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Lecture

EOSC 114 Lecture Notes - Haiyuan County, Ningxia


Department
Earth and Ocean Sciences
Course Code
EOSC 114
Professor
All

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Landslidesthatoccurinpopulatedareascandamagepropertyandinfrastructureandtrigger
secondarydisastersthataredifficulttodealwith.
SocioEconomicImpact
Valleysinmountainousregionshaveexperiencedacceleratedeconomicdevelopmentin
responsetogeneralpopulationgrowthandassociateddemandsforincreasedmining,
forestry,andagriculturalactivities.
Vulnerabilitytolandslidesisincreasedby:
● increasingpopulationdensity
● useofmarginalland
● rapidlandusechange
● globalwarming
ElmEvent,Switzerland,1881
Becamefamousinlandslidestudiesbecausetheventsleadinguptoandaccompanyingfailure
werewelldocumentedoneeyewitnessusedastopwatchtotimetheinitialfailure.
LaConchita,California,1995and2005
Twomassivelandslideshaveoccurred.Bothoftheselandslideswerecausedbyheavyrainfall
duringtherainyseasoninthearea.
1995and2005disasterscouldhaveeasilybeenpreventedhadthenecessaryandproper
geologicengineeringworkweredonepriortothedevelopmentofthearea.
Haiyuan,Ningxia(Gansu),China,EarthquakeandLandslides,1920
EarthquakeandlandslidesinChinawereresponsiblefor200,000deathsduetothedestruction
ofhomes(directdamage)andexposuretotheharshwinterclimate(indirectdamage).
OtherMassMovementEvents
Ice/rockAvalanche
RockfallandSlideEvents
Involveasmallerrockmassthatdisintegratesintonumerousblocksthatfall,bounce,androllon
steepslopesafterdetachment.
RockfallsareamajorhazardforhighwaysandrailwaysthroughrockyterraininCanadaand,
thoughsmall,haveresultedinnumerousdeathsinthiscentury.However,rockfallsdonotpose
thesamelevelofeconomicriskaslargescalefailureswhichcananddoclosemajor
transpirationroutesfordaysatatime.
MudslideoccurredinVancouverin2005:weliveinamountainousregionwithsteepslopes,
areaswherelandslidesoccurmorefrequentlythaninothers.Heavyrainandsnowfalladdto
thehazardousconditions.
Effectsofgravity
Massmovement(masswasting)referstothedownslopemotionofsoiland/orrockmaterials
undertheinfluenceofgravity.
Landslideisageneraltermformassmovements.

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Landslidesoccurwhenthegravitationalforcepullingtheslopematerialdownexceedstheslope
material'sinternalresistancetoshearorfailure.Materialsmovedownslopebyslidingorshearing
alongaweakfailureplaneoranumberoffailureplanes.
GravityandShearstress
Gravityisthemainforceresponsibleforlandslides.
Gravityforce:objectpullingeverythingtowardthecentreoftheEarth.
Gp:thecomponentofgravitythatactsperpendiculartotheslope.Ithelpstoholdanobjector
slopematerialsinplaceontheslope.
Gt:thecomponentofgravitythatactstangentialtotheslopesurfaceorparalleltoanddownthe
slopeshearstress.
Whentheslopeissteeper,theshearstressortangentialcomponentofgravity,Gtincreases,
andtheperpendicularcomponentofgravity,Gpdecreases.
ShearStrength
Asshearstresspullsslopematerialdownslope,forcesresistingthismovementwillbeinduced.
Theseforcesincludedfrictionalresistanceandcohesionamongtheparticlesoftheslope
materialandaregroupedunderthetermshearstrength.Wedefineshearstrengthasthe
internalresistanceofabodytoshearstressoramaterial'sinternalshearingresistance.
Byincreasingfrictionandcohesion,shearstrengthincreases.
Shearstrengthbyanumberofdifferentfactorsthataffectfrictionandcohesion:
● typeofsoil/rock
● conditionofgeologicmaterials
● presenceofweaksurfaces
● porewaterpressurewithintheslope
TheFactorofSafety,Fs
Slopefailuresoccurswhentheforceofgravityparalleltotheslope.Landslidesoccurwhen
shearstressatthetimeoffailureisgreaterthantheshearstrengthofthegeologicmaterial.
Fs=shearstrength/shearstress
if)Fs>1:shearstrengthisgreaterthanorequaltoshearstress,theslopeisstable
Fs<1:shearstressisgreaterthanshearstrength,slopefailureisexpected.
Downslopemovementfavouredby:
1. Steeperslopeangleswhichincreasetheshearstress
2. Anythingthatreducestheshearstrengthofmaterials,suchasloweringthecohesion
amongtheparticlesorloweringthefrictionalresistance.
Twofactorsdeterminethisresistancetofailure:
1. theweightofthematerialitselfactingperpendiculartotheslope,Gpwhichhelpstohold
theslopematerialsinplace.Anothertermforthisisnormalstress.
2. factorsthataddstothespacesbetweengrains(primarilywaterpressure),which
decreasethestrengthofthematerial.Wewillrepresentthisfactorsimplyasporewater
pressure.

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effectivestress=normalsstressporewaterpressure
FactorofSafety=shearstrength/shearstress=factorresistance+cohesion/shear
stress
Discerningacausevs.atrigger.
Acauseisafactorthatmakesslopessusceptibletomovementwithoutactuallyinitiatinga
landslide.
AtriggeristheeventthatactuallyinitiatesmovementbydrivingtheFs<1.0.
Earthquakes,volcaniceruptions,intense&shortperiodrainfall,miningandquarrying,irrigation,
vegetationremoval,excavationofaslopeatitstoe,loadingoftheslopeatitscrest,andloud
noisesorvibrationTRIGGERS
WhenshearstrengthisgreaterthanshearstressareconsideredCAUSESFs<1.0
InsomecasesslopestabilitycanincreasethroughtimeasdenotedbyanincreaseinFs.
Slopestabilityanderosion
Erosionisthetransportofmaterialawayfromitssource.Removingmaterialatthebaseofa
slopecancauseslopefailureintwoways:
1. thedrivingmassbecomesgreaterthantheresistingmass
2. theslopesteepnessatthebaseoftheslopeincreasessuchthatshearstressbecomes
greaterthanshearstrength
Slopestabilityandvegetation
Vegetationisafactorthatinfluencesslopestabilitybyaffectingtheshearstrengthofthe
materialscomprisingtheslope.
Plantrootsincreasesoilcohesionandstrengthbyhelpingtobindloosematerials.Roots
decreasesurfaceerosionandhelptoabsorbexcesswaterthroughevapotranspiration.
Removalofvegetationcanmakeslopesunstable.
Slopestabilityandwatercontent
● Excessiveamountofwaterwithinaslopeincreasesitsmass,increasingshearstress
withinpartsoftheslope.
● Anotheraspectofwaterthataffectsslopestabilityisfluidpressure.Insomecasesfluid
pressurecanbuildinsuchawaythatwatercansupporttheweightoftheoverlyingrock
mass.Whentheslopeiscomposedofsedimentwheregrainsarenotcementedtogether
andexcesswatersetsthegrainsapart.Whenthisoccurs,frictionisreducedandthus
theshearstrengthholdingthematerialontheslopeisalsoreducedresultinginslope
failure.
● Waterincreasestheweatheringofrockswhenitflowsintoeventhenarrowestofrock
fractures.Thevolumechangeisaverypowerfulforcethatcanwedgeapartrocks,often
causingthemtofallfromsteepslopesinmountainsandcanyons.
● Insedimentswaterhelpsorhinderscohesion.Ifthesandistotallydry,itisimpossibleto
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