Geology Applied to Civil Engineering 土木工程地質(zhì) 英文課件 第六章 Rock Weathering and Soil Properties

6RockWeatheringandSoilPropertiesGeologyAppliedtoCivilEngineering123contentsFactorscontrollingweatheringWeatheringdegreeevaluationTypesofrockweathering4Engineeringclassificationofrockandsoil5Engineeringpropertiesofspecialsoil1TypesofrockweatheringAccordingtothefactorscausingrockweathering,itcanbedividedintothreetypes,andeachtypehasdifferentweatheringformsandcharacteristicsofweatheringproducts.PhysicalweatheringChemicalweatheringBiologicalweathering1.1Physicalweathering(1)ExpansionwithheatandcontractionwithcoldThisactioniscausedbytemperaturechanges,whichisverypopularintheareaswithlargetemperaturedifferencebetweendaytimeandnight.Thetemperaturedifferenceontherocksurfaceismuchlargerthanthatinsidetherock,whichcausesunevenexpansionandcontraction(2)FrostwedgingFrostwedgingreferstotherockfracturecausedbywaterfreezing.Whenthetemperaturedroppedbelowzero,waterintherockfracturesisfrozentoicefollowedbyitsvolumeexpansionofabout9%.Thevolumeexpansioninducesbigpressureontherockaroundthefracturesandmakesthefractureswideranddeeper.(3)crystallizationexpansionItreferstotherockfracturecausedbythecrystallizationofsalts.Thesaltsinthesolutioninrockvoidsorfracturesmaycrystallizeduetowaterevaporationortemperaturereduction.1.2Chemicalweathering(1)DissolutionAsthemineralsinrockaredissolved,theporosityoftherockincreasesandthebondbetweenparticlesisweakened.(2)HydrationHydrationistheprocessthatmineralscombinewithwatertoformnewminerals.(3)OxidationOxidationreferstothereactionofmineralswithoxygentoformnewminerals.(4)CarbonationItisasolutionweatheringprocessthatiscausedbythemineralsintherocksreactingwithwatercontainingaconsiderableamountofcarbondioxide.1.3BiologicalweatheringItreferstotheweatheringofrocksinducedbytheactionoforganismsincludinganimals,plantsandmicroorganisms.Forexample,rootsofplantscanwidenthejointsandfracturesinrocksastheygrowup,causingcracksandultimatelythebreakingupoftherockmass.Thisactivityisobservedinsomeofthemostmassiverocktypessuchasgranite,quartzite,andsandstonewhererootsoftreespenetratewhatseemstobesolidrocktoobtainsupportandsustenance.Thisprocessiscalledrootsplittingthatisonekindofbiologicalphysicalweathering.Exudationonthesurfaceofrocksfromcreatureandorganicremains(suchasrhizobia)cancorrodeanddecomposetherocks,whichisonekindofbiologicalchemicalweathering.2FactorscontrollingweatheringLithologyandgeologicalstructureClimateandfrostfireTopographyWater2.1Lithologyandgeologicalstructure(1)PetrogenesisThepetrogenesisofrockreflectstheenvironmentalconditioninwhichrockforms.Ingeneral,iftheenvironmentandconditionwherearockformedaresimilartothoseoftheEarthsurface,therockhasstrongweatheringresistance.Onthecontrary,thegreaterthedifferencebetweenthetwoenvironmentsandconditions,themoreeasilytherockisweathered.(2)MineralcompositionTheweatheringresistanceofarocklargelydependsontheweatheringresistanceofitsconstitutedminerals,especiallychemicalweatheringandbiochemicalweathering.Theweatheringresistanceofdifferentmineralsvaries,sometimesgreatly.2.1Lithologyandgeologicalstructure(3)TextureandstructureTheweatheringresistanceofcryptocrystallinerocksisstrongerthanthatofphanerocrystallineones,becausethelatterexhibitobviousmineralcrystalsandconsequentlyhavediscontinuitiesbetweenthecrystals.(4)GeologicalstructureGeologicalstructures,includingfoldsandfaults,resultinthedevelopmentofalargenumberofstructuralplanesinrocks.Thisincreasescontactofrockswithwaterandairpromotingtheweatheringoftherocks.2.2Climateandfrostfire(1)ClimateThetemperature,precipitationandbiologicaltypesvaryaccordingtodifferentclimaticregions，whichresultindifferenttypesandcharacteristicsofrockweathering.Physicalweatheringdominatesinpolarandalpineregions,whilechemicalweatheringandbiologicalweatheringaremorenotableinthehumidtropics.(2)ForestfirePhysicalbreakdownofrockcanbecausedbyforestfirebecauseofthegreatexpansionduetotheelevatedtemperatures.2.3TopographyTopographyhasinfluencesonthespeedanddepthofrockweathering,weatheringtypeandaccumulationofweatheringproducts.Inhighsteepanddeepincisionareas,physicalweatheringprevails.Inflatregionswithlittletopographicrelief,chemicalweatheringprevailsduetotheslowrunoffandshorttransportdistance,resultinginathicklayerofsoil.2.4WaterTheamountofwateravailabletotherockistheprimaryingredientcontrollingthetypeofrockweathering.Generally,ifthewaterisabundant,chemicalorbio-chemicalweatheringwillprevail,whileintheabsenceofwater,physicalorbio-physicalweatheringtakecontrol.Water’sroleinchemicalweatheringisthatofuniversalsolvent.MostchemicalreactionsontheEarth’ssurfacetakeplaceinwaterbecauseitprovidesoxygenforreactionandmobilityfortheions.Withpresenceofwater,anincreasedtemperaturetypicallyyieldsanincreasedrateofreaction.Thisexplainsthehighchemicalweatheringrateofthetropics,butwithoutwater,elevatedtemperaturesdonothavethiseffect.Therefore,physicalweatheringprevailsindesertareas.3WeatheringdegreeevaluationTheengineeringpropertiesofrocksdeterioratewiththeadvanceofweathering.Itisimportanttodeterminethedegreeofrockweatheringbecauseitisofgreatsignificanceforthedesignandconstructionofcivilengineering.Theevaluationofrockweatheringdegreecanbedonethroughfieldobservation(suchaschangesonmineral,textureandstructure)andlabtests(suchasLongitudinalwavevelocity,uniaxialcompressivestrength).Thecomprehensiveapplicationofthetwomethodsisrecommended,andtheweatheringdegreeisusuallyclassifiedintofivelevelsincludingun-weathered,slightlyweathered,moderatelyweathered,stronglyweatheredandcompletelyweathered.4EngineeringclassificationofrockandsoilTheengineeringclassificationofrockandsoilisofgreatsignificancetothedesignandconstructionforcivilengineering.Forexample,rockswithdifferenthardnesshavedifferentadaptabilitytobridgefoundationtypeandtunnelexcavationtechnology.Also,astheclassificationbasisofsoil,particlesizeandsizedistributiondeterminethebehaviorofsoiltoagreatextentunderengineeringload.4.1ClassificationofrockhardnessThehardnessofrockdependsonthehardnessofthefreshrock(un-weathered)anditsweatheringdegree,whichcanbequalitativelyclassifiedbyfieldobservationorquantitativelyclassifiedbylabtestresult(suchasuniaxialsaturatedcompressivestrength,σc)4.2ClassificationofsoilsTheformationofasoilprofileisoneconsequenceofrockweathering.Soilisamaterialsovitaltohumanexistenceandengineering.Thetermsoilisusedinseveraltechnicalfields,eachwithadifferentdefinitionaccordingtoitsrole.4.2Classificationofsoils(1)Classificationbyparticlegradation,particleshapeandplasticityindexIncivilengineering,soilcanbeclassifiedintoBIM-soil,sand,siltandclayaccordingtoitsparticlegradation,particleshapeandplasticityindex.(2)ClassificationbyoriginBasedontheoriginofsoil,itisclassifiedintoresidual,deluvial,colluvial,proluvial,alluvial,siltingandaeoliansoil.(3)SpecialsoilsSpecialsoilsrefertothesoilswithspecialengineeringpropertiesorgeotechnicalproblemsduetotheirspecialcomposition,textureandstructure.Atpresent,specialsoilsincludeloess,expensivesoil,softsoil,salinesoil,permafrost,laterite,mollisolandfillingsoil.5EngineeringpropertiesofspecialsoilLoessExpansivesoilSoftsoilFrozensoil5.1LoessLoessischaracterizedbysiltparticleswithcarbonateminerals,largeporesandcollapsibility.Itisakindofyellowterrestrialsedimentwithobviousverticaljointandisnotstratified.(1)Particlecomposition:Siltprevailsinloesswiththemasspercentageofabout60%-70%,followedbysand(1-29%)andclay(8-26%)particles.(2)MassDensity:Thespecificgravityofloessparticlesis2.54-2.84.Thenaturaldensityofloessisabout1.5-1.88g/cm3,whilethedrydensityisabout1.3-1.6g/cm3.(3)Compressibility:Bytheuseofcoefficientofcompressibilitywhichcanbereferredtosoilmechanics,loessismostlymediumcompressible.(4)Collapsibility:Thecollapsibilityofloessisrelatedtoitsnaturalwater.Loessdoesn’tcollapseasthewatercontentexceeds25%,butthecollapsibilityincreaseswiththedecreaseofmoisturecontent.5.2ExpansivesoilExpansivesoilisakindofhighplasticclaywhichisrichinhydrophilicclayminerals(suchasmontmorilloniteandillite)andexpandsorshrinkssignificantlywiththeincreaseordecreaseofitswatercontent.Therepeatedexpansionandcontractionoftheclaymineralswillreducethestrengthofthesoil,resultingincracking,subsidence,instabilityandfailureofslopes,subgradesandbuildings.Expansivesoilismostlygraywhite,brownyellow,brownredandbrownduetoitsmineralcomposition.Theparticlesinvolvedaremainlyclay,withacontentof35-50%orevenmore,followedbysiltandlittlesand.Anexpansivesoilisover-consolidated,thatis,itwassubjectedtoagreaterpressurethanthecurrentoverburdenself-weight.5.3SoftsoilSoftsoilismostlygrayishgreenandgrayishblackandhasafishysmellincaseofhighorganicmattercontent.Theparticlesinvolvedinsoftsoilaremainlyclay

andsilt,ofwhichthecontentofclayis60-70%.Softsoilhastypicalspongy,honeycomborflocculentstructure,whichisanimportantreasonforitslargevoidratio.Theextremelysmallvoids,stronghydrophilicityofclaymineralsleadtothepoordrainage,slowconsolidationandhighcompressibilityofsoftsoilSoftsoilischaracterizedbyitslowstrengths.Underthelong-termactionofload,thedeformationofsoftsoilcanlastforalongtimeandeventuallycausethefailureofsoil,whichiscalledrheology.5.4FrozensoilFrozensoilreferstosoilwithatemperatureequaltoorlowerthan0℃andcontainingice,whichisdividedaspermafrostandseasonallyfrozensoil.Permafrostreferstothesoilthathaskeptbeingfrozenforminimum2consecutiveyears,whileseasonalfrozensoilreferstothesoilthatisperiodicallyfrozenandthawedwithseasonalchanges,andtheannualfreezingtimeshallbemorethan15days.SeasonallyfrozensoilThedamageofseasonallyfrozensoiltobuildingsismainlyduetofrostheavingandthawingsettlement.Duringfreezing,waterconcentratestothefrozenpartandexpandsinitsvolume,makingthegroundsurfacehavingafrozenheavingtoendangerthesafetyofbuildings.Duringmelting,thewatercontentinsomelocalareasincreases,andthesoilisinsoftplasticorflowplasticstate,resultinginmeltingsettlement,whichwillcrackanddeformthebuildinginseriouscases.Soilfreezingandthawingcancreatestressfracturesandaltersoilphysicalproperties,includingporecontinuityandaggregatestability;th