2. Components of a Gravity
Dam and their Functions
Drainage galleries
Shafts
Overflow section
Non-overflow section
Power house
Energy dissipation works
Outlets
Joints
1/28/2014
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3. Drainage galleries :
A gallery is a small passage in a dam for
providing an access to the interior of the
dam.
The gallery is usually rectangular in shape
with its top and bottom either flat or semi
circular.
For a gallery with its top and bottom flat,
it is necessary that all the corners should
be rounded. The width of gallery generally
varies from 1.5 to 1.8 m. The height of the
gallery in between 2.2 to 2.4 m, so that a
person can easily walk inside it.
1/28/2014
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5. Types of gallery based on
uses :
A Foundation gallery
b. Drainage gallery
c. Inspection gallery
d. Grouting gallery
e. Gate gallery
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6. Functions of a gallery :
1. To provide drainage of the dam
section.
2. To provide space for equipment
required for drilling holes and
grouting the hole to form a grout
curtain in the foundation.
3. To provide space for header and
return pipes for post cooling of
concrete.
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7. 4. A gallery provide an access to the
interior of the dam for inspection
ard maintenance.
5. A Gallery also provides space for
installing various instruments in
the dam to study its structural
behaviour.
6. A gallery can provide space for
the mechanical and electrical
equipment required for the
operation of gates for spillways and
outlets.
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8. 2. Shafts :
A shaft is a vertical opening
provided in a dam. Shafts are
required for locating headers of the
post cooling system and for locating
measuring devices.
Shafts are also required for the
movement of elevators and the
hoisting equipment. Sometimes
shafts are constructed inclined to
connect two galleries or the same
gallery at two different elevations
by a staircase or a lift arrangement.
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9. A plumb line shaft is constructed at
the maximum section of the dam to
make observations of the deflection
of the dam under loads.
A plumb bob is suspended by a wire
fixed at the top of the shaft. As the
dam deflects relative to the base,
the plumb bob also moves by the
same amount.
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10. A stilling well shaft is a special
shaft used to record fluctuations of
the water level in the reservoir. The
shaft is connected to the reservoir
at a point below the minimum
reservoir level.
There is a floating mechanism in
the stilling well shaft which records
fluctuations in the water level.
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11. 3. overflow Section :
The spillway in a gravity dam is called
overflow section. Spillway is provided
to dispose of surpluswater from the
reservoir to the downstream.
Spillways are provided for all dams as
a safety measure against overtopping
and the consequent damages, and
failure. spillway may be located either
in the middle of the dam or at the end
of the dam near abutment.
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12. Requirements of Spillway :
It must have adequate discharge
capacity.
It must be hydraulically and
structurally safe.
The surface of the spillway must be
erosion resistant.
It should be provided with some
device for the dissipation of excess
energy.
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13. Non-overflow section :
The portion of the gravity dam
other than the spillway is a non-
overflow section, a road is located
on the non-overflow section of the
dam.
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14. Power house :
At the one end of a gravity dam a
power house is located. Water from
the reservoir passes tnrough
penstock and rotates the turbine
provided at power elevations to
produce electricity.
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15. Energy dissipation works :
Water flowing over a spillway has a
very high kinetic energy because of
the conversion of the entire potential
energy to the kinetic energy. If the
water flowing with such a high
velocity is discharged directly into the
channel down stream, serious scour of
the channel may be occur. If the scour
is not properly controlled it may
extend backward and may damage the
spillway and the dam.
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16. The energy dissipation devices can be
broadly classified into two types.
Devices using a hydraulic jump
Devices using a bucket
Following constructions are provided
for energy dissipation :
11 Deflector bucket
1) Sharply upturned bucket
5 End sill with baffles
- Stilling basin,
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17. 7. Outlets :
A major portion of the storage volume
in the reservoir on the upstream of a
dam is below the spillway crest level.
Dam outlets are provided in the body
of the dam or its abutments below the
crest level of the spillways so that the
water can be withdrawn from the
reservoir.
Sluiceways are the special type of
outlets provides in the body of a
concrete gravity dam
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18. Classification of outlets
based on purpose :
River outlet - discharge water into the
river
Canal outlet - discharge water into the
canal.
Pipe outlet - delivers water into a
closed pipe.
Classification of outlet based on gates :
• Gated outlets
• Ungated outlets
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20. (a) Construction Joints
Construction joints are providedfor ease in
construction of the dam. They also permits
systematic, convenient and economical
construction of the dam. It is usual
practice to place the concrete in a gravity
dam in lifts so as to avoid excessive heat of
hydration of cement.
These are the horizontal joints. The
thickness of each layer of concrete should
not be more than 1.5 m. The thickness of
the first layer of concrete above rock
foundation should not be more than 0.75
m.
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21. (b) Contraction joints :
Contraction joints are provided to
avoid cracks caused due to
shrinkage of concrete due to
temperature changes.
Contraction joints are of two types :
(i) Transverse joints
(ii) Longitudinal joints
1/28/2014
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