岩石力学50原则.docx
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岩石力学50原则.docx
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岩石力学50原则
The50PrinciplesofRockMechanics
forRockEngineering
1. Rockisanaturalmaterial:
itspropertiescannotbespecifiedaswithafabricatedmaterial;thepropertieshavetobemeasuredonsite. Thegeologicalhistoryofarockmasswilldetermineseveralkeycharacteristics.Theseincludetheinhomogeneity(differentpropertiesatdifferentlocations),theanisotropy(differentpropertiesindifferentdirections),thepresenceandmechanicalcharacteristicsofthediscontinuities(pre-existingfractures),andthehydraulicproperties.
2. Thenatureoftheintactrockwilldependonitsgeologicaltypeandthedegreeofweatheringtowhichithasbeensubjected. Rockmechanicsstartedwithdetailedstudiesofthedeformationandfailureofintactrock.Theengineeringpropertiesofintactrockarefunctionsoftherockmicrostructurewhichinturnisafunctionofthegeologicalformationandhistory.
3. Usingastifforservo-controlledtestingmachine,thecompletestress-straincurveforintactrockinuniaxialcompressioncanbeobtained.Thetestisconductedwithaxialstrainastheindependentvariable(thecontrolledvariable)andaxialstressasthedependentvariable(themeasuredvariable). Thecompletestress-straincurverepresentsthestructuralcollapseoftherockmicrostructurefrominitialloadingtocompletedisintegration. Themostwidely-usedcharacteristicsofthecompletestress-straincurvearethemodulus(measuredat50%maximumstress)andthecompressivestrength(themaximumstresssustained).
4. Thecompletestress-straincurveforintactrockdependsonthespecimengeometryandloadingconditionsofthetestandtheenvironmentalconditions.Infact,neitherthecompressivestrengthnorthetensilestrengthisamaterialproperty–becausetheybothdependonthespecimengeometryandtheloadingconditionsofthetest.Amaterialpropertydoesnotdependonthesefactors.
5. Whenaconfiningpressureisalsoappliedduringacompressiontestonintactrock,therockwillexhibiteitherbrittleorductilebehavior.Inbrittlebehavior,thestressdecreasesafterthecompressivestrengthhasbeenreached. Inductilebehavior,thestresscontinuestoincrease.Theconfiningpressureassociatedwiththebrittle-ductiletransitionis,forexample,0MPaforrocksalt,20-100MPaforlimestone,andmorethan100MPaforsandstoneandgranite.
6. ThemostwidelyusedfailurecriteriaforintactrockaretheMohr-Coulomb,Griffith,andHock-Brownfailurecriteria.TheMohr-Coulombcriterionconsidersthecohesionandangleoffrictionassociatedwithshearfailure.TheGriffithcriterionconsiderstheenergyrequiredbyapropagatingcrackintermsofaninitialcracklength.TheHoek-Browncriterionisanempiricalcriterionusingtwoparameterswhichcanbeestimatedfromtherockdescription.Manyotherfailurecriteriaforintactrockhavebeendeveloped.
7. Inmostcases,thepropertiesandengineeringbehaviourofrockmassesaregovernedbythediscontinuities.Thediscontinuitiesareanybreaksinthemechanicalrockcontinuum-whichcanoccuratavarietyofscales,fromfaultstobeddingplanestojointstofissuresandmicro-fissures. Themostimportantdiscontinuitiesforengineeringarefaultsorothershearfeatures,butjoints-whichhavebeencreatedbynormaltensilestresses-canbeverysignificantaswell.Discontinuitieshavelittleorzerotensilestrength.
8. Tenmaincharacteristicsareusedtodescribediscontinuities:
spacing;orientation;persistence;roughness;aperture;numberofsets;blocksize;filling;wallstrength;andseepage.
9. ThemostwidelyusedparametertodescribediscontinuityoccurrenceistheRockQualityDesignation(RQD).Thisisthepercentageofpiecesinaborebolecoreorlengthsalongaseanlinethataregreaterthanl0mmor4inches.TheRQDcanberelatedtothediscontinuityfrequencyifthenatureofthediscontinuityspacinghistogramisknown.
10. Becausediscontinuitiestendtooccurinsets(ofparallelorsub-paralleldiscontinuities),thediscontinuityfrequencyvalueisdifferentalonglinesindifferentdirectionsthrougharockmass.ItfollowsthattheRQDwillalsobedifferentindifferentdirectionsthroughtherockmass.
11. Thepersistence,orextentofadiscontinuity,isanimportantcharacteristicformanyengineeringcharacteristics,suchasthemodulusofdeformationoftherock,thedegreetowhichrockblocksareformed,andthehydraulicconnectivityofthediscontinuitynetwork.Theroughness,apertureandfillingofthediscontinuitiesarealsoimportantforthemechanicalandhydrologicalcharacteristics.
12. Themainmechanicalpropertiesofadiscontinuityforengineeringarethestiffnessandstrength.Thestiffnessshouldbeconsideredasthenormalstiffnessandthetwoshearstiffnesses;thestrengthisspecifiedbytheshearstrength,i.e.theangleoffriction(rememberingthatthediscontinuityhasessentiallynotensilestrength,andisalsoassumedtohavelittleornocohesion). Theangleoffrictionisacomplexcombinationofthebasicfrictionangle,thestrengthoftheasperitesandthediscontinuityroughness.
13. Arockmasswillcontainapre-existingnaturalstressstate,theinsitustress,whichiscausedbygeologicalprocesses,mainlytectonic.Thequantity‘stress’isnotascalarorvectorquantitybutatensorquantitywhichhastobecharacterizedbysixindependentvalues----usuallythemagnitudesanddirectionsofthethreeprincipalstresses.Theserockstressesaremainlycausedbytectonicactivitybutold,residualstressescanalsobepresent.
14. Therearefourmainmethodsofmeasuringtheinsitustress:
theflatjack,hydraulicfracturing,theUSBMovercoringtorpedo,andtheCSIROovercoringgauge.TheCSIROgaugeisthemostreliableandhydraulicfracturingistheonlymethod,thatcanbeusedasignificantdistancefrom'man-access'.Itistheruleratherthantheexceptionthatthemaximumhorizontalstresscomponentisgreaterthantheverticalstresscomponent. Becausediscontinuitieshaveasignificanteffectonthelocalprincipalstressmagnitudesanddirections,measuredstressesareexpectedtovaryattheprojectlocation.
15. Watercanbepresentintheporesoftheintactrockandinthediscontinuities.Thewaterpressureissubtractedfromthenormalstresscomponentsofthestresstensortogiveeffectivestresses.
16. Strainisatensorquantitylikestress.Assumingtherockisbehavingelastically,thesixcomponentsofthestraintensorcanberelatedtothesixcomponentsofthestresstensorbytheelasticcompliancematrix. Forisotropy,twoelasticconstantsareneeded:
Young'smodulusandPoisson'sratio.Fortransverseisotropy,fiveelasticconstantsareneeded:
twoYoung'smoduli,twoPoisson'sratiosandashearmodulus.Fororthotropy,nineelasticconstantsarerequired:
threeYoung'smoduli,threePoisson'sratiosandthreeshearmoduli. Forcompleteanisotropy,the21independentconstantsoftheelasticcompliancematrixarerequired.
17. The'ideal'rockmassisaCHILEmaterial:
Continuous,Homogeneous,Isotropic,andLinearlyElastic. TheactualrockmassisaDIANEmaterial:
Discontinuous,Inhomogeneous,Anisotropic,andNotElastic. Rockmassesarediscontinuousbecausetheycontaindiscontinuities. Theyareinhomogeneousandanisotropicbecausetheyarecomposedofdifferentgeologicalstrataanddifferentdiscontinuitygeometriesatdifferentlocationsandwhichhavedifferentpropertiesindifferentdirections.Theyarenotelasticbecausetheenergygiventotherockmassduringdeformationcannotgenerallyberecoveredcompletely.
18. Thedeformabilityofarockmassresultsfromdeformationofboththeintactrockandthediscontinuities.Becausetheintactrockcanbeanisotropicandbecausethediscontinuitiesoccurinsetscausingthediscontinuitycontributionstobeanisotropic,thedeformationmodulusoftherockmasswillbedifferentindifferentdirections.
19. Thestrengthofarockmasswilldependonwhetherfailureoccursthroughtheintactrockoralongoneormorediscontinuities.
20. Theeasewithwhichwaterflowsthrougharockmassisexpressedbythepermeability.Likestressandstrain,permeabilityisasecondordertensorwithsixindependentcomponents-usuallycharacterizedbythemagnitudesanddirectionsoftheprincipalpermeabilities.Thepermeabilityoffracturedrockmassescanvarygreatly.
21. TheRepresentativeElementalVolume(REV)isanimportantconceptforthepermeabilityofrockmass.Inarockmasssample,thenumberofdiscontinuitiespresentisafunctionofthesamplesize,stabilizinginaveragepropertieswhenthesamplesizeislargeenough.TheREVistherockmasssamplesizebelowwhichthepermeabilitycanvarysignificantlyandatandabovewhichthepermeabilityisessentiallyconstant. ThisREVconceptalsoappliestoallpropertiesgovernedwhollyorpartlybythediscontinuities.
22. Inordertoestablishthepropertiesofrocks,testingtechniquesareused. Thesetestingtechniquescanbestandardized.However,differentpropertiesarerequiredfordifferentprojects.Becausetherearemanydifferentrockengineeringobjectives,eventhoughthetestingtechniquesthemselvescanbestandardized,therecanbenostandardizedsiteinvestigation.
23. BecausetheREVsizeisgenerallyoftheorderoftensofmeters,itisgenerallynotpossibletoconductmeaningfultestsdirectlyontherockmass.Testsareconductedontheintactrockandthediscontinuitiesseparatelyandtheirsignificancefortherock
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