Arch and Buttress Dams


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Arch and Buttress Dams

  1. 1. Arch and Buttress Dams Unit-III BTCI09007
  2. 2. Syllabus Unit-III Arch and buttress dams: • Types of arch dams, forces on an arch dam, design. Types of buttress dams.
  3. 3. Arch Dam • An Arch Dam is just a Curved Beam, the ends of which are restrained and the way in which the loads are resisted is termed a arch action. • An arch dam may be defined as a solid wall, curved in plan, standing across the entire width of the river valley, in a single span. This dam body is usually made of cement concrete, although rubble and stone masonry has also been used in the past.
  4. 4. Arch Dam • This wall will structurally behave ; partly as a cantilever retaining wall standing up from its base, and partly, the load will be transferred to the ends of the arch span by horizontal arch action. • The arch load will, thus be transferred to the side walls of the canyon, which must be strong, stable and rocky.
  5. 5. Arch Dam
  6. 6. Arch Dam • The distribution of part of the load to the side walls of the canyon, reduces the load on the cantilever wall, thereby reducing the thickness, as compared to that in an ordinary gravity dam; and that is the only benefit we derive from an arch dam in comparison to a gravity dam. • Evidently, the grater is the wall curvature (in Plan), the greater will be the load that will be transferred to the sides of the canyon, and hence greater will be the economy on the dam thickness.
  7. 7. Arch Dam
  8. 8. Arch Dam • This economy in dam thickness can be further increased considerably by making the dam body not only curved in plan, but also curved in section, Such a non vertical dam is known as double curvature arch dam or a shell-arch dam.
  9. 9. Double Curvature Arch Dam
  10. 10. Types of Arch Dams • Simple arch dams, which transfer a larger part of their loading by cantilever action may also be of different types, since their face may be either vertical or curvilinear • Depending upon the shape, the simple arch dam can be divided into three types • (i) Constant Radius Arch Dam • (ii) Variable Radius Arch Dam • (iii) Constant Angle Arch Dam
  11. 11. Types of Arch Dams • A Constant radius arch dam is the simplest in design as well as construction, but uses the maximum concrete. A constant angle arch dam on the other hand, uses about 43 % of the concrete used by a constant radius arch dam. The variable radius arch dam is an intermediate choice, using 58 % of the concrete used by constant radius arch dam.
  12. 12. Constant Radius Arch Dam • A constant radius arch dam is an arch dam, the arch centre of which for the upstream face, the downstream face and the centre line are all coincident with the axis centre at all elevations. In simple types the upstream face is vertical so that its radius at all elevations is constant but thickness is proportional to the height.
  13. 13. Constant Radius Arch Dam
  14. 14. Constant Radius Arch Dam • This type of dam is triangular in crosssection with increased thickness at the crest. It is particularly adapted to U-shaped canyon where cantilever action carries a relatively large proportion of water load at lower elevations. • However it is less economic as compared to constant arch dam.
  15. 15. Variable Radius Arch Dam • Also termed as variable centre arch dam. In this type of dam, both radius and centre angle are not constant. It is a compromise between the constant radius and the constant angle arch dam. The characteristic of this type of dam is variable radius. • The radii gradually decrease as the depth below the crest increases. • Central angle is different for different arches and usually varies from 180 0 to 150 0.
  16. 16. Variable Radius Arch Dam
  17. 17. Variable Radius Arch Dam • The upstream face is generally vertical or even overhanging at the upstream side near the abutment and that at the downstream side near the crown. Advantages • • • • Greater arch efficiency Saving in concrete over 50 % Useful to meet foundation requirements, and Preferred than constant radius arch dam
  18. 18. Constant Angle Arch Dam • It is type of arch dam in which all the arch rings have a constant angle. It is thus an improvement on variable radius type arch dam in that the central angle of the horizontal arch rings at all elevation is constant. • However, it is economical, 30 to 40 % saving in concrete compared to constant radius arch dam.
  19. 19. Constant Angle Arch Dam
  20. 20. Forces Acting on Arch Dam • Various forces acting on an arch dam are the same as for gravity dam but their relative importance is different in arch dams • The temperature changes are very important as they cause deflection and stresses in arch dams. Reservoir water load • It is the principal live load acting on the dam. Most of the water load is carried horizontally to the abutments by arch action depending on the amount of the curvature of the arch. The balance water load is transferred to the foundation by cantilever action.
  21. 21. Forces Acting on Arch Dam
  22. 22. Forces Acting on Arch Dam Uplift Pressure • Unlike gravity dams, uplift pressure in arch dams is not so important because arch dam are very thin in section. However, if cracks occur, uplift in cracks result in increase in downstream deflection, changes in load distribution accompanied by increase in maximum compressive stresses in the arch and cantilever elements.
  23. 23. Forces Acting on Arch Dam Ice Pressure • Ice Pressure cause heavy concentration loading on the top arch, more precisely along the arch element at the level of ice. The internal stresses caused by ice pressure are important in arch dams.
  24. 24. Forces Acting on Arch Dam
  25. 25. Forces Acting on Arch Dam Temperature Changes • Internal Stresses caused by temperature changes are important, their effect being to • (i) move the dam upstream during summer • (ii) to move the dam downstream during winter, thereby augmenting reservoir loads • In stress analysis, the latter condition i.e. moving the dam downstream during winter is very important as it acts in the direction in which the reservoir water load acts.
  26. 26. Forces Acting on Arch Dam Yielding of Abutment • In order that the arch section is more effective, the end of the arch shall not yield. It follows that the rock of the canyon constituting the abutments of the arch shall not yield under any force and movement developed at that section. Slight yielding of abutment cause internal stresses in the arch.
  27. 27. Forces Acting on Arch Dam
  28. 28. Buttress Dams • In a gravity dam the quantity of concrete is determined by the dead weight required for the stability of the dam, rather than by the strength of the concrete. Thus in gravity dams there are lower stresses in the dam body in central part of the dam and the high strength concrete there is not stressed as it could be, and hence lesser efficient use of concrete strength, large uplift force on dam base and hence increased dam section for stability.
  29. 29. Buttress Dams
  30. 30. Buttress Dams • A buttress dam envisages to reduce the total volume of concrete by constricting the material where the stresses are higher and other undesirable features of gravity dam but has higher cement content per cubic metre of concrete than a gravity dam.
  31. 31. Buttress Dams • A buttress dam is defined as a dam consisting of a relatively thin water supporting facing or deck supported by buttress generally in the form of equally spaced triangular walls or counter forts that transmit the water load and deck weight to the foundations.
  32. 32. Buttress Dams
  33. 33. The Essential Components of Buttress Dam • Deck: The upstream sloping membrane or deck which supports the water load and transmit it to a series of buttress or counter forts. • Buttress The buttress or counter forts are provided at the certain intervals at right angle to the axis of the dam to support the deck and for transference of the load to the foundation.
  34. 34. The Essential Components of Buttress Dam
  35. 35. The Essential Components of Buttress Dam Lateral Braces • A system of cross bracing between buttress is provided to effect lateral stiffness and resistance to buckling of counter forts. Foundation Mat • foundation provides distribution of the pressure onto the soil.
  36. 36. The Essential Components of Buttress Dam
  37. 37. Buttress Dams Advantages • Thin section with one-third to one-half economy in concrete compared to gravity dam of similar height • No problem of uplift or foundation drainage • Can be constructed on weak foundations, • Vertical component of water resting against upstream sloping deck add to the stability of the dam.
  38. 38. Buttress Dams
  39. 39. Buttress Dams Disadvantages • The thinner section is more vulnerable due to deterioration of concrete • Susceptible to damage or destruction by sabotage during war • Saving in concrete is largely offset by more reinforcement required • Higher cost of form work.
  40. 40. Buttress Dams
  41. 41. Forces Acting on Buttress Dam • Forces acting on buttress dam are similar to those acting on gravity dam and arch dam except that • Downward component of water pressure is greater • Uplift pressure is less due to lateral escape of the uplift pressure under the buttress • Ice pressure is no important as ice tend to slide up the sloping deck. • Wind pressure is important if a diagonal wind of higher velocity can be reach the downstream side. • Because of small mass relative to rigidity of individual units, slab and round head buttress dams are efficient in resisting earthquake forces
  42. 42. Types of Buttress Dams • Rigid Buttress Dams • Deck Slab Buttress Dams • Bulk head buttress Dams
  43. 43. Rigid Buttress Dams Rigid Buttress Dams • Rigid buttress dams have rigidity of construction. The upstream sloping deck is made of monolithic with the buttresses. Rigid buttress dams are suitable as high dams. The shapes of buttress and layout of arches can be made to get best advantage of the foundation • Two types of rigid buttress dams are • (i) Multi Arch Dams • (ii) Multiple dome buttress dam
  44. 44. Rigid Buttress Dams
  45. 45. Multi Arch Dams Multi Arch Dams • It comprises a series of inclined arches supported by buttresses. The water load is transferred from arches to the foundation through the buttresses. The arch may be of uniform thickness or of variable thickness. The main consideration are slope of upstream face, central angle of each arch and buttress spacing.
  46. 46. Multi Arch Dams
  47. 47. Deck Slab Buttress Dams • Deck Slab buttress dam is made of a flat reinforced concrete slab spanning across a series of parallel buttresses which rest directly on the foundation or upon a concrete slab resting on the foundation material. • The water pressure is transmitted by the slab to the buttresses. The main layout considerations are spacing of the buttresses and shape of buttress head to support the slab. • The buttresses spacing, depending on the height of dam, varies from 5 to 15 m. The upstream slope of the slab may vary between 35 0 to 45 0 with the horizontal.
  48. 48. Deck Slab Buttress Dams
  49. 49. Deck Slab Buttress Dams
  50. 50. Deck Slab Buttress Dams Fixed Deck Slab Buttresses Dam • In fixed or continuous deck slab buttress dam, the deck slab is cast monolithically with the buttresses. With the provision of expansion joints at convenient place.
  51. 51. Deck Slab Buttress Dams Simple Deck Slab Buttress Dam • Also termed as free deck type or Amburson dam. The reinforcement is placed at the downstream face of the deck slab and as such is susceptible to rusting. This type of dam is suitable for wide valley with poor foundation.
  52. 52. Deck Slab Buttress Dams Cantilever Deck Buttress Dam • In this type of dam, as the name implies, the slab is laid monolithic with the buttress but cantilevered at both ends, with proper joints at both ends or at the centre. The water load is transferred to the supporting buttress by cantilever action of the deck and reinforcement is provided at upstream face in view of cantilever action. It is subjected to high stresses due to temperature, shrinkage and foundation settlement.
  53. 53. Bulk Head Buttress Dam Bulk Head Buttress Dam • It is a type of buttress dam in which the face slab is replaced by flaring the upstream edge of the buttress with massive head forming the water supporting member. The massive head and buttresses, through independent structural units, behave as one monolithic unit for stability of the dam section.
  54. 54. Bulk Head Buttress Dam
  55. 55. Selection of Type of Buttress Dam The selection of various types of buttress dam Foundation Condition • For good foundation any type of buttress dam is suitable. Requirement of Spillway • Spillway can be conveniently provided at the top and outlet at the base of the buttress dam. • Other Consideration are economy governed by the availability of construction material, Skilled labour.
  56. 56. Selection of Type of Buttress Dam Foundation Rock Suitable Type of a Dam Soft Foundation Any buttress type of dam with widespread footing. Permeable Foundation Slab and buttress dam with bottom slab under buttresses. Percolation and uplift are considered. Unequal Settlement may occur especially Slab and buttress dam with buttresses strutted. Jointed or faulted Rock Slab and buttresses dam Good foundation no unequal settlement Multiple arch dam Variety of foundation Massive head buttress dams.
  57. 57. References • Modi P.N. (2011), “Irrigation water resources and water power engineering”, Standard Book House • Garg S.K. (2010), “Irrigation Engineering and Hydraulic Structures”, Khanna Publishers • Internet Websites
  58. 58. Thanks…