Geology Applied to Civil Engineering 土木工程地质 英文课件 第五章 Engineering properties of rocks

5EngineeringpropertiesofrocksGeologyAppliedtoCivilEngineering123contentsHydraulicpropertiesofrocksMechanicalpropertiesofrocksPhysicalpropertiesofrocks4Rockmassstrength1PhysicalpropertiesofrocksSpecificgravityofarockisadimensionlessnumberexpressingoftheratiobetweenthemass(orweight)ofthesolidandthatofanequalvolumeofwateratthetemperatureof4℃.Thespecificgravity(SpG)ofrockdependsontheSpGvalueofmineralsmakinguptherockandtheirproportion.Thespecificofrockisabout2.6,upto3.3.Massdensityisequaltothespecificgravitymultipliedbythedensityofwater,whichyieldsthemaximummassperunitvolumethesolidcanexhibitasitassumesthatnoporesarepresent.Unitweightisusedinmanycalculationsratherthandensity,whichcanbeobtainedfromdensitybymultiplyingthevalueofgravity.Unitweightofarockisdeterminedbythespecificweightofmineralsintherock,theporosityandwatercontentoftherock.Ifthereisnowaterintherockpores,wegetdryunitweight,whilewegetsaturatedunitweightiftheporesintherockarefullofwater.1PhysicalpropertiesofrocksVoidratio(notedase)oftherockisthevolumeofporesinarockdividedbythevolumeofsolid.Porosity(notedasn)isthevolumeoftheporesdividedbythetotalvolumeofsolidincludingtheporevolume.

Calculate(5-1)and

(5-2)：n=e/(1+e),e=n/(1-n)(5-3)1PhysicalpropertiesofrocksTheporosityofarockmainlydependsonthestructureandtextureoftherock,whileaffectedbyweatheringdegree,magmaactivity,tectonicmotion,metamorphismanddiageneticage.Forexample,theclasticsedimentaryrocksatmorerecentdiageneticagewithworsecementationhavebiggerporosity.Generally,theporosityofcrystallinerockislessthanthatofamorphousrock,usuallynomorethan3%.RockSpecificgravitydsIgneousGranite2.50-2.8423.0-28.00.04-2.80Syenite2.50-2.9024.0-28.5Diorite2.60-3.1025.2-29.60.18-5.00Gabbro2.70-3.2025.5-29.80.29-4.00Porphyry2.60-2.8027.0-27.40.29-4.00Porphyrite2.60-2.9024.0-28.62.10-5.00Diabase2.60-3.1025.3-29.70.29-5.00Basalt2.50-3.3025.0-31.00.30-7.20Andesite2.40-2.8023.0-27.01.10-4.50Tuff2.50-3.3025.0-31.00.30-7.20SedimentaryConglomerate2.67-2.7123.0-25.60.80-10.00Sandstone2.60-2.7522.0-27.11.60-28.30Shale2.57-2.7723.0-27.00.40-10.00Limestone2.40-2.8023.0-27.70.50-27.00Marlite2.70-2.8023.0-25.01.00-10.00Dolomite2.70-2.9021.0-27.00.30-25.00Metamorphicgneiss2.60-3.1023.0-30.00.70-2.20granitegneiss2.60-2.8023.0-33.00.30-2.40schist2.60-2.9023.0-26.00.02-1.85slate2.70-2.9023.1-27.50.10-6.00marble2.70-2.9026.0-27.00.10-6.00quartzite2.53-2.8428.0-33.00010-8.70serpentine2.40-2.8026.00.10-2.50quartzschist2.60-2.8028.0-29.00.70-3.00Table5-1Physicalpropertiesofcommonrocks2HydraulicpropertiesofrocksWaterabsorptionofarockreferstotheabilityoftherocktoabsorbwater,whichcanbecharacterizedbywaterabsorption(ω1)andsaturatedwaterabsorption(ω2).Alargerwaterabsorptionvalueusuallyshowsthatthemorelikelythespecimenistobeerodedandsoftenedbywater,andthattheinfluenceofwateronrockstrengthandstabilityismoresignificant.

Saturatedwaterabsorptionreferstothewaterabsorptionwhenallvoidsarefilledwithwater.Inordertoobtainsaturatedwaterabsorption,therocksampleshallbeimmersedinwaterunderthevacuumconditionoratahighpressureof15MPasothatalltheopenvoidswillbefilledwithwater.

2HydraulicpropertiesofrocksWaterpermeabilityofrockreferstotheabilityofrocktoallowwatertopassthrough,expressedbypermeability(K).Itdependsmainlyonthesizeandconnectivityofvoidsintherock.Permeability(K)isalsocalledhydraulicconductivity.Inisotropicmedia,itisdefinedastheseepagevelocityofwaterthroughunitareaofacrosssectionundertheunithydraulicgradient.

RockFromlaboratorytestFromfieldtestgranitebasaltsandstoneshalelimestonedolomiteschistTable5-3Thepermeabilityofrocks3MechanicalpropertiesofrocksThemechanicalpropertiesofarockarereflectedinitsstrengthanddeformationindices.Deformationisdeterminedbythestressandstrengthoftherock.Idealconditionsofasinglestressenvironment,arockcanbesubjectedtothreeprimarytypesofstress:compressive,shearandtensile.Idealconditionsofasinglestressenvironment,arockcanbesubjectedtothreeprimarytypesofstress:compressive,shearandtensile.Fig.5-3Diagramsofcompression,tensionandshearstress3.1StrengthindicesCompressivestrengthisthecompressivestressrequiredtobreaktherockspecimen.Theunconfinedcompressivestrength(alsocalleduniaxialcompressivestrength)pertainstorocksunconfinedattheirsideswhiletheloadisappliedverticallyuntilfailureoccurs.Thecompressivestrengthvariesgreatly,whichmainlydependsonthestructureandtextureofrocksandisaffectedbytheirmineralcompositionandrock-formingconditions.ThestrengthismeasuredinNewtonpersquaremeterbythefollowingequation:

Tensilestrengthisthetensilestressrequiredtobreaktherockspecimen.Itistheabilityofrocktoresisttensilefailureunderuniaxialtension.Thetensilestrengthsareconsiderablylessthantheircompressivestrengths-ontheorderofonly10%asgreat(refertoTerryR.West,1995).So,whenarockstratumiscompressedintoafold,itisoftendamagedbytensionatthepartswithlargebendingdeformation,resultingintensilefractures.Shearstrengthistheabilityoftherocktoresistshearfailure,expressedbyinternalfrictionangle(φ)andcohesion(c)whichcanbeobtainedbydirectionalshearingtestsandtriaxialcompressiontests.Triaxialcompressiontestsareperformedunderdifferentconfiningpressures.Inthetest,theendsofthesample,usuallyarockcorewithalengthtwiceitsdiameter,aresubjectedtoincrementsofincreasingload,whileconfiningpressureisappliedandheldconstant.Theconfiningpressureandmaximumaxialloadarerecordedwhentherocksamplefractures.Theaxialloadrequiredtocausefailureofarockincreaseswithincreasingconfiningpressureforthistest.Byperformingthreeorfourtestsatdifferentconfiningpressures,therelationshipbetweentheaxialcompressivestrengthandconfiningpressurecanbedetermined,andthedataareplottedonashear-compressivestressdiagramknownasaMohrdiagram.Indirectsheartests,shearingpressuresareappliedtocausefailureofrockspecimenunderdifferentnormalload,andthenormalloadandcorrespondingmaximumshearingpressurearerecorded.3.1StrengthindicesFig.5-4Mohrdiagramsforrockintriaxialcompression,inwhichA,BandCmarkthepointsoffailure,indicatingtheshearstressatfailureforthoseconditionsofconfiningpressureandaxialpressure.Fig.5-5Fittingdiagramsforrockindirectionalshearing,inwhichA,B,CandDmarkthepointsoffailure,indicatingtheshearstressatfailurefordifferentaxialpressures.

3.1Strengthindices

3.1Strengthindices

Recoverable

deformationistermedelastic(εe)；Non-recoverablepartistermedplastic(εp)deformation.3.2DeformationindicesFig.5-6TypicaldiagramforrockinuniaxialpressiontestAninitialbeddingdownandcrackclosurestage(OA);Elasticdeformationstage(AB);Anaxialstressofσciisreachedatwhichastablecrackstage(BD)isinitiated;Theaxialstressreachesσcdwhenanunstablecrackgrowthandirrecoverabledeformationstage(DE)begins;Untilthepeakoruniaxialcompressivestrength(σc)isreached.3.2DeformationindicesTheaxialYoung’smodulusofarockspecimenvariesthroughouttheloadinghistoryandsoisnotauniquelydeterminedconstantforthespecimen.

TangentYoung’smodulus,Et,istheslopeoftheaxialstress–axialstraincurveatsomefixedpercentage,generally50%,ofthepeakstrength.AverageYoung’smodulus,

Eav,istheaverageslopeofthemore-or-lessstraightlineportionoftheaxialstress–straincurve.SecantYoung’smodulus,

Es,istheslopeofastraightlinejoiningtheoriginoftheaxialstress–straincurvetoapointonthecurveatsomefixedpercentageofthepeakstrength.3.2DeformationindicesIfthestress-strainresponsesbeforethepeakstrengthexhibitasamore-or-lessstraightline,theYoung’smodulusistheslopeofthelinewhichissocalledelasticmodulus(Ee).

3.2DeformationindicesAlsoofinterestregardingcompressiontestingofrockistheextensionthatoccursperpendiculartothedirectionofloading,thatisthelateralstrain(εr)ofarockspecimen.εristypicallyexpressedasafractionoftheaxialstress(εa).ThetermisknownasPoisson’sratioandcorrespondstoanyvalueofYoung’smodulus,avalueofPoisson’sratiomaybecalculatedas:

Rockstrengthisaconsequenceofpetrographywhichinvolvesthedetaileddescriptionoftherocksincludingtheirmineralcomposition,thetextureandthestructure.Thehardnessofmineralshasadecisiveinfluenceonthestrengthoftherockstheyform.Thesize,shapeandconnectionmodeofminerals,socalledthetextureofrocks,alsohaveanimportantimpactonthestrengthofrocks.Thestructureofrocks,thatis,thespatialarrangementcharacteristicsoftheconstitutedminerals,isoftenrelatedtothetypeofrocksandwillalsoaffecttherockstrength.3MechanicalpropertiesofrocksMechanicalpropertiesofcommonvarietiesofrockaregiveninTable5-5andTable5-6.(153)Thetermrockusedinthismajorreferstointactrockblocksandexcludesthediscontinuitiessurroundingtherockblocks.4RockmassstrengthHowever,naturalrocksarealwayssurroundedbydifferenttypesofdiscontinuities.Thecombinationofrockblocksanddiscontinuitiessiscalledrockmass.Fig.5-7IllustrationofrockmassesinwhichthephotoshowstheinterlayerofsandrockandclayrockinLongquanmountaininChengdu,China.4RockmassstrengthIftherockisstrong,suchasthesand,thestrengthandbehavioroftherockmassaredeterminedbythenatureofitsdiscontinuities.Whilethestrengthoftherockmaydeterminethestrengthandbehaviorofarockmassinwhichtherockisveryweak,suchastheclayrock.Whenthelatteristhecase,thediscontinuitiesareoftenverydense,ortherockblockiseasytocollapseincaseofwaterimpact.Fig.5-7IllustrationofrockmassesinwhichthephotoshowstheinterlayerofsandrockandclayrockinLongquanmountaininChengdu,China.Thestrengthofboththerockblocksandthediscontinuitiesjointlydeterminesthestrengthandbehaviorofarockmass.sandrockclayrock4RockmassstrengthFordiscontinuitiesinrockmasses,theimportantconcernsaretheattitude

oftheplanesrelatedtothefreeface(cutslopeandtunnelexcavationfacearethecases),theextentofcontinuityofthepl