕｅｓｅｔｓ・Oncompletionofthisstep，theinsertion deviceisremovedfromtheborehole・Ｉｔｉｓｉｍｐｏｒｔａｎｔｔｏｍeasuretheexactdistancefromapoint onthedrillingmachinetothestraincell（asareferenceforthedistancesoftheovercoring measurement)．Theelectricalconnectionsfromthestraincellarepassedthroughthethin walledovercoringdrillbit，thedriUrods，andthewaterswivel・Theelectricalcircuitsand straingaugeresistanceareagainchecked，inadditiontothemonitoringanddatalogging equipment・
Theovercoringisconductedusingthethinwalled（3mm）７６mm-diameferbit，ｗｈｉｃｈｉｓｏｆｔｈｅ ｓａｍｅｄｉａｍｅｔｅｒａｓｔｈｅｂoreholeitself・Adisplacementtransducerisusedtomonitortheadvance ofovercorlng，whichiscontinuouslyconducteduntilthestraincelliscompletelyovercoredover theminimumovercoringdistancelOOmmtoabout300mm、Ｉｆａｊｏｉｎｔｉｓencountered，thestrain cen／corerecoveryiseasier・Itisnotedthatthestramcellisnotreused
Duringovercoring，thedataaregenerallyrecordedforevery5mnovercoringadvance・When higherprecisionisrequired，thereadingismadeevery2nm、Thedataarerecordedoncomputer diskandtabulatedlngeneral，asacheckonovercoringprogress，onechannelismonitored，
concerningarelationofgaugeoutputversusdistance
Theequipmentspecificationandtolerancesarenotgivenhereindetail，becausemanyofthese mteractandthelengthofthestraingaugesmthestraincellhasbeenoptimizedthroughtheory andexperiencaSomeofinformationisgivenmthereferences［８，９]・Theequipmentcanbe operatedeasily,asmentionedabove・Also,severalpapershavebeenpublishedｏｎｉｔｓｕｓｅ［10, 11lTheconicalstraincellofcommonuseisavailableatarockfemperaturerangefrom0℃ｔｏ
60℃．
3．ＤａｔａReduction 3.1Principleofin8it皿rockstregsdetermination
lizsimrockstressesactingonrockmasssurroundmgastressmeasuringstatioｎａｒｅａｓｓｕｍｅｄ ｔｏｂｅｕｎｉｆｏｒｍｐｒｉｏｒｔｏｔheboring，andthemagnitudesoftheircomponentsarecalculatedfrom thestrainsonaconicalsocket,basedonthetheoryofelasticity[9-151
－２２－
3.2Co-ordinatesandexpressionofm8jtmstresstensorofrock
Forcalculationofmsitustresstensorofrockfromthestrains，thecylindricalco-ordinates
(応６，ｚ)，thesphericalco-ordinates（p,ａＯ）andtheCartesianco-ordinates（z,9,ｇ）are
definedrespectivelyasillustratedinFiguro3，makingthez-axiscomcidentwiththeaxisofthe borehole・ThemsitzJstresstensorofrocklo｝isexpressedasfoUows：（O}＝{Ox’０h,’０２，乃透，聡，Tky}Ｔ （１）
whereox，Ｄｙ，ｏｂ１ｒｉ'z,恥andTkyarethestresscomponentsintheOartesianco-ordinates．
Figure3Co-ordinatessystemsfixedtotheconicalboreholesocketand ｔｈｅｓｔｒａｉｎｓｔｏｂｅｍｅａｓｕｒｅｄｂｙｔｈｅｌ６ｅlementmethod、
3.3Strainstobemeasured
Thestrainsarerequiredtobemeasuredatthespecifiedeightpointsontheconicalsocketof 76Imndiameter・Thesepomtsareaxisymmetricallyarrangedalongameasurmgcircleofl9mn radius，byrotating45degreesatastepStrainmeasuringpointshasbeenspecifiedthrough theoryandexperience[９１Intheusuall6elementmethod，thetangentialstrameoandthe radialstraingparemeasuredateachstrainmeasuringpoints，ｕｓｍｇａｎｌ６ｅｌｅｍｅｎｔｓｔｒａｉｎｃｅｌ1．
The24elementmethodrequirestheadditionalstrainateachpoint，thatistheobliquestrain EP,asshowninFigure4､TheangLbetweenerandePis45degreesandthatbetweencoand epis45degreeaThus，thestramsmeaBuredonaconicalboreholesocketcanbedenotedby
（β}＝{β,,β2,β3,…,β､}Ｔ
（２）where〃：numberofstrains；ｉ､e，〃＝l6forthel6elementmethod：〃＝２４ｆｏｒｔｈｅ２４ｅｌｅｍｅｎｔ method．
灘
動Figure4Thestraingaugearrangemenｔｆｏｒｔｈｅ２４ｅｌｅｍｅｎｔｍｅｔｈｏｄ
－２３－
3.4Relationsbetweenthestrainsandthem8ifuzstresstensorofrock
Thestrains｛eB,９，,ＥＣ｝atastrainmeasuringpointofatangentialangleOaregiveninthe isotropiccase，asfollows：
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Ｄ１Ｉｃｏｓｅ Ｄ２ｌｃｏｓＯ
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、u１ｓｉｎＯ Ｄ２ｌｓｉｎＯ
Ｄ３１ｓｉｎＯ－Ｄ３２ｃｏｓＯ
(3)
ｗｈｅｒｅＥｉｓｔｈｅＹｏｕｎｇ，smodulusofrock，ａｎｄＡｌｌ，Ａ１３，….，D32aretheconstantsandare termedasthestraincoefficients．
3.5StraincoefficientsdependinguponPoisson,ｓｒａｔｉｏ
ＴｈｅｖａｌｕｅｓｏｆｔｈｅｓｔｒａｉｎCoefficientsaredependentuponPoisson1sraｔｉｏｏｆｒｏｃｋＴｈｅｓｅｖａｌｕｅｓ ｈａｖｅｔｏｂｅｅｖaluatedbythenumericalanalysis，sincethereisnoanalyticalsolutionThestrain coefficientsoftheisotropiccasecomputedbytheBEManalysisaresummarizedinTablel・The straincoefficientsfortransverselyisotropicrockaregiveninthereferences［l6l
TablelStraincoefficientsintheisotropiccase
Poisson,ｓｒａｔｉｏＡＩｌ Ａ１３ Ａ２，Ａｚ３ Ａ３ｌ Ａ３ｚ Ａ３３
0.343 0.365 0.373 0.380 0.386
0.562 0.519 0.496 0.474 0.426 1.002
1.000 0.999 0.997 0.989
郡、羽叫Ⅱ
７７７７６１１１ｌ１ｌ一一’一 0.1090022 -0.021 -0.065 -0.154
２８１２３０１２２２８８８８８●ｃ●●じ０００００一一一’一
浜、兜ね西
７７６６６℃幻・幻幻幻
、、お扣扣
●●●●■０００００Ｕ・●
Poisson，ｓｒａｔｉｏＣｌ ＣｈＧＤ１，Ｄ2，Ｄ3１、3２
５３７１１５６１７８１２３３４●●◆ｂ●０００００一’一｜’ 0.246
0.185 0.155 0.126 0.071
0.082 0.095 0.101 0.108 0.123
1.542 1.627 1.673 1716 1.787 0.10
0.20 0.25 0.30 0.40
0.655 0.641 0636 0.632
4‐０６３０
0.802 0.860 0.886 0.911 0.953
-1.725 -1.860 -1.923 -1983 -2.091
Remarks:ＡＩ2＝Ａ22＝DI2＝Ｄ22＝0
－２４－
３．６０bservationequationoftheinSituLrockstress
Observationequationofthei几sjtustresstensor｛o）isexpressedbythefollowingmatrix equation．
[Ａ]{◎}＝Ｅ･(β） (4)
where[Ａ］isａｎ几x6elasticcompliancematrixnormalizedbytheYoung'smodulusELThe elementsof[Ａ］arecomputedbysubstitutingthetangentialangleOateaChstrainmeasuring pointinequation（３）
3.7ThemostprobablevaluesOfi〃situzrockstresses
Themostprobablevaluesoftherockstressesaredeterminedbythemeansquaremethod，
providingthenormalizedexpressionofequation（４）asfollows：
[B]{o}＝Ｅ･{β１ (5)
where［B]＝[Ａ]T･[Ａｌ(β*}＝[Ａ]T･(β｝・Themostprobablevaluesoftherockstress：｛o*｝
areexpressedas
（｡.}＝Ｅ･[C]･{β*） (6)
where[C]istheinversematrixof[B]．
3.8Standarddeviationsofthemostprobablevalues
Thestandarddeviations鳥ofeachstresscomponentisingeneralevaluatedbyassumingthat theerrorofmeasuredstrainobeysthenormalprobabilitydistribution，asfollows：
鳥2＝c裡駕，‘＝1,2,.…,６ (7)
where爵isthevarianceofmeasuredstrainsgmdqMisacorrespondingdiagonalelementofthe
matrix［Cl
3.9Simulationofthecompactovercoringprocess
Equation（３）isavailabletodescribethevariationofstrainduringthecompactovercoring，
However，fortheintermediatesteps，whenovercoringisnotcomplete，FEMandBEManalyses aｪ℃ｕｓｅｄｔｏｄｅｔｅｌｍｉｎｅｔｈｅｃｏｍｐｏｎｅｎｔｓｏｆｔｈｅｅlasticcompliancematrixforthedifferentdegrees ofovercoringThus，onesoftwareisrequiredfortheprocesssimulation[１２，１４，１５]．
3.10E1evatedrocktemperature
lnthecaｓｅｗｈｅｒｅｔｈｅｒｏｃｋｉｓｏｆａｄｉｆｆｅｒｅｎｔｔｅmperaturefromtheovercormgdrillingwater，
thebondedcellwhichinitiallyisoftherocktemperatureiscooledbythedrillingwater、Then，
thestrainshavetobecorrected1usingthecoefficientofthermalexpansion・Ｉｔｉｓnormally esseIrtialtoknowtherocktemperatureandthecoefficientofthermalexpansion、Ｔｈｉｓｉｓ，
however，acomplexsubjectbecausethecorrectionmayalsodependonthelengthofthe boreholaAcompensationtechniquehasbeendevelopedtoaccountforthetemperatureeffect [l7lStramcellswithatemperaturesensorhavebeendeveloped．、
－２５－
3.11Comparisonofstrainsensitivity
Accuracyofthemethodiscloselyrelatedwiththestrainsensitivityatstrainmeasuring pomts・h1otherwoIds，itisassociatedwiththeboreholegeometryandstraingauge ampangement・Ｗｈｅｎtheobservationerrorofstrainfollowsthenormalprobabilitydistribution asgiveninequation(7)，thevarianceofeachstresscomponentisindirectproportiontothe magnitudeofthecorrespondingdiagonalelementcjiofthematrix［Clandalsototheerror
variance爵ofthestrainmeasuredlntheCCBOmethod,themagnitudeofc樹isdependent
upontheradiusofthestrainmeasurmgcircle，ｔｈｅｎｕｍｂｅｒｏｆthestraingaugesandPoisson's ratioofrock、ＩｎｏｒｄｅｒｔｏｉｍｐｒｏｖｅｔｈｅａｃｃｕｒａｃｙｏｆｔhBmethod，thustheminimizationofthe maximumvaluecmaxofqiisrequiredfordeterminingtheoptimumstramgaugearrangement・
ThevaluesofcmaェoftheCCBOmethodaresummarizedinTable２，comparingwiththoseofthe conventionalstrainmeasurementovercoringmethods[2,8,15]、
Thesmanestvalueofcmaxisobtainedbythehemispherical-endedboreholetechnique・Inthe CCBOmethod，thevalueofcmaxdecreaseswithincreasingthenumberofstraingauges、Inthe casethatPoisson,sratioisl／４，thevaluesofcmaェｏｆtheCOBOmethodarealmostthesame asthatofthedirectstrainmeasurementbythreerosettetypedstraingaugesoncylindricalwall
ofaborehole．
Tnble2Comparisonofcin型
RadiusIntioof st【Ｙｍ１ｍ函動F ｍｇｃｉにles：●
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ＴｂｃＣＣＢＯ ２４ｅｌｅｍｆｍＴ ｍｅＵｍｄ
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p[巳sentcdby Kobayashi etal.[21］
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〃３0.467
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0.127 0.133 0148
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－２６－
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Theevolutionofthestraingaugesforevery5mm（or2Inm）overcoringadvancecanbeplotted，
withtheterminalstramdistributionsaroundthestraincell・Ｆｉｇｕｒｅ５ｓｈｏｗｓａｎｅｘａｍｐｌｅｏｆｔｈｅ ｅｖｏlutionofthestraingaugesforevery5mmovercoringadvance，comparingtothetheoretical curveｓ［l21Thelateralaxisofthefigurerepresents,theovercorlngadvance，thatisthe distanceintheaxialdirectionbetweBntheheadofthecompactovercoringandthestrain measuringcircleontheconicalsocket・Thechangesinstrainaｪ℃ｒａｐｉｄｉｎａｌｌｃａｓｅｓａｆｔｅｒｔｈｅ compactovercoringpassedthroughthesectionofthestrainmeasurmgcircle．
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１１⑩ユ・巨肩舅の
ｍＩｕ
、
ユ、
Ⅷ
０
０，degrees
Figure6Finalstraindistributionontheconicalsocket，
comparmgtothetheoreticalcurves．
Thestressmatrixwithreferencetothe（工,ソ,ｚ）co-ordinatessystemcanbepresentedThis isusefulifaseriesofmeasurementsaremadeandthevariationisbeingstudied・A1tematively，
theprincipalstressmagnitudesandtheirdirectionscanbepresented，bothinanisometricplot and／oronalowerhemisphericalprojectionTheisometricplotisdemonstratedinFigure7 (a)．Thisistheresultobtainedbyfourtimesstressmeasurementsusingasinglehorizontal borehole，ａｔｔｈｅｄｅｐｔｈｏｆ３２４ｍｂｅｌｏｗｔｈｅｓｕｒｆａｃe，withinaverticalpillarofzincore，４２min width，100ｍｉｎｈｅｉｇｈｔａｎｄ８０ｍｉｎｌｅｎｇｔｈ［18]．Thelowerhemisphericalprojectionofthe principalstressdirectionsofthecaseispresentedinFigure7（b)．
(日） (b） Ｎ
1３ 1４
ｄＴ Ｗ
Ｃ
Figure7Principalstressesmeasurｅｄｉｎｔｈｅｐｉｌｌａｒｂｙｆｏｕｒｔｉｍｅｓｓｔｒｅｓｓmeasurement usingaborehole；（a）isometricplot,numeralsrepreselitthemagnitudeof theprincipalstressinMPa；（b）lowerhemispherestereographic projectionoftheprmcipaldirection．
－２８－
ｌｎｔｈｅｃａｓｅｗｈｅｒｅｔｈｅｓｔｒｅｓｓｅｓａｒｅｂｅmgstudiedmaspecificplane，possiblytocorrelatewith geologicalfeature，ｉｔｉｓａｌｓｏａｖａｉｌａｂｌｅｔｏｓｈｏｗｔｈｅ２－Ｄｐrincipalstressmagnitudesand directionsalongtheboreholeinanappropriateplane・Figulre8showsthe2-Dprincipalstress magnitudesanddirectionsalongasingleborehole，whichisdrilledhorizontanyfromtｈｅｗａｌｌ ｏｆａｇａｌｌｅｒｙａｔａｄｅｐｔｈｏｆ５２０ｍindioriteandgranodiorite･Theboreholeintersectswiththe faultmof0.25minwidthdippingabout80degrees，ａｎｄａｎｉｍｍｅｄｉａｔｅｓｋａｒｎｏｆＬ５ｍｉｎｗｉｄｔｈ [１３，１９，２０］This2-Dexpressionisusefulforunderstandmgthevariationsofthestress magnitudesanddirectionsalongtheborehole．
(８）
臺吊念･(:二F褐＝二三藻蕊団目司昌
Figure82-Dprincipalstressmagnitudesanddirectionsalongtheboreholein aparticularoraspecificplane；（a）elevationｖｉｅｗ；（b）planview．
Ｗｈｅｎthe24elementstraincellisused，thestressdistributionontheboreholesocketcanbe presented，ａｓｗｅｌｌａｓｔｈｅ３－Ｄｉ〃siturockstresses・Theprincipalstressmagnitudesand directionsonthesocketsurfacecanbeplottedonthecrosssectionalviewoftheborehole［14, 15lOnecaseisshowninFigure9､The2-Dprojectionofthesurfacestressonaconicalsocket ispresentedforeveryovercoringadvance，ａｎｄｃａｎｂｅｃｏｍｐｍ巳dwiththetheory．
●の
Ｘ
－－－－＝￣
Figure9Stressdistributionontheconicalboreholesocketsurface，comparingto themaximumcompressionintheplaneperpendiculartotheborehole axis（solidaITows)．
－２９－
5．nlustrativeCaseExample
DzsiturockstressmeasurementbymeansoftheCCBOtechniquehasbeenconductedin Kamaishimine，Japan，toevaluatethevariationoftheregionalstressmagnitudes，
orientationsandthestressgradient，includingtheeffectofjointsandfaultsonthestress distribution・FigurelOshowstheregio､alprincipalstressmagnitudesanddirectionsinthe horizontalplanes，evaluatedbythemulti-timesstressmeasurementandsubsequentaveraging ofeachstresscomponent[2２，２３１
亀 、 Ｂｏ
燃
ぴびEＣ
９
550ｍＬ
Ｕｍｐｅ１Ｆ１０
250ｍし ０１kｍ Ｏ１ｋｍｕⅡ
■Ⅱ
FigurelORegionalprincipalstressmagnitudesanddirectionsinthe horizontalplanes，Kamaishimine，Japan．
Threestations:Ga-1，Ga-2andGa-3havebeenarrangedinGanidakediorite／granodioriteat +550mleveLandthreestations:Ku-1,Ku-2andKu-3areinKurihashigranodioriteat
＋250mleveLItisnoteworthythatthedirectionofthemaximumhorizontalcompressionis approximatelyinnorth-southdirectionandthestressmagnitudesincreasewithincreasingthe depthbelowthesurface
AtthestationGa-2，theCCBOmethodhasbeenappliedtoclarifytheeffectofjointsand faultsonthestxもssdistribution［13,19,20]・StI℃sseshavebeenmeasured21timesmtotalin asingleboreholeintherangeｆｒｏｍｑ6ｍｔｏ29.5mapartfromthegallery，butthemeasurement isconcludedbygivmgthel8reliableresultsinFigure8Theboreholehasintersectednotonly Faultmbutalsomanyjoints・However,theresultsobtainedclearlyindicatethatanoteworthy
differenceofthestressstateexistsbetweeninfronta､。intherearofFaultmThismeansthat
Faultmplaysanimportantroleonthestressdistribution，whiletheinfluenceofthejoint systemisminor．
－３０－
ThetimerequiredforthetotalmeasurementatthestationGa-2hasbeenreportedtobeabout ll2hours・ThetimerequiredforeachoperatioｎｈａｓｂｅｅｎｓｕｍｍａｒｉｚｅｄｍＴａｂｌｅ３，exceptthetime forconventionaldrining[20l
ComparlsonoftheCCBOmethodtoothermethodshasbeenconductedinseveralsitesin Japａｎ[26,27］
THble3AveragetimerequiredfOrlhemeasurement Ｔｉｍｅ(minutes）
Operation
CIeaUngthecomcalboreholesocket C1eaningandthecameraoperation Gluingthestraincellintheplace Overcormgandstrainmeasurement Recovenngcorewiththestraincell
22.8 10.0 ４０．３ 22.4 10.4
6．InterpretationofResults
6.1Determinntionofelasticmodu2li
lnordertocalculatethestresscomponentsfromthestrains，Young,ｓｍｏｄｕｌｕｓｏｆｒｏｃｋｉｓ ｒｅｑｕｉｒｅｄａｓｗｅｎａsPoisson'sratioofrockForthedeterminationofthesetwovalues，thetwo schemeshavebeenapplied・OneisalaboratorytestusingthecoresampleandtheotherismsitzJ loadingexperimentusingtheconicalsocket・Theformerisusuallyused，andthelatteris mainlyconductedtore-confirmthereliabilityofthelaboratorytestresults・
Thelaboratorytestisindispensabletoestimatetheelasticity，thenonlinearityandthe anisotropｙｏｆｒｏｃｋ・Ingenera１，theconventionalmulti-stageuniaxialcompressionexperlment isconductedAsillustratedinFigurell，threecylindricalspecimensofperpendiculartoeach other，２５mmindiameterand50mminlength，aretakenbydrillmgtherecoveredcorehavinga conicalsocketwithin・Fourcross-typedstraingaugesareusedtomeasmもthestrainresponseof eachspecimenThemulti-stageloadingpatternisdesignedtoreproducetheaxialstrainasthe sameasthemaximumstramontheconicalsoOket･Themaximumloadisusuanysetlowerｔｈａｎ 60%oftheuniaxialcompressivestZengthofrock・IngeneraLYoung'smodulusandPoisson，s ratioaredeterminedfromthelinearrelationbetweentheaxialstressandthestramrecoveZy・
Ｓｔｒ３ｌｎｃｅｌｌ
FigurellCoresamplingforthemulti-stageuniaxialcompression experiment，inthelaboratory．
－３１－
ThemsimloadingexperimentisconductedaftergluingthestrainceUonaconicalsocket，
justbeforethecommencementofthecompactovercoring・AsshowninFigurel2,aflat-ended ringof6mminwidthisformed，andtheaxialpressureisappliedonit，usingasteelringplaten Thepressursstrainrelationismonitored，andbothYoung,smodulusandPoisson'sratioof rockareevaluatedusingthespecialchartsgivenbytheBEManalysis、Theprocedurehasbeen presentedelsewhere[11]．
oIe
Figurel2Schematicviewofinsjtuaxialloadmgexperimentfor
thedetermiT1ationofelasticmoduli．
6.2Evaluationofrockmassstrength
Themulti-timesCOBOstressmeasurementisapromisingmethodpresentlyavailableforthe evaluationoftherockmassstrength，sinceitenablesonetomeasurethe3-Dstressstatewithin thegroundarchandthestressconcentrationaroundthecavity，Whenitisappliedtothelarge rockcavem，suchasundergroundpowerhousecaverns，thestressdistributioninthegrouｎｄ ａｒｃｈｃａｎｂｅｍｅａｓｕｒｅｄａｓｗｅｌｌａｓｔｈｅstresseswithinapostfailuI℃regionneighboring immediatelytothecavityface，providingthedatacloselyrelatedtothestrength characteristicsofrockmass・Thestrengthevaluationbasedonmsimstressmeasurementsis 亜portedinthereferences[5,6,241
6.3Evaluationoffrictionalcharacteristicsofjoints
Themulti-timesOCBOstressmeasurementisalsoavailablefortheevaluationofstresses
actingonjointsandtheirfrictionalcharacteristicsForthispurpose，the3-Dstressstateis requiredtobemeasuredatseveralstationswithmrockblocksseparatedbyjoints・Whenthe jointorientationaredeterminedusingtheborehole-camerasystemand／orothermethods,the 2-Dstressesactingoneachjointplanecanbeestimatedfromadjacentstressdata,applymgthe stresstransformationlaw,Oaseexamplesarereportedinthereference[251
－３２－
7．SummaryandConclusions
Thecompactconical-endedboreholeovercoring（CCBO）techniqueispromismglyavailable forflzsitzJrockstressmeasurementinthefaultedandhighlyjointedrockformations・The apparatusandoperatingprocedｕｒｅｆｏｒｔｈｅＣＣＢＯｔｅｓｔｉｌｚｓｉｔｕｈａｖｅｂｅendescribed，andthe observationequationofthestresstensortobeusedinpracticehasbeenpresented，aswellasthe recommendedprocedureforpresentingandinterpretingtheresults･Thus，ithasbeenclarified howthethreedimensionalstateofi〃simrockstresscanbedeterminedfromthestrainsonthe conicalendsurfaceofasingleboreholeinanisotropicandtransverselyisotropicrockmass，
togetherwiththeerrorinstress、Itisremarkablethattheovercoringisconductedusingthe thinwalled（3mm）７６nml-diameterbit,whichisofthesamediameterastheboreholeitself・The necessarystlBssreliefiscompletedbytheovercoringadvanceoflOOmm・Thisistheadvantageof theCCBOtechnique,whichenablestomeasurerockstressesinhighlyjointedrockformations ThereliabilityandtheapplicabilityoftheCOBOtechniquehavebeenverifiedfromthecase examples・IthasbeenconfirmedthattheinducedstressinapillarcanbemeasuredbytheCCBO technique，ａｎｄｔｈａｔａｎｕｎｉｆｏｒｍｉｔｙｏｆｔｈｅｓｔｒｅｓｓｓｔａｔｅｍｔｈｅｐｉｌｌａｒｉｓａｂｌｅｔｏｂｅｅｘａｍｍedbythe reproducibilityofthemeasurements・Ｎａｍｅｌｙｔｈｅｓａmeresultisobtainedmorethanoncebythe subsequentmeasurements・Anotherｃａｓｅｅｘａｍｐｌｅｈａｓｓｈｏｗｎｔｈａｔｔｈｅｆａｕｌtdominatestherock stressfield,thenthestressjumpanddiscontinuityappearsalongthefaultplane・Inthiscase，
the2-Dexpressionoftheresultsiseffectiveforunderstandingthevariationsofthestress magnitudesanddirectionsalongtheborehole・Fromthelastcaseexample，ithasbeen concludedthatthemulti-timesstressmeasurementbymeansoftheCCBOtechniqueand subsequentaveragingofeachstresscomponentareeffectivetoevaluatethevariationofthe regionalstressmagnitudes，orientationsandthestressgradient、Additionally，through interpretationofresults，ｉｔｈａｓｂｅｅｎｃｏｎｃｌｕｄｅｄｔｈａｔｔｈｅＣＯＢＯｔechniqueisavailableforboth initialandinducedrockstressmeasurements，ａｎｄａｌｓｏｆｏｒｔｈｅｅｖａｌｕａｔｉｏｎｏｆｒｏｃｋｍassstrength andthefrictionalcharacteristicsofjoints．
8．Acknowledgments
TheauthorswishtoacknowledgetheencouragementandsupportgivenbyＰｒｏｆＪ.Ａ、Hudson oflmperialCoUegeofScience，TechnologyandMedicinemUK・Theauthorsarealsomost gratefultｏＤｒ・ToshiroAoki，TokyuConstructionOqLtd.，Ｄｒ・KiyotoshiSakaguchi，
ResearchAssociateofTohokuUniversity，，r・Hyun-KukJang，MessersYoshifumiNoguchi andNaoakiNakamura，NittetuMiningCo・Ｌｔｄ，ａｎｄＤｒ・KatsuhikoKaneko，Professorof HokkaidoUnivelsityforgoingourresearch
－３３－
References
ＬＫｉｍＫ・andFranklinJ.Ａ、Suggestedmethodsforrockstressdetermination，hzt・ｊｂＲｏｃｂ Ａ化ch.ＭＺ".Sbi.＆Geomec/i・A6str､２４(1)，53-73（1987)．
2．SugawaraK.，ＯｂａｒａＹ.，ＯｋａｍｕｒａＨ・ａｎｄＷａｎｇＹ・TheDeterminationoftheComplete StateofStressinRockbytheMeasurementofStrainsonaHemisphericalBorehole-bottom j8Mi几MbtaJZ.」)zst・jtZpaJzlO1，277-282（1985)．
3．SugawaraK.，ＯｂａｒａＹ.，ＯｋａｍｕｒａＨ、andAokimMeasurementofStrainsona Hemi-sphericalBoreholeBottombytheStressReliefTecbnique.』.〃、､MEtqJJ.mst・ｊｑｐａ几 102,463-468（1986)．
4．SugawaraK.，ＯｂａｒａＹ.，ＡｋｉｍｏｔｏＭａｎｄＡｏｋｉＴ・StabilityEstimationofLargeRock Cavernbyln-SituStressMeasurements.Ｐ７℃c・ｍｔ・Symp.ｏ几助gmee冗晤ilzCompZerRodb FbmmtioJzs,Ｖ０１．１，ｐｐ､135-141．Beijing（1986)．
5．SugawaraK.，KanekoK.，ＯｂａｒａＹ、ａｎｄＯｋａｍｕｒａＨ、DeterminationoftheStateofStresｓ ｉｎＲｏｃｋｂｙｔｈｅＭｅａｓｕｒｅｍｅｎｔｏｆＳｔｒａｉ､ＳｏｎtheHemisphericalBoreholeBottom・Ｐｍｃ・ｈｔ・
Symp・皿'gBRodbCbuem,Vol､２，ｐｐ､1039-1050.Helsinki（1986)．
6．SugawaraK.，ＯｂａｒａＹ.，KanekoK・ａｎｄＡｏｋｉＴ・Hemispherical-endedBoreholeTechnique forMeasurementofAbsoluteRockStlCss．Ｐ７℃c・ｍｔ、Symp・ＲＯＣ虎８t花ssmdRoc虎８t７℃ｓｓ Ｍｂ[zsu7w7ze几ts,pp207-216.､Stockholm（1986)．
7．SugawaraK・ａｎｄＯｂａｒａＹ・MeasurementofDzSituRockStressbyHemispherical-ended BoreholeTechnique.、2.ｊＵＭｊ"mgScie'zcea7zdT1edb"oJogy3,287-300（1986)．
8．SugawaraK.，SakaguchiK.，ＯｂａｒａＹ.，NakayamaT・ａｎｄＪａｎｇＨ・RockStress MeasurementandNumericalApproachforCavernDesigning・ｊｂＫｂ1℃ｍＲｏｄｂＭｂｃﾉmJzjcs Sbciety2(1)，164-176（1992)．
9．SakaguchiK.，ＯｂａｒａＹ.，NakayamamandSugawaraK・AccuracyofRockStress MeasurementbymeansofConical-endedBoreholeTechnique・ｊｋＭｉ几．Mbtcz"．〃２８t・jqpα〃
108,455-460（1992)．
10．NakayamaT.，ＯｂａｒａＹ.，SakaguchiK･ａｎｄＳｕｇａｗａｒａＫ・Conical-endedBoreholeTechnique forRockStressMeasurementanditsApplications・Ｐｍｃ､ｍｔ、Symp．ｏ〃Assessme"ｔα'2ｄ Ｂ℃uelztio〃q/FmJ…Ｐhe"omemzinRodbEngj7zee元rzg,ｐｐ､295-300.Istanbul（1993)．
11．SakaguchiK.，TakeharaT.，ＯｂａｒａＹ.，NakayamaT・ａｎｄＳｕｇａｗａｒａＫ、RockStress MeasurementbymeansoftheCompactOvercoringMethod．ｊ・雌"・Metα〃・bzst・ｊｔｍＱｎ 110,331-336（1994)．
12．SakaguchiK.，ＯｂａｒａＹ.，ＪａｎｇＨ・ａｎｄＳｕｇａｗａｒａＫ・ProcessSimulationofStressRelieving forRockStressMeasurement.』・Mjlz､MbtaZJ､hzst,ｊｎｐα〃110,601-606（1994)．
13．SakaguchiK.，ＪａｎｇＨ.，NoguchiY、ａｎｄＳｕｇａｗａｒａＫ・ApplicationofConical-ended BoreholeTechniquetoDiscontinuousRockandConsideration．‘.〃、､MbtcJJ・hzst・ｊｑｐＱ几 111,283-288（1995)．
14．ＯｂａｒａＹ・ａｎｄＳｕｇａｗａｒａＫ・ImprovementinAccuracyoftheConical-endedBorehole Technique・Ｐｍｃ・ｈｔ・Symp.ｏ几RoclBSt７℃ss,ｐｐ､77-82.Kumamoto，（1997)．
15．ＯｂａｒａＹ.，ＩｍａｉＫ.，NakamuraN,ａｎｄＳｕｇａｗａｒａＫ,ImprovementoftheConical-ended BoreholeTechniqueforRockStressMeasurementwithaHighAccuracy、ｊ８ＭＺ凪Mbtα〃・
hst,jt1pQ几113,825-831（1997)．
16．ＯｂａｒａＹ.，ＪａｎｇＨ.，MurakamiK・ａｎｄＳｕｇａｗａｒａＫ・ApplicabilityoftheComcal-ended BoreholeTechniquetoAnisotropicRocks.ｊ１ＭピルM2ZczJJ.hst､ｊｔ１ｐｑ〃111;919-924（1995）．
17.ＯｂａｒａＹ.，SugawaraK.，SakaguchiK・andMizuochiY・ApplicationofHemispherical-ended BoreholeTechniquetoHotRock，Ｐ７℃０．７fhmtemQtjoJzczZCbng1℃ＳＳＱ／LSRMpp､587-590.
－３４－
Aachen（1991)．
18．ＯｂａｒａＹ.，SugawaraK・andSakaguchiK・RockStreBsMeasurementsbytheConical-ended BoreholeTechniqueUsingtheCompactOvercoring・Ｐｍｃ、８thmtematio7zczZCmg形ＳＳＱ／
ＥＲＭ)ppl45-148・Tokyo（1995)．
19．ＯｂａｒａＹ.，SugawaraK、andTakeharaT・RockStressMeasurementbyStressRelievmgin Japan・Proc・ＭＭＬﾉ／Ａ凹sZM11`jOi"tSymposizzmW,ｐｐ､425-432.Ｕｂｅ（1994)．
20．ＯｂａｒａＹ.，ＪａｎｇＨ.，SugawaraK、andSakaguchiK、MeasurementofStressDistribution aroundFaultandConsiderations・Ｐｍｃ,２"dmt・ＣＯけ.ｏ几Afbcﾉmlzicsq/jOi"te[ZcmdFbuZted Rodb,pp495-500､Vienna（1995)．
21．KobayashiS.，NishimuraN・ａｎｄＭａｔｕｍｏｔｏＫ，DisplacementsandStrainsarounda Non-flat-endBorehole,Ｐｍｃ.」>2t・symp．ｑ/FYeJdmeCzsuだ、B）ztj几Geomech.,ｐｐ､1079-1084．
Ｋｏｂｅ（1987)．
22．ＪａｎｇＨ・ａｎｄＳｕｇａｗａｒａＫ・MacToRockStressMeasurementattheKamaishiDistrict・日･oc・
ﾋﾉteKbl1e[z-jQp(zJzJOi"tSymp.ｏ"Ｒｏ疏醜gmeemZg,ｐｐ､207-215.Seoul（1996)．
23．ＪａｎｇＨ.，ＯｂａｒａＹ，ａｎｄＳｕｇａｗａｒａＫ・RockStressMeasurementmaGraniticMassifby meansoftheConical-endedBoreholeTechnique・Pro０．ｍt､Ｆｂｍｍｑ/Resou1℃es団?gmee｢ing，
ｐｐ､pp256-261、Seoul（1994)．
24．ＯｂａｒａＹ，andSugawaraK・FieldstressMeasurementsinJointedRock・ｐｍｃ・hzt、Ｃｂｎｆ ｏ几Ｍ２ｃﾉiCmjcsq/jbmted(mdFtzuJtedRoc虎,ｐｐ､827-834.Vienna（1990)．
25.SugawaraK・InitialStress・ＭＥｃｈＱ几jcsq/Rodb,ｐｐ､357-383.Maruzen,Tokyo（1993)．
26．DenboyaN.，FukuharaA.，ＯｂａｒａＹ・ａｎｄＳｕｇａｗａｒａＫ、ApplicabilityoftheCompact OvercoringMethodforlnitialStressMeasurementinHighlyCrackedBedrockpmc．〃t・
Symp,o7zRoclBStl1Bss,ｐｐ､83-88.Ｋｕｍａｍｏｔｏ（1997)．
27．IshiguroY.，NishimuraH.，NishinoK・ａｎｄＳｕｇａｗａｒａＫ、RoCkStressMeasurementfor DesignofUndergroundPowerhouseandConsiderations・ｐｍｃ．ｍt・日y血p、ｏ〃RodbStｱ℃ss，
pp491-498・Kumamoto（1997)．
－３５－
