The document discusses the design of gravity dams. It begins with basic definitions related to gravity dam geometry and forces that act on gravity dams, such as water pressure, weight of the dam, uplift pressure, and pressure due to earthquakes. It then covers stability analyses to prevent overturning, sliding, crushing, and tension. Finally, it addresses designing the dam section to be economical while satisfying stability requirements, and categorizing dams as low or high based on height.
3. A gravity dam is a solid structure, made of concrete or masonry,
constructed across a river to create a reservoir on its upstream.
The section of the gravity dam is approximately triangular in
shape, with its apex at its top and maximum width at bottom.
The section is so proportioned that it resists the various forces
acting on it by its own weight.
Where good foundations are available, gravity dams can be
built upto any height. It is the most permanent one, and
requires little maintenance.
The most ancient gravity dam on record was built in Egypt more
than 400 years B.C. of uncemented masonry.
INTRODUCTION
5. 1. Axis of the dam: It is the line of the upstream edge of the top
(or crown) of the dam. The axis of the dam in plan is also
called the base line of the dam. The axis of the dam in plan is
usually straight.
2. Length of the dam: It is the distance from one abutment to
the other, measured along the axis of the dam at the level
of the top of the dam.
3. Structural height of the dam: It is the difference in elevations
of the top of the dam and the lowest point in the excavated
foundation. It, however, does not include the depth of special
geological features of foundations such as narrow fault zones
below the foundation. In general, the height of the dam
means its structural height.
BASIC DEFINITIONS
6. 4. Toe and Heel : The toe of the dam is the downstream edge
of the base, and the heel is the upstream edge of the
base.
5. Maximum base width of the dam: It
is the maximum horizontal distance between the heel and
the toe of the maximum section of the dam in the middle
of the valley.
6. Hydraulic height of the dam: It is equal to the difference
in elevations of the highest controlled water surface on
the upstream of the dam (i. e. FRL) and the lowest point
in the riverbed.
BASIC DEFINITIONS
7.
8. FORCES ACTING ON GRAVITY DAM
• Water pressure
• Weight of the dam
• Uplift pressure
• Silt pressure
• Wave pressure
• Ice pressure
• Pressure due to earthquake forces
9. WATER PRESSURE
• It is the major external force acting on a dam.
• The intensity of the pressure varies
triangularly, with a zero intensity at the water
surface, to a value “wh” at depth h below the
water surface.
• Force due to water pressure
P = W H / 2
• This acts at a height of h/3 from base of the
dam.
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11. WEIGHT OF THE DAM
• Weight of the dam is the major resisting force.
• Unit length of the dam is consider.
• Total weight of the dam acts at the center of
gravity of this section.
13. UPLIFT PRESSURE
• Uplift pressure is the upward pressure exerted
by water as it seeps through the body of the
dam or its foundation.
• Seeping water exerts pressure on the base of
the dam and it depends upon water head.
15. SILT PRESSURE
• Silt gets deposited against the upstream face
of the dam.
• If h is the height of the silt deposited, then the
force exerted by this silt in addition to external
water pressure, can be
Psilt = γsub .h2 . Ka / 2
• It acts at h/3 from base.
16. WAVE PRESSURE
• Waves are generated on the surface of the
reservoir by the blowing winds, which cause
pressure towards the downstream side.
• Waves pressure depends upon the wave height.
Hw =
Pw = 2.4 γw . Hw
• It acts at hw/2 above the still water surface.
17. ICE PRESSURE
• The ice may be formed on the water surface
of the reservoir in cold countries, may
sometimes melt and expand.
• The dam face has to resist the thrust exerted
by the expending ice.
• The magnitude of this force varies from 250 to
1500 kN/m depending upon the temperature
variations.
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18. EARTHQUAKE FORCES
• If the dam is to be designed, is to be located in
a region which is susceptible to earthquakes,
allowance must be made for stresses
generated by the earthquakes.
• An earthquake produces waves which are
capable of shaking the earth upon which the
dam is resting , in every possible direction.
28. STABILITY ANALYSIS
1) OVERTURNING
– If the resultant of all the force acting on a dam at any
of the section, passes outside the toe, the dam shall
rotate and overturn about the toe.
• Its value generally varies between 2 to 3.
29. STABILITY ANALYSIS
2) SLIDING
– A dam may fail in sliding at its base.
– Sliding will occur when the net horizontal force
exceeds the frictional resistance developed at that
level.
Where µ = coefficient of static earth pressure
= 0.65 to 0.75
30. STABILITY ANALYSIS
3) COMPRESSION OR CRUSHING
– A dam may fail by the failure of its materials.
– The compressive stress may exceed the allowable stress
and the dam material may get crushed.
4) TENSION
– Masonry and concrete gravity dam are usually designed in
such a way that no tension is developed anywhere,
because the materials can not withstand sustained tensile
stresses.
– If it subjected to such stresses, these materials may crack.
31. DESIGN OF GRAVITY DAMS
• The section of gravity dam should be chosen in such a way
that it is the most economical section and satisfies all the
conditions and requirements of stability. Hence, after the
section of dam has been arrived at, the stability analysis for
the dam must be carried out.
• TO DECIDE WHETHER THE DAM IS LOW OR HIGH- First of all,
the height of the dam to be constructed, should be checked
so as to ensure whether it is a low gravity dam or a high
gravity dam.
• If the ht. of the dam is less than that given by
𝑓
γw(𝑆𝑐+1)
(where 𝑓 is the permissible compressive stress of the dam
material and Sc is the Sp. Gravity of the dam material) then
the dam will be a low gravity dam otherwise vice versa.
