Geological Considerations - Dam

Geological Considerations - Dam

1. GEOLOGICAL CONSIDERATIONS -DAM
ENGR. MD. SHAMIM AHMED
LECTURER
DEPT. OF CIVIL ENGINEERING
SOUTHERN UNIVERSITY BANGLADESH

2. DAMS
Adammaybedefinedasasolidbarrierconstructedatsuitablelocation
acrossrivervalleywithaviewofimpoundingwaterflowingthroughthat
river.
Damsarecreatedforthefollowingobjectives:
Generationofhydropowerenergy
Providingwaterforirrigationfacilities
Fishfarming
Fightingdroughts
Controllingoffloods
Watersupplyfordomesticconsumption
Providingnavigationalfacilities

4. Terminologies
•Heel:contactwiththegroundontheupstreamside.
•Toe:contactonthedownstreamside.
•Abutment:Sidesofthevalleyonwhichthestructureofthedamrest.
•Galleries:smallroomslikestructureleftwithinthedamforchecking
operations.
•Diversiontunnel:Tunnelsareconstructedfordivertingwaterbefore
theconstructionofdam.Thishelpsinkeepingtheriverbeddry.
•Spillways:Itisthearrangementnearthetoptoreleasetheexcess
waterofthereservoirtodownstreamside.
•Sluiceway:Anopeninginthedamnearthegroundlevel,whichis
usedtoclearthesiltaccumulationinthereservoirside.

5. TypesofDams
Onthebasisofdesign:
oGravitydams
oButtressdams
oArchdams
oEarthdams

6. 1.Gravitydam
•Asolidconcreteormasonrystructures,thatwithstandsthewater
pressure,byvirtueofitsweight
•Allforcesactingonthedamareassumedtobedirectlytransmitted
tothefoundationrocks
•Theyaregenerallyoftriangularprofileandareamongthesafest

7. Gravitydam

8. 2.Buttressdam
•Theyhaveacontinuousupstreamface,supportedatregular
intervals,bybuttresswallsonthedownstreamside.
•Theyarelighterthansoliddams.
•Likelytoinducegreaterstressesatthefoundation,sincemostofthe
loadpassesthroughthebuttresswallsandisnotspreaduniformlyover
thefoundation.

9. Buttressdam

10. 3.Archdam
•Arch-shaped,convexattheupstreamside.
•Majorportionofthethrustforcesactingonthedamaretransmitted
byarchaction,ontotheabutmentrocks.
•Structuralefficiencyishigherthanthatofgravitydams,thepresence
ofsoundabutmentsisaprimenecessity.
•Useslessamountofconcrete

11. Archdam

12. 4.Earthdam
•Non-rigidstructures,builtwithnaturallyavailablematerialssuchas
earthandrock.
•Ideal,wherethedamsiteisweaktosupportconcretedams,or
wherecompetentrocksarefoundatgreatdepths.
•Theyaretrapezoidalinshape,relativelysmallerinheightandbroad
atthebase.

13. Earthdam

14. 4.Compositedam
•Compositedamsarecombinationsofone
ormoredamtypes.
•Mostoftenalargesectionofadamwill
beeitheranembankment orgravitydam,
withthesectionresponsibleforpower
generationbeingabuttressorarch.

15. GeotechnicalConsiderationsforDamSiteSelection
•Topography-aplaceisselectedideallyinnarrowgorgeorsmallvalley
withenoughcatchmentareabehinddamissothatcalculatedamount
ofwatercanbeeasilystoredinthereservoircreatedupstream..
•Technically-asiteshouldhavestrong,impermeableandstablerock
body.
•Constructionally-siteshouldnotbemuchawayfromthedepositsof
materialwhichrequiredforconstruction.
•Economical
•Environmental

16. Geologicalinvestigation
•Generalgeologyofarea-
–Indetailedmappingofthearearevealsthefactslike
Topographicfeatures
Naturaldrainagepatterns
Generalcharactersandstructuresofrockformationlike
stratification,foldingandfaulting
Trendandrateoferosioninthearea.

17. Geologicalinvestigation…
•Lithology-
–Itisthemostimportantfactorindamconstructioninvestigation.
–Lithologyinandaroundareawithsurfaceandsubsurfacelithologyis
investigated.
–Thesestudiesrevealsthecompositionandtexturesofrockatsite.
–Itgivestheideaabouttherocktypeasigneous,sedimentaryor
metamorphic.
–Italsoshowsthattheareaismadeupofsinglethicklayerofsame
rocktypeorwithmultiplelayers.

18. Geologicalinvestigation…
•Structures-
–Theseinvolvesdetailstudyofandmappingofplanesofweaknesslikefaults,
fold,joints,beddingplaneetc.
–Dipandstrike:
Strengthofunfracturedstratifiedrockisgreaterwhenstressesareacting
normaltothebeddingplanes.
Horizontalbedsarestablebase
Dippingupstreamarestable
Dippingdownstreamunstable

19. Geologicalinvestigation…
–Fault
Thefaultedrocksaregenerallyshatteredandweakalongtherupture
surface
Faultedlandsurfacesarethefavorablesitesforshocksduring
earthquakes.
Smallfaultsandshearzonesare
treatedwithsomemethods
Deepandmajorfaultsareavoided
atconstructionsite.

20. Geologicalinvestigation…
–Folds:
Insynclinebendsdamsplacedontheupstreamlimbswouldcauseleakage
frombeneaththedam.
–Joints:
Accordingtothenatureofjointstheappropriateengineeringtechniqueis
usedtofeelupthejoints.

21. RESERVOIR
Areservoirisalarge,artificiallakecreatedbyconstructingadamacrossa
river.

22. SELECTIONOFSITEFORARESERVOIR:
•Atthesiterivervalleyshouldbenarrowsothelengthofthedamtobeconstructedisless.
•Basinshouldbewatertightandfreefromfissures,cracks,sothatthereisnolossofwater
duetopercolation.
•Thesiteshouldbesuchthatasfaraspossibleminimumlandandpropertyissubmergedin
thereservoir.
•Itshouldbesuchthatitavoidswaterfromthetributarieswhichcarryusuallyhighcontentof
sediment.
•Thesitemustbesuchthatadequatereservoircapacityisavailableforthedesiredpurpose.
•Thesiteshouldbesuchthatadeepreservoirmaybeformedsothatthelandcostsperunit
ofcapacityarelow,evaporationlossislessandthereislesslikelihoodofweedgrowth.
•Thesoilandmassatthereservoirsiteshouldnotcontainanyobjectionablesoluble
mineralsandsaltswhichmaygetdissolvedinwateranddeterioratewaterquality.

23. SELECTIONOFSITEFORARESERVOIR:
•Thequalityofwaterstoredinthereservoirmustbesatisfactoryavailableforitsintended
use.
•Thesiteshouldbeeasilyaccessiblebyroadorrailway.
•Theconstructionmaterialsforthedamshouldbeavailableinthevicinityofthesite.
•Thesiteshouldbesuchthatthecostsofassociatedworkssuchasroads,rails,housing
coloniesforworkersandstaff,etc.shouldbelow.

24. THANK YOU