What is Gravity Dam?
A gravity dam depends on its own weight for stability and is usually straight in plan although sometimes slightly curved.
A gravity dam can hold a large amount of water. As they rely on their own weight, it is necessary to construct them on a solid foundation of bedrocks.
Construction of Gravity Dams.
Before construction work in a river channel can be started, the stream flow must be diverted.
In two-Stages construction, the flow is diverted to one side of the channel by a cofferdam while working proceeds on the other side.
After work on the lower portion of one side of the dam is complete, flow is diverted through outlets in this portion or may even be permitted to over top the completed portion while work proceeds in the other half of the channel.
If geologic and topographic conditions are favorable, a tunnel or diversion channel may be used to convey the entire flow around the dam site.
A tunnel is particularly advantageous if it will serve some useful purpose after completion of the dam.
Four 50 feet circular concrete-lined tunnels were used for diversion at Hoover Dam and later converted to outlet works.
A diversion channel or tunnel should be capable of carrying a flow selected by frequency analysis as a reasonable risk in view of the hazards on each particular job.
It is advantageous to schedule construction of the lower portion of a dam during normal low-flow periods to minimize the diversion problem.
The foundation must be excavated to solid rock before any concrete is poured.
After excavation, cavities or faults in the underlying strata are sealed with concrete or grout.
Read Also: Setting of Cement – Process, Tests, & Time.
Frequently a grout curtain is placed near the heel of the dam to reduce seepage and uplift.
Grouting at pressures up to about 40 psi may be done before concrete is placed for the dam, but high-pressure grouting (200 psi) is done from permanent galleries in the dam after the dam is complete so that the weight of the dam can resist the grouting pressures.
Concrete for the dam is usually placed in blocks depending on the dimensions of the dam, with a maximum width of about 50 ft on large dams.
The maximum height of a single pour is usually about 5 ft.
Sections are poured alternately so that each block is permitted to stand several days before another one is poured next to it or on top of it.
After individual sections are poured, they are sprinkled with water and otherwise protected from the drying effect of the air.
After the form work is removed, the lateral surfaces of each section are painted with a paint (asphaltic emulsion) to prevent adherence to adjoining sections and to form construction joints to reduce cracking of the concrete.
Keyways are provided between sections to carry the shear forces from one section to the adjacent one and make the gravity dam act as a monolithic structure.
Metal water stops are also placed in the vertical construction joints near the upstream face to prevent leakage.
Inspection galleries to permit access to the interior of the dam are formed as the concrete is placed.
These galleries may be necessary for grouting operation, for operation and maintenance of gates and valves, and as intercepting drains for water which seeps into the gravity dam.
When concrete sets, a great deal of heat is liberated, and the temperature of the mass is raised.
As the concrete cools, it shrinks, and cracks may develop. To avoid cracks, a special type of cement (low-heat cement) may be used.
Very lean mixes are also used for the interior of the dam. Two sacks of cement per cubic yard of concrete are not uncommon.
In addition, the materials which go into the concrete may be cooled before mixing concrete.
For best results, the temperature of the concrete mix should be between 50° and 80°F.
Occasionally, further cooling is accomplished by circulating cold water through pipes embedded in the concrete, although this is expensive and is generally used only on large gravity dams.
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