Dam engineering

concept of Dam Engineering

1. Dams and Spillways Prof Hole G.R. ME-(Hydraulics) Lecturer in Civil Engineering Department J.S. Polytechnic, Hadaspar, Pune

2. Dam:- Structure built across a stream or a River to store water is called Dam. Purpose of Dam: Hydropower Irrigation Water for domestic consumption Drought and flood control For navigational facilities Other additional utilization is to develop fisheries Introduction

3. Foundation Should Strong Should be located in narrow valley. Should have sufficient space for spillway. Should have impervious bed of reservoir. Should fulfill the purpose of Reservoir i.e. Irrigation, Drinking. Material Should be locally available. Should have less area under submergence. Site should be easily accessible. Length of dam should be minimum because length is directly affects cost of construction. Site selection criterion for Dam

4. There are mainly Five types of Dam Gravity Dam Earthen Dam Rock and Fill Dam Arch Dam Buttress Dam Types of Dam

5. 1.Gravity Dam It is type of dam in which the whole water pressure is resisted by its own Self Weight. It is more durable and required less maintenance. Material used for construction is Concrete and steel. Also known as Concrete or Masonry Dam. It requires strong foundation, Seepage through body of dam is less.

6. Heel: contact with the ground on the upstream side Toe: contact on the downstream side Galleries: small rooms like structure left within the dam for checking operations. Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side . Free Board : It is the vertical distance provided between Top of Dam and HF L Components of Gravity Dam

7. 1. Bhakra Dam: Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world . Bhakra Dam is across river Sutlej in Himachal Pradesh The construction of this project was started in the year 1948 and was completed in 1963 . It is 740 ft. high above the deepest foundation as straight concrete dam being more than three times the height of Qutab Minar . Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30 feet) Bhakra Dam is the highest Concrete Gravity dam in Asia an d Second Highest in the world Practical Examples of Gravity Dam in India

8. The forces that act on a gravity dam are – Self Weight . Hydraulic Pressure. Uplift Force . Silt Pressure Wave Pressure. Wind Pressure. Seismic Force . Ice Pressure Forces Acting on Gravity Dam

9. The self weight of the gravity dam is the main stabilizing force which counter balances all the external forces acting on it. For construction of gravity dams the specific weight of concrete & stone masonry shouldn’t be less than 2400 kg/m3 & 2300 kg/m3 respectively. The self weight of the gravity dam acts through the center of gravity of the. Its calculated by the following formula – W = γm * Volume of Dam Where γm is the specific weight of the dam’s material . 1. Self Weight

10. Water Pressure is the most major external force acting on a gravity dam. On upstream face pressure exerted by water is stored up to the full reservoir level. The upstream face may either be vertical or inclined. On downstream face the pressure is exerted by tail water. The downstream face is always inclined. It is calculated by the following formula – Where γw is the unit weight of water & h is the height of water . It is acting about H/3 from Heel and 2/3 from Top of Dam. 2 . Hydraulic Pressure

11. The water stored on the upstream side of the dam has a tendency to seep through the soil below foundation. While seeping, the water exerts a uplift force on the base of the dam depending upon the head of water. This uplift pressure reduces the self weight of the dam. To reduce the uplift pressure, drainage galleries are provided on the base of the dams. It is calculated by the following formula :- Where B is the base width of the dam and H is the height up to which water is stored. This total uplift acts at B/3 from the heel or upstream end of the dam . 3. Uplift Pressure

12. It is the pressure exerted by the silt deposited in the reservoir on U/S face of Dam . It is Similar to the earth pressure on vertical face It is calculated by following formula :- 4 . Silt Pressure

13. Waves are generated on the surface of the reservoir by the blowing winds, which exert a pressure on the upper part of the dam above the water level. The magnitude of waves depend upon – The velocity of wind. Depth of Reservoir. Area of Water Surface . It is calculated by the following formula – Where ‘ hw ’ is the wave height. Yw is Unit Weight of water The maximum pressure intensity due to wave action occurs when it acts at 0.5 meters above the still water surface. 5. Wave Pressure

14. During EQ Dams are subjected to Seismic Vibration and it will act is any direction. For Designing of dam for Seismic load this vibration are divided in to two components one is Horizontal Acceleration and Other One is Vertical Acceleration . Horizontal Acceleration Creates most Unstable Conditions This Seismic Load generates two forces Inertia Force(Body Force) and Hydrodynamic Force(Due to Water) Body Force(Inertia Force): Body Force acts Horizontally on Body of Dam and it is calculated as following:- Pe = α * W α = Earthquake Coefficient W = Self Weight of Dam Hydrodynamic Force(Due to Water ):- Water vibration produces a force on the dam acting horizontally & calculated by P= Ce * α * Yw *h P= Hydrodynamic Pressure KN/m2 Ce = Coefficient depend on shape of dam and depth of reservoir H = Height of water in reservoir α = Coefficient of EQ Yw = Unit wt of Water KN/m3 6. Earthquake Load

15. When the dam has been newly constructed, and the reservoir has not yet been filled, then the worst combination of vertical and horizontal inertia forces would have to be taken that cause the dam to topple backward Under the reservoir full condition, the worst combination of the inertia forces is the one which tries to topple the dam forward Hu: Horizontal earthquake force acting In the upstream direction HD: Horizontal earthquake force acting in the downstream direction Vu: Vertical earthquake force acting upwards VD: Vertical earthquake force acting downwards Continue

16. The top exposed portion on the dam is small & hence the wind pressure on this portion of dam is negligible. But still an allowance should be made for the wind pressure at the rate of about 150 kg/m2 for the exposed surface area of the upstream & downstream faces. 7 . Wind Pressure

17. his pressure is considered only when dams are located either at very high altitudes or in very cold regions. In very cold conditions, the reservoir gets covered with a layer of ice. When this layer is subjected to expansion and contraction due to variation in temperature, a force is developed which acts on the dam at level of water in the reservoir. This force acts linearly along the length of the dam at the reservoir level. The average magnitude of this force may be taken as 5 kg cm2 of the contact area of the ice layer with U/S face of the dam. The thermal expansion of ice is about five times that of concrete. 8. Ice Pressure

18. Elementary Profile of Gravity Dam When water is stored against any vertical face, then it exerts pressure perpendicular to the face which is zero at top & maximum at bottom. The required top thickness is thus zero & bottom thickness is maximum forming a right angled triangle with the apex at top, one face vertical & some base width. Two conditions should be satisfied to achieve stability – When empty - The external force is zero & its self weight passes through C.G. of the triangle. When Full - The resultant force should pass through the extreme right end of the middle- third . The limiting condition is – where, σc =allowable compressive stress . S = Sp. Gravity of Dam Material Y = Unit Weight of Dam Material Free Board is not Provided on top. Working Platform is not available at top .

19. Theoretical Profile of Gravity Dam Various parameters in fixing the parameters of the dam section are – Free Board –IS 6512, 1972 specifies that the free board will be 1.5 times the wave height above normal pool level. Top Width – The top width of the dam is generally fixed according to requirements of the roadway to be provided. The most economical top width of the dam is 14 % of its height. Base Width – The base width of the dam shall be safe against overturning, sliding & no tension in dam body. For elementary profile – When uplift is considered When uplift isn’t considered, Free Board is Provided on top. Working Platform is available at top .

20. Definition of Gallery: It is the horizontal passage provided in body of dam which may run parallel or transverse to the axis of Dam at different level. Usual Size of Galleries are Width = 2 to 2.5m Height = 2.5 to 4m All Galleries are interconnected by vertical shaft. Galleries In Gravity Dam

21. Following are the Functions of Galleries:- Inspection of Dam from Inside To Drain off Seepage water from body of Dam To provide Access to Spillway and Gates To Access for Grouting and Interior Maintenance. Types of Galleries :- Foundation Galleries Inspection Galleries Functions of Galleries

22. Joints In Gravity Dam

23. Construction Joints: These are horizontal joints provided for ease in construction of the gravity dams. They also permits systematic, convenient & economical construction of the dam. The thickness of each layer of concrete shouldn’t exceed 1.5m. The thickness of first layer of concrete above rock foundation shouldn’t exceed 7.5 m. Joints In Gravity Dam

24. Earthen Dams

25. Introduction An Earthen Dam is an embankment dam, constructed primarily of compacted earth materials The foundation requirements are not as hard as other dams Local available soil is the main construction material Special skills are not required No special plants & equipments are required, mostly earth-moving machines can be used

26. Advantages Design procedures are straight forward Comparatively small plant and equipment are required Foundation requirements are less stringent than that of other Earthfill dams resist settlement and movement better than rigid dams Suitable for areas where earth movements are common.

27. Disadvantages An earth embankment is easily damaged or destroyed by water flowing on, over or against it. Spillway and adequate upstream protection are essential for any earth dam. Adequate compaction required during construction, otherwise the dam will offer weak structural integrity, offering possible pathways for preferential seepage. Earth dams require continual maintenance to prevent erosion, tree growth, subsidence, animal and insect damage and seepage.

28. Classification of Earthen Dam 1. Based on mechanical characteristics of earth materials Homogeneous earth dam Non-Homogeneous (zoned) earth dam Dam with a Diaphragm 2 .  Based on the method of construction Rolled fill earth dam Hydraulic fill earth dam

29. Homogeneous earth dams

30. Homogeneous earth dams It is mainly composed of only one material. Therefore it is constructed where one type of material is available in bulk. These dams are constructed with uniform and homogeneous materials. It is suitable for low height dams (up to 10m). These dams are usually constructed with soil and grit mixed in proper ratios. Seepage is quite high in such dams.

31. Non-Homogeneous (Zoned) earth dams

32. Non-Homogeneous (Zoned) earth dams A zoned earth dam usually consists of a central impervious core The central core is made up of clay, silt or clayey silt. The thickness of the core wall is made sufficiently thick to prevent leakage of water through the body of the dam. Also it distributes the load over larger area of foundation . The pervious shell gives the stability to the dam and it is made up of sand, gravel or a mixture of these materials. Filter is placed in between them to prevent piping Transition filters prevents the migration of the core material into the pores of shell material.

33. Dam with a Diaphragm

34. Dam with a Diaphragm This type of dam is constructed with pervious materials, with a thin impervious diaphragm in the central part to prevent seepage of water. The thin impervious diaphragm may be made of impervious clayey soil, cement concrete or masonry or any impervious material. The main difference in zoned and diaphragm type of dams depend on the thickness of the impervious core or diaphragm.

35. Rolled Fill Earth dams In this type of dams, successive layers of moistened or damp soils are laid one over the other. Each layer not exceeding 45 cm in thickness is properly consolidated at optimum moisture content required for optimum density, only then is the next layer laid. Generally sheep foot rollers and pneumatic tired rollers are used for compaction. Most of the modern dams are the rolled fill dams.

36. Hydraulic Fill dams

37. Hydraulic Fill dams In this type of dams, the construction, excavation, transportation of the earth is done by hydraulic methods by means of the water. Outer edges of the embankments are kept slightly higher than the middle portion of each layer. During construction, a mixture of excavated materials in slurry condition is pumped and discharged at the edges. This slurry of excavated materials and water consists of coarse and fine materials. When it is discharged near the outer edges, the coarser materials settle first at the edges, while the finer materials move to the middle and settle there. Fine particles are deposited in the central portion to form a water tight central core. In this method, compaction is not required.

38. Typical section of earthen dam Shell Core Cut off Trench Transition zone Drainage System Rip rap Sod OR Turfing U/S Blanket

39. Failure of earth fill dams Hydraulic failure: By overtopping , Erosion of upstream surface, Erosion of downstream toe, Erosion of downstream face by gully formation, Failure due to Frost action etc Seepage failure: Piping through dam body, Piping through foundation, Conduit leakage, Sloughing of downstream side of dam Structural failure: Sliding due to weak foundation, Sliding of upstream face due to sudden drawdown, Sliding of the downstream face due to slopes being too steep, liquefaction, burrowing animals , settlement, Faulty construction

40. 1. Overtopping flood water pass over the dam flood that exceeded the design flood for the spillway faulty operation of the spillway gates

41. 2. Erosion of upstream surface During winds, the waves developed near the top water surface may cut into the soil of upstream dam face which may cause slip of the upstream surface leading to failure. For prevent this upstream face should be protected with stone pitching or riprap

42. 3. Erosion of downstream toe Downstream side may be eroded due to i) heavy cross-current from spillway buckets, or ii) tail water. Failure is prevented by downstream slope riprap up to a height above the tail water depth. Also the side wall of the spillway should have sufficient height and length to prevent possibility of cross flow towards the earth embankment.

43. 4. Erosion of downstream face by gully formation During heavy rains, the flowing rain water over the downstream face can erode the surface, creating gullies, which could lead to failure. To reduce this failure berms could be provided at suitable heights and surface is well drained.

44. 5.Failure due to Frost action If the earth dam is located at place where the temperature is falls below the freezing point, frost may form in the pores of the soil in earth dam . When there is heaving the cracks may form in the soil .This may lead to dangerous seepage and consequent failure. To avoid this, soil having good resistance to frost action should be used.

45. 1. Piping through dam body This is the progressive backward erosion starting from the exit point and subsequent removal of the soil from within the body of the dam It begins at point where the water seeping through the dam emerges at a D/S face Common causes of piping are poor construction, differential settlements, Burrowing animals, Surface cracks, Presence of roots etc

46. 2. Piping through foundation When highly permeable cavities or fissures or strata of gravel or coarse sand are present in the dam foundation, it may lead to heavy seepage. The concentrated seepage at high rate will erode soil which will cause increase flow of water and soil. As a result, the dam will settle or sink leading to failure.

47. 3. Conduit leakage This is caused due to seepage taking place by the surface of a conduit enclosed within an embankment dam. Cracks may develop in the conduits due to the deterioration of the conduit itself when the soil mass lying below it settles and the conduit is not sufficiently strong to support the soil mass lying above

48. 4. Sloughing of downstream side of dam This phenomena take place due to the dam becoming saturated due to the presence of highly pervious layer in the body of the dam This causes the soil mass to get softened and a slide of the downstream face takes place

49. 1. Sliding due to weak foundation Due to the presence of faults and seams of weathered rocks, shales , soft clay strata, the foundation may not be able to withstand the pressure of the embankment dam. The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides causing large mud waves to form beyond the toe.

50. 2.Sliding of upstream face due to sudden drawdown If the reservoir water is suddenly depleted, say due to the need of emptying the reservoir in expectation of an incoming flood, then the pore pressure cannot get released, which causes the upstream face of the dam to slump.

51. Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and / or too steep in relation to the shear strength of the shoulder material. This causes a sliding failure of the downstream face of the dam. 3. Sliding of the downstream face due to slopes being too steep

52. Triggered by a shock or a movement, as during an earthquake, some portion of the dam or foundation may destabilize due to the phenomena called liquefaction. This causes excess pore water pressure to develop, where both the effective stress and the strength decrease. Under circumstances when the effective stress drops to zero, which means the soil loses all its shear strength, it behaves like a dense liquid and slides down, and the dam slumps. 4. Flow slides due to liquefaction

53. 5. Embankment and foundation settlement Excess settlement of the embankment and/or the foundation causes loss of free board . The settlement may be more in the deeper portion of the valley, where the embankment height is more.

54. Sufficient spillway capacity and freeboard should provided Seepage flow through the embankment is controlled The slopes of the embankment are stable under all conditions of reservoir operation, including rapid drawdown .The stresses imposed by the embankment upon the foundation are less than the strength of material in the foundation with a suitable factor of safety The upstream face is properly protected (stone pitching, riprap, revetment) against erosion caused by wave action, and the downstream face is protected (counter-booms, turfs) against the action of rain Criteria for the design of earth dams(Check List)

55. Location of the phreatic line 2. Quantity of seepage discharge (Flow net) Seepage through dam

56. Location of phreatic line Phreatic line, also called as saturation line, top flow line, seepage line, etc. is defined as the line within a dam in a vertical plane section below which the soil is saturated and there is positive hydraulic pressure. On the line itself, the hydrostatic pressure is equal to atmospheric pressure, that is, zero gauge pressure. Thus phreatic line is upper boundary of flownet.

57. The flow of the seepage water below the phreatic line can be approximated by the Laplace Equation: Where ‘φ' is the potential head, and x and z are the coordinates in the horizontal and vertical dimensions, respectively Location of phreatic line

58. Fig. Phreatic surface D-P-G for dam with horizontal drainage blanket. Determination of phreatic through a homogeneous dam with horizontal drainage blanket (filter)

59. L = Horizontal projection B-D of the upstream face length A-D Mark point C as CD = 0.3 L Taking C as center draw circle of radius CF to point E. Draw vertical tangent from E to H (E-H line is directrix ) G point midway between F and H. This is extremity of seepage line D-P-G Draw vertical line at Q (F-Q = x) With F as center, Q-H as radius R, cut PQ vertical at P. The distance P-Q = y. are the coordinates of the seepage line parabola. Draw other points similar to P. The seepage line meets at C. U/S end part of the seepage line is redrawn to meet the water surface at D at right angle .   Determination of phreatic through a homogeneous dam with horizontal drainage blanket (filter)

60. Determination of phreatic through a homogeneous dam without horizontal drainage blanket (filter) Fig. Phreatic surface B-I-K-F for dam without horizontal drainage blanket.

61. The focus (F) on the parabola, in this case, will be the lowest point of the D/S slope as shown in fig. The base parabola BIJC will cut the D/S slope at J and extend beyond the dam toe up to point C (Vertex of Parabola). Then by taking distance AF draw the arc cutting the extension of line AB .The downward extension of this point is nothing but point D . However seepage line will emerges at K meeting the downstream face tangentially there .The portion KF is known as discharge face and always remains wet. Determination of phreatic through a homogeneous dam without horizontal drainage blanket (filter)

62. Flow Net in Earthen Dam

63. The solution of the Laplace equation gives the two sets of the curves, called the equipotential lines & Flow lines (Stream lines), which are mutually orthogonal to each other. The Equipotential lines represents contours of equal potential (total head).The direction of the seepage flow is always perpendicular to equipotential lines. The path along which the seepage flow occurs is called as the flow lines or stream lines. Flow Net in Earthen Dam

64. 1.Quantity of seepage flow where ‘h’ is the total hydraulic head and N d is the number of potential drops The discharge passing through two streamlines of the field ( Δq ) is given as: Where, ‘ K’ is the coefficient of permeability and ‘ b’ is the width of one flow channel Uses of Flow Net

65. 2.Determination of Pore water pressure Pressure head at A + Datum head at A = Pressure head at B+ Datum head at B Point ‘B’ is on the phreatic line, the pressure head is Zero. Therefore, Pressure head at A= Datum head at B - Datum head at A The pressure head at A is thus equal to difference between datum head (Vertical intercepts) between A & B. The pore water pressure is indicated by difference in pressure indicated by piezometeres inserted at point A & B.  Uses of Flow Net

66. 3.Determination of the hydraulic gradient The hydraulic gradient ( i ) at any point can be determined from the flow net as follows, ΔҺ (h/ N d ) i = ------ = ---------- l l Where, Δ Һ =Loss of head between two successive equipotential lines. l= Length of flow field h= Total head N d = Total No of equipotential lines Uses of Flow Net

67. Drainage of earth Dam(Seepage Control) Seepage control through embankment Rock Toe or Toe filter Horizontal drainage blanket Chimney Drain Strip Drain Seepage control through foundations 1. Toe Drains 2. Relief Wells 3. Vertical Sand drains

68. 1.Rock Toe or Toe filter The principal function of the rock toe is to provide drainage Rock available from compulsory excavation may be used in construction of the rock toe. It also protects the lower part of the downstream slope of an earth dam from tail water erosion Generally height is kept 25 to 35 % reservoir head.

69. 2.Horizontal drainage blanket used for earth dams of moderate depth. 1/3 of the base width of the dam. In case of the zoned section it is extended up to core. provided in D/S foundation core where high upward seepage forces exist made up of pervious material

70. 3.Chimney Drain Vertical or a nearly vertical drain which is located inside the dam so that it intercepts all layers of the dam in the seepage zone Increases the stability of the D/S slope Some cases it combined and placed with the rock toe.

71. 4.Strip Drain Network of inner longitudinal and inner cross drains is preferred to inclined/vertical filters Generally adopted for small dams, where the quantity of seepage to be drained away is comparatively small If there is choking of an individual drain a significant length of the D/S face of the dam would undrained & sloughing will be there

72. 1.Toe Drains To collect seepage from the horizontal filter or inner cross drains, through the foundation as well as the rain water falling on the face of the dam Provided at outside the toe of rock toe, to facilitate visual inspection. The section of the toe drain should be adequate for carrying total seepage from the dam, the foundation and the expected rain water.

73. 2.Relief Wells

74. 2.Relief Wells They are sometimes used in with upstream impervious blankets, along with to provide additional assurance that excess hydrostatic pressures do not develop in the downstream portion of the dam Relief wells are extended deep enough into the foundation Relief wells are generally used for the drainage of the foundation if it consist of deep pervious stratum which is stratified and whose permeability increases with depth . Relief wells are provided at or near the D/S toe of the dam to collect water seeping through the foundation and reduce the pore pressure in the foundation.

75. 3.Vertical Sand drains The sand drain consists of vertical holes drilled in the foundation all along the base of the dam. These holes are filled with clean, coarse sand of high permeability to form sand columns. These sand drains reduce the path of drainage in the horizontal direction and help in the drainage of the foundation. The diameter of the drains is usually between 15 and 30 cm. The spacing of the drains depends on the several factors such as characteristics of foundation soil, depth of hard strata etc.

76. Slope stability analysis by Swedish Circle Method Where ,‘c’ = unit cohesion between soil ΔL = curved length of slice Φ = Angle of internal friction of soil Shear resisting moment is M R = r [cΔL + N.tan φ] Total Shear resisting moment over the entire arc AB is , M R = r [c∑ΔL +∑N .tan φ] Where ,∑N = Sum of all normal components ∑ΔL =Length of circle AB= Let ‘ r’ be the radius of the possible slip surface N = W cos α and T = W sin α M D = ∑T . r = r . ∑T Shearing resistance of the soil along the actual surface AB is = cΔL + N tan φ Factor of safety = Factor of safety = M R / M D

77. Graphical method of Normal Component (N) & Tangential Component (T) 4.Then all points are joined by smooth curve 5.The same procedure is adopted for drawing the T-Diagram ∑N=A N . Ύ ∑T =A T . Ύ Where, A N & A T are the areas of the N diagram & T-diagram rspt . , Ύ is unit wt.of soil . If the scale is 1 cm = X metres then A N = a n . x 2 A T = a t . x 2 Where, a n & a t are the actual measured area of plots in cm 2 Horizontal base line is drawn below the slip surface N-components of the various slices are plotted vertically above the base line Determine the areas of N-diagram & T-diagram

78. Design of Filter.. Design of filters require to prevent the mitigation of soil particles into the drains. The first layer of the filter which comes in contact with the seeping water consists of fine material. Subsequent layers of filter are made of increased coarseness. The last layer of the filter is made of gravels. The soil of the earth dam and the foundation material surrounding the filter are known as the base material.

79. Design of Filter… Guidelines to Design of filter Filters are so graded that the finer layers are adjacent to the soil in the dam and foundations and the coarser layers are adjacent to drain. The filter material should be coarse and pervious in relation to the base material The filter material should be coarser than the perforations of openings in the drain pipes, so that filter material is not lost in the drains. The D 15 size of the filter material must not be more than 4 to 5 times the D 85 of the base material. This prevents the foundation material from carrying through the pores of the filter material.

80. Design of Filter Guidelines to Design of filter D 15 of filter material = 5 to 40 D 15 of base material 6. D 85 of filter materials = 2 or more Max opening of perforations of pipes 7. The compaction for layer is same as that of other materials 8. The grain size curve of the filter material should be about parallel to the curve of the base material.