Classification of Dams
The classification of a dam is based upon certain
parameters like construction material, purpose etc. The following table illustrates the classification. It should be noted that any kind of dam
(material-wise) could serve any of the purposes:
|
|
Parameters |
Types |
Examples |
|
DAM |
Construction Material |
Masonry/concrete/gravity |
|
|
Earth fill |
|||
|
Buttress |
Stony Gorge, USA |
||
|
Steel |
Ash Fork, USA |
||
|
Combined |
|||
|
Purpose |
Irrigation |
||
|
Hydro power |
|||
|
Flood Control/ diversion |
|||
|
Water Supply |
Nearly All |
||
|
Combined |
Hirakund |
||
|
Shape |
Straight |
||
|
Arch |
|||
|
Combined |
Trend in
Material of Dams The graph shows that the older concept of building more masonry dams
has gradually changed. And in order to
attain homogeneity between the dam material and foundation, earth dams have
become more predominant.
Trend in
Height of Dams The figure shows that India
is significantly (nearly 20 yrs) behind in the dam building technology than
rest of the world trend.
Dams built by using concrete and cement are of his
type. The mechanics of a dam changes
with its construction material. The
following section is devoted to this type of dam:
PROBLEMS
& FAILURES
ii)
Failure
Problem (Figure):
iii)
Measures
to Prevent (Figure):
iv)
Sliding
problem: Sliding can occur
in three ways which is mainly due to action of water
a)
Lubrication of dry rock
surface thus reducing the co-efficient of friction.
b)
Water moving between starta
dissolves, corrodes and erodes.
c)
Saturation of
interstices results into increase in pore pressure.
v)
Uplift
Problem: According to
Terzaghi the uplift acts on the whole area of the base of the dam whereas
others believe that the uplift acts to the 2/3rd area of the base of
the dam.
Geological Investigation for a Concrete
Dam
1.
Rock should be sound
and resistant to static and dynamic forces.
2.
Valley slopes should be
stable.
3.
Dam foundation should
be safe from sliding.
4.
The foundation rocks
should be of same geological properties.
5.
Reservoir should be
free from seepage/leakage.
6.
Rocks should be
resistant to corrosion, erosion, solution, drying and wetting.
7.
Reservoir drainage and
overburdens should be resistant to erosion and sliding.
8.
In case of arch dams
the abutments should be extremely strong.
9.
The site should have a
permissible (a) spillway and (b)
diversion tunnel and/or outlet works, if any.
10.
Construction material
should be economically available.
11.
Overall economy and
safety.
EARTH DAMS
A generalized X-section of an earth dam is shown (Figure). The
change in design of earth dams to that of masonry dams is well visualized
there:
Safety requirement for earth dams:
i)
There is no danger of
overtopping – sufficient freeboard and spillways.
ii)
The seepage line is
well within the downstream face.
iii)
The upstream slope is
safe from sudden draw down (rip-rap).
iv)
The upstream and
downstream slopes are flat enough that, with the materials utilized in the
embankment, they will be stable and show a satisfactory factor of safety by
recognized methods of analysis.
v)
The upstream and
downstream faces of the dam are flat enough that the shear stresses induced in
the foundation is enough less than the shear strength of the material in
foundation to ensure suitable factor of safety.
vi)
Water which passes
through and under the dam, when it reaches the discharge surface has a pressure
and velocity so small that it is incapable of moving the materials of which the
dam or its foundation is composed.
vii)
The upstream face is
properly protected from the wave action and the downstream face is protected
against action of rain.
viii)
Clay core and a filter
at the toe to regulate the flow from upstream to downstream.
Failure of
Earth Dams: As construction
material, the failure of earth dams is also very different from the masonry
ones. The failure statistics has been
given in the figure. The examples are also given therein.
Frequency
of Failure vs. age of the dam: The figure
shows that the frequency of failure of earth dams decreases with its age. Also, a sizable number of dams have failed
during their construction periods.
Earth
Dams on varying foundations and availability of construction material : The design and construction of each earth dam depends
upon the above mentioned two factors. The figure shows how the
cross-section of an earth dam changes with different kinds of foundation and
availability of construction materials.
A timber or buttress dam is made up of timber though
a good practice in old days now such dams are not being built for reasons more
than one. Figure shows various types of timber dams.
These dams are made up of steel. However, live timber dams, these dams have
to become rare in recent days. Some
examples can be given as Ask Fork Dam in Arizona and Red-ridge Dam in
'Michigan' shows various types of steel dams.
Advantages and disadvantages of steel dams:
1.
Greater speed in
construction.
2.
Claimed less cost.
3.
Stresses more
determinate.
4.
Greater flexibility to
resist unequal settlement without excessive leakage.
5.
Not affected by frost
action.
6.
Modern welding
processes permit leaky joints to be more easily repaired than in hollow
concrete dams.
1.
Steel not as permanent
as concrete.
2.
Requires greater and
more constant maintenance.
3.
Being lighter, are not
as adaptable to absorb the shock from vibration from spilling water and
seismisity
4.
Anchoring required
These are the main purposes of dams, but there are
other benefits. Their reservoirs provide recreation, such as fishing and
swimming. They become refuges for fish and birds. Dams conserve soil by
preventing erosion. They slow down streams so that the water does not carry
away soil
Dams also create problems. Their
reservoirs may cover towns or historic and scenic places. Dams may impair
fishing. At Bonneville Dam, fish ways help the salmon around the dam as they
swim upstream to spawn. In fish ladders, the fish jump from one ascending pool
to another. Fish locks lift the fish like locks lift ships. Another problem of
dams is silting. Some rivers pick up clay and sand and deposit them behind the
dam, thereby lessening its usefulness.
There are structures related to dams. Cofferdams are
temporary dams built to hold water back so that work can be done. Dikes keep
the sea off land that is below sea level. Levees are artificial riverbanks
constructed high enough to prevent flooding. A dam across a river intended to
permit flow, once a certain depth of water has been reached, may be called a
barrage. A small dam that
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