Geology in civil engineering

GeologyinCivilEngineeringWhythetoweristilting?Whythebarnissliping?AlandslideinNingboAlandslideinCalifornia,USA,1995WhatisGeology?Geologyisthestudyoftheplanetearth.Itisconcernedwiththeoriginoftheplanet,thematerialandmorphologyoftheearth,anditshistoryandtheprocessacted/actingonit.Theword“geology”wasfirstusedbyaSwissscholar-H.B.DeSaussure(1740-1799)in1779.Theprefix“geo-”means“earthorland”,whilethesuffix“logy”means“subject-asourceorareaofstudy”.ChapterOneRockTypesandStratigraphyIgneousRocksMetamorphismandMetamorphicRocksSedimentaryRocksStratigraphyandStratificationAccordingtotheirorigin,rocksaredividedintothreegroups,namely,theigneous,metamorphicandsedimentaryrocks.(1)IgneousRocksIgneousRocksareformedwhenhotmoltenrockmaterialcalledmagmasolidifies.Igneousrocksarecomposedprincipallyofsilicateminerals.Igneousrocksmaybedividedintointrusiveandextrusivetypes,accordingtotheirmodeofoccurrence.IgneousIntrusionsTheformthatintrusionsadoptmaybeinfluencedbythestructureofthehostorcountryrocks.Dykesarediscordantigneousintrusions,thatis,theytraversetheirhostrocksatanangleandaresteeplydipping.Sills,likedykes,areparallel-sidedigneousthatcanoccuroverrelativelyextensiveareas.Themajorintrusionsincludebatholiths,stocksandbosses.Certainstructuresareassociatedwithgranitemassifs,tendingtobebestdevelopedatthemargins.VolcanicActivityandExtrusiveRocksVolcaniczonesareassociatedwiththeboundariesofthecrustalplates.Platescanbelargelycontinental,oceanic,orboth.VolcanicactivityisasurfacemanifestationofadisorderedstatewithintheEarth’sinteriorthathasledtothemeltingofmaterialandtheconsequentformationofmagma.Eruptionsfromvolcanoesarespasmodicratherthancontinuous.Steammayaccountforover90%ofthegasesemittedduringavolcaniceruption.Athighpressures,gasisheldinsolution,butasthepressurefalls,gasisreleasedbythemagma.Thesizeofpyroclastsvariesenormously.Thetermlapilliisappliedtopyroclasticmaterialthathasadiametervaryingfromapproximately10to50mm.Thefinestpyroclasticmaterialiscalledash.Afterpyroclasticmaterialhasfallenbacktothegroundsurface,iteventuallybecomesindurated.Itthenisdescribedastuff.Whencloudsorshowersofintenselyheated,incandescentlavasprayfalltotheground,theyweldtogethertobecomeweldedtuff.Inothercases,becausetheparticlesbecomeintimatelyfusedwitheachother,theyattainalargelypseudo-viscousstate,especiallyinthedeeperpartsofthedeposit.Thetermignimbriteisusedtodescribetheserocks.TextureofIgneousRocksThedegreeofcrystallinityisoneofthemostimportantitemsoftexture.Anigneousrockmaybecomposedofanaggregateofcrystals,ofnaturalglass,orofcrystalsandglassinvaryingproportions.HolocrystallineHolohyalineMerocrystallineWhenreferringtothesizeofindividualcrystal,theyaredescribedascryptocrystalline.AphaniticPhanerocrystallineThreegradesofmegascopictextureareusuallydistinguished,namely,fine-grained,medium-grainedandcoarse-grained.Leucocraticrocks,whichcontainlessthan30%darkmineralsMeasocraticrocks,whichcontainbetween30and60%darkmineralsMelanocraticrocks,whichcontainbetween60and90%darkmineralsHypermelanicrocks,whichcontainover90%darkmineralsIgneousRockTypesGranitesandgranodioritesarethecommonestrocksoftheplutonicassociation.Theyarecharacterizedbyacoarse-grained,holocrystalline,granulartexture.Ingranodiorite,theplagioclaseisoligoclaseorandesineandisatleastdoubletheamountofpotashfeldsparpresent,thelatterforming8to20%oftherock.IgneousRockTypesThetermpegmatitereferstocoarseorvery-coarse-grainedrocksthatareformedduringthelaststagesofcrystallizationfromamagma.(2)MetamorphismandmetamorphicRocksMetamorphicrocksarederivedfrompre-existingrocktypesandhaveundergonemineralogical,texturalandstructuralchanges.Thesechangeshavebeenbroughtaboutbychangesthathavetakenplaceinthephysicalandchemicalenvironmentsinwhichtherocksexisted.Thechangingconditionsoftemperatureand/orpressurearetheprimaryagentscausingmetamorphicreactionsinrocks.Whenmetamorphismoccurs,thereisusuallylittlealterationinthebulkchemicalcompositionoftherocksinvolved.Twomajortypesofmetamorphismmaybedistinguishedonthebasisofgeologicalsetting.Onetypeisoflocalextent,whereastheotherextendsoveralargearea.Anothertypeofmetamorphismisdynamicmetamorphism,whichisbroughtaboutbyincreasingstress.MetamorphicTextureandStructuresMostdeformedmetamorphicrockspossessomekindofpreferredorientation.Preferredorientationsmaybeexhibitedasmesoscopiclinearorplanarstructuresthatallowtherockstosplitmoreeasilyinonedirectiongthaninothers.ThermalorContactMetamorphismThermalmetamorphismoccursaroundigneousintrusionssothattheprincipalfactorcontrollingthesereactionsistemperature.RegionalMetamorphismRegionalmetamorphisminvolvesboththeprocessessofchangingtemperatureandstress.Regionalmetamorphismisaprogressiveprocess,thatis,inanygiventerrainformedinitiallyofrocksofsimilarcomposition,zonesofincreasinggrademaybedefinedbydifferentmineralassemblages.DynamicMetamorphismDynamicmetamorphismisproducedonacomparativelysmallscaleandisusuallyhighlylocalized.Stessisthemostimportantfactorindynamicmetamorphism.MetasomatismMetasomaticactivityinvolvestheintroductionofmaterialinto,aswellasr