CHAPTER 3 Dams and Spillways

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1 CHAPTER 3 Dams and Spillways
Ercan Kahya Department of Civil Engineering, I.T.U. Figure uses by courtesy of Prof. Recep YURTAL

2 I.T.U., Department of Civil Eng.

3 I.T.U., Department of Civil Eng.

4 Embankment (Fill) Dams
I.T.U., Department of Civil Eng.

5 Environmental Effects of Dams
Social and economic effects Ecologic effects Regional climate effects Vegetation effects Fishery Navigation effects Upstream and downstream navigation effects Tourism effects

6 3.1 Classification of Dams
According to dams height If crest elevation and foundation level greater than 15 m  Large Dam If dam height less than 15 m  Small Dam If dam height greater than 50 m  High Dam 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

7 Classification of Dams
According to construction purpose Single purpose ■ Storage Dams ■ Diversion Dams ■ Detention Dams ■ Hydropower Dams Multiple purpose (Serves for all or most of the above purposes)

8 I.T.U., Department of Civil Eng.
Drinking water Navigation Flood control Recreational purposes Irrigation Energy I.T.U., Department of Civil Eng.

9 Classification of Dams
According to Hydraulic Design ■ Overflow Dams (i.e., diversion dams) ■ Non-overflow Dams (i.e., earth fill & rock fill dams)

10 Classification of Dams
According to Materials of Construction ■ Embankment Dams Earth-Fill Dams Rock-Fill Dams ■ Masonry and Rubble Dams ■ Concrete Dams ■ Steel and Timber Dams

11 Classification of Dams
According to Structural Design Gravity Dams Arch Dams Buttress Dams Earth-Fill Rock-Fill Pre-stressed Concrete Dams

12 According to Structural Design

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16 Recep YURTAL Ç.Ü., İnş.Müh.Böl.

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20 Recep YURTAL Ç.Ü., İnş.Müh.Böl.

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22 Recep YURTAL Ç.Ü., İnş.Müh.Böl.

23 Embankment (Fill) Dams

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25 3.2 Parts of Dams Structural components: - Body - Spillway
- Body - Spillway - Outlet Facilities (i.e., sluiceways & water intake tower) - Others (i.e., hydropower stations, roads, fish ladder, etc.)

26 3.3 Planning of Dams Three steps: - Reconnaissance surveys
(Infeasible alternatives eliminated) - Feasibility study - Planning study

27 Planning 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).

28 Planning of Dams D) Determination of dam site
FEASIBILITY STUDY D) Determination of dam site ◘ Factors should be taken into consideration: Topography Geology and dam foundation Available of construction materials Flood hazard Seismic hazard Spillway location and possibilities Construction time Climate Diversion facilities Sediment problem Water quality Transportation facilities Right of way cost

29 Planning of Dams E) Determination of type of dam F) Project design
◘ 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) ◘ Failure of the dam  “Dam Break” It is rapid for a concrete dam. See the textbook for the examples.

30 Planning of Dams 3.3.2 PLANNING STUDY
◘ Followings need to be done in planning certain type of dam, since dimensions are already determined: Topographic surveys (1:5000 scaled map) Foundation study (seepage permeability etc. tests) Materials study (quantity of materials) Hydrologic study (measurements of hydrologic parameters) Reservoir operation study (is to be performed periodically)

31 3.4 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:

32 Completed portion of dam Construction zone Flow through sluiceway
Flow in stream bed Construction zone Cofferdam Diversion tunnel First stage Second stage Upstream Downstream Cofferdam Construction zone Diversion tunnel Diversion by tunnel (a) (b) Completed portion of dam Construction zone Flow through sluiceway (c) River Diversion facilities

33 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

34 Formation of the Dam Body
For 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 Keyways Downstream face Downstream face

35 “Inspection galleries” permit access to the interior of concrete
“Waterstops” are placed near upstream face to prevent leakage Copper strip Waterstops “Inspection galleries” permit access to the interior of concrete Dams and are needed for seepage determination, grouting operations and etc.

36 For Earth-fill dams 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) Protect the downstream face against rainfall erosion (i.e., planting grass or riprap)

37 Cross section of typical earth dams
Silt Silt clay 1 on 2.5 1 on 2 Sandy gravel (a) Simple zoned embankment Clay core Silt 1 on 2.5 Transition zone Pervious strata Pervious foundation Rock-fill toe (b) Earth dam with core extending to impervious foundation

38 (c) Earth dam on pervious material
Cross section of typical earth dams Silt Silt clay 1 on 3.1 1 on 2 Sandy gravel 1 on 3.8 Clay blanket Concrete cutoff wall Pervious material (c) Earth dam on pervious material

39 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

40 Cross-section of typical Rock-fill dams
Select Compacted Rock 1.3 1 Coarse Dumped Rock Reinforced Concrete Membrane Cutoff wall (a) Impermeable face Rolled rock Clay core Dumped or Grout curtain (b) Impermeable earth-core Rolled Medium Size Rock Cross-section of typical Rock-fill dams Graded transition sections 1.4 1.4 1 1 (0.2m) (1.5m)

41 GRAVITY DAMS Recep YURTAL Recep YURTAL Ç.Ü. İnş.Müöl.

42 Resist the forces by their own weight
Concrete Gravity Dams Resist the forces by their own weight Recep YURTAL Ç.Ü. İnş.Müöl.

43 Concrete Gravity Dams Recep YURTAL Ç.Ü. İnş.Müöl.

44 Concrete Gravity Dams Recep YURTAL Ç.Ü. İnş.Müöl.

45 Concrete Gravity Dams Recep YURTAL Ç.Ü. İnş.Müöl.

46 Concrete Gravity Dams Why & Where we prefered?
Sağlam ve geçirimsizliği sağlanabilecek yeterli kalınlıkta kaya temellerin uygun bir derinlikte bulunduğu orta genişlikteki vadilerde Yeterli miktarda ve istenen özellikte agrega malzemesinin bulunduğu, çimento naklinin ekonomik olduğu yerlerde Büyük taşkın debilerinin baraj gövdesi üzerinden mansaba aktarılması gereken durumlarda Baraj üzerinden bir ulaşım yolu geçirilmesi gereken durumlarda tercih edilir Savaş ve sabotaja karşı daha dayanıklı olması da ayrıca bir tercih nedeni olabilir. Recep YURTAL Ç.Ü. İnş.Müöl.

47 Types: Straight Gravity Dams Arch Gravity Dams
Concrete Gravity Dams Types: Straight Gravity Dams Arch Gravity Dams Baraj ekseni, iki yamaç arasındaki en kısa bağlantıyı sağlayacak şekilde bir doğru ile birleştirilir. Temel kayasının yapısına, derzlere veya emniyet ihtiyacına bağlı olarak kavisli de yapılabilir. Recep YURTAL Ç.Ü. İnş.Müöl.

48 Concrete Gravity Dams Design Criteria:
En uygun kesit, etki eden en önemli dış kuvvet olan haznedeki hidrostatik su basıncı dağılımına uyum sağlayan, tabana doğru genişleyen üçgen kesit seçilir. Üçgenin tepesi genellikle haznedeki en yüksek su seviyesidir. Memba yüzeyi düşey veya %10 ‘u geçmeyecek şekilde eğimli yapılır. Baraj boş haldeyken çekme gerilmelerini önlemek, dolu haldeyken kayma ve devrilme emniyetini artırmak için yüksek barajlarda memba yüzeyi genellikle eğimli planlanır. Üçgenin tepe kısmında, duvar kalınlığını artırmak, yamaçlar arası ulaşımı sağlamak gibi nedenlerle dikdörtgen kesitli bir başlık bulunur. Recep YURTAL Ç.Ü. İnş.Müöl.

49 Concrete Gravity Dams Design Criteria: Recep YURTAL Ç.Ü. İnş.Müöl.

50 Design Principles: Concrete Gravity Dams
Ağırlık barajı hesaplarında üçgen profil gözönüne alınır. Üçgen kesitin minimum boyutları, barajın kendi ağırlığı, hidrostatik su basıncı ve taban su basıncının etki ettiği normal yükleme durumunda çekme gerilmeleri meydana gelmeyecek şekilde belirlenir. Bunun için: b H  Recep YURTAL Ç.Ü. İnş.Müöl.

51 For the dam dimensions: Check out the safety for
Concrete 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

52 Overturning Check 1/md H B Recep YURTAL Ç.Ü. İnş.Müöl.

53 Overturning Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

54 Overturning Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

55 Overturning Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

56 Overturning Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

57 Overturning Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

58 Sliding Check 1/md H B Recep YURTAL Ç.Ü. İnş.Müöl.

59 Sliding Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

60 Sliding Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

61 Sliding Check H B Recep YURTAL Ç.Ü. İnş.Müöl.

62 Sliding Check 1/md H B Recep YURTAL Ç.Ü. İnş.Müöl.

63 Bearing Capacity Check
1/md H Recep YURTAL Ç.Ü. İnş.Müöl.

64 3.5.1 FORCES ON GRAVITY DAMS Free body diagram showing forces acting on a gravity dam

65 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:

66 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.

67 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 75 115 160 100 140 180

68 F) EARTHQUAKE FORCE (Fd):
Acting horizontally and vertically at the center of gravity k (earthquake coefficient): Ratio of earthquake acceleration to gravitational acceleration.

69 G) DYNAMIC FORCE (Fw) : In the reservoir, induced by earthquake as below  Acts at a distance 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 '

70 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.

71 Considered when a long fetch exists
I) WAVE FORCES : Considered when a long fetch exists LOADING CONDITIONS: 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

72 Dam must be safe against
3.5.2 STABILITY CRITERIA Dam must be safe against (1) Overturning for all loading conditions  resisting moments  overturning moments Safety factor: F.SO  2,0 (usual loading) F.SO  1,5 (unusual loading)

73 STABILITY CRITERIA (2) Sliding over any horizontal plane f = friction coef. btw any two planes Safety factor: FSS  1,5 (usual loading ) FSS  1,0 (unusual or severe loading)

74 (3) Shear and sliding together
STABILITY CRITERIA (3) Shear and sliding together A : Area of shear plane (m²) τs : Allowable shear stress in concrete in contact with foundation Safety factor: FSss  5,0 (usual loading) FSss  3,0 (unusual or severe loading)

75 STABILITY CRITERIA (4) Between foundation and dam contact stresses (σ) > 0 at all points There are two cases for the base pressure:

76 Base Pressure Check CASE 1: e  B/6 Ph  s Pt ΣV B Ph DAM BASE e
Pt  s Ph  s Recep YURTAL Ç.Ü. İnş.Müöl.

77 Base Pressure Check CASE 2: e > B/6 Pt Pt  s ΣV B e DAM BASE
Recep YURTAL Ç.Ü. İnş.Müöl.