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CHAPTER 3 Dams and Spillways. Ercan Kahya. Department of Civil Engineering, I.T.U. Figure uses by courtesy of Prof. Recep YURTAL. Embankment (Fill) Dams. Environmental Effects of Dams. Social and economic effects E cologic effects Regional climate effects Vegetation effects - PowerPoint PPT Presentation
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CHAPTER 3
DamsDams and Spillwaysand Spillways
Ercan KahyaErcan Kahya
Department of Civil Engineering, I.T.U.Department of Civil Engineering, I.T.U.
Figure uses by courtesy of Prof. Recep YURTAL Figure uses by courtesy of Prof. Recep YURTAL
I.TI.T..UU., ., Department of Civil Eng.Department of Civil Eng.
I.TI.T..UU., ., Department of Civil Eng.Department of Civil Eng.
Embankment (Fill) Dams
I.TI.T..UU., ., Department of Civil Eng.Department of Civil Eng.
Environmental Effects of DamsEnvironmental Effects of Dams Social and economic effects Social and economic effects EEcologic cologic effectseffects Regional climate effects Regional climate effects Vegetation effects Vegetation effects Fishery Fishery Navigation effects Navigation effects Upstream and downstream navigation effectsUpstream and downstream navigation effects Tourism effectsTourism effects
3.1 Classification of Dams3.1 Classification of Dams
According to dams height According to dams height
If crest elevation and foundation level greater than 15 m If crest elevation and foundation level greater than 15 m
Large DamLarge Dam
If dam height less than 15 m If dam height less than 15 m Small Dam Small Dam
If dam height greater than 50 m If dam height greater than 50 m High DamHigh Dam
More specifically More specifically
The height of the dam > 15 m The crest width of the dam > 500 m “LARGE DAM” The storage volume of the dam > 106 m3
Classification of Dams Classification of Dams
According to construction purposeAccording to construction purpose
Single purpose Single purpose
■ Storage Dams
■ Diversion Dams
■ Detention Dams
■ Hydropower Dams
Multiple purpose Multiple purpose (Serves for all or most of the above purposes)
Drinking water
Navigation
Flood control
Recreational purposes
Irrigation
Energy
I.TI.T..UU., ., Department of Civil Eng.Department of Civil Eng.
Classification of DamsClassification of Dams
According to Hydraulic DesignAccording to Hydraulic Design
■ Overflow Dams
(i.e., diversion dams)
■ Non-overflow Dams
(i.e., earth fill & rock fill dams)
Classification of DamsClassification of Dams According to Materials of Construction According to Materials of Construction
■ Embankment Dams
• Earth-Fill Dams Earth-Fill Dams • Rock-Fill DamsRock-Fill Dams
■ Masonry and Rubble Dams
■ Concrete Dams
■ Steel and Timber Dams
Classification of DamsClassification of Dams
According to Structural DesignAccording to Structural Design
Gravity Dams Gravity Dams Arch Dams Arch Dams Buttress Dams Buttress Dams Earth-Fill Earth-Fill Rock-Fill Rock-Fill Pre-stressed Concrete DamsPre-stressed Concrete Dams
According to Structural Design
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Embankment (Fill) Dams
3.2 Parts of Dams
Structural components:
- Body
- Spillway
- Outlet Facilities (i.e., sluiceways & water intake tower)
- Others (i.e., hydropower stations, roads, fish ladder, etc.)
3.3 Planning of Dams
Three steps:
- Reconnaissance surveys (Infeasible alternatives eliminated)
- Feasibility study
- Planning study
Planning of DamsPlanning of Dams
3.3.1 FEASIBILITY STUDY A) Determination of water demand B) Determination of water potential
C) Optimal plans
◘ Check out the relation D (demand) versus S (supply).
Planning of DamsPlanning of Dams
D) Determination of dam site
◘ Factors should be taken into consideration:
TopographyGeology and dam foundationAvailable of construction materialsFlood hazardSeismic hazardSpillway location and possibilitiesConstruction time ClimateDiversion facilitiesSediment problemWater qualityTransportation facilitiesRight of way cost
FEASIBILITY STUDY
Planning of DamsPlanning of Dams
E) Determination of type of dam
◘ Comparative characteristics of dams should be considered
F) Project design
◘ involves the computation of dimensions of the dam.
- Hydrologic design (max. lake elevation + spillway cap. + crest elevation)- Hydraulic design (static & dynamic loads + spillway profile + outlet
dimensions)- Structural design (stress distribution + required reinforcement)
Planning of DamsPlanning of Dams
3.3.2 PLANNING STUDY ◘ Followings need to be done in planning certain type of dam, since dimensions are already determined:
a)Topographic surveys (1:5000 scaled map)
b)Foundation study (seepage permeability etc. tests)
c)Materials study (quantity of materials)
d)Hydrologic study (measurements of hydrologic parameters)
e)Reservoir operation study (is to be performed periodically)
3.4 Construction of Dams3.4 Construction of Dams
Four principal steps are followed during the construction: 1) Evaluation of Time Schedule and Equipment
◘ a work schedule is prepared using CPM.
(characteristics of dam site; approx. quantities of works; diversion facilities; urgency of work)
2) Diversion
◘ before the construction, river flow must be diverted from the site◘ see the below figure for two possible ways to divert water:
Upstream
Downstream
Cofferdam
Cofferdam
Construction zone
Diversion by tunnel
(a) (b)
Cofferdam
Diversion tunnel
First stage
Second stage
Completed portion of dam
Flow through sluiceway
(c)
River Diversion facilities
3) Foundation Treatment ◘ Concrete & Rock-fill dams hard formations
Earth-fill dams most of soil conditions ◘ Highly porous foundation excessive seepage, uplift,
settlement “Grouting Operation” is applied to solidify the foundation
& to reduce seepage
Formation of the Dam BodyFor Concrete Gravity dams:
• Low-heat cements to reduce shrinkage problem
• Concrete is placed in “blocks”
• “Keyways” are built between sections to make the dam act as a monolith
Upstream face Upstream face
Downstream face Downstream face
Keyways
• “Waterstops” are placed near upstream face to prevent leakage
Copper strip
Copper strip
Waterstops
“Inspection galleries” permit access to the interior of concrete Dams and are needed for seepage determination, grouting operations and etc.
• Constructed in multi-layer formation (Layers: impervious, filter and outer)
• First place the materials in layers of 50 cm and then compact these materials.
• For high dams, horizontal berms are constructed to enhance slope stability
• Protect the upstream face of dam against wave action (i.e., concrete or riprap)
For Earth-fill dams
• Protect the downstream face against rainfall erosion (i.e., planting grass or riprap)
Silt
Silt clay
1 on
2.5
1 on 2
Sandy gravel
Clay coreSilt
1 on
2.5
1 on 2.5
Silt
Transition zonePervious strata
Pervious foundation
Rock-fill toe
(a) Simple zoned embankment
(b) Earth dam with core extending to impervious foundation
Cross section of typical earth dams
SiltSilt clay
1 on
3.1
1 on 2
Sandy gravel
1 on 3.8Clay blanket
Concrete cutoff wall
Pervious material
(c) Earth dam on pervious material
Cross section of typical earth dams
For Rock-fill dams:• Core and filter zones are similarly constructed as the earth dam
• Due to heavy rocks on the sides, these dams
• have steeper slopes • have less materials • are economic
• Construction period is shorter and easy to increase the crest elevation
Width of dam crest: There are two traffic lanes
Elevation of dam crest: There is no overtopping during design flood
Freeboard: See the table for recommendations
Select Compacted Rock
1.3
1
1.3
1
CoarseDumped Rock
Reinforced Concrete Membrane
Cutoff wall (a) Impermeable face
Dumped RockRolled rock
Cla
y co
re
Dumped or Rolled rock
Grout curtain(b) Impermeable earth-core
Graded transition sections
(1.5m)(0.2m)
1.4
1
1.4
1
Cross-section of typical Rock-fill dams
RolledMediumSize Rock
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GRAVITY DAMSGRAVITY DAMS
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Resist the forces by their own weight
Concrete Gravity DamsConcrete Gravity Dams
Recep YURTALRecep YURTAL Ç.Ü. İnş.Müöl.Ç.Ü. İnş.Müöl.Concrete Gravity DamsConcrete Gravity Dams
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Concrete Gravity DamsConcrete Gravity Dams
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Concrete Gravity DamsConcrete Gravity Dams
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Why & Where we prefered?Why & Where we prefered?
Sağlam ve geçirimsizliği sağlanabilecek yeterli kalınlıkta kaya temellerin Sağlam ve geçirimsizliği sağlanabilecek yeterli kalınlıkta kaya temellerin uygun bir derinlikte bulunduğu orta genişlikteki vadilerdeuygun bir derinlikte bulunduğu orta genişlikteki vadilerde
Yeterli miktarda ve istenen özellikte agrega malzemesinin bulunduğu, Yeterli miktarda ve istenen özellikte agrega malzemesinin bulunduğu, çimento naklinin ekonomik olduğu yerlerdeçimento naklinin ekonomik olduğu yerlerde
Büyük taşkın debilerinin baraj gövdesi üzerinden mansaba aktarılması Büyük taşkın debilerinin baraj gövdesi üzerinden mansaba aktarılması gereken durumlardagereken durumlarda
Baraj üzerinden bir ulaşım yolu geçirilmesi gereken durumlarda tercih Baraj üzerinden bir ulaşım yolu geçirilmesi gereken durumlarda tercih ediliredilir
Savaş ve sabotaja karşı daha dayanıklı olması da ayrıca bir tercih Savaş ve sabotaja karşı daha dayanıklı olması da ayrıca bir tercih nedeni olabilir.nedeni olabilir.
Concrete Gravity DamsConcrete Gravity Dams
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Types:Types:• Straight Straight Gravity DGravity Damsams• Arch Gravity DamsArch Gravity Dams
Baraj ekseni, iki yamaç arasındaki en kısa Baraj ekseni, iki yamaç arasındaki en kısa bağlantıyı sağlayacak şekilde bir doğru ile bağlantıyı sağlayacak şekilde bir doğru ile birleştirilir.birleştirilir.
Temel kayasının yapısına, derzlere veya Temel kayasının yapısına, derzlere veya emniyet ihtiyacına bağlı olarak kavisli de emniyet ihtiyacına bağlı olarak kavisli de yapılabilir. yapılabilir.
Concrete Gravity DamsConcrete Gravity Dams
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Design Design CriteriaCriteria:: En uygun kesit, etki eden en önemli dış kuvvet olan haznedeki En uygun kesit, etki eden en önemli dış kuvvet olan haznedeki
hidrostatik su basıncı dağılımına uyum sağlayan, tabana doğru hidrostatik su basıncı dağılımına uyum sağlayan, tabana doğru genişleyen üçgen kesit seçilir. Üçgenin tepesi genellikle haznedeki genişleyen üçgen kesit seçilir. Üçgenin tepesi genellikle haznedeki en yüksek su seviyesidir. en yüksek su seviyesidir.
Memba yüzeyi düşey veya %10 Memba yüzeyi düşey veya %10 ‘‘u geçmeyecek şekilde eğimli u geçmeyecek şekilde eğimli yapılır. yapılır.
Baraj boş haldeyken çekme gerilmelerini önlemek, dolu haldeyken Baraj boş haldeyken çekme gerilmelerini önlemek, dolu haldeyken kayma ve devrilme emniyetini artırmak için yüksek barajlarda kayma ve devrilme emniyetini artırmak için yüksek barajlarda memba yüzeyi genellikle eğimli planlanır. memba yüzeyi genellikle eğimli planlanır.
Üçgenin tepe kısmında, duvar kalınlığını artırmak, yamaçlar arası Üçgenin tepe kısmında, duvar kalınlığını artırmak, yamaçlar arası ulaşımı sağlamak gibi nedenlerle dikdörtgen kesitli bir başlık bulunur.ulaşımı sağlamak gibi nedenlerle dikdörtgen kesitli bir başlık bulunur.
Concrete Gravity DamsConcrete Gravity Dams
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Concrete Gravity DamsConcrete Gravity Dams
Design Criteria:
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Design Principles:Design Principles:
Ağırlık barajı hesaplarında Ağırlık barajı hesaplarında üçgen profilüçgen profil gözönüne alınır. gözönüne alınır.
Üçgen kesitin minimum Üçgen kesitin minimum boyutları, boyutları, barajın kendi ağırlığı, barajın kendi ağırlığı, hidrostatik su basıncı ve taban hidrostatik su basıncı ve taban su basıncının su basıncının etki ettiği normal etki ettiği normal yükleme durumunda çekme yükleme durumunda çekme gerilmeleri meydana gerilmeleri meydana gelmeyecek şekilde belirlenir.gelmeyecek şekilde belirlenir.
Bunun için: Bunun için:
b
H
mHb
tgb
1
Concrete Gravity DamsConcrete Gravity Dams
For the dam dimensions:
Check out the safety for
• Overturning
• Shear & sliding
• Bearing capacity of foundation
• No tensile stresses are allowed in the dam body
Concrete Gravity DamsConcrete Gravity Dams
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H1/md
B
Overturning CheckOverturning Check
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Overturning CheckOverturning Check
H
B
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H
B
Overturning CheckOverturning Check
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H
B
Overturning CheckOverturning Check
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H
B
Overturning CheckOverturning Check
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H
B
Overturning CheckOverturning Check
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Sliding CheckSliding Check
H1/md
B
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H
B
Sliding CheckSliding Check
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H
B
Sliding CheckSliding Check
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H
B
Sliding CheckSliding Check
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H1/md
B
Sliding CheckSliding Check
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Bearing Capacity CheckBearing Capacity Check
H1/md
3.5.1 FORCES ON GRAVITY DAMS3.5.1 FORCES ON GRAVITY DAMS
Free body diagram showing forces acting on a gravity dam
The following loads should be considered:
A) WEIGHT (WC): Dead load and acts at the centroid of the section
B) HYDROSTATIC FORCES:
Water in the reservoir + tailwater causes Horizontal Hu Hd &
Vertical Fh1v Fh2v
C) UPLIFT FORCE (Fu): acts under the base as:
D) FORCE OF SEDIMENT ACCUMULATION (Fs):
Determined by the lateral earth pressure expression
where
• Fs : the lateral earth force per unit width, • γs : the submerged specific weight of soil, • hs : the depth of sediment accumulation relative to reservoir
bottom elevation, • θ : the angle of repose.
This force acts at hs /3 above the reservoir bottom.
E) ICE LOADS (Fi): considered in cold climate
Ice force per unit width of dam (kN/m) can be determined from the following table:
Thickness of ice sheet (cm)
Change in temperature (oC/hr)
2.5 5 7.5
25 30 60 95
50 58 90 150
75 75 115 160
100 100 140 180
F) EARTHQUAKE FORCE (Fd):
Acting horizontally and vertically at the center of gravity
k (earthquake coefficient): Ratio of earthquake acceleration to gravitational acceleration.
G) DYNAMIC FORCE (Fw) :
In the reservoir, induced by earthquake as below
Acts at a distance 0.412 h1 from the bottom
• Fw : the force per unit width of dam• C : constant given by
• θ’ : angle of upstream face of the dam from vertical (oC)
• For vertical upstream face C = 0.7
'
H) FORCES ON SPILLWAYS (∑F):
Determined by using momentum equation btw two successive sections:
• ρ : the density of water• Q : the outflow rate over the spillway crest• ΔV: the change in velocity between sections 1 and 2 (v2-v1) Momentum correction coefficients can be assumed as unity.
I) WAVE FORCES :
Considered when a long fetch exists
Usual loading
B &Temperature Stresses at normal conditions + C + A + E + D
Unusual loading
B & Temperature Stresses at min. at full upstream level + C + A +D
Severe loading
Forces in usual loading + earthquake forces
LOADING CONDITIONS:
3.5.2 STABILITY CRITERIA
Dam must be safe againstDam must be safe against
(1) (1) OverturningOverturning for all loading conditions for all loading conditions
resisting momentsresisting moments
overturning momentsoverturning moments
Safety factor: Safety factor:
F.SF.SOO 2,0 ( 2,0 (usual loadingusual loading))
F.SF.SOO 1,5 ( 1,5 (unusual loadingunusual loading))
FSM
MO
r
o
(2) Sliding over any horizontal plane
f f = friction coef. btw any two planes= friction coef. btw any two planes
Safety factor: Safety factor: FSS FSS 1,5 ( 1,5 (usual loading )) FSS FSS 1,0 ( 1,0 (unusual or severe loading))
STABILITY CRITERIA
STABILITY CRITERIA
(3) Shear and sliding together
AA : Area of shear plane (m²) : Area of shear plane (m²)
ττss : Allowable shear stress in concrete in contact with foundation : Allowable shear stress in concrete in contact with foundation
Safety factor: Safety factor: FSFSssss 5,0 ( 5,0 (usual loading))
FSFSssss 3,0 ( 3,0 (unusual or severe loading))
STABILITY CRITERIA
(4) Between foundation and dam contact stresses (σ) > 0
at all points
There are two cases for the base pressure:
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Base Pressure CheckBase Pressure Check
CASE 1:CASE 1: e e B/6 B/6B
ΣV
PhPt
e x
DAM BASE
Pt s
Ph s
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x
Pt
B
e
DAM BASE
CASE 2: e > B/6
Pt s
Base Pressure CheckBase Pressure Check
ΣV