3. GRAVITY DAM
It is a masonry or concrete dam
which resists the forces acting on it
by its own weight.
These dams are heavy and massive
wall-like structures of concrete in
which the whole weight acts
vertically downwards. Its c/s is
approximately triangular in shape.
As the entire load is transmitted on
the small area of foundation, such
dams are constructed where rocks
are competent and stable.
4. Gravity dams are rigid concrete dams which ensure
stability against all loads by virtue of their weight alone.
They transfer all the loads to the foundation and hence
are built when the foundation is strong rock. A typical
section of a gravity dam is shown.
13. ADVANTAGES OF GRAVITY DAM
• Strong, Stable and Durable
• Suitable for moderately wide valleys having steep
slope
• Loss of water by seepage in gravity dams is less.
• Can be constructed to very great heights
• Suitable for an overflow spillway section
• Maintenance cost is very low
• Does not fail suddenly
14. Gravity dams of great height can be constructed only on sound
rock foundations
Initial cost is more than earth dam
Takes longer time in construction
Require more skilled labor than earth dam
Subsequent raise is not possible in a gravity dam
Design of gravity dams is very complicated
DISADVANTAGES OF GRAVITY DAM
15. Forces Acting On Gravity
Dam
Water Pressure
Weight of the Dam
Uplift Pressure
Silt Pressure
Wave Pressure
Wind pressure
Ice pressure
Earthquake Pressure
16. Water Pressure is the most major external force acting on a
gravity dam.
On upstream face pressure exerted by water is stored upto the ful
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
Water Pressure
17. Weight of the Dam
The self weight of the gravity dam is the main stabilising
force which counter balances all the external forces
acting on it.
18. Uplift Pressure
Uplift pressure is the
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.
19. Silt Pressure
Sediment deposition in the reservoir results in a force
acting horizontally on the upstream face. This force is
assumed to have a hydrostatic distribution.
20. Wave Pressure
Waves are generated on the surface of the reservoir by
the blowing winds, which exert a pressure on the
upstream side. Wave pressure depends upon wave
height
21. Wind pressure
• The top exposed portion on the dam is small & hence
the wind pressure on this portion of dam is negligible.
22. The ice which may be formed on the water surface of
the reservoir in cold countries may sometimes melt and
expand. The dam face is subjected to the thrust exerted
by the expanding ice.
Ice pressure
23. Dams are subjected to vibration during earthquakes.
Vibration affects both the body of the dam as well as the water
in the reservoir behind the dam.
The most danger effect occurs when the vibration is
perpendicular to the face of the dam.
Earthquake Pressure
24. Sliding – It may take place on a horizontal joint above formation, on
the foundation. Sliding takes place when total horizontal forces are
greater than the combined shearing resistance of the joint and the
static friction induced by total vertical forces.
Causes of gravity dams Failure
25. Overturning – A dam fails in overturning when total horizontal
forces acting on the dam section are quite great in comparison with
total vertical forces. In such cases the resultant of two passes
outside the limits of the dam.
Causes of gravity dams Failure
26. Dam may fail when tension is produced in the concrete.
Dam may fail in crushing due to compression.
Causes of gravity dams Failure
27. To prevent overturning, the resultant of all forces acting on
the dam should remain within the middle-third of the base
width of the dam.
In the dam, the sliding should be fully resisted when the
condition for no sliding exists in the dam section.
In the dam section, the compressive stresses of concrete or
masonry should not exceed the permissible working stress
to avoid failure due to crushing.
There should be no tension in the dam section to avoid the
formation of cracks.
The factor of safety should be maintained between 4 to 5
Precaution against dams Failure
