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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 showsthat India is significantly (nearly20 yrs) behind in the dam building technology than rest of the world trend.
GRAVITY/CONCRETE/MASONRY DAMS
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
i) Forces acting on a masonry dam: A combination of vertical, horizontal and multidirectional forces (both static and dynamic) act on a dam fig.4. The divergence between the resultant and the vertical forces at the base of the dam is known as eccentricity (Figutre). For a gravity dam this should tend to zero. And in no case the resultant should pass within1/3rd of the base of the dam on toe side. If this requirement is not met than the dam will be vulnerable to topple downstream due to horizontal forces. Another safety factor for the dam is that the value of co-efficient of friction should be more than P/W. The coefficient of friction for masonry vs. masonry is0.6 –0.7 whereas for masonry vs. gravel/sand is0.4 –0.5.
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 the2/3rd 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 thefigure. 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.
TIMBER OR BUTRRESS DAM
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.
STEEL DAM
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
Benefits and Problems
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.
