CHAPTER 3 Dams and Spillways

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.


Embankment (Fill) Dams


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


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).


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


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)


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

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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.



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.



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.




Design Criteria:


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


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



H1/md

B

Overturning CheckOverturning Check



H

B


H

B



H

B



H

B



H

B



Sliding CheckSliding Check

H1/md

B


H

B



H

B



H

B



H1/md

B



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:


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


x

Pt

B

e

DAM BASE

CASE 2: e > B/6

Pt s

Base Pressure CheckBase Pressure Check

ΣV

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CHAPTER 3 Dams and Spillways