CVE 471 WATER RESOURCES ENGINEERING DAMS. Assist. Prof. Dr. Bertuğ Akıntuğ

Transcription

1 CVE 471 WATER RESOURCES ENGINEERING DAMS Assist. Prof. Dr. Bertuğ Akıntuğ Civil Engineering Program Middle East Technical University Northern Cyprus Campus CVE 471 Water Resources Engineering 1/101

2 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 2/101

3 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 3/101

4 Classification of Dams A dam is an impervious barrier built across a watercourse to store water for several purposes: water supply, creating head (energy generation), forming a lake, sediment control, flood control, recharging of groundwater, etc. There are disadvantages of dams as well: imbalance of ecosystem, decrease amount of downstream water, reduction in the fertility of farmlands, etc. Therefore, detailed survey should be carried out to ensure that the relative weights of advantages over disadvantages are higher. CVE 471 Water Resources Engineering 4/101

5 Classification of Dams Dams can be classified into a number of different categories depending upon the purpose of classifications. A classification based on the type and materials of construction: Gravity Dams Concrete gravity dams Prestressed concrete gravity dams Roller compacted concrete (RCC) gravity dams Arch Dams Constant-angle arch dams Constant-center arch dams Variable-angel, variable-cemter arch dams Buttress Dams Flat-slab buttress dams Multiple-arch buttress dams Embankment (Fill) Dams CVE 471 Water Resources Engineering 5/101

6 Classification of Dams Gravity Dams Concrete gravity dams Pre-stressed concrete gravity dams Roller compacted concrete (RCC) gravity dams Karun Dam, Iran Shasta Dam, California, USA CVE 471 Water Resources Engineering 6/101

7 Classification of Dams Arch Dams Constant-angle arch dams Constant-center arch dams Variable-angel arch dams Variable-center arch dams Monticello Dam, California, USA Gordon Dam, Tasmania CVE 471 Water Resources Engineering 7/101

8 Classification of Dams Buttress Dams Used mainly in wide valleys, it consists of an impermeable wall, which is shored up by a series of buttresses to transmit the thrust of the water to the foundation. Flat-slab buttress dams Multiple-arch buttress dams CVE 471 Water Resources Engineering 8/101

9 Classification of Dams Buttress Dams Flat-slab buttress dams Lake Tahoe Dam, California, USA CVE 471 Water Resources Engineering 9/101

10 Classification of Dams Buttress Dams Multiple-arch buttress dams Bartlett Dam, Phoenix, Arizona, USA CVE 471 Water Resources Engineering 10/101

11 Classification of Dams Embankment (Fill) Dams Earth-fill dams Simple embankment Zoned embankment Diaphragm type embankment Upstream of Ataturk Dam, Turkey Embankment (Fill) Dams Rock-fill dams Downstream of Ataturk Dam, Turkey Impermeable-face Impermeable-earth core CVE 471 Water Resources Engineering 11/101

12 Classification of Dams A classifications based on purpose, such as storage diversion flood control hydropower generation A classification based on hydraulic design such as overflow dams, non-overflow dams Gilboa Dam, New York State, USA CVE 471 Water Resources Engineering 12/101

13 Classification of Dams A timber crib dam in Michigan, USA Liberty Dam, USA CVE 471 Water Resources Engineering 13/101

14 Classification of Dams A classification based on dam height: According to the International Commission on Large Dams (ICOLD): Large Dam if height > 15 m Large Dam if 10 m < height < 15 m reservoir storage > 10 6 m 3 crest length > 500 m High Dam height > 50 m Small Dam height < 10 m Distribution of dam heights in Turkey as of CVE 471 Water Resources Engineering 14/101

15 Classification of Dams Percent distribution of dams in Turkey according to purpose CVE 471 Water Resources Engineering 15/101

16 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 16/101

17 Parts of Dams A dam is composed of the following structural components Body forms the main part of a dam as an impervious barrier. Reservoir is the artificial lake behind a dam body. Spillway is that part of a dam to evacuate the flood wave from the reservoir. Water intake is a facility to withdraw water from a reservoir. Outlet facilities are those appurtenances to withdraw water from the reservoir to meet the demands or to discharge the excess water in the reservoir to the downstream during high flows. sluiceways, penstocks, diversion tunnels, bottom outlets, and water intake structures Others: Hydropower station, site installations, roads, ship locks, fish passages, etc. CVE 471 Water Resources Engineering 17/101

18 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 18/101

19 Planning of Dams There are commonly three steps in the planning and design: reconnaissance survey, feasibility study, and planning study. In reconnaissance surveys, the alternatives, which seem infeasible without performing intensive study, are eliminated. Feasibility Study: Estimation of water demand Determination of water potential Optimal plans Determination of dam site Topography Geologic information Foundation conditions Flood hazard CVE 471 Water Resources Engineering 19/101

20 Planning of Dams Feasibility Study: Determination of dam site (cont d) Spillway location and possibility Climate Diversion facilities Sediment problem Water quality Transportation facilities Right of way cost Determination of type of dams Project design Hydrologic design Hydraulic design Structural design CVE 471 Water Resources Engineering 20/101

21 Planning of Dams Planning Study: Topographic surveys Foundation studies Details on materials and constructional facilities Hydrologic study Reservoir operation study CVE 471 Water Resources Engineering 21/101

22 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 22/101

23 Construction of Dams Details of dam construction are beyond the scope of this course. The principal steps to be followed during the construction of any type of dam briefly: Evaluation of time schedule and required equipment. Diversion of river flow Foundation treatment Evaluation of Time Schedule and Required Equipment. Items to be considered: the characteristics of dam site the approximate quantities of work the preservation of construction equipment and materials diversion facilities and urgency of work CVE 471 Water Resources Engineering 23/101

24 Construction of Dams Diversion of River Flow Diversion of the river flow is may be accomplished in one of the following ways 1. Water is diverted through a side tunnel or channel. (Applicable for low flow depths ~1.5 m) Diversion by side tunnel or channel CVE 471 Water Resources Engineering 24/101

25 Construction of Dams Diversion of River Flow (cont d) Typical cross-section of earth cofferdams f: free board f=0.2(1+h) h: flow depth (meters) G=z/5 + 3 (meters) Cofferdams should be constructed during the low flow season. For fill type dams, embankment cofferdam may be kept in place as part of the embankment (e.g. Keban Dam and Ataturk Dam). For concrete dams, embankment cofferdam should be demolished after the dam has been constructed. Earth cofferdam on impervious foundation Earth cofferdam on pervious foundation CVE 471 Water Resources Engineering 25/101

26 Construction of Dams Diversion of River Flow (cont d) Hoover Dam, USA CVE 471 Water Resources Engineering 26/101

27 Construction of Dams Diversion of River Flow (cont d) Hoover Dam Overflow Tunnels (spillways), USA CVE 471 Water Resources Engineering 27/101

28 Construction of Dams Diversion of River Flow (cont d) Hoover Dam Overflow Tunnels (spillways), USA CVE 471 Water Resources Engineering 28/101

29 Diversion of River Flow (cont d) Construction of Dams Hoover Dam Overflow Tunnels (spillways), USA CVE 471 Water Resources Engineering 29/101

30 Construction of Dams Diversion of River Flow (cont d) 2. Water is discharged through the construction, which takes place in two stages. This type of diversion is normally practiced in wider valleys. Two-stage diversion CVE 471 Water Resources Engineering 30/101

31 Construction of Dams Diversion of River Flow (cont d) Two-stage diversion CVE 471 Water Resources Engineering 31/101

32 Construction of Dams Diversion of River Flow (cont d) A cofferdam on the Ohio River, Illinois, USA, built for the purpose of constructing the lock and dam. CVE 471 Water Resources Engineering 32/101

33 Construction of Dams Diversion of River Flow (cont d) Selection of a proper diversion scheme is based on the joint consideration of hydrologic characteristics of river flow, type of dam and its height, availability of materials, characteristics of spilling arrangements. The optimum design is based on cost minimization. The cost analysis is carried out for various sizes of diversion tunnels or channels to determine the corresponding total costs. The optimum tunnel diameter or bottom width of a lined trapezoidal channel is then determined according to the minimum total cost of the facility. CVE 471 Water Resources Engineering 33/101

34 Construction of Dams Foundation Treatment Foundation treatment for dams is essential to achieve less deformation under high loads, to decrease permeability and seepage, to increase shearing strength, and to satisfy slope stability for the side hills. Highly porous foundation material causes excessive seepage, uplift and considerable settlement. Such problems can be improved by a grouting operation. In this operation, the grout mix is injected under pressure to decrease the porosity, and hence to solidify the formations underlying the dam and reservoir. CVE 471 Water Resources Engineering 34/101

35 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 35/101

36 Gravity Dams Gravity dams are satisfactorily adopted for narrow valleys having stiff geological formations. Their own weight resists the forces exerted upon them. They must have sufficient weight against overturning tendency about the toe. The base width of gravity dams must be large enough to prevent sliding. These types of dams are susceptible to settlement, overturning, sliding and severe earthquake shocks. CVE 471 Water Resources Engineering 36/101

37 Gravity Dams Concrete Gravity Dams Concrete gravity dams area built of mainly plain concrete to take compressive stresses. Shasta Dam, California, USA CVE 471 Water Resources Engineering 37/101

38 Gravity Dams Concrete Gravity Dams (cont d) Concrete gravity dams have lower maintenance and operation costs compared to the other types of dams. In the design of these structures, the following criteria should be satisfied: Dimensions of the dam are chosen such that only compressive stresses develop under all loading conditions. The dam must be safe against overturning, shear and sliding. CVE 471 Water Resources Engineering 38/101

39 Gravity Dams Concrete Gravity Dams (cont d) In the construction of concrete gravity dams special care is required for the problems due to shrinkage and expansion. Formation of the body of the concrete gravity dam CVE 471 Water Resources Engineering 39/101

40 Gravity Dams Concrete Gravity Dams (cont d) Forces Acting on Gravity Dams The weight: W c = dead load Hydrostatic forces: Uplift Force: φ: uplift reduction coefficient Moment arm of F u =B(2h 1 +3h 2 ) / 3(h 1 +h 2 ) Actual uplift pressures are determined by pressure gauges installed at the bottom of the dam. Free body diagram. Forces acting on a concrete gravity dam CVE 471 Water Resources Engineering 40/101

41 Gravity Dams Concrete Gravity Dams (cont d) Forces Acting on Gravity Dams Sediment Force: γ s : submerged specific weight of soil K a : active earth pressure coefficient according to the Rankine theory. K a = (1-sinθ)/(1+sinθ) Ice Load (F i ): Free body diagram. Forces acting on a concrete gravity dam CVE 471 Water Resources Engineering 41/101

42 Gravity Dams Concrete Gravity Dams (cont d) Forces Acting on Gravity Dams Earthquake Force: F d = kw c k: earthquake coefficient Dynamic Force in the reservoir induced by earthquake F w = 0.726Ckγh 2 1 θ C = Dynamic Force acting on a spillways ΣF = ρq u obtained using momentum equation Free body diagram. Forces acting on a concrete gravity dam CVE 471 Water Resources Engineering 42/101

43 Gravity Dams Concrete Gravity Dams (cont d) Forces Acting on Gravity Dams Wave Force may be considered for wide and long reservoirs. Temperature Loads may be severe during construction because of hydration reactions Free body diagram. Forces acting on a concrete gravity dam CVE 471 Water Resources Engineering 43/101

44 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria Stability analyses are performed for various loading conditions The structure must prove its safety and stability under all loading conditions. Since the probability of occurrence of extreme events is relatively small, the joint probability of the independent extreme events is negligible. In other word, the probability that two extreme events occur at the same time is relatively very low. Therefore, combination of extreme events are not considered in the stability criteria. Floods (spring and summer) Ice load (winter). No need to consider these two forces at the same time. CVE 471 Water Resources Engineering 44/101

45 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria Usual Loading Hydrostatic force (normal operating level) Uplift force Temperature stress (normal temperature) Dead loads Ice loads Silt load Unusual Loading Hydrostatic force (reservoir full) Uplift force Stress produced by minimum temperature at full level Dead loads Silt load Extreme (severe) Loading Forces in Usual Loading and earthquake forces CVE 471 Water Resources Engineering 45/101

46 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The ability of a dam to resist the applied loads is measured by some safety factors. To offset the uncertainties in the loads, safety criteria are chosen sufficiently beyond the static equilibrium condition. Recommended safety factors: (USBR, 1976 and 1987) F.S 0 : Safety factor against overturning. F.S s : Safety factor against sliding. F.S ss : Safety factor against shear and sliding. However, since each dam site has unique features, different safety factors may be derived considering the local condition. CVE 471 Water Resources Engineering 46/101

47 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The factor of safety against overturning: F. S 0 = M M r 0 where ΣM r : total resisting moment about the toe. ΣM 0 : total overturning moment about the toe. The factor of safety against sliding: F. S s = f V H where f: coefficient of friction between any two planes ΣV: vectorial summation of vertical forces. ΣH: vectorial summation of horizontal forces. CVE 471 Water Resources Engineering 47/101

48 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The factor of safety against sliding and shear: f V + raτ s F. Sss = (in the dam) H F. S ss = f V + ca H (at foundation level) where A: Area of the shear plane, τ s : shear strength of concrete r: factor to express max allowable average shear stress r=0.33, 0.50, and 1.0 for usual, unusual, and extreme loading, respectively. f: coefficient of friction between any two planes ΣV: vectorial summation of vertical forces. ΣH: vectorial summation of horizontal forces. CVE 471 Water Resources Engineering 48/101

49 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The contact stress between the foundation and the dam or the internal stress in the dam body must be compressive: V σ = ± A Mc I Normal stress Bending or flexural stress Base pressure distribution where σ : vertical normal base pressure A: Area of the shear plane, M: net moment about the centerline of the base (M = ΣV.e) e: eccentricity ( B / 2 x) c: B/2 I : Moment of inertia (B 3 /12) ΣV: vectorial summation of vertical forces. CVE 471 Water Resources Engineering 49/101

50 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The contact stress between the foundation and the dam or the internal stress in the dam body must be compressive: In order to maintain compressive stresses in the dam or at the foundation level, the minimum pressure σ min 0. This can be achieved with a certain range of eccentricity. σ = ΣV ± A Mc I for a unit width σ min = ΣV A ΣV e B / 2 B 3 /12 = ΣV B 1 6e B 0 σ min 0 can be achieved if e B/6 Full reservoir σ max at the downstream face Empty reservoir σ max at the upstream face Base pressure distribution CVE 471 Water Resources Engineering 50/101

51 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The contact stress between the foundation and the dam or the internal stress in the dam body must be compressive: Tension along the upstream face of a gravity dam is possible under reservoir operating conditions. z = 1.0 (if there is no drainage in the dam body) z = 0.4 (if drains are used) P: hydrostatic pressure at the level under consideration CVE 471 Water Resources Engineering 51/101

52 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria Concrete gravity dams have varying thickness. Hence the inclined compressive stresses parallel to the face of the dam need to be computed. For a concrete gravity dam with slopes of 1V:mH at the upstream face and 1V:nH at the down stream face, the major principle compressive stresses, σ iu (parallel to the upstream face) and σ id (parallel to the downstream face) are obtained from the static equilibrium of forces in the vertical direction as: (ΣF y =0) where σ u and σ d vertical normal compressive stresses and p u and p d hydrostatic pressures CVE 471 Water Resources Engineering at the upstream and downstream faces, respectively. 52/101

53 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria Internal horizontal and vertical shear stresses at the upstream and downstream faces are obtained by equating the total moment to zero as (ΣM A =0, ΣM B =0): where τ hu, τ hd, τ vu, and τ vd are the horizontal and vertical internal shear stresses at the upstream and downstream faces, respectively. CVE 471 Water Resources Engineering 53/101

54 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria The maximum compressive stress, σ max,must be smaller than a certain fraction of the compressive strength of concrete, σ c, and foundation material, σ f. Safety criteria for concrete gravity dams Unconfined compressive strength, σ f for foundation materials CVE 471 Water Resources Engineering 54/101

55 Gravity Dams Concrete Gravity Dams (cont d) Stability Criteria Excessive care must be taken during the filling of the reservoir. Initially 1/3 of the dam height may be filled first. After waiting for several weeks and assuring that the dam is safe, further filling is performed. Since safety levels change with respect to upstream water depth, gravity dams must be analyzed for various operating levels and empty reservoir cases, separately. For the empty reservoir case, the overturning tendency must be checked with respect to the toe and heel, separately. The stability against sliding may be improved by providing a cut off wall in the foundation at the upstream side. CVE 471 Water Resources Engineering 55/101

56 Gravity Dams Prestressed Concrete Gravity Dams In a prestress concrete dam, forces are applied to the dam before the reservoir is filled in order to counter undesirable stress that would develop in the absence of the prestressing forces. For prestressing, either small-diameter high-tensile wires or hightensile steel bars can be used. Roller Compacted Concrete (RCC) Gravity Dams RCC dam is constructed using cement, water, fine and course aggregates, and fly ash which are mixed in certain proportions to have a no-slump, rather dry composition. Construction is based on the compaction of this mixture by heavy static or vibrating rollers. Construction period of RCC dams is shorter than that of conventional concrete gravity dams. CVE 471 Water Resources Engineering 56/101

57 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 57/101

58 Arch Dams Arch dams are thin concrete structures. Gokcekaya, Oymapinar, Karakaya, Gezende, and Berke dams in Turkey. Gokcekaya Dam Berke Dam Karakaya Dam CVE 471 Water Resources Engineering 58/101

59 Arch Dams Arch dams: Oymapinar Dam, Turkey CVE 471 Water Resources Engineering 59/101

60 Arch Dams Hoover Dam, USA CVE 471 Water Resources Engineering 60/101

61 Arch Dams Arch dams are thin concrete structures. Stability of an arch dam is based on its self weight and its ability to transmit most of the imposed water loads into the valley walls. At the sites of arch dams, the side formations and foundations should be very stiff to resist the applied load. For effective arching action, the radius of the arch should be as small as possible. They are formed by concrete blocks having base dimensions of approximately 15 m by 15 m and height of 1.5 m Reinforcement is not generally required in thick arch dams because it increases the cost drastically. Arch dams have normally higher structural safety than conventional gravity dams. CVE 471 Water Resources Engineering 61/101

62 Arch Dams Types of Arch Dams Arch dams are classified according to geometric characteristics of the valley where they are adopted. Arch dams are classified according to the location of the center and magnitude of the central angle Constant-center (variable angle) arch dams are suitable for medium-high dams in U-shape valleys. They have single curvature in plan with vertical upstream face. CVE 471 Water Resources Engineering 62/101

63 Arch Dams Types of Arch Dams Variable-center (constant angle) arch dams are suitable for V-shape valleys. Radius of the arc reduces with respect to depth. So arching action is more pronounced at low depths. Since these types of dams are normally thinner than constant-center dams, they are more elastic and safer. Variable-center (constant angle) arch dams CVE 471 Water Resources Engineering 63/101

64 Arch Dams Types of Arch Dams Variable-center (variable angle) arch dams are composed of the combination of two types described above. Load distribution in vertical direction governs the cross-sectional shape of the dam. This type has a pronounced double curvature They utilized the concrete strength more compared the other types resulting in thinner and more efficient structure. However, tensile stresses may develop in the dam body. Variable-center (variable angle) arch dams Gokcekaya Dam CVE 471 Water Resources Engineering 64/101

65 Arch Dams Types of Arch Dams Variable-center (variable angle) arch dams Gokcekaya Dam Cross-section of Gokcekaya Dam CVE 471 Water Resources Engineering 65/101

66 Arch Dams Design of Arch Dams Structural design of an arch dam requires the determination of load distribution in the dam body using the trial load method and applications of the theory of elasticity and the theory of shells. Structural design is beyond the scope of this course. Simplified design: The determination of the thickness at any elevation of an arch dam whose crest elevation has already been determined in the hydrologic design step. In the arch dams, the total load is shared by arch and cantilever actions and transmitted to the sides and foundation, respectively. Therefore, the base width of arch dams is usually much narrower than that of concrete gravity dams having almost the same height. Hence, the effect of uplift pressure can be ignored. CVE 471 Water Resources Engineering 66/101

67 Arch Dams Design of Arch Dams However, effect of temperature stresses should be checked to ensure that they are smaller than tensile strength. Near the crest of the dam, most of the loads taken by arches and transmitted to the side abutments. Near the bottom of the dam, cantilevers take most of the load and transmit to the foundation. Gokcekaya Dam CVE 471 Water Resources Engineering 67/101

68 Arch Dams Design of Arch Dams In the following analysis, the water thrust induced by hydrostatic pressure is assumed to be taken by arch action only and transmitted to the sides. The differential force acting on a differential element having a central angle of dφ is df v = P r dφ The vertical component of this force is df' v = P r dφ sinφ Free-body diagram for arch dam analysis CVE 471 Water Resources Engineering 68/101

69 Arch Dams Design of Arch Dams Integration of this force along the arc length gives the total horizontal force, H h. H h π 2 π = 2 γhr sinφ dφ = 2γhr cos 2 π θ a 2 2 π θ cos a 2 2 θ = 2γhr sin a 2 Free-body diagram for arch dam analysis where h: the height of the arch rib relative to the reservoir surface r: the radius of arch θ a : the central angle CVE 471 Water Resources Engineering 69/101

70 Arch Dams Design of Arch Dams The equilibrium of forces in y- direction involves where Therefore H h = 2R y θ R R a y = sin 2 θ hr a θ 2γ sin = R a 2 sin 2 2 R = γhr where R: the reaction offered by the sides against the transmission of water thrust. Free-body diagram for arch dam analysis As observed from the R = γhr, the reaction at the sides is directly proportional to the arc radius at a given height. Therefore, narrow valleys having stiff geological formations and small r-values are suitable for arch dams. CVE 471 Water Resources Engineering 70/101

71 Arch Dams Design of Arch Dams If the thickness of the arch rib, t, is relatively small as compared with r, there is small difference between the average and maximum compressive stresses in the rib and σ R/t. The required thickness of the rib is then t γhr σ = (the thickness varies linearly with depth.) all where σ all : the allowable working stress for concrete in compression. CVE 471 Water Resources Engineering 71/101

72 Arch Dams Design of Arch Dams The volume of concrete per unit height of a single arch rib across a canyon of width of B a is V=Lt where L is the arch length which is equal to rθ a (θ a in radians). Inserting the values of L and t into the equation above V h = σ γ 2 all r θ a CVE 471 Water Resources Engineering 72/101

73 Arch Dams Design of Arch Dams The optimum central angle θ a for a minimum volume of arch rib can be determined as 133º34 by differentiating V with respect to θ a and equating the result to zero. This is the reason why a constant-angle arch dam can be design to require less concrete than a constant-center dam. In practice, the central angles of arch dams vary from 100º to 140º. However, the formwork of a constant-angle dam is more difficult. CVE 471 Water Resources Engineering 73/101

74 Arch Dams Design of Arch Dams The optimum central angle θ a for a minimum volume of arch rib can be determined as 133º34 by differentiating V with respect to θ a and equating the result to zero. This is the reason why a constant-angle arch dam can be design to require less concrete than a constant-center dam. In practice, the central angles of arch dams vary from 100º to 140º. However, the formwork of a constant-angle dam is more difficult. CVE 471 Water Resources Engineering 74/101

75 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 75/101

76 Buttress Dams A buttress dam consists of a sloping slab. Depending on the orientation of slab, a buttress dam may be classified as flat-slab buttress dam multiple-arch buttress dam Elmali Dam construction, Istanbul, 1941 A typical buttress dam. CVE 471 Water Resources Engineering Elmali Dam 76/101

77 Buttress Dams Flat-slab buttress dams Lake Tahoe Dam, California, USA CVE 471 Water Resources Engineering 77/101

78 Buttress Dams Multiple-arch buttress dams Bartlett Dam, Phoenix, Arizona, USA CVE 471 Water Resources Engineering 78/101

79 Buttress Dams Some advantages of buttress dams over conventional gravity dams: They can be constructed on foundations having smaller bearing capacity then required for gravity dams. Since they have thinner slabs, possibility of development of vertical cracks is less. Problems encountered during the setting of concrete are reduced. Unless a mat foundation is used, uplift forces are negligibly small because of hollow spaces provided between the buttresses. Ice pressures are also small as the ice sheet slides up the inclined slab. Main disadvantage of buttress dams: May have comparable costs, because of increased formwork and reinforcement. There is only one buttress dam in Turkey (Elmali 2 Dam). Elmali 2 Dam CVE 471 Water Resources Engineering 79/101

80 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 80/101

81 Embankment (Fill) Dams They composed of fill of suitable earth materials at the dam site. Coarse-grained soils (gravel and coarse sand) relatively pervious, easily compacted, resistant to moisture, Clay is considered as a core material (impermeable) unstable when saturated (expands due to wetting, hard to compact) Therefore, clay mixed with sand and fine gravel is used as a core. Core must be compacted in thinner layers with fairly accurate moisture control. Compacted asphalt may also be used as an economical core material in case of loose foundations. Asphalt can absorb earthquake shocks effectively. CVE 471 Water Resources Engineering 81/101

82 Embankment (Fill) Dams Embankment dams are usually safer against deformations and settlements. Embankment dams Earth-fill dams Rock-fill dams (More than 50% of the total material is of rock.) Earth-fill dams in Turkey Seyhan Dam Demirkopru Dam Cubuk 2 Dam Bayindir Dam Rock-fill dams in Turkey Keban Dam Ataturk Dam Hasan Ugurlu Dam Hasan Ugurlu Dam CVE 471 Water Resources Engineering 82/101

83 Embankment (Fill) Dams Body volume of embankment dams is relatively greater than the other types of dams. Normally cheaper than the other types where there is enough fill material in the close vicinity. Fill dams comprise more than 70% of the dams in the world and 90% in Turkey. Keban Dam CVE 471 Water Resources Engineering 83/101

84 Embankment (Fill) Dams Earth-fill Dams Construction: Placement of selected material on layers of 50 cm thick and compaction. Non-organic and non-plastic soils are needed. The embankment soil is usually irrigated at the borrow area. Piezometers can be placed in the embankment at various depths during the construction to measure the pore water pressure. A typical earth-fill dam is constructed in a multi-layer formation. CVE 471 Water Resources Engineering 84/101

85 Embankment (Fill) Dams Earth-fill Dams A typical earth-fill dam is constructed in a multi-layer formation. Earth dam on pervious foundation CVE 471 Water Resources Engineering 85/101

86 Embankment (Fill) Dams Earth-fill Dams Seepage through an earth-fill dam. The flow rate, q, between two flow lines can be expressed using the Darcy law as q = The total flow rate, q h KAi = K D L Kh q = N N K: the hydraulic conductivity i : the hydraulic gradient h: head loss (h/n) N : number of equipotential drops N : the number of stream tubes CVE 471 Water Resources Engineering 86/101

87 Embankment (Fill) Dams Earth-fill Dams Drainage systems in an earth-fill dam. Chimney drains, in the embankment as well as enlarged toe drains are effective in controlling the seepage through the dam. CVE 471 Water Resources Engineering 87/101

88 Embankment (Fill) Dams Rock-fill Dams Having relatively high pore space Can be adopted to weaker foundations where a gravity dam cannot be constructed. Cross-sections of typical rock-fill dams CVE 471 Water Resources Engineering 88/101

89 Embankment (Fill) Dams Rock-fill Dams (Ataturk Dam) Largest dam in Turkey Reservoir Volume: 48.7 x 10 9 m 3 Installed capacity: 2400 MW Annual energy production: 8.9 x 10 9 kwh Irrigated land: ha (with the completion of the project) A cross-sections of the Ataturk Dam CVE 471 Water Resources Engineering 89/101

90 Embankment (Fill) Dams Rock-fill Dams (Ataturk Dam) A cross-sections of the Ataturk Dam CVE 471 Water Resources Engineering 90/101

91 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 91/101

92 Cross-sectional Layout Design of Dams A suitable dam cross-section should be provided such that both safety and desired functionality concerning service requirement are attained. Sufficient crest width, t c must be provided. a width of two lane traffic may be selected. For small embankment dams up to H f =15 m. t c =0.2H f +3 For large embankment dams up to H f =150 m. t c =3.6 (H f ) 1/3 where t c and H f are in meter. CVE 471 Water Resources Engineering 92/101

93 Cross-sectional Layout Design of Dams By examining some existing muti-purpose concrete gravity dams throughout the world, Yanmaz et al. (1999) proposed the following regression equations to define the shape of a gravity dam. H * = H t t c = H t where all variables are in meter CVE 471 Water Resources Engineering 93/101

94 Cross-sectional Layout Design of Dams United States Bureau of Reclamation (USBR) propose the following formulas for cross-sectional layout of arch dams: t t t c b 0.45H t = 0.01 = 3 = 0.95t ( H + 1.2B ) H b t t B a a B 0.15 Ht 400 H t / 400 All the dimensions are in ft where B a : the span width at the crest B 0.15 : the span width at 15% of the dam height above the base t 0.45Ht : the dam thickness at 45% of the dam height above the base. CVE 471 Water Resources Engineering 94/101

95 Cross-sectional Layout Design of Dams The crest elevation of a dam is to be determined such that there is almost no overtopping danger of the flood wave during the occurance of the design flood. Freeboards on flood levels for concrete and embankment dams Greater freeboards are required for embankment dams since they are susceptible to erosion at the downstream face due to overtopping from their crest. The required side slopes of concrete gravity dams are determined from stability analyses. The maximum downstream slope of gravity dams is 45. Side slopes of embankment dams are determined on the basis of seepage and slope stability analyses. CVE 471 Water Resources Engineering 95/101

96 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 96/101

97 Local Scour at the Downstream of Dams Excessive kinetic energy of the flowing water at the downstream of outlet works (spillways, sluiceways etc.) should be dissipated in order to prevent the erosion of the streambed and the banks below the dam. Local scour at the downstream of the dam and sluice gates Excessive scours at the downstream of Keban Dam have resulted in serious foundation stability problems (depth of approx 30 m). CVE 471 Water Resources Engineering 97/101

98 Local Scour at the Downstream of Dams Some of the scour prediction equations are given in the table. Scour prediction equations for the downstream of dams. d s : the maximum depth of scour hole in m. b: the thickness of the jet in m. Φ: the side inclination for the scour hole, Fr: Jet Froude number. U: the velocity of the jet in m/s =(γ s - γ)/γ, γ s: : specific weight of sediment in kn/m 3 γ : specific weight of water in kn/m 3. W f : Fall velocity in m/s q: unit discharge in m 3 /s/m H g : gross head in m h: tailwater depth in m D 50 : median size of bed material in m. CVE 471 Water Resources Engineering 98/101

99 Overview Classification of Dams Parts of Dams Planning of Dams Construction of Dams Concrete Gravity Dams Arch Dams Buttress Dams Embankment (Fill Dams) Cross-sectional Layout Design of Dams Local Scour at the Downstream of Dams Dam Safety and Rehabilitation CVE 471 Water Resources Engineering 99/101

100 Dam Safety and Rehabilitation Excessive care must be taken in planning, design, and construction stages of a dam. Major causes for a dam break: Inadequate spillway capacity, Improper construction of any type of dam, Insufficient compaction of embankment dams or compaction with undesirable water content, Improper protective measures, Excessive settlements, etc Continuous inspection and monitoring are required to assess the safety level of the dam throughout the lifetime. CVE 471 Water Resources Engineering 100/101

101 Dam Safety and Rehabilitation Upon periodic inspection, the following deficiencies may be observed that are indicators of problems: Large horizontal and vertical movements of crest, Tilting of the roadway along the crest, Deformation of embankment slope, Higher than usual pore water pressure in embankment dams, Unusual seepage at the toe or edges of an embankment dam, Seepage flows with not decreasing with low flow conditions, Turbit outflow through the embankment, Tilting of the spillway crest Increased leakage into inspection galleries in concrete dams, etc. CVE 471 Water Resources Engineering 101/101