Types of Gravity Dam Forces Acting on a Gravity Dam Causes of failure of Gravity Dam Elementary Profile of Gravity Dam Practical Profile of Gravity Dam Limiting height of Gravity Dam Drainage and Inspection Galleries

1. IRRIGATION AND HYDRAULIC STRUCTURES
(GRAVITY DAM)
UNIT – II
MOOD NARESH, Assistant ProfessorPETW

2. Learning Objectives
1. Types of Gravity Dam
2. Forces Acting on a Gravity Dam
3. Causes of failure of Gravity Dam
4. Elementary Profile of Gravity Dam
5. Practical Profile of Gravity Dam
6. Limiting height of Gravity Dam
7. Drainage and Inspection Galleries

3. Types of Gravity Dam
A Gravity dam is a structure so proportioned that its own weight
resists the forces exerted upon it.
Types:
1. Masonry Dam
2. Concrete Dam
 Suitable across gorges with very steep slopes where earth
dams might slip.

4. Advantages of Gravity Dam
Advantages:
1. Strong, Stable and Durable
2. Suitable for moderately wide valleys having steep
slopes
3. Can be constructed to very great heights
4. Suitable for an overflow spillway section
5. Maintenance cost is very low
6. Does not fail suddenly

5. Disadvantages of Gravity Dam
Disadvantages:
1. Gravity dams of great height can be constructed
only on sound rock foundations.
2. Initial cost is more than earth dam
3. Takes longer time in construction
4. Require more skilled labor than earth dam
5. Subsequent raise is not possible in a gravity dam

6. F m
1. Wate
2. Weig
3. Uplift
4. Pres
5. Ice P
6. Wav
7. Silt P
8. Wind
orces acting on Da
r Pressure
ht of Dam
Pressure
sure due to earthquake
ressure
e Pressure
ressure
Pressure
1
1
1
2
3
4
6
7
1

8. 1. Water Pressure
This is the major external force acting
on dam
Pressure Components on both
upstream and downstream are:
1. Vertical Component
2. Horizontal Component
Unit weight of water, γw=1000 kg/m3

9. 2. Weight of Dam
This is the major resisting force
 Generally unit length of dam is considered
 The cross section of dam may be divided
into several triangles and rectangles and
weights W1, W2, W3 etc., may be
computed
 The total weight W of the dam acts at the
C.G. of its section.
Weight = Volume per unit length x Density of material

10. 3. Uplift Pressure
The uplift pressure is defined as the
upward pressure of water as it flows or
seeps through the body of the dam or its
foundation.
Uplift Pressure
(No Gallery)

11. 4. Pressure due to Earthquake
 Earthquake waves imparts accelerations to the
foundations under the dam and causes its movement
 This earthquake wave may travel in any direction
 For design purpose, Horizontal and Vertical
directions are considered.
 Seismic Force = Mass x Earthquake Acceleration
 According to IS 1893-2002, India was divided into
Four zones: zone II, III, VI, and V.

12. Earthquake Acceleration
 Earthquake Acceleration is usually
designated as fraction of the acceleration
due to gravity
 It is expressed as α.g where α is known as
Seismic Coefficient

13. Seismic Coefficient
Seismic coefficient is divided into
1. Horizontal Seismic coefficient, αh
2. Vertical Seismic Coefficient, αv = .75 αh
αh can be determined by one of the two methods
1. Seismic Coefficient Method < 100m height of the dam
2. Response Spectrum Method > 100m height of the dam

14. Seismic Coefficient Method
As per IS: 1893-1984,
Horizontal Seismic coefficient, αh = 2α0
Where α0 = Basic Seismic Coefficient
Basic Seismic Coefficient as per IS 1893:1984
Seismic Zone II III IV V
Basic Seismic Coefficient 0.02 0.04 0.05 0.08

15. Response Spectrum Method
As per IS: 1893-1984,
Horizontal Seismic coefficient, αh = 2 F0 (Sa/g)
Where F0 = Seismic Zone Factor
Seismic Zone Factor as per IS 1893:1984
Seismic Zone II III IV V
Seismic Zone Factor, F0 0.10 0.20 0.25 0.40
IS: 1893-1984 recommends a damping of 5% for dams
*Damping is an influence within or upon an oscillatory system that
has the effect of reducing, restricting or preventing its oscillations.

16. Effect of Earthquake Acceleration
Inertia Force = Force x EarthquakeAcceleration
= (W/g) (αh g) = W.αh
2. Hydrodynamic Pressure of water
Pey = Cy αh w h  Equation 8.16 (Page 370)
where Cy is a dimensionless pressurecoefficient
Effect of Horizontal Earthquake Acceleration, αh g
1. Inertia Force in the body of the dam
The Inertia Force is the product of mass and acceleration and this
force acts in the direction opposite to that of the ground motion.
If Reservoir is Full  Inertia Force acts in downstream direction
If Reservoir is Empty  Inertia Force acts in upstream direction

17. Effect of Earthquake Acceleration…
Effect of Horizontal Earthquake Acceleration, αh g
a) Horizontal Shear, Peh = 0.726 Pey h
b) Moment, Meh = 0.299 Pey h2
c) Vertical Component of Shear,
Wh = (Pe2 - pe1) tan φ
where φ = actual slope of the u/s face
Pe2 = Horizontal shear at elevation of the section beingconsidered
Pe1 = Horizontal shear at elevation at which the slope of the dam facecommences

18. Effect of Vertical EarthquakeAcceleration
1. Due to Vertical Earthquake Acceleration, αvg,
the dam as well as reservoir water are
accelerated vertically upwards or downwards.
2. An acceleration upwards increases the weighs
and an acceleration downwards decreases the
weighs.
3. Altered weighs = w(1+ αv) or w(1- αv)

19. 5. Ice Pressure
1. The ice formed on water surface of the
reservoir is subjected to expansion and
contraction due to temperature variations
2. Coefficient of thermal expansion of ice is 5
times more than concrete
3. The dam face has to resist the force due to
expansion of ice

20. 5. Ice Pressure
4. This force acts linearly along the length of the
dam, at reservoir level
5. IS: 6512-1984 recommends 250 kN/m2 applied
to the face of dam over the anticipated area of
contact of ice with the face of the dam.

21. 6. Wave Pressure
Waves are generated on the reservoir surface
because of wind blowing over it.
1
hw
 0.0322 VF  0.763 0.271F4 for F 32  32 km
hw
 0.0322 VF for F 32  32 km
Where hw= height of wave in m, V = wind velocity in KMPH,
F = Fetch or straight length of water expanse in Km
Wave Pressure, Pw = 2 w hw
2
and it acts at a distance of 3hw/8 above
the reservoir surface

22. 7. Silt Pressure
 The river brings silt and debris along with it.
 The dam is, therefore, subjected to silt pressure,
Ps, in addition to water pressure
Where γ’ = submerged unit weight ofsilt
h = height of silt deposit
Φ = Angle of internal friction
According to IS : 6512-1972, the silt pressure and
water pressure exist together in submerged silt.
The following are recommended for calculating
forces:
Psh = 1360 Kg/m3
Psv = 1925Kg/m3

23. 8. Wind Pressure
 It is a minor force acting on dam
 Acts on Superstructure of the dam
 Normally, wind pressure is taken as 1 to 1.5
kN/m2

24. Combination of Loading for Design
2. Extreme Load Combination
3. Reservoir Empty Conditionpressure and normal uplift
 Normal water surface elevation, earthquake force, silt
pressure and normal uplift
USBR Recommendations:
1. Normal Load Combination
 Normal water surface elevation, ice pressure, silt
 Maximum flood water elevation, silt pressure and
extreme uplift with no drain in operation to release the
lift
 Maximum water surface level, silt pressure and
normal uplift

25. Combination of Loading for Design
t
IS R
Gravi
adver
 Co
 No
 Fl
 Ot
ecommendations: (IS: 6512-1984)
ty dam design shall be based on the mos
se conditions A, B, C, D, E, F, and G
nstruction Condition (A)
rmal Operating Condition (B)
ood Discharge Condition (C)
her Load Combinations such as D,E,F and G

26. Causes of Failure of Dam
1. Overturning
2. Sliding
3. Compression or Crushing
4. Tension

30. Overturning Failure
The overturning of the dam section takes place
when the resultant force at any section cuts the
base of the dam downstream of the toe.
Factor of Safety (F.S.) should not be less than 1.5

31. Sliding Failure
A dam will fail in sliding at its base, or at any other
level, if the horizontal forces causing sliding is more
than the resistance available to it that level
For Low Gravity Dams, Factor of Safety against Sliding (F.F.S.) should
be greater than 1
Coefficient of friction μ varies from 0.65 to 0.75

32. Sliding Failure
For Large Gravity Dams, Shear Strength of joint
should also be al design.considered for economic
Factor,
th of joint varies from 1300 to 4500 kN/m2,
Shear Friction
Where,
c = Shear streng
b = Width of the joint

33. Compression or Crushing
t
ss
Th
the
gre
for
e maximum compressive stress occurs a
toe and for safety, this should not be
ater than the allowable compressive stre
the foundation material.

35. Tension Failure
If eccentricity e >b/6, then Tension will be
developed at the heel of the dam.
Since concrete can not resist Tension, No
tension is permitted at any point of the
dam under any circumstances

36. Principal Stresses
Where
Compre
Intensity
p may c
p = p-pe
ssure
ream
ssive Stress,
of water Pressure, p = γ h
hange depends upon Earthquake Pre
for downstream and p = p+pe for upst
Intensity of Wate
normal to the fa
r Pressure
ce of dam
Uplift Pressure
Principal Stress

38. Elementary Profile of Gravity Dam
1. No any other force except
forces due to water such as
weight of dam, water pressure
and uplift Pressure
2. Same shape as hydrostatic
pressure distribution diagram
3. e = b/6
If Reservoir is empty, then c= 0

40. Practical Profile of a Gravity Dam
1. Practical profile has a provision of
roadway at top, addition loads due to
roadway and free board.
2. Resultant force of the weight of dam
and water pressure falls outside the
middle third of the base of the dam
and causes tension at upstream when
the reservoir is full
3. To eliminate tension, some masonry is
to be provided to the upstream.
Free Board:
Free board is the margin provided between top of dam
and HFL in the reservoir to prevent the splashing of
the waves over the non-overflow dam.
Fee Board = 3/2 hw
Where hw is wave pressure
Modern Practice is to provide a maximum Free Board
equal to 3 to 4% of the height of dam and it should
not be less than 1m in any case
Top Width = 14% of height of water level

41. Limiting Height of Gravity Dam
If the height of dam is greater than H,
then it is known as High Gravity Dam

42. Drainage and Inspection Galleries
1. A gallery i
2. It runs in l n a slope
Purpose:
constantly
e behavior of
of outlet
1. To drained
through th
2. To provide
structure
3. To provide
gates and
s a formed opening left in a dam
ongitudinal direction horizontally or o
off the seepage water which occurs
e upstream face of the dam
access to observe and measure th
an access needed for the operation
spillway gates

44. Galleries are openings or passageways left in the dam
body. They may be provided parallel or normal to dam axis
at various elevations The galleries are interconnected by
steeply sloping passages or by vertical shafts fitted with
lifts. The shape and size of the gallery depends on the size
of the damned and the function served.
The functions for which the galleries are provided are:
1. Drainage: To cater for the drainage of dam section by
intercepting seepage from the water face and carry it
away from the downstream face.
2. Inspection: To provide access to the interior of the mass
comprising the dam with a view to inspect the structure
and study the structural behaviour of the dam in post-
construction period.

46. Previous Questions
1. What is a Gravity Dam? Write down the profile of a Gravity Dam?
2. Explain the merits and demerits of Gravity Dam?
3. List out the various forces acting on a Gravity Dam?
4. Explain overturning and sliding of a Gravity Dam?
5. What is the elementary profile of a Gravity Dam and how it is deduced? What
should be the maximum depth of a elementary profile of a gravity dam, if safe
limit of stress on the Masonry should not exceed 1500 KN/m2?
6. State the general conditions of stability of gravity dams. Explain step wise
procedure of analyzing High Gravity Dams?
7. What is a Gallery in a Dam? List out the various purposes for which a gallery is
formed in the dams?