This document discusses arch dams and buttress dams. It describes the key components and design considerations for each type of dam. For arch dams, the main points are that they function as curved beams to transfer water loads to the canyon walls, reducing required thickness compared to gravity dams. Types include constant radius, variable radius, and constant angle arch dams. Forces acting on arch dams include water pressure, uplift, ice pressure, temperature changes, and potential yielding of abutments. Buttress dams consist of a thin deck supported by triangular buttresses to transmit loads to foundations. Types are rigid, deck slab, and bulkhead buttress dams. They offer concrete savings compared to gravity dams but require more reinforcement.

1. Arch and Buttress Dams
Unit-III
BTCI09007

2. Syllabus
Unit-III
Arch and buttress dams:
• Types of arch dams, forces on an arch
dam, design. Types of buttress dams.

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. 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. Arch Dam

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. Arch Dam

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. Double Curvature Arch Dam

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. 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. 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. Constant Radius Arch Dam

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. 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. Variable Radius Arch Dam

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. 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. Constant Angle Arch Dam

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. Forces Acting on Arch Dam

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. 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. Forces Acting on Arch Dam

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. 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. Forces Acting on Arch Dam

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. Buttress Dams

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. 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. Buttress Dams

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. The Essential Components of
Buttress Dam

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. The Essential Components of
Buttress Dam

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. Buttress Dams

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. Buttress Dams

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. Types of Buttress Dams
• Rigid Buttress Dams
• Deck Slab Buttress Dams
• Bulk head buttress Dams

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. Rigid Buttress Dams

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. Multi Arch Dams

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. Deck Slab Buttress Dams

49. Deck Slab Buttress Dams

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. 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. 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. 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. Bulk Head Buttress Dam

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. 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. 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. Thanks…