Connecticut Water Trails Association
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Connecticut Water Trails Association |
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Connecticut Water Trails Program Dams
Types Of Dams -
Dam Structure And Design
Earthfill Dams
The development of modern construction equipment has made massive earthfill dams economical. The Rogun and Nurek dams in Tajikistan, the world's highest, are earthfill structures. Canada's Syncrude Tailings, which will be the world's most massive, is also an earthfill structure.
Earthfill dams typically have a water-impermeable clay core, and a water cut-off wall from their base to bedrock to prevent underground seepage. During construction, the stream or river must be diverted either through the dam-site by means of a conduit, or around it by means of a tunnel.
Earthfill dams require supplementary structures as spillways for discharging water from behind the dam. If sufficient spillway capacity is not provided, an earthfill dam may be damaged or even destroyed by the erosive water flowing over its crest. Unless special precautions are taken, such dams are also subject to serious damage or even failure, due to water seepage.
Embankment Or Rockfill
Dams
The rockfill dam uses rock instead of earth to provide stability. It has an impervious, watertight membrane, usually an upstream facing of impervious soil, concrete paving, or steel plates; or it may have a thin interior core of impervious soil.
Rockfill embankment dams and zoned-embankment dams are the most common embankment dams. Rockfill embankment dams have a mound of loose rock covered with a waterproof layer on the upstream side. The waterproof layer may be made of concrete, flat stone panels, or other impervious materials. Zoned-embankment dams include an impervious core surrounded by a mound of material that water can penetrate. The supporting mound is usually made of loose rock or earth. The core might be built from concrete, steel, clay, or any impervious materials.
Embankment dams hold back water by the force of gravity acting upon their mass. Embankment dams require more material because loose rock and earth are less dense than concrete. Engineers often choose to build them if the materials are readily available.
A cross-section (or slice) through an embankment dam shows that it is shaped like a bank, or hill. Most embankment dams have a central section, called the core, made from an impermeable material to stop water passing through the dam. Clayey soils, concrete or asphaltic concrete can be used for the core.
Embankment Dam: Forces
Water pushes against the embankment dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.
The
Tarbela Dam contains more than 126 million cubic meters earth and rock.
This amounts to more than 15 times the volume of concrete used in the
Grand Coulee Dam.
Water pushes against the gravity dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.
Most gravity dams are made from concrete, a mixture of port land cement, water, and aggregates. Concrete is well suited for dam construction. A concrete gravity dam uses a triangular cross-section and steep upstream face. Its shape differs from that of the earthfill or rockfill dam in that its inward, water-facing side is perpendicular to the water surface; in profile, the dam forms a right-angled triangle.
A cross-section (or slice) through a gravity dam will usually look roughly triangular.
Gravity dams are suited to sites with either wide or narrow valleys, but they do need to be built on sound rock.
The
Grand Coulee Dam contains nearly 8 million cubic meters concrete. It is
one of the most massive structures ever built, standing 168 meter high
and 1,592 meter long.
Arch Dam Forces
The arch squeezes together as the water pushes against it. The weight of the dam also pushes the structure down into the ground.
Concrete Arch
Concrete arch dams are built in narrow, steep-walled canyons. The canyon walls take up the thrust exerted by the arch and the pressure of the water. Such dams can be extraordinarily thin. Vaiont Dam is 265 meters high, but only 22.7 meters thick at its base. In comparison, Hoover Dam is 221 meters high and 201 meters thick at its base and has a partial arch effect.
Glen Canyon Dam, which spans the Colorado River in Arizona, is the highest arch dam in the United States. It is 216 meters high and 475 meters long but contains less than four million cubic meters of concrete. Arch dams can be less expensive to build than gravity dams.
Arch-Gravity Dam
An arch-gravity dam, such as the Hoover Dam, is a dam with the characteristics of both an arch dam and a gravity dam. It is a dam that curves upstream in a narrowing curve that directs most of the water against the canyon rock walls, providing the force to compress the dam. It combines the strengths of two common dam forms and is considered a compromise between the two. A gravity dam requires a large volume of internal fill. An arch-gravity dam can be thinner than the pure gravity dam and requires less internal fill.
Arch-gravity dams are massive dams of reinforced concrete that resist the thrust of water by their weight pushing down using the force of gravity. A gravity dam is constructed so that the dam's massive weight resists the pressure of the water against it.
At the same time an arch-gravity dam incorporates the arch's curved design that is so effective in deflecting the water in narrow, rocky locations where the gorge's side are of hard rock and the water is forced into a narrow channel. Therefore the span needed for the dam is narrow; the dam's curved design effectively holds back the water in the reservoir using a lesser amount of construction material.
When properly situated on an appropriate site, the arch-gravity dam, combining the load resisting qualities of both a gravity and arch type dam inspires the most confidence in the public at large because of its appearance of massive permanence. However, curving a gravity dam may make it look stronger, but some of this effect may be psychological factors in operation in making a choice of dam style.
Buttress Dam
A buttress dam consists of a face supported by several buttresses on the downstream side. Buttress dams are made of concrete reinforced with steel. Buttresses are typically spaced across the dam site every 6 to 30 meter, depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.
Buttress dams require less concrete than gravity dams, but are not necessarily less expensive to build. Costs associated with the complex work of forming the buttresses or multiple arches may offset the savings in construction materials. Buttress dams may be desirable, however, in locations with foundations that would not easily support the massive size and weight of gravity dams.
Buttress Dam: Forces
Water pushes against the buttress dam, but the buttresses push back and prevent the dam from toppling over. The weight of the buttress dam also pushes down into the ground.
Spillways
All dams -- whether they're embankment, buttress, arch, or gravity -- must be maintained as they get older. Without proper maintenance, spillways can clog and concrete can crack. Some dams are even removed because they block the migration of fish.
When should dams be taken down? When should they be repaired? Engineers must consider the services that each dam provides and the environmental impact that each dam creates before they make this decision -- and this isn't easy. Oftentimes, there is no right answer.
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