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Welcome  Brian's compendium website on Dam Construction and the History of Dams
Important words found on this site. Dam, History Dams, Construction, Buttress, Embankment, Gravity, Aswan, Edwards,  Folsom, Grand Coulee, high, vital statistics, Reservoir, Capacity,  Purpose, Materials. Engineers, Flood Control, Columbian Basin Project, Hoover, Hydroelectric, Concrete, Gravity, South Fork, Three Gorges Dam, Diversionary, Timber, Embankment, Rockfill, Masonry, Eder, Arch, Steel, Coffer, Beaver, Spillway, Teton Failure, Pablo, Outlet, Tunnelling, Power PLant, Road Tunnel Lyaskovo, Environment Planning, Stress, Soil Behaviour, parameters, impervious core, FEM, and  Mechanism.

 Important words found on this site.

You can find this site again  by typing in the  Google search engine  the unique word " 1smaD "  which is  OR " Dams1 " backwards. or  "1noitcurtsnoCmaD "  which is  OR " DamConstruction1 " backwards.

Article Word Count 26,868

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Dam Basics

With the exception of the Great Wall of China, dams are the largest structures ever built. Throughout history, big dams have prevented flooding, irrigated farmland, and generated tremendous amounts of electricity. Without dams, modern life as we know it would simply not be the same.

Since the first large-scale dam was built in Egypt more than 5,000 years ago, engineers have devised various types of dams to withstand the forces of a raging river.

El Atazar Dam
Arch dam:
El Atazar Dam

_

Arch dams...
are good for narrow, rocky locations. They are curved, and the natural shape of the arch holds back the water in the reservoir. Arch dams, like the El Atazar Dam in Spain, are thin and require less material than any other type of dam.

Sneak a peek at the forces that affect arch dams.
Image of the Bartlett Dam
Buttress dam:
Bartlett Dam

_

Buttress dams...
may be flat or curved, but one thing is certain: a series of supports, or buttresses, brace the dam on the downstream side. Most buttress dams, like the Bartlett Dam in Arizona, are made of reinforced concrete.

Check out the forces that affect buttress dams.
Image of an embankment dam
Embankment dam:
New Waddell Dam

_

Embankment dams...
are the most commonly built dams in the United States. They are massive dams made of earth and rock. Like gravity dams, embankment dams rely on their heavy weight to resist the force of the water. But embankment dams are also armed with a dense, waterproof core that prevents water from seeping through the structure. Tailings dams -- large structures that hold back mining waste -- are a type of embankment dam.
Check out the forces that affect embankment dams.
Grand Coulee Dam
Gravity dam:
Grand Coulee Dam

_

Gravity dams...
are massive dams that resist the thrust of water entirely by their own weight. Most gravity dams, like the Grand Coulee Dam in Washington, are expensive to build because they require so much concrete. Still, many people prefer its solid appearance to the thinner arch and buttress dams.

Take at look at the forces that affect gravity dams.

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.

Now that you know more about different types of dams, make some of your own decisions about troubled dams in the Dam Challenge!

The Dam Challenge
Dams don't last forever. Hot and cold weather makes them crack. Water erodes their foundations. They create environmental problems. Eventually, every dam must be repaired, removed, or replaced.

You are a consulting dam engineer, and today, four dams need your attention. It is your job to advise the dam owners whether their dams must be repaired, taken down, or simply left alone. Listen to what the supporters and opponents of each dam have to say.

Like all dam engineers, you must weigh the pros and cons before you make your final decisions. Good luck!

The Problem Dams:

Arch dam   Embankment dam at a gold mine
Arch dam near a big city   Embankment dam at a gold mine
Gravity dam holding back a reservoir   Buttress dam
Gravity dam holding back a reservoir   Buttress dam on a scenic river

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Aswan High Dam
Aswan High Dam
Click photo
for larger image.

Vital Statistics:
Location: Aswan, Egypt
Completion Date: 1970
Cost: $1 billion
Reservoir Capacity: 5.97 trillion cubic feet
Type: Embankment
Purpose: Flood control, hydroelectric power, irrigation
Reservoir: Lake Nasser
Materials: Rock, clay
Engineer(s): planned by a team of British engineers; built by a team of Soviet engineers
In the middle of the arid Egyptian desert lies one of the largest embankment dams in the world. It is called the Aswan High Dam, or Saad el Aali in Arabic, and it captures the mighty Nile River in the world's third largest reservoir, Lake Nasser. Before the dam was built, the Nile River overflowed its banks once a year and deposited four million tons of nutrient-rich silt on the valley floor, making Egypt's otherwise dry land productive and fertile. But there were some years when the river did not rise at all, causing widespread drought and famine. In 1952, Egyptian president Gamal Abdal-Nasser pledged to control his country's annual flood with a giant new dam across the Nile River. His plan worked.

Aswan High Dam
Click photo
for larger image.

The Aswan High Dam captures floodwater during rainy seasons and releases the water during times of drought. The dam also generates enormous amounts of electric power -- more than 10 billion kilowatt-hours every year. That's enough electricity to power one million color televisions for 20 years!

Unfortunately, the dam has also produced several negative side effects. In order to build the dam, 90,000 Egyptian peasants had to move. To make matters worse, the rich silt that normally fertilized the dry desert land during annual floods is now stuck at the bottom of Lake Nasser! Farmers have been forced to use about one million tons of artificial fertilizer as a substitute for natural nutrients that once fertilized the arid floodplain.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)
 

Chart showing the relative size of the biggest dams in the world Aswan High Dam
5.97 trillion cubic feet
Fast Facts:
  • About 95 percent of Egypt's population lives within 12 miles of the Nile River.
  • Since the dam was completed in 1970, the fertility of Egypt's farmland has gradually decreased. Today, more than half of Egypt's soil is rated medium to poor.
  • Enough rock was used in the construction of the Aswan High Dam to build 17 Great Pyramids at Giza, one of the Seven Wonders of the Ancient World.
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Edwards Dam
Edwards Dam
Click photo
for larger image.

Vital Statistics:
Location: Augusta, Maine, USA
Completion Date: 1837
Reservoir Capacity: 740 million cubic feet
Type: Embankment (timber crib)
Purpose: Hydroelectric power
Misc: removed 1999
Materials: Timber, rock, concrete
Engineer(s): Kennebec Dam Company
 

On the morning of July 1, 1999, thousands of people lined the banks of the Kennebec River in Augusta, Maine, to watch workers punch a hole in the Edwards Dam. Water began trickling through the hole, and within minutes, a raging torrent gushed past the 915-foot-long, 20-foot-tall wall of rock, log, and concrete. Crowds cheered as the Kennebec River flowed freely past the Edwards Dam out to the Atlantic Ocean for the first time in more than 160 years.

Edwards Dam
Click photo
for larger image.

The Edwards Dam was the first hydroelectric dam in the country removed by the federal government for environmental reasons. It was built in 1837 to supply power to mills along the river's banks. But by the mid-1990s, the old dam was generating only 3.5 megawatts of electricity in 1990 -- an amount equal to .1 percent of Maine's annual energy usage.

Even before the dam was built, environmentalists argued that a dam on the Kennebec would block salmon, shad, herring, and other fish from reaching their spawning grounds upstream. They were right. Soon after the dam was built, these fish all but disappeared from the river.

For the first time in history, the federal government decided that the damage the Edwards Dam caused far outweighed its benefits. The dam was removed against the owner's will at a cost of $7.3 million.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)
 

Chart showing the relative size of the biggest dams in the world Edwards Dam
740 million cubic feet
Fast Facts:
  • Since the dam was removed in 1999, more than two million alewives, striped bass, shad, sturgeon, and Atlantic salmon have returned to the Kennebec River.
  • Hundreds of logs have been salvaged from the bottom of the dam and are being recycled into furniture, musical instruments, and other products.
  • The removal of the Edwards Dam has sparked the removal of other river-damaging dams across the country. Since the dam's removal, 25 small dams have been removed in the United States. At least 18 more will be removed by the end of 2000.
BUILDING BIG Home Page BUILDING BIG Site Map BUILDING BIG Labs BUILDING BIG Databank BUILDING BIG Glossary



 		 _   Folsom Dam
Folsom Dam
Click photo
for larger image.

Vital Statistics:
Location: Folsom, California, USA
Completion Date: 1956
Cost: $81.5 million
Reservoir Capacity: 43.9 billion cubic feet
Type: Gravity
Purpose: Hydroelectric power, irrigation, recreation
Reservoir: Folsom Lake
Materials: Concrete
Engineer(s): U.S. Army Corps of Engineers
On July 17, 1995, a spillway gate on the Folsom Dam broke open as it was being raised, causing an uncontrolled five-story cascade to gush down the face of the dam. Nearly 40 percent of Folsom Lake drained out past the broken gate before it could be repaired. Normally, when a reservoir becomes too full -- like after a heavy rainstorm -- engineers open spillway gates, which allow the excess water to drain out of the reservoir at a controlled rate of speed. When these gates open suddenly and engineers lose the ability to control the flow, disaster can result.

Folsom Dam
Click photo
for larger image.

Luckily, no major flooding occurred as a result of the failure, and the Folsom Dam was fully repaired at a cost of $20 million. After a year of thorough investigation, the United States Bureau of Reclamation blamed the disaster on a design flaw. Some spillway gates, like the ones at Folsom Dam, roll up and down on giant brass and steel pins, like a big garage door. As the gate opens and closes, the pins can become hot with friction. Friction reduced the strength of the pins in the Folsom Dam spillway gate and caused the gate to break.

Today, engineers trained in rope-climbing techniques can inspect these difficult-to-reach spillway gates and help prevent similar disasters from happening.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)
 

Chart showing the relative size of the biggest dams in the world Folsom Dam
43.9 billion cubic feet
Fast Facts:
  • When the spillway gate broke, more than 40,000 cubic feet per second of water gushed through the dam.
  • So much freshwater poured into the San Francisco Bay when the dam burst that salmon and striped bass were fooled into thinking that fall had arrived. The fish began their annual fall migrations way ahead of schedule.
  • The Folsom Dam came close to flooding half a million people in 1986, when engineers were forced to open the spillway gates after heavy rains.
Misspelled words used to find this page 3 of 6.nginear, enginer, engineel, iegngeigneel, eigngiegneel, eigngineel, engeignel, iegngiegneer, iegngineer, eignginel, engiegneer, iegngiegner, iegnginer, eigngeigneer, engiegner, iegngiegnear, iegnginear, eigngeigner, engiegnear, iegngiegneel, iegngineel, eigngeignear, engiegneel, iegnginel, eigngeigneel, engiegnel, engeigneer, iegngeigneer, eigngiegneer, eigngineer, engeigner, iegngeigner, eigngiegner, eignginer, engeignear, iegngeignear, eigngiegnear, eignginear, engeigneel, enginels, enginears, enginers, engineels, engineers, egineers, enineers, engneers, engieers, enginees, 3ng1n3rs, 3mg1n3rs, eng1ners, engineesr, engineres, engieners, engnieers, enigneers, egnineers, negineers, flood, frood, floud, froud, fod, phod, f1od, flodo, folod, lfood, flod,control, contol, contrl, cntrol, cotrol, conrol, contror, contlol, contlor, contro, contlo, contro1, comtrol, contrlo, contorl, conrtol, cotnrol, cnotrol, ocntrol, ontrol, clumbia, coumbia, colmbia, colubia, columbia, columia, columba, corumbai, corumbya, columbai, columbya, corumbia, corumbea, columbea, columbien, columbyen, columbian, corumbien, clumbian, corumbyen, coumbian, colmbian, columbiin, colubian, columbyin, columian, corumbiin, columban, corumbyin, columbin, columbain, columbyan, corumbian, corumbain, corumbyan, co1unb1am, colunb1am, colunbiam, columbiam, columbina, columiban, colubmian, colmubian, coulmbian, cloumbian, oclumbian, olumbian, basin, bacin, bacen, basiegn, basan, baciegn, bacan, bahsin, bahsen, bahcin, bahcen, bahseign, bahsan, bahceign, bahcan, bahsiegn, bahciegn, baseign, basen, baceign, bas1n, basim, basni, baisn, bsain, absin, project, poject, prject, proect, projct, projet, ploject, perject, porject, plojet, perjet, porjet, projec, plojec, perjec, porjec, projetc, projcet, proejct, prjoect, rpoject, roject,hover, hovur, hovel, hobur, hoover, hoovel, houvel, hoobur, houbur, hoovur, houvur, houver, hoove, houve, hov3r, hovre, hoovre, hooevr, hovoer, ohover,hidroelectrik, hidloleectric, hydroeelctlik, hydroeelctrik, hydrelectric, hidroeelctrik, hidroerectric, hydroleectlik, hydroleectrik, hydrolectric, hidroleectrik, hidroeerctric, hydloelectlik, hydloelectrik, hydroeectric, hydloerectric, hidloelectrik, hidloerectric, hydroerectlik, hydloeelctrik, hydroelctric, hydloelectlic, hidroerectrik, hidroelectlic, hidroelectric, hydloleectrik, hydroeletric, hydroerectlic, hidroelectlik, hidroeelctlic, hidroeelctric, hydroerectrik, hydroelecric, hydloerectlic, hydroelectric, hidroleectlic, hidroleectric, hydroeerctrik, hydroelectic, hdroelectric, hidloelectlic, hidloelectric, hydloerectrik, hydroelectrc, hyroelectric, hidroerectlic, hidloeelctric, hydroelectlik, hydroelectrik, hydoelectric, hydloleectlic, hydloeelctric, hydroeerctlic, hydloleectric, hydroerectric, hydroeerctric, hydloeerctric, hydroelectlic, hydroeelctlic, hydroeelctric, hydroleectlic,
Grand Coulee Dam
Grand Coulee Dam
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Vital Statistics:
Location: Grand Coulee, Washington, USA
Completion Date: 1942
Cost: $300 million
Reservoir Capacity: 421 billion cubic feet
Type: Gravity
Purpose: Flood control, hydroelectric power, irrigation
Reservoir: Franklin D. Roosevelt Lake
Materials: Concrete
Engineer(s): Bureau of Reclamation
The Grand Coulee Dam, located on the Columbia River in central Washington, is the largest single producer of electricity in the United States. Made from 12 million cubic yards of concrete, the Grand Coulee Dam is also one of the largest concrete structures in the world. But engineers were confronted with a unique problem when building such a massive concrete dam.

Grand Coulee Dam
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When concrete is made, it produces a chemical reaction that gives off heat. As concrete cools, it gradually shrinks. If the shrinkage is not controlled, cracks can form -- and cracks are disastrous in dams. The solution? Engineers pumped cold water through an intricate network of pipes in the concrete to help cool the concrete as it hardened. It's a good thing they did this, because it would have taken 200 years for the concrete to cool naturally, and many cracks would have formed!

The Grand Coulee Dam is the largest producer of hydroelectric power in the United States and the third largest hydroelectric facility in the world. With its 28 generators producing up to 23,860,944,469 kilowatt-hours annually, it is the primary source of electric power to states in the Northwest.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world Grand Coulee Dam
421 billion cubic feet
Fast Facts:
  • The base of the Grand Coulee Dam is almost four times as large as the base of the Great Pyramid of Giza.
  • If all of the pipes used to cool the cement in the Grand Coulee Dam were laid end to end, they would stretch from New York City to the Grand Canyon!
  • There is enough concrete in the Grand Coulee Dam to build a highway across the United States.

  • History of the Columbia Basin Project


     

    Overview

    Grand Coulee Dam is a large hydroelectric dam located on the Columbia river in Central Washington. Made from 12 million cubic yards of concrete, Grand Coulee Dam is the largest concrete structure in the United States and the third largest hydroelectric facility in the world. Sharing the river with 10 other U.S. dams, Grand Coulee is the first dam encountered on the Columbia after the river enters the U.S from Canada. Lake Roosevelt, the reservoir created by the dam, contains 9 million acre-feet of water and streches over 150 miles back to the border.

    Grand Coulee Dam fills three primary rolls. First, with its 24 generators providing up to 6.5 million kilowatts of power, it is a major provider of electrical power to the Northwest. Secondly, water pumped from behind the dam provides irrigation for over half a million acres of the Columbia basin from Coulee City in the north to Pasco, WA in the south. Finally, by strictly regulating the Columbia's highly variable flow rate, the dam provides much needed flood control to the river basin.

    History

    Grand Coulee Dam and the Columbia Basin Project are managed by the U.S. Bureau of Reclamation, an agency of the Department of the Interior. The Bureau was established by congress in 1902 and was tasked with boosting development in the West by developing water storage and irrigation networks west of the 100th meridian. The Bureau took a tentative interest in providing Central Washington with irrigation water pumped from the Columbia as early as 1904. For one reason or another this idea was not followed up for several years.

    The idea for damming the Columbia below the Grand Coulee was first proposed by Ephrata attorney William M. Clapp in the spring of 1917. The idea gained rapid support from the citizens of Ephrata and the surrounding area. Eventually the state government took an interest in the project as well.

    Preliminary feasibility studies were carried out in the 1920s. Initially the primary purpose of the dam was to provide irrigation water. Although the dam idea had a great deal of local support there were other irrigation proposals in the works. In particular, a plan to build a long canal to carry water down from the Pend Oreille river in Northern Idaho was under serious consideration.

    A final report favoring the construction of the dam was produced by the Corp of Engineers in late 1931 and presented to the 73d Congress of the United States as House Document #103. This was supplemented in January of 1932 by a report from the Bureau of Reclamation outlining the details of a dam-based irrigation project.

    $377,000 was comitted to the project by the State of Washington in 1933. This was followed soon afterward by a promise from president Franklin D. Roosevelt to provide initial funds to the tune of 63 million dollars to begin work on the dam as a project under the Public Works Administration.

    Not wanting to back up water into Canada, it was decided that the Canadian border would be the ultimate limiting factor as to how high the dam could be built. However, at the time there was a surplus of electric power in the Northwest and no major increase predicted for the foreseeable future. For this reason the original proposal called for a LOW dam. This dam would be 200 feet lower than the maximum height allowed by the Canadian border restriction. It would provide irrigation and flood control with the possibility of a reduced amount of power generation. However, it was decided to design the structure in such a way that it could be raised to its full height providing a corresponding increase in generation capacity if the need ever presented itself.

    Initial excavation of the dam site began in December of 1933 with work toward improving the local infrastructure proceeding in parallel. On August 30, 1935 congress authorized the construction of the full high dam and no low dam version was ever completed. By 1941 the main dam was essentially finished with construction of the powerhouses and pumping plant underway.

    Ironically, because of the Second World War and the importance of the Northwest's aluminum industry to that effort, the production of electricty became the overriding priority for the dam. Irrigation was deferred until later. During the war six Grand Coulee generators were brought on line as well as two generators borrowed from the yet to be completed Shasta dam.

    After the war an emphasis was put back on irrigation. Construction was resumed on the pumping plant in 1946. By 1951 the plant and its six 65,000 horsepower pumps were ready for operation. The first water was delivered to the Banks Lake equalizing reservoir above the dam that same year. The first year only 66,000 acres were irrigated. Since then this figure has steadily increased as more canals, siphons, reservoirs and auxilary pumping plants have been added to the project.

    In 1973 the pumping plant was extended to the south and two of six 67,500 horsepower pump-generator units were installed. Unlike the existing six pumps, these pump-generators can be reversed during periods of high power demands and operated as generators. In their generation mode each unit can produce 50,000 kW of electrical power. The remaining four pump/generator units were installed and operating by late 1983.

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Hoover Dam
Hoover Dam
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Vital Statistics:
Location: Arizona and Nevada, USA
Completion Date: 1936
Cost: $165 million
Reservoir Capacity: 1.24 trillion cubic feet
Type: Gravity
Purpose: Hydroelectric power
Reservoir: Lake Mead
Materials: Concrete
Engineer(s): Bureau of Reclamation
In 1931, during the height of the Depression, thousands of American workers came to the Black Canyon on the Arizona-Nevada border to tame the Colorado River. They began construction on what would be the largest dam of its time -- the Hoover Dam.

Hoover Dam
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for larger image.

But before the dam could be built, workers had to divert the wild Colorado River away from the construction site. How did they do this? They blasted tunnels -- as big as four-lane highways -- right through the canyon walls. For the next five years, the Colorado River gushed through these diversion tunnels while 8,000 workers toiled in the harsh, dry canyon bottom. Amazingly, they completed the dam in less than five years -- ahead of schedule and under budget.

The Hoover Dam is a curved gravity dam. Lake Mead pushes against the dam, creating compressive forces that travel along the great curved wall. The canyon walls push back, counteracting these forces. This action squeezes the concrete in the arch together, making the dam very rigid. This way, Lake Mead can't push it over.

Today, the Hoover Dam is the second highest dam in the country and the 18th highest in the world. It generates more than four billion kilowatt-hours a year -- that's enough to serve 1.3 million people!

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world Hoover Dam
1.24 trillion cubic feet
Fast Facts:
  • At its base, Hoover Dam is as thick (660 feet) as two footballs fields measured end to end.
  • Hoover Dam is 726 feet tall. That's almost 200 feet taller than the Washington Monument in Washington, D.C.
  • During peak periods of electrical demands, enough water runs through the generators to fill 15 average-size swimming pools (20,000 gallons each) in one second.
  • There is enough concrete in Hoover Dam (4.5 million cubic yards) to build a two-lane road from Seattle, Washington, to Miami, Florida, or a four-foot-wide sidewalk around the Earth at the Equator.
  • The Hoover Dam is so thick and heavy, it doesn't even need to be curved! It's heavy enough to resist the weight and thrust of the water pushing behind it, but designers thought people would feel safer with a curved design.
Itaipu Dam
Itaipu Dam
Click photo
for larger image.

Vital Statistics:
Location: Brazil and Paraguay
Completion Date: 1991
Cost: $18 billion
Reservoir Capacity: 1.02 trillion cubic feet
Type: Gravity
Purpose: Hydroelectric power
Reservoir: Itaipu Reservoir
Materials: Concrete
Engineer(s): International Engineering Company; Itaipu Binacional
Eighteen was a lucky number for engineers working on the Itaipu Dam. The 4.8-mile-long complex of concrete and rockfill dams on the Upper Parana River at the Brazil-Paraguay border has 18 generators, and it took 18 years and $18 billion to build. The main structure, a hollow, concrete gravity dam, has a powerhouse capable of generating 12,600 megawatts of electricity. That's enough to power most of the state of California. In fact, the enormous dam provides 25 percent of Brazil's energy supply and 78 percent of neighboring Paraguay's energy supply. But building one of the largest hydroelectric dams in the world was not easy.

Itaipu Dam
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for larger image.

Engineers actually had to shift the course of the seventh largest river in the world, the Parana River, around the construction site before building the Itaipu Dam. It took almost three years for workers to carve a 1.3-mile-long, 300-foot-deep, 490-foot-wide diversion channel for the river. Fifty million tons of earth and rock were removed in the process. The American Society of Civil Engineers recognized this amazing feat and named the Itaipu Dam one of the "Seven Wonders of the Modern World."

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world Itaipu Dam
1.02 trillion cubic feet
Fast Facts:
  • Engineers chose a hollow gravity dam because it required 35 percent less concrete than a solid gravity dam. The hollow dam is still heavy and sturdy enough to resist the thrust of water entirely by its own weight.
  • The volume of iron and steel used in the dam would be enough to build 380 Eiffel Towers.
  • The dam is a major tourist attraction. More than nine million visitors from 162 countries have visited the structure since it was completed in 1991.
Misspelled words used to find this page 5 of 6 dyversionery,timber, timbre, thymber, thymbel, timbel, thymbre, thymble, timble, timor, timer, thymer, thymel, thymor, timel, tiber, tibel, imber, imbel, t1nber, tinber, tinbre, timebr, tibmer, tmiber, itmber, timbe, timbr, tmber,rockf11, rockf1l, rockfill, rockflil, rockifll, rocfkill, rokcfill, rcokfill, orckfill, rockfil, masonry, msonry, maonry, masnry, masory, masony, masonrie, nasomry, masomry, masonyr, masorny, masnory, maosnry, msaonry, amsonry, masonr, asonry,deer, del, dear, der, deel, eder, edar, edel, edre, eedr, arch, arsh, arkh, alch, rach, lach, arhc, acrh, steel, stel, ster, stle, stere, stele, steal, steer, steul, staul, stlee, stale, st31, ste1, setel, tseel,offer, coffer, ougher, ofer, offel, ofel, cougher, coffel, coffe, cofe, cofer, cofel, cof3r, cofre, coffre, cofefr, cfofer, ocffer,offer, coffer, ougher, ofer, offel, ofel, cougher, coffel, coffe, cofe, cofer, cofel, cof3r, cofre, coffre, cofefr, cfofer, ocffer,spilweigh, spylwaie, sillway, spyllweigh, spyllwhaie, spllway, spyrlway, spirlwaie, spillay, spyrlweigh, spirwaie, spillwy, spylweigh, spirlwhaie, spillwaie, spyrlwhay, spilwaie, spyrlwaie, spillwhaie, spillweigh, spilwhaie, spirlweigh, spyllwaie, spillway, spylway, spyllwhay, spylwhay, spirlway, spirway, spirlwhay, spilway, spirwhay, spillwhay, spilwhay, spyllway, sp11way, sp1lway, spillwya, spillawy, spilwlay, splilway, sipllway, psillway,teton, titon, titun, tetun, tetom, tetno, teotn, tteon, etton, failure, failre, failue, filure, falure, faiure, fialure, fairure, failule, fialule, fairule, f11uer, ph11uer, fa11uer, fa1luer, failuer, failrue, faiulre, faliure, afilure, failur, ailure,pablo, pabro, pab1o, pabol, palbo, pbalo, apblo,outlet, outret, ootret, outelt, ootlet, ootelt, out1et, outlte, oultet, otulet, uotlet, outle, outlt, outet, oulet, otlet, utlet, tuneriegng, tunleeigng, tunnelig, chunneriegng, chunering, tunleiegng, tuneleignt, tunneleigng, chunnleiegng, chunleing, tuneliegnt, tunneliegng, tunnereigng, tunneling, chunneliegnt, chunelint, chunneling, tunneriegng, tunnleeigng, chuneliegng, chunneleigng, chunnering, tunnleiegng, tunneleignt, chunnereigng, chunnleing, tunneliegnt, tunnereignt, tnneling, chunnleeigng, chunnelint, tunneriegnt, tunnleeignt, tunnling, chunneleignt, chunnerint, tunnleiegnt, tuneleigng, tunneing, chuneleigng, chunnleint, tuneliegng, tunereigng, tunnelng, chunneliegng, chuneling, tunleint, tunnering, tunering, tunnerint, tunnleing, tunerint, tuneling, tunleing, tunnelint, tunnleint, tunelint, tun311ng, tum311ng, tune11ng, tunel1ng, tunnelign, tunnelnig, tunneilng, tunenling, tnuneling, utnneling, power, powher, powel, powhel, powre, poewr, pwoer, opwer, plant, plente, prente, plent, plante, pliant, pliante, prant, prante, priant, priante, pland, pliand, prand, priand, prent, p1ant, plamt, platn, plnat, palnt, lpant, road, rowed, rode, oard, load, orad, olad, roda, loda, raod, tunnel, chunel, chuner, chunle, chunnel, chunner, chunnle, tunnle, tunel, tunle, tunner, tuner, tunne, tune, chunne, chune, tun31, tum31, tune1, tunenl, tnunel, utnnel,1yaskovo, lyaskoov, lyaskvoo, lyasokvo, lyaksovo, lysakovo, layskovo, ylaskovo, lyaskov, lyaskoo, lyaskvo, lyasovo, l
 
South Fork Dam
South Fork Dam
Click photo for larger image.

Vital Statistics:
Location: Johnstown, Pennsylvania, USA
Completion Date: 1852
Cost: $166,647
Reservoir Capacity: 2.7 million cubic feet
Type: Embankment
Purpose: Water supply for canal, recreation
Reservoir: Lake Conemaugh
Misc: collapsed 1889
Materials: Rock, clay
Engineer(s): Sylvester Welsh
On the afternoon of May 31, 1889, a private dam in western Pennsylvania burst, sending 20 million gallons of water and debris into the unsuspecting town of Johnstown with the force of a tidal wave. The catastrophe killed 2,209 people, left thousands homeless, and transformed the prospering city of Johnstown into a virtual wasteland.

South Fork Dam
Click photo
for larger image.

Before it burst, South Fork Dam held back Lake Conemaugh, the pleasure lake of the South Fork Fishing and Hunting Club. It was an embankment dam made of clay, boulders, and dirt. Through the years, the spillway became clogged with trees and other floating debris. When it started raining on Memorial Day in 1889, the lake swelled and seeped over the top of the structure. The earth-and-rock structure collapsed, releasing a thunderous wave 40 feet high and half a mile wide into the valley. Water slammed into Johnstown with the force of Niagara Falls. It carried huge amounts of debris, including houses, barns, animals, and people. The wave destroyed the city in 10 minutes.

The South Fork Dam collapsed because the spillway was poorly maintained. Today, large dams and their spillways are inspected frequently by qualified engineers.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world South Fork Dam
2.7 million cubic feet
Fast Facts:
  • The average speed of the wave on its trip to Johnstown was 40 miles per hour.
  • The wave was 35 to 40 feet high at its crest as it hit Johnstown.
  • The volume of water that destroyed Johnstown was equal to the volume that goes over Niagara Falls in 36 minutes.
Three Gorges Dam
Three Gorges Dam
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for larger image.

Vital Statistics:
Location: Three Gorges, China
Completion Date: 2009
Cost: $17-$100 billion
Reservoir Capacity: 1.39 trillion cubic feet
Type: Gravity
Purpose: Flood Control, hydroelectric power, irrigation, navigation
Reservoir: not named
Materials: Concrete
Engineer(s): Changjiang Water Resources Commission; Zhongnan Investment & Design Institute; Huadong Investment & Design Institute
Construction is now under way in China on what will be the world's largest hydroelectric dam. When it is completed in 2009, Three Gorges Dam will stretch more than one mile across the Yangtze River and soar 600 feet above the valley floor. It will be the largest concrete dam in the world, and will produce 18,000 megawatts of electrical energy, nudging Brazil's Itaipu Dam to second place.

Three Gorges Dam
Click photo
for larger image.

China's Three Gorges Dam is years from completion, but environmentalists and human rights advocates are already concerned about the consequences of such a huge structure. To make way for the enormous project, more than one million people living on the banks of the Yangtze River will have to move to higher ground. The 350-mile-long reservoir will submerge villages, ancient temples, burial grounds, and the spectacular canyons that tourists from all over the world come to see. Environmentalists also argue that the dam will wipe out a number of rare species, including the Yangtze River dolphin, and that the reservoir will trap millions of tons of raw pollutants spewing from China's largest industrialized city, Chongqing.

When finished, Three Gorges Dam will generate one-ninth of China's power. Unfortunately, the dam may be remembered not for its hydroelectric power, but for its drastic social and environmental impact.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world Three Gorges Dam
1.39 trillion cubic feet
Fast Facts:
  • About 20,000 people are working nearly round the clock to complete the 1.24-mile-wide structure by 2009.
  • The lake that will form behind Three Gorges Dam will stretch for about 350 miles -- the distance from San Francisco to Los Angeles.
  • When it is completed in 2009, the enormous Three Gorges Dam reservoir will actually be visible from the moon!

Here are some useful links to sites about Dams.

Individual Dams

Grand Coulee Dam
Want to learn more about how the Grand Coulee Dam was built? how much power it produces? All of your answers are right here.
Hoover Dam
The Bureau of Reclamation tells the story of the Hoover Dam -- from construction facts to brief explanations of how the dam works.

Johnstown Flood Museum
This virtual museum displays the photos and tells the shocking story of the 1889 Southfork Dam disaster.

Dams -- General

United States Bureau of Reclamation
Search this government site for information on the Hoover, Grand Coulee, and Folsom Dams -- and thousands of other large dams throughout the western United States.
World Commission on Dams
Learn more about the great dam debate. Read articles by opponents and proponents of large dams throughout the world.

Webography Index
Bridges | Domes | Skyscrapers | Dams | Tunnels | Other Structures
Engineering Resources

Dam

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This article discusses structures for water impoundment. For other meanings, see dam (disambiguation).
Scrivener Dam, in Canberra, Australia, was engineered to withstand a once-in-5000-years flood

Scrivener Dam, in Canberra, Australia, was engineered to withstand a once-in-5000-years flood

A dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a reservoir, lake or impoundment. In Australian and South African English, the word "dam" can also refer to the reservoir rather than the structure. Most dams have a section called a spillway or weir over which or through which it is intended that water will flow either intermittently or continuously.

Contents  below:

History

Some of the first dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, for Mesopotamia's weather effected the Tigris and Euphrates rivers and could be quite unpredictable. The earliest recorded dam is believed to have been on the Nile river at Kosheish, where a 15m high masonry structure was built about 2900 B.C. to supply water to capital of Memphis.

Types of dams

The Hoover Dam, a concrete gravity-arch dam in the Black Canyon of the Colorado River

The Hoover Dam, a concrete gravity-arch dam in the Black Canyon of the Colorado River

Dams can be formed by human agency, natural causes, or by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their structure, intended purpose or height.

Based on structure and material used, dams are classified as timber dams, embankment dams or masonry dams, with several subtypes.

Intended purposes include providing water for irrigation or town or city water supply, improving navigation, creating a reservoir of water to supply industrial uses, generating hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and containing effluent from industrial sites such as mines or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one.

According to height, a large dam is higher than 15 metres and a major dam is over 150 metres in height. Alternatively, a low dam is less than 30 m high; a medium-height dam is between 30 and 100 m high, and a high dam is over 100 m high.

A saddle dam is an auxiliary dam constructed to confine the reservoir created by a primary dam either to permit a higher water elevation and storage or to limit the extent of a reservoir for increased efficiency. An auxiliary dam is constructed in a low spot or saddle through which the reservoir would otherwise escape. On occasion, a reservoir is contained by a similar structure called a dike to prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake. This is similar to a levee, which is a wall or embankment built along a river or stream to protect adjacent land from flooding.

An overflow dam is designed to be overtopped. A weir is a type of small overflow dam that can be used for flow measurement.

A check dam is a small dam designed to reduce flow velocity and control soil erosion. Conversely, a wing dam is a structure that only partly restricts a waterway, creating a faster channel that resists the accumulation of sediment.

A dry dam is a dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.

Diversionary dams

A diversionary dam is a structure designed to divert all or a portion of the flow of a river from its natural course.
  Timber dams

A timber crib dam in Michigan, photographed in 1978.

A timber crib dam in Michigan, photographed in 1978.

Timber dams were widely used in the early part of the industrial revolution and in frontier areas due to ease and speed of construction. Rarely built in modern times by humans due to relatively short lifespan and limited height to which they can be built, timber dams must be kept constantly wet in order to maintain their water retention properties and limit deterioration by rot, similar to a barrel. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly or difficult to transport, and either a low head diversion dam is required or longevity is not an issue. Timber dams were once numerous, especially in the North American west, but most have failed, been hidden under earth embankments or been replaced with entirely new structures. Two common variations of timber dams were the crib and the plank.

Timber crib dams were erected of heavy timbers or dressed logs in the manner of a log house and the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water.

Timber plank dams were more elegant structures that employed a variety of construction methods utilizing heavy timbers to support a water retaining arrangement of planks.

Very few timber dams are still in use. Timber, in the form of sticks, branches and withes, is the basic material used by beavers, often with the addition of mud or stones.

Embankment dams

Embankment dams are made from compacted earth, and have two main types, rock-fill and earth-fill dams. Embankment dams rely on their weight to hold back the force of water, like the gravity dams made from concrete.

Rock-fill dams

A rockfill dam

A rockfill dam

Rock-fill dams are embankments of compacted free-draining granular earth with an impervious zone. The earth utilized often contains a large percentage of large particles hence the term rock-fill. The impervious zone may be on the upstream face and made of masonry, concrete, plastic membrane, steel sheet piles, timber or other material. The impervious zone may also be within the embankment in which case it is referred to as a core. In the instances where clay is utilized as the impervious material the dam is referred to as a composite dam. To prevent internal erosion of clay into the rock fill due to seepage forces, the core is separated using a filter. Filters are specifically graded soil designed to prevent the migration of fine grain soil particles. When suitable material is at hand, transportation is minimized leading to cost savings during construction. Rock-fill dams are resistant to damage from earthquakes. However, inadequate quality control during construction can lead to poor compaction and sand in the embankment which can lead to liquefaction of the rock-fill during an earthquake. Liquefaction potentical can be reduced by keeping susceptible material from being saturated, and by providing adequate compaction during construction. An example of a rock-fill dam is New Melones Dam in California.

Earth-fill dams

A Farmer's Dam

A Farmer's Dam

Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed of well compacted earth. A homogeneous rolled-earth dam is entirely constructed of one type of material but may contain a drain layer to collect seep water. A zoned-earth dam has distinct parts or zones of dissimilar material, typically a locally plentiful shell with a watertight clay core. Modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve the integrity of the downstream shell zone. An outdated method of zoned earth dam construction utilized a hydraulic fill to produce a watertight core. Rolled-earth dams may also employ a watertight facing or core in the manner of a rock-fill dam. An interesting type of temporary earth dam occasionally used in high latitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight region of permafrost within it. Examples of earth-fill dams include Nurek Dam in Tajikistan, the tallest dam in the world, and Oroville Dam, the tallest dam in the United States.

 Masonry dams

Masonry dams are of either the gravity or the arch type.

 Gravity dams

The Eder dam in Germany, built around 1910.

The Eder dam in Germany, built around 1910.

In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. The dam will not overturn provided that the moment around the turning point, caused by the water pressure is smaller than the moment caused by the weight of the dam. This is the case if the resultant force of water pressure and weight falls within the base of the dam. However, in order to prevent tensile stress at the upstream face and excessive compressive stress at the downstream face, the dam cross section is usually designed so that the resultant falls within the middle at all elevations of the cross section (the core). For this type of dam, impervious foundations with high bearing strength are essential.

When situated on a suitable site, a gravity dam inspires more confidence in the layman than any other type; it has mass that lends an atmosphere of permanence, stability, and safety. When built on a carefully studied foundation with stresses calculated from completely evaluated loads, the gravity dam probably represents the best developed example of the art of dam building. This is significant because the fear of flood is a strong motivator in many regions, and has resulted in gravity dams being built in some instances where an arch dam would have been more economical.

Gravity dams are classified as "solid" or "hollow." The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct. Gravity dams can also be classified as "overflow" (spillway) and "non-overflow." Grand Coulee Dam is a solid gravity dam and Itaipu Dam is a hollow gravity dam.

With a height of 285m the tallest gravity dam in the world is the Grande Dixence Dam in Switzerland.

 Arch dams

In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal hydrostatic pressure between vertical cantilever and arch action will depend upon the stiffness of the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. The normal component of the weight of the arch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. For this type of dam, firm reliable supports at the abutments (either buttress or canyon side wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock. The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the character of the rock should be carefully inspected.

Two types of single-arch dams are in use, namely the constant-angle and the constant-radius dam. The constant-radius type employs the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom of the dam the central angle subtended by the face of the dam becomes smaller. Jones Falls Dam, in Canada, is a constant radius dam. In a constant-angle dam, also known as a variable radius dam, this subtended angle is kept a constant and the variation in distance between the abutments at various levels are taken care of by varying the radii. Constant-radius dams are much less common than constant-angle dams. Parker Dam is a constant-angle arch dam.

A similar type is the double-curvature or thin-shell dam. Wildhorse Dam near Mountain City, Nevada in the United States is an example of the type. This method of construction minimizes the amount of concrete necessary for construction but transmits large loads to the foundation and abutments. The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream.

The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments. The multiple-arch dam does not require as many buttresses as the hollow gravity type, but requires good rock foundation because the buttress loads are heavy. See Geotechnical engineering.

 Steel dams

Red Ridge steel dam, b. 1905, Michigan

Red Ridge steel dam, b. 1905, Michigan

A steel dam is a type of dam briefly experimented with in around the turn of the 19th-20th century which uses steel plating (at an angle) and load bearing beams as the structure. Intended as permanent structures, steel dams were an (arguably failed) experiment to determine if a construction technique could be devised that was cheaper than masonry, concrete or earthworks, but sturdier than timber crib dams. Only two examples remain in the US.
 Cofferdams

A cofferdam during the construction of locks at the Montgomery Point Lock and Dam.

A cofferdam during the construction of locks at the Montgomery Point Lock and Dam.

A cofferdam is a (usually temporary) barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, concrete or steel sheet piling, cofferdams are used to allow construction on the foundation of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam may be demolished or removed. See also causeway and retaining wall. Common uses for cofferdams include construction and repair of off shore oil platforms. In such cases the cofferdam is fabricated from sheet steel and welded into place under water. Air is pumped into the space, displacing the water allowing a dry work environment below the surface. Upon completion the cofferdam is usually decontructed unless the area requires continuous maintenance.

 Beaver dams

Beaver dams are made by beavers and are made out of wood. they are also usually the beaver's home at the same time, becuase they are in a dome shape.
Spillways

Spillway on Llyn Brianne dam, Wales soon after first fill

Spillway on Llyn Brianne dam, Wales soon after first fill

A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have floodgates designed to control the flow through the spillway.

A service spillway or primary spillway passes normal flow. An auxiliary spillway releases flow in excess of the capacity of the service spillway. An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway. A fuse-plug spillway is a low embankment designed to be overtopped and washed away in the event of a large flood.

Any cavitation or turbulence of the water flowing over the spillway slowly erodes the dam's wetted surfaces. To minimize that erosion (especially with maximum water elevation at the crest), the downstream face of the spillway is ordinarily made an ogee curve.

It was the inadequate design of the spillway that caused the overtopping of a dam that caused the infamous Johnstown Flood.

Other considerations

The best place for building a dam is a narrow part of a deep river valley; the valley sides can then act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.

Significant other engineering and engineering geology considerations when building a dam include:

  • permeability of the surrounding rock or soil
  • earthquake faults
  • landslides and slope stability
  • peak flood flows
  • reservoir silting
  • environmental impacts on river fisheries, forests and wildlife (see fish ladder)
  • impacts on human habitations
  • compensation for land being flooded as well as population resettlement
  • removal of toxic materials and buildings from the proposed reservoir area
The reservoir emptying through the failed Teton Dam

The reservoir emptying through the failed Teton Dam

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine monitoring of seepage from drains in, and around, larger dams is necessary to anticipate any problems and permit remedial action to be taken before structural failure occurs. Most dams incorporate mechanisms to permit the reservoir to be lowered or even drained in the event of such problems. Another solution can be rock grouting - pressure pumping portland cement slurry into weak fractured rock.

Examples of failed dams

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Dam Engineering

International papers of technical excellence

Published quarterly by International Water Power & Dam Construction, Dam Engineering is the industry's best vehicle to promote the exchange of technical information on dams and associated structures.

Dam Engineering features fully-reviewed quantitative papers on aspects of the planning, design, construction and maintenance of reservoirs, dams and barrages, and scientific aspects of the design, analysis and modelling of dams, hydro power plants and associated structures. A subscription to this highly technical journal can be ordered online.



An independent journal, with a well-respected board of reviewers, Dam Engineering represents a much needed forum, in an organized context, which will appeal not only to the specialist but also to the discipline in general. First published in January 1990, topics covered in Dam Engineering include:
  • Methods of analysis and modeling aspects of: loads for natural and man-made environments; structure, reservoir and foundation domains; transient and steady state fluid flow; joints, cracks and other discontinuities; transient and steady state temperature effects.
     
  • Behavioral characteristics and material modeling of: mass concrete; embankment material; new materials (including asphaltic concrete and RCC); rock foundations (including jointed rock); fracture in concrete and rock.
  • Case studies of the application of advanced analysis and correlation with numerical model and prototype measurements.
  • Modeling and analysis for failure and safety criteria.
  • Correlation of surveillance data with safety criteria (including expert systems).
  • Geometrical design and dam optimization.
  • Quantitative aspects of dam siting and construction.
  • Construction: analysis of existing practice; new methods and materials.
  • Interaction between civil and mechanical structures.
  • Refurbishment of dams and hydro plants, including: analysis of civil structures, and quantitative comparison of structures before and after repair.
Tackling conduit replacement
15 June 2006

The removal and replacement of existing outlet works at Pablo dam in the US demonstrates a good example of the complexities of conduit replacement at an embankment dam

Generally, removal and replacement of an existing conduit through an embankment dam consists of excavating the dam down to the existing conduit, stockpiling the material, removing the existing conduit, constructing a new conduit and possibly new entrance and terminal structures, installing a filter around the downstream portion of the conduit, and replacing the embankment material. A cofferdam may also be required if the reservoir cannot be drained during construction.

Removal and replacement of a deteriorating conduit can be time consuming and expensive compared to other renovation methods. Typically, construction costs for removal and replacement may be five to 10 times higher than for other renovation methods, such as sliplining of the conduit. Construction costs rapidly rise as the height of the embankment dam increases. However, if the embankment dam is small and the downstream impacts to users are acceptable, this method may be more advantageous. Often, removal and replacement is the alternative of choice for low hazard embankment dams, since it is generally less expensive. This is especially true on older low hazard embankment dams, where they may have been built without the benefit of modern design and construction techniques and often lacked proper quality control. The safer and more efficient solution may be to remove and replace the conduit and possibly the entire embankment dam.

Modern principles should be employed for design and construction of the new conduit. A new publication is available through the Federal Emergency Management Agency (FEMA) titled ‘Technical Manual: Conduits through Embankment Dams’ which provides extensive information on the best practices for design, construction, problem identification and evaluation, inspection, maintenance, renovation, and repair of conduits.

Removal of an existing conduit

The advantages of removal and replacement of an existing conduit through an embankment dam include:

• Evaluation – the exposed foundation of the conduit can be fully examined and evaluated.

• Repairs – areas along the existing conduit that may have been damaged by internal erosion or backward erosion piping can be repaired.

• Seepage – extensive seepage control measures along the conduit can be installed.

• Design modifications – the new conduit can be designed to provide increased discharge capacity to meet current or future operational and emergency release requirements.

The disadvantages of removal and replacement of an existing conduit through a high embankment dam include:

• Cofferdam – unless the reservoir can be drained, the construction of a cofferdam is generally required. Inflows into the reservoir will need to be diverted. In some special cases a downstream cofferdam may also be required.

• Costs – construction costs for removal and replacement are generally higher than for other renovation methods.

• Reservoir operations – construction may impact reservoir operations and add risk to the downstream community.

• Seepage paths – if proper compaction of the embankment closure section is not obtained, potential seepage paths may exist along the junction of the closure section and existing embankment.

The first step in removal and replacement of the existing conduit is usually to excavate the embankment dam to the invert of the conduit and remove it. An excavation transverse to an existing embankment dam centreline increases the potential for hydraulic fracture of the replacement embankment material from arching. Because hydraulic fracture poses special hazards when the reservoir is subsequently refilled, special care is required for designs that involve excavation transverse to the existing embankment dam. Excavations should be wide enough at the bottom to ensure adequate working room for removal of the existing conduit and replacement with the new conduit, and compaction of earthfill materials.

A qualified professional engineer or engineering geologist should carefully observe and document the excavation required for the removal of the existing conduit to verify that any damaged embankment or foundation materials have been fully removed and/or treated prior to construction of the new conduit and replacement of embankment materials. Often, removal of the entrance structure, terminal structure, or other structures may be required due to age or deterioration, or to ease construction of the replacement structures. Occasionally, where removal of the existing conduit is difficult and expensive, the existing conduit may not be removed, but will be abandoned by backfilling the conduit with grout and installing a new conduit at a separate location.

A new filter should be designed to extend upstream into the embankment dam. Frequently, the filter installed in this situation is larger than that used for first time construction of an embankment dam. The filter should extend to both sides of the new conduit and key into the existing embankment dam. If the existing dam has a chimney filter, the filter should be designed to be a part of that system where feasible.

If the conduit is being replaced in a zoned earth fill embankment dam where a central core is substantially different in properties than the outside embankment shells, backfill for the conduit should coincide with the zoning for the dam. Core zone backfill should only be used around the conduit through the core section, with shell backfill soils used through those sections of the conduit. An exception to this recommendation is where rock shell zones include large angular rocks that could impose point loads on the conduit that exceed its strength. For that condition, cushioning soil with small sand and gravel should encircle the conduit to prevent the problem.

The soil removed from the embankment dam as the existing conduit is excavated is frequently reused to backfill the notch in the dam. Designers should carefully evaluate the water content of these soils and determine if drying or wetting is required for satisfactory reuse. The excavated slopes in the existing embankment dam may remain exposed for a period of time before they are backfilled. The time over which the excavation made to replace the conduit is left exposed may be hot, dry weather. In this case, the exposed soils on the face of the excavation may desiccate to considerable depths. Before commencing backfilling of the excavation in the dam, any desiccation cracks in the existing dam must be removed, and the earthfill surface disked and moistened. This process will probably have to be delayed until immediately before backfill of an interval of the dam is ready to commence. If backfilling of the excavation is interrupted during hot weather, the surface of the reconstruction backfill also should be closely inspected for desiccation features before placing new fill. Poorly bonded lifts can occur during interruptions of fill placement. They provide an avenue for possible internal erosion.

Designers should consider these important points:

• Testing – soils used to rebuild the embankment dam should be evaluated by the same tests that would be used to evaluate soils for a new embankment dam. The water content, plasticity, gradation, compaction properties, and dispersivity of clay fines are important evaluations. If the replacement fill is in a zoned embankment dam, similar zoning should be used.

• Water content – soils used to rebuild the dam should usually be placed wet of Standard Proctor optimum water content to improve their flexibility and resistance to cracking and arching. Compacting soils at water contents that are 1 to 3% wet of optimum significantly improves their flexibility. At the same time, the likelihood that pore pressures could be generated in medium to high plasticity clays in fills of significant height should also be evaluated. Designers must weigh the advantages of compacting soils wet of optimum against the disadvantages of this wetter compaction water content. The lower shear strength and potential pore pressures generated by wetter compaction water contents must be considered in the design stability evaluations. Many designers consider excessive pore pressures to be a lesser long term danger to the successful performance of an embankment dam than the danger of arching and hydraulic fracture if the soils are placed dry.

• Exposed filler – special care to remove desiccation cracks in exposed fill surfaces is important. This applies to the exposed excavation slopes and to layers of fill used in reconstructing the embankment dam.

Generally, the construction period for a complete removal and replacement of a conduit will require more time than other renovation methods. Mitigating the impacts of a longer construction period may require consideration of: (1) diversion and downstream water requirements (i.e. irrigation); (2) traffic control measures (lighting, signs, etc.), road closures, construction of detours (such as detouring dam crest traffic); (3) larger disturbance areas and potential environmental issues; and (4) draining or drawing down of the reservoir.

An example of the replacement of a conduit is the Pablo dam modification.

Pablo dam

Pablo dam is located on the Flathead Indian Reservation near Polson in Montana, US. The dam is operated and maintained by the US Bureau of Indian Affairs. The embankment dam is an earthfill structure consisting of a main dam and dikes, which flank both sides of the dam, south and north. The crest elevation of the main dam is at 981.5m, and the dikes are at 980.5m. The main dam has a structural height of 13.1m, a crest length of 3215.6m, a crest width of 6.1m, a 3:1 upstream slope and a 2:1 downstream slope. The north dike has a crest length of 1783.1m, and the south dike has a crest length of 3124.2m. The crest width of both dikes is 3.7m.

Pablo dam was constructed in three phases over 24 years. In 1911, the embankment was constructed to elevation 976m. The second construction in 1918 raised the embankment dam to elevation 978.1m, and the final construction in 1934 raised the dam to the present elevation of 981.5m. Pablo dam is an offstream structure that is fed by the Pablo Feeder canal, and its purpose is to impound water for irrigation. The reservoir has a capacity of 35Mm3 at elevation of 978.7m.

The original outlet works were situated at the maximum section of the dam and consisted of a 12.8m high concrete intake structure with two 0.9m by 1.52m slide gates. The original outlet works consisted of three box shaped conduits; the middle and south conduits were 52m long and 1.37m wide by 1.52m high. The north conduit was about 41.4m long and 31.37m wide by 1.52m high, but was abandoned prior to the third phase of original construction.

Differential settlement between the intake tower and the outlet works conduits caused some offset in ‘sliding joints’. This settlement was expected, as sliding joints (no reinforcement crossing the joint) were included in the original design. However, continued settlement of the intake structure and the first 4m of the conduits required grouting of the foundation shortly after construction. No further settlement had been detected in the last 50 plus years. The first sliding joint is displaced vertically about 5cm and sprays water at high reservoir head. Mortar filling in all sliding joints was disbonded, cracked, and deteriorating; tensile cracks were also discovered along the length of the conduit. Water was commonly leaking from both the cracks and the sliding joints, and there are signs of possible internal erosion of embankment material occurring in a few areas. Furthermore, spalling concrete had been discovered in the walls of the conduits. The concrete in the centre wall at the downstream end of the conduits was deteriorated, resulting in exposed aggregate and rebar.

Commencing work

Dam safety modifications began in 1993, consisting of the injection of polyurethane grout into cracks and conduit joints. A two-man crew from McCabe Brothers Drilling of Idaho Falls, Idaho, mobilised to the job site. They installed ventilation ductwork into the two outlet works conduits and began drilling injection holes in the south conduit. Existing cracks (mostly at construction joints) upstream of station 0+38.71 were injected with polyurethane resin grout to stop leakage through the cracks. This was done prior to repairing spalled concrete in the conduits. The subcontractor used a ratio of polyurethane to water of 1.3:1, which effectively stopped 90% of the seepage. However, after completing injection of cracks in the south conduit, seepage began to migrate downstream and appear in cracks that were previously dry.

During drilling of the injection holes, two voids were discovered, one in the crown of each conduit at station 0+3.96. The voids were approximately 30.5cm deep and 61cm wide and seemed to be connected to each other. Old construction drawings showed this as the location where concrete counterfort walls, which support the intake tower, meet the conduits. No voids were found behind any of the other cracks. The voids at station 0+3.96 were injected with polyurethane. As injection of the south conduit was completed, some migration of polyurethane was noted through the crown and divider wall of the middle conduit.

In mid-November, McCabe Brothers Drilling finished injecting polyurethane resin into cracks in the outlet works conduits. They injected a total of 1154.5 litres into the two conduits (the specified quantity was 189.3 litres). As the injection operation progressed from upstream to downstream, cracks that had been previously dry near the canal outlet began to seep water. Therefore, these cracks were injected also. Because the seepage appeared to be following the exterior of the conduits and exiting farther downstream, the seepage continued to be unfiltered and may increase the internal pressures in the embankment. A decision was made to install weep drains in the conduit and to construct a filter collar about the exterior of the walls. A modification to the contract was issued to provide for this additional work. After the polyurethane injection was completed, the conduits were unwatered and inspected. Repair areas were marked, and the contractor began chipping out and preparing the surfaces of the repair areas for epoxy-bonded concrete. Approximately 30 small repairs and one large repair at the conduit outlet (splitter wall) were done to complete the conduit repairs option of the work. Smaller and shallow areas were repaired using an approved two-part epoxy material. Larger areas were repaired with epoxy-bonded concrete.

Areas of concern

During an inspection of the interior of the conduits in April 2001, it was discovered that material had been deposited inside the middle conduit near an opening in a construction joint. This was occurring through a hole in the floor of the middle conduit at a construction joint near station 0+39.62. Approximately 2.83cm3 of silt and fine sand were deposited on the floor. However, this deposit was observed during the winter when no irrigation releases are made. More deposition may have occurred during irrigation season that was washed downstream and not observed. Consequently, the total volume of material could have been much greater than the 2.83cm3 observed in 2001. The US Bureau of Reclamation (USBR) theorised that plugging this opening could result in redirecting the erosion through a different hole or crack in the conduit. Also, redirecting the erosion might cause a more dangerous path to develop along the foundation contact of the conduits, and an erosion exit might develop downstream of the embankment dam. If the exit point was located within the outlet channel, early detection would be very difficult.

Another area of concern was the condition of the north conduit that was reportedly plugged at each end prior to the final raise of Pablo dam in 1932, but was never confirmed. Therefore, it could be possible that a nearly full reservoir head could exist at the end of the north conduit, which was less than 31m from the downstream toe of the dam. After much discussion between all involved parties, it was decided to completely remove and replace the original outlet works.

As an interim measure, a temporary patch was installed over the opening to prevent additional material from being eroded into the conduit while allowing for relief of water pressures. The patch consisted of filter fabric under a metal screen. During March 2002, the geotextile portion of the patch ruptured and approximately 1.42cm3 of silt and fine sand were deposited into the conduit. The patch was repaired soon after the rupture was discovered. Reservoir level restrictions were implemented in April 2003 and were to be kept in place until the removal and replacement modifications could be completed.

New outlet works

The USBR prepared designs, drawings, and specifications for the conduit replacement. The construction of a new outlet works began in November 2004 and was completed by the spring of 2005. The major aspects of the work included:

• Construction of a cofferdam to maintain an area free of water during construction.

• Clearing, grubbing, and stripping prior to excavation.

• Removing existing embankment dam slope protection.

• Excavating embankment materials to accommodate construction of the new outlet works (Slopes transverse to the dam centerline were excavated at 4H:1V).

• Removal of the existing reinforced concrete intake structure, conduits, retaining walls, and apron. A forensics investigation was conducted during the excavation to better understand the causes and mechanisms on how embankment materials were transported into the conduits. Provisions were included within the specification paragraphs requiring the Contractor to facilitate such efforts.

• Constructing a lean concrete mudslab, on which to found the new outlet works.

• Constructing reinforced cast-in-place intake structure, conduit, retaining walls, and apron. The new conduit was double barreled with each barrel having a 1.83m, 7.5cm inside diameter. The exterior surface of the conduit was sloped at 1H:10V below spring line and was curved above spring line to provide a good surface to compact earth fill against. Each conduit joint was a treated as a control joint with longitudinal reinforcement extending across the joint and 15cm polyvinyl chloride (PVC) water stop.

• Installing two emergency guard gates and two regulating gates within the upstream intake structure.

• Constructing a chimney filter and drain system. The filter extended downstream and encased the outlet works conduit. Filter materials encasing the conduit consisted of sand processed to a specified gradation from an approved offsite source.

• Placing and compacting zoned earth fill in the embankment dam closure section.

• Replacing the embankment dam slope protection.

Lessons learned

Sometimes repairs alone are not fully robust enough to address all the unknown erosional mechanisms existing within an embankment dam. Due to continued dam safety concerns, more extensive measures may be warranted.

The absence of fines within the embankment facilitated seepage and internal erosion of organic materials along the conduits.

Dissimilar foundation materials under the conduits at about Station 0+39.62 probably allowed for differential settlement. This settlement could have caused the conduit to crack. At this same location, a hole between the floor and the south wall allowed the internal erosion of embankment materials into the conduit.

Antiseep collars did not stop seepage or internal erosion of fines along the conduit. There was really no benefit from the collars because the embankment materials adjacent to the conduit were so pervious.

The polyurethane resin was able to travel along the sides and around the collars of the conduits. However, the polyurethane did not travel more than about 5 feet (1.52m) perpendicular to the conduits. Although there was limited opportunity to observe the foundation material under the conduits, what was seen
contained no polyurethane
Tunnelling at Tsankov Kamak
18 July 2006

Four tunnels were excavated as part of the Tsankov Kamak hydro power plant in Bulgaria

Groundbreaking began earlier this year for the Lyaskovo Road tunnel, the last tunnel at the Tsankov Kamak hydro power plant in Bulgaria. This project, which started in 2004, is located at the Vacha hydro power cascade on the Vacha river in the Rhodope Mountains, near the town of Smolyan on the border with Greece, about 250km south east of Sophia.

Tsankov Kamak is designated as a CO2 reduction project within the scope of the Kyoto Protocol mechanisms. Emission certificates generated by the plant will be transferred to the Austrian Carbon Credit programme via the Joint Implementation Mechanism (JI).

The client and project investor is the Natsionalna Elektricheska Kompania (NEK), Bulgaria’s national electric company. The work will be carried out by an Austrian industrial group led by VA Tech Hydro, and includes Verbundplan (now Pöyry Energy GmbH) for engineering/project management and Alpine Mayreder for the civil works, including tunnelling.

VA Tech Hydro, now a member of the Andritz group, is responsible for the supply of the electro-mechanical equipment like penstocks, gates, water turbines and generators, including mechanical and electrical balance of plant. The total investment is about h200M (US$250.8M), which covers civil works, engineering and the complete electromechanical package. Financing was arranged by VA Tech Finance and is entirely funded by the Austrian side.

The contract involves the construction of a 125m-high arch dam and an 80MW hydro power plant, with a standard production of 185GWh/year. The reservoir will have a volume of 111Mm3 and the power station will be located downstream, above ground on the left bank. The intake water tunnel, approximately 600m long, will be driven conventionally and will partly cut across a geologically problematic zone. An area of approximately 60,000m2 must be sealed off around the intake structure in the Gashnia Valley due to this area’s geological features.

The whole project will provide work for up to 1200 people during the four-year construction period.

Tunnel programme

The design of this project called for the building of four tunnels: the by-pass road tunnel as part of the temporary road connection, the diversion tunnel, the penstock tunnel with a vertical shaft, and the Lyaskovo Road tunnel.

With a height of 125m and a crest length of 457m, the double curvature concrete arch dam will close off the Vacha Valley. The river will be diverted through a 500m tunnel and the public road will run through a 200m road tunnel in the dam area during the construction period. In addition to the intake structure, the intake waterway consists of a 60m vertical shaft and a 530m horizontal pressure tunnel with a 10% inclination. Steel lining will be required for the entire intake waterway. The water will power two Francis turbines and flow through a stilling section back into the Vacha river, which will be in the tailrace channel.

The 197m by-pass road tunnel was completed in accordance with the Verbundplan design. It was executed by the New Austrian Tunnelling Method in only 100 days. Drilling, blasting, excavating and lining operations were carried out as a continuous process, on a two-shift programme by fifteen workers on each shift.

The tunnelling and construction equipment for all tunnelling work was sourced by Alpine Mayreder Bau GmbH, Salzburg and Alpine Bulgaria AD from many locations around the world including Finland, Sweden, Germany, Switzerland, Russia, US and Bulgaria.

Diversion tunnel (length: 49m)

To reach the portal of the tunnel it was necessary to construct an access road. First a temporary pipe bridge was erected on the right-hand side of the river, close to the diversion tunnel outlet, to obtain access to the portal. Daily maintenance of the pipe bridge is necessary due to output from operations at the Devin hydro power plant.

Where it was not possible for excavation works for the diversion tunnel outlet portal to be carried out conventionally, drilling and blasting methods were used. Slope protection consisting of a 6cm layer of sulphatresistant shotcrete C20/25 was installed. While excavation of the portal was being carried out, the protection dam was built. Rock material from the portal excavation and the blasting operations at Road No.8 was used as fill for the dam. The dam was constructed in accordance with the technical specifications. After finishing excavation works at the tunnel outlet portal, the first face drilling was completed as follows:

•Survey and mark tunnel face and axis.

•Horizontal drilling and grouting of 3m anchors in a single row.

•Complete face drilling with a drilling length of 2m.

•Loading and blasting.

After excavating approximately 5m, the steel trusses were installed. After installation and support of the steel truss, drilling and blasting works were carried out according to the New Austrian Tunnelling Method. The stability of rock formations after blasting operations in the area of the tunnel face was checked before further support measures were carried out.

In normal rock conditions it was possible to proceed with conventional drilling and blasting. In bad rock conditions, support measures such as pre-shotcreting and spot bolting with an anchoring system were necessary. The support measures were executed approximately 10-15m behind the actual tunnel face. They consisted of pre-shotcreting caves and geological overbreaks, fixing steel mesh, and applying a second shotcrete layer through the steel mesh. Processing the second pre-shotcrete layer was carried out wet-in-wet. Finally, anchorage according to the design was completed. This is the proposed working process until the breakthrough. It was not necessary to overwork the surface of the tunnel in respect of the hydrogeological conditions. The mix design for the shotcrete is the same as used in the road by-pass tunnel.

After erection of the temporary road, excavation works for the inlet portal were completed in the same way as described for the outlet portal. After finishing the excavation works for the portal, the underground excavation works were then started from the inlet towards the outlet. Materials for the shotcrete and concrete were delivered from the small batching plant located next to the old diversion tunnel outlet. Blasting materials were stored and managed in the blasting depot next to the small batching plant.

Penstock tunnel

To reach the Intake it was necessary to excavate the intake – open cut, approx 30.000m3 – and parts of Road Nr. 4. To reach the tunnel outlet area, the landslide and power house excavations had first to be completed. The works were carried out with excavators and trucks. Where those methods were not possible, drilling and blasting was used. Slope protection, from el 673.45m asl down to 659m, was carried out with shotcrete C20/25 in a thickness of 17-30cm. Mesh 6.5mm dia, 15/15 and rock bolts dia 25mm and 28mm were also used. After finishing the open cut excavation works at the inlet, work started immediately on the site installation and excavation for the shaft and bend. This began on 1 August 2005. Drill and blast operations were executed. After the blasting, checking and validation of rock formations, instant support measures for the met rock conditions were carried out. Subsequent processes were similar to those used on the diversion tunnel.

Following the power house excavation and support of the slopes, work started immediately on the site installation and tunnel excavation from the power house site. The underground excavation works then began at the outlet towards the intake.

Lyaskovo road tunnel (length: 882m)

To reach the entrance (South) portal of the Lyaskovo tunnel, it was necessary to first erect a temporary access road. On the exit (North) portal, two temporary access pipe bridges were also erected.

As with the other tunnels, excavation works for the portals will be carried out with excavators and bulldozers, but where that is not possible, drilling and blasting will be used. It is anticipated that excavation of the portal area will include the removal of a diluvium layer and strongly weathered rocks.

Slope protection will include two layers of shotcrete C20/25, with a thickness of 5cm for each layer, steel mesh dia 6.5mm; 15/15 cm; Steelclass A-I and reinforcement concrete rockbolts 25mm dia. Steelclass A-III, L = 3.00m will be used at intervals of 2m in both directions. The execution sequence for the portals is as follows:
•Excavation works to the top of the tunnel portal.
•Support for the slopes with shotcrete, wire mesh and rockbolts.
•Erection of the protective ditch.
•Mount first support ring. Then the next three rings will be added outside. Steel mesh, sieve and shotcrete will be mounted over.
•Begin excavation of the tunnel calotte (top heading of the tunnel profile) and support.
•After excavation of approx 50m of the upper side of the tunnel profile, begin excavation of the invert, the lower side.
•After excavation of the lower side, the lower parts of the steel rings and shotcrete to the bottom of the tunnel profile will be applied.
•For access to the face, a temporary loading platform will be erected from excavated material after each step.

Dewatering of the portal areas will be through a drainage system from river gravel. Under it, a drainage pipe ø100 will be mounted. The drainage pipes pass by the lowest part of the protective fence and reach to the dewatering ditches of the road.

After installation and support of the steel truss, normal drilling and blasting works will be carried out according the New Austrian Tunnelling Method. Installation of the support measures depends on the stability of the rock formations after blasting in the tunnel face area. The contractor’s engineer is responsible for determining the stability of the rock formations and the support measures required for the met rock conditions.

Excavation works in the portal from the Devin side should be finished completely before the tunnel breakthrough. After breakthrough, it may be necessary for the shotcrete to be restored according to the Execution Design.

Overcoming hazards

Projects like this are rarely accomplished without encountering difficulties at some stage. Tunnelling in the Rhodope Mountain range produces its own problems. Challenging geological issues, coupled with the unique design of the dam added further complications. For example, at some points, instead of finding rock – as indicated in the drawings – gravel was discovered. In other instances, it was the other way round – rock where gravel was expected. Things like this cannot always be predicted accurately during the design and budgeting phase of a project. Also, weather conditions in the mountainous locations were very severe and changeable. During the winter months, temperatures plummeted to 20º below zero!

In addition to coping with these problems, all works were, and are, being carried out with due regard to the safety and security of the environment – the plants, the trees, the wildlife and of course the air and the water. All machinery is equipped with the necessary filters, engines are controlled to prevent incidents like oil losses, and operations are confined to the area of the future lake as much as possible, to avoid spoiling the surrounding region.

Energy boost

The rehabilitation and integration of all works will improve the operation and water management of the other cascade hydro power stations. Environmentally, the plant will have no added impact on water flows, as the new construction site is located within the existing cascade of power stations. The construction of the missing interim section of the Dospat-Vacha hydro power cascade will contribute to the effective utilisation of the hydro energy potential along the Vacha river.

The existence and high flexibility of this hydro power plant will boost the operational efficiency of the power generating facilities connected to the electricity grid. It will contribute to the peak and peak-following load demand and frequency regulation. It will provide a fast reserve in case of a major unit failure and, above all, it will substantially reduce emissions potential.

Tsankov Kamak hydro power plant, with an estimated production of 185GWh/A, is extremely important to Bulgaria for energy-political reasons. It has already been decided to shut down the Bobov, Maritsa 3 and Rousse coal-fired power plants in 2008. They are to be replaced by the Tsankov Kamak plant, which is designated as a pilot project. The Austrian Ministry of Environment and Bulgaria have signed a bilateral agreement for the construction of the hydro power station as part of the Kyoto Protocol. The reduction of CO2 emissions caused by shutting down three coal-fired power stations in Bulgaria and the construction of the Tsankov Kamak HPP by 2008 will be used by Austria as a basis to verify the reduction of emissions under the Kyoto Protocol. On this basis, approximately 250,000 certificates per year will be generated by this power station from 2008 to 2012.
Click on Images

View to the cofferdam: upstream protection against 20 years flood ...
 


Further view of the shaft water intake
 


Tsankov Kamak penstock bifurcation piece in the workshop during fabrication ...
 


The south portal of the street tunnel on the road ...
 


Shaft for the water intake: From here the water will ...
 


View of the penstock outlet: from here the water will ...
 


Excavation for the Dam body: you can see the excavation ...
 

 
Large steps in small hydro
21 July 2006

The 18th Hidroenergia conference took place in Crieff, Scotland, UK, on 8-9 June. The biennial European event continued its trend for interesting and diverse papers and presentations on the topic of small hydro power, writes Jon Last

Set in the extravagant Crieff Hydro hotel in the beautiful Perthshire region of Scotland, the country that is the main source of the UK’s hydro power (160 sites, 120 of them small hydro), the Hidroenergia 2006 conference was another memorable entry to the series of symposiums that began back in 1989 in Madrid, Spain. Papers presented at the conference – which was organised by the British Hydropower Association (BHA) and the European Small Hydropower Association (ESHA) – were separated into three categories, and delivered in sessions accordingly: Policy & New Opportunities; Environmental & Planning; and Engineering Solutions. These sessions looked at the key issues affecting small hydro developers today – and the following paragraphs take a look at a paper from each.

Policy and new opportunities

‘Community Hydropower In Somerset and Dorset – Mill Owners Working Together’ by Keith Weaton-Green of South Somerset District Council is the tale of a ‘trail blazing’ mini hydro project in the southwest of England, UK, where owners have worked together to create a small hydro network across the region’s historic mills.

In 2001, Weaton-Green, the environmental projects officer for the council, had the dilemma of trying to appease a community that was keen to embrace renewable energy (required by the council’s renewable energy objectives) but had shown disdain for the ‘obvious’ choice, wind power. As a result, Weaton-Green began investigating the idea of developing small-scale hydro power. He decided the most straightforward way of producing this new energy would be to consult owners of the Somerset region’s several mills, and see if they would develop hydroelectricity at their sites. The general consensus was that they would, provided they were supported both financially and technically.

A first meeting was set up at Gants mill, with mill owners informed about the process of getting abstraction licences from the Environmental Agency; the availability of feasibility studies from consultants Hydrogeneration; and how the council would provide its own administrative support. This satisfied a number of candidates, and 12 mill owners agreed to put £100 (US$184) each towards feasibility studies at their own mills, with the council matching that amount again.

Since then, the organisation dubbed the Somerset Mills Group has flourished. It meets every six weeks at a rotation of mills, has received grants from central government and an electrical supply company has installed turbines at seven of the sites – with the remaining five locations set to be equipped by the end of 2007.

The small-scale nature of the operation means that even with grant funding it has been difficult to achieve a return on the mill owners’ investment of less than eight years. Similarly, in this size project, crossflow turbines have proved to be the best value equipment; a more efficient Kaplan model would doubtless put the sites beyond financial viability. But equipment choices vary across the mills – in fact, at one site, a low cost Vietnamese fixed flow propeller turbine has been used; at another, the mill owner has designed and built his own 10kW double regulated Kaplan turbine and cleared the silt from 200m of clogged, brick lined, underground leat by hand. The complete line-up of turbine types at the seven sites is as follows: Vietnamese propeller (producing 2kW); Valley H crossflow (3.7kW); Ossberger crossflow (3.7kW); plastic Francis + Armfield (1.6kW+); Valley H crossflow (8.2kW); Valley H crossflow (12kW); double regulated Kaplan (10kW).

The scheme in Somerset has been successful and well-publicised, and has left something of a legacy – its mill owner/civil society partnership model has inspired similar groups to form across the UK.

One such group is the Stour and Vale Hydropower Group, based on the Stour catchment in Dorset, a potentially lucrative area geographically adjacent to the south Somerset group. The spirit of openness and communication has transferred to this newer group, and personal contact is made between them, resulting in fast progress being made. The Stour and Vale set has been granted similar support from its own council, the North Dorset District, and has received a healthy £30,000 (US$55,600) in grants to date. Following intense lobbying, the group has attracted the interest of a major electricity supply company, whose board of directors is currently discussing a package for site owners that will give them returns proportionate to their investment – a financial model that promises a productive future. Feasibility studies for most of the viable Stour sites are now complete and detailed design and abstraction licence negotiation is underway for the first tranche of sites for development.

Further small-scale hydro power developments are emerging in the south and west of England, in areas such as Mendip, West Dorset, Wiltshire, Dartmoor, Exmoor, Kent and South London. It seems that the success in Somerset has lead to an endowment of new hydro in the UK – an example of what can happen when people work together.

Environment and planning

This section bought with it a paper titled ‘Rivers Exempt From Damming – Case Study of Lowlands’, by Petras Punys of the Lithuanian Hydropower Association (LHA). This piece chronicled a situation that occurred recently in Lithuania when a law was introduced that actively subdued hydro power development in the country, despite the nation’s strong potential, and a subsequent study on two of its largest rivers to investigate the consequences of this dam prohibition.

Lithuania is located in Europe, sandwiched between Poland, Belarus and Latvia. An amendment was made to the country’s national Water Law in 2004, producing a subsequent act that made it illegal to construct a dam on any of its largest rivers, along with a very high number of medium and small watercourses. The new law decided that dams would affect the rivers ‘from an ecological and cultural point of view.’ This naturally decreased Lithuania’s economically feasible potential of small hydro power, and basically eliminated the potential for large hydro (classified as more than 10MW).

This, the paper infers, comes as decidedly odd considering the government’s attitude a mere two years earlier. Lithuanian renewable energy policy is consistent with the EU RES-E relevant policy documents, and fosters the use of hydro power. The National Energy strategy of 2002 emphasised the use of the largest rivers of the country – Nemunas and Neris – for hydro power purposes. Furthermore, the government’s ‘master plan’ for the country’s development is to further modernise the main inland waterway on the lower Nemunas (just downstream of Kaunas, the second largest city in Lithuania), as well as constructing a new waterway on the lower Neris river, between Kaunas and Jonava, including the establishment of a river port. Banning the construction of dams – a list of 169 ‘forbidden rivers’ was compiled – undermines this strategy.

As a reaction, the LHA commissioned a study of those two largest rivers, the Nemunas and Neris.

Nemunas river is the largest river in Lithuania, flowing through Kaunas city and discharging to Curonian bay (the Baltic sea). It has a roughly 100,000m2 catchment with mean flow of 630m3/sec, and currently has only one hydro power plant operating, producing 100MW.

The concern for the LHA study was how to improve the technical conditions of the waterway on the Nemunas. To do this, the lower part of the river was subject to a feasibility study, one of the main aims of which was to determine the cost effective means for providing sufficient depth. All through the investigation, as the consultant involved carried out studies including river bed and valley digital terrain modelling, it became clear that building dams on the river would certainly benefit the waterway and make it more commercially attractive.

A range of possible depths for the navigation fairway were considered: 1.5m (zero option); 2m; 2.5m; and 3m. It was concluded that a more significant depth was required: at least 3m. This level could be assured by building low head dams. In fact, the paper says that many advantages of damming were taken into account: multipurpose utilisation of water resources including power generation, the opportunities to develop recreational activities, improvement of the landscape surrounding the river, the status of which is currently poor, and so on. But, of course, the new Water Act forbids the construction of any such structures.

The second LHA study was of the second largest river in Lithuania – the Neris river’s catchment area and mean flow are 24,942km2 and 179m3/sec respectively. The study of this river was more focused on how to use it to its best potential; its water resources are not being as intensively used for economic or recreational purposes as they could be. The list of advantages of using dams to regulate the river and its depths include: protecting the Neris’ wide fish population; the regulation of depth to allow water tourism (such as navigation by cruise ships) in the three short summer months of the summer tourist season; prospects for allowing recreational fishing; developing drinking water supply through reservoirs; and flood control. But since dams are not permitted on the Neris, these opportunities are going to waste.

All this is before generating hydroelectricity has been mentioned – a factor which, the paper says, could go some way towards justifying any large costs. Naturally, the LHA’s reaction to the 2004 Water Act seems to be one of frustration, and Punys is confident in his paper that if the national environmental legislation is reviewed then it would find that building dams does not necessarily have to hinder the country’s economic and social welfare – in fact it may be found that quite the opposite will be true.

Engineering solutions

‘Vasocompact – A European Project For the Development of a Commercial Concept for Variable Speed Operation of Submersible Compact Turbines’ is a many-authored paper from the Engineering Solutions section of the event. It is written by Jochen Bard (from the Institute for Solar Energy Supply Technology, Germany); Heikki Pirttiniemi (Waterpumps Oy, Finland); Prof. Dr. Eberhard Goede (Institute of Fluid Mechanics and Hydraulic Machinery, University of Stuttgart, Germany); Albin Mueller (Elmotec Antriebstechnik AG, Switzerland); Dr. Drona Upadhyay (IT Power, UK); and Martin Rothert (SMA Technologie AG, Germany).

The project, developed between 2001-2005, was to design mechanically unregulated submersible turbines operating at a variable speed. It was based on an existing turbine concept, modified using a permanent magnet synchronous generator (PMSG) directly coupled on the runner shaft and a frequency converter (FC) for variable speed operation. After a new runner was designed using computational fluid dynamics (CFD) and tested in the hydraulic test rig, Tirva hydro power plant in Finland played host to a 50kW field test version of the new turbine concept, which was developed manufactured and tested at the facility.

The paper reports that variable speed operation in small hydro power plants has already been tested in a number of projects (some of which can be observed in the ‘Status Report on Variable Speed Operation in SHP’ document, published on the DGTREN website at http://ec.europa.eu/energy/res/sectors/doc/small_hydro/statusreport_vspinshp_colour2.pdf) – but has been more often than not considered unfeasible. But these past tests, usually reliant on using a modified Kaplan turbine design, were not, according to the paper, efficient enough to give a significant cost reduction.

The project presented in the paper offered what its calls an ‘optimised technical concept’ for VSO of compact turbines, and yielded an ‘innovative turbine concept’, based on a submersible unregulated compact turbine concept of the Finnish manufacturer Waterpumps Oy, whose turbine runner and intake, PME-generator and frequency converter are optimised for VSO. The paper also states that speed variation is used for optimising hydraulic performance at different heads and flows; the permanent magnet generator avoids any kind of mechanical speed increaser and is optimised for operation with a frequency converter; and a special frequency converter for the speed control and grid connection eliminates the need for power factor correction and other additional grid connection requirements.

The development goals of the project were as follows:

•Achieve a high system performance over a wide operating range.

•Reduce the overall investment cost for small hydro power.

•Reduce electricity production cost at low head hydro power sites.

•Increase the flexibility and operating range of the compact turbine series in order to maximise the advantages of the concept for a wide range of sites.

Using CFD, three options for runner geometries were investigated: divergent radial, convergent radial and axial, with the latter chosen. The final design specifications for the axial runner were as follows: design net at optimum 3.5m (from demonstration site Tirva); design discharge 1.4m3/sec (required to achieve 50kW); design speed 600rpm; operational speed range ± 25% from design speed; number of blades: four (a better option than five in terms of cost, soiling tendency etc.); and an outer diameter of 0.6m (determined by available draft tube).

The speed–torque characteristics of the runner and the next design phase, the generator, had to match. The following specifications for the generator were agreed for the laboratory test machine:

•Rated speed: 600rpm.

•Rated power: 54kW (56kVA).

•Rated torque: 840Nm.

•Number of poles: 40 (rated frequency 200Hz).

•Rated voltage: 313V (open circuit).

•A sinusoidal voltage shape (at no load).

•Rated current 92.4 A (eff).

•The use of standard plate design for the stator, but improved materials (higher efficiency).

•Runaway speed >300% of rated speed (has been set to 2000rpm).

•IP44, external water cooling.

•Integrated temperature sensors (2 Pt100).

Finally, after the generator design was in place, detailed specifications of the frequency converter (FC) could be made – the generator is directly connected to the FC. A controlled rectifier was selected over an uncontrolled diode rectifier, partly as it represented a cheaper option for a 100 to 200kW energy range. The FC’s functions are to convert the variable generator frequency into 50Hz, to compensate the variable voltage from the generator; to control the speed of the runner by controlling the generator torque through the electric current; and to supervise the generator as well as the grid with respect to frequencies and voltages.

Careful attention was paid to the torque control of the FC, as it and the PMSG were tested intensively in the laboratory. Following the successful field tests, the turbine was set up to operate continuously at the Tirva site – for about 2849 hours between November 2005 to April 2006. About 87MWh, with an average output of 30.6kW, was produced during this test phase, where three different speeds of 450, 600 and 750rpm were trailed at three different heads of 3.37, 3.81 and 4.44m (resulting in nine different operating points). The maximum mean power output in a one hour interval during the test phase was about 45kW.

The project was an encouraging endeavour for those involved; total ‘water to wire’ efficiencies in the range of 80% were found during the tests, and the paper insists that that, as in any new concept, room still remains for optimisation in the future.

Lack of recognition

It was an opportune time to be meeting in the UK to talk about small hydro, since the long-anticipated UK Energy Policy Review has just recently been carried out by the Government’s Department of Trade and Industry. Along with predictable talk of nuclear development, the review encourages the acquisition of power through renewable means, with an obligation to push this towards 20%, which would mark a 5x increase, in order to reduce carbon emissions.

However, any mention of hydro power in the official press report is conspicuous by its absence; wave and tidal is the closest reference. This lack of recognition is a sometimes frustratingly common theme where hydro is concerned; it was pointed out in the opening address of Hidroenergia 06 that hydroelectricity is often overlooked as a power source, even when talking about renewables. The reasoning is not certain – suggested factors include that it may not be new enough – but, as we have seen at times in the featured papers above, often when initial ignorance or apprehension are tackled, the dam and hydro power route can prove to be a truly worthwhile option.
Teton Dam
The reservoir behind the Teton Dam was emptied within hours of the initial breach.   
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The reservoir behind the Teton Dam was emptied within hours of the initial breach.

The Teton Dam was a federally-built dam on the Teton River in southeastern Idaho in the United States which suffered a spectacular failure on June 5, 1976. The collapse of the dam resulted in the loss of 14 lives. The dam cost about USD $100 million to build, and the federal government paid over $300 million in claims related to the dam failure. The dam was never rebuilt.

Failure report - the case of Teton dam
16 August 2006

On 5 June 1976, the Teton dam in Idaho, US, failed during its first filling. The failure received considerable scrutiny from engineering experts, with several failure mechanisms suggested. Here, N Sasiharan, B Muhunthan and V S Pillai revisit the failure, using the modern framework of fundamental state-based soil mechanics as an investigation tool

Teton dam was designed and built to the modern standards of the 1970s; however, the 123m high zoned earthfill dam failed during its first filling on 5 June 1976. The dam was constructed on the Teton river (a tributary of the Snake river) as part of a major irrigation-power-flood control scheme in the high plateau of southeastern Idaho, US (Figure 1). The failure of the dam resulted in 14 fatalities and a very large economic loss. Its failure was one of the most publicised events at that time involving a large earthfill dam built using current standards. Therefore, it received considerable attention from engineering experts around the world. However, the failure assessment and prognosis by experts including those by the Independent Panel (IP, 1976) and the Interior Review Group (IRG, 1980) failed to arrive at a conclusive explanation. Failure mechanisms suggested included hydraulic fractures, internal erosion, the wet-seam theory, and defects in the abutment rock. However, there remained an unanswered question as to why the dam breached when the reservoir reached el. 5301.7ft (1616m) and initiated only in the vicinity of Sta.14+00 on the right abutment.

The impervious core/water barrier (Zone-1) of Teton was constructed of uniform clayey silt of low plasticity and low liquidity index. Highly compacted soils of low plasticity tend to crack in an environment of low liquidity index, low confining stresses and high shear stresses. None of the previous investigations focused on the possibility of the presence of cracks in the upper portions of the dam. This critical feature is investigated here using the fundamental concepts of the modern framework of ‘state based soil mechanics’ (Pillai and Muhunthan, 2001, 2002). The investigation consisted of laboratory tests on Zone-1 material to determine the physical and mechanical parameters and finite element analysis conducted using ABAQUS to simulate field stress conditions.

Background

The design cross section of the Teton dam at the river valley and the right abutment are as shown in Figure 2 and Figure 3, respectively. The construction of the dam began in June 1972 and was completed in November 1975. The dam was conservatively designed to have a wide impervious core with a head to width ratio of about 1.5 (Figures 2 and 3). The bedrock consisted of open-jointed rhyolite and basalt but was well treated with blanket and curtain grouting. The abutment rock was trenched to provide a large core-rock contact and a long flow path to have a low seepage gradient (Figure 3).

The impervious core (Zone-1) of the dam consisted of clayey silts of aeolion origin with low plasticity (PI ~ 4) and USCS classification of CL- ML. As per the design and specifications Zone-1 material was placed at average water contents of 1% dry of optimum and compacted to a maximum dry density of 98-102% of the Standard Proctor test (Figure 4). Similarly, the support zone (Zone-2) (chimney filter/drain) was compacted to a high relative density of the order of 65-70% (IRG, 1980).

The first filling of the reservoir began on 3 October 1975. The rate of filling of the reservoir was about a foot per day (0.3m) in the early stages; however, it was increased to about 3ft (0.9m) per day for the most part of May and June 1976. When the dam breached on 5 June 1976 the reservoir had reached only el. 5301.7ft (1616m), which was about 22ft (6.7m) less than the design full pool elevation.

Surface manifestations

On or before 3 June 1976 (reservoir level was at or below el. 5297ft [1614.5m]), no unusual signs of distress or springs or other water seepage were noticed downstream of the dam. On 4 June, minor evidence of clear seepages appeared downstream, a good 1300-1500ft (396-457m) distance from the toe, which was consistent with the raising of the ground water regime due to rising reservoir water level. Late in the evening of 4 June (reservoir el. 5300ft [1615m]), some dampness was noticed in the downstream dam slope at the right abutment at el. 5200ft (1585m). The following morning on 5 June, shortly after 7:00AM (reservoir el. 5301.3ft [1615.8m]), some muddy water was first observed to be flowing from the junction of the embankment and the abutment at el. 5200ft [1585m]. At 10:30AM, a large leak of about 15cfs (0.4m3/sec) appeared with a ‘burst’ on the downstream at el 5200ft (1585m). The leak appeared to emerge from a tunnel of about 6ft (1.8m) in diameter from inside the embankment and roughly perpendicular to the dam axis at Sta. 15+25. At about 11:00AM, a vortex appeared in the reservoir near Sta. 14+00 above the upstream slope of the embankment. At 11:30AM, a sinkhole on the downstream slope (el. 5315ft [1620m]) developed near the crest and above the leaky tunnel. At 11:55AM, the crest of the dam began to collapse between the vortex and the sinkhole, leading to a full breach at 11:59AM (IP 1976).

Fracture, rupture and ductile behaviour of soils

Aggregates of grains that form natural and man-made soil deposits exhibit three distinct classes of behaviour (Figure 5); at large depths, high pressures cause ductile yielding of the aggregates and the layer of sediments to fold; above these depths and at lower pressures aggregates rupture and a layer of sediment faults with the presence of gouge material along the slip planes; near the surface where the pressure is even lower, a layer of sediment fractures or cracks (Muhunthan and Schofield, 2000).

Critical state soil mechanics (Schofield and Wroth, 1968) captures these simple depositional and structural phenomena of folds, faults, and fractures in soil and sedimentary as well as man-made deposits in a scientific manner. It explicitly recognises that soil is an aggregate of interlocking frictional particles and the regimes of soil behaviour depend in a major way on its density and effective pressure.

In the critical state framework, the state of soils is defined in 3D: mean effective normal stress (p), shear stress (q) and void ratio or specific volume (v) space. Limits to stable states of yielding are defined by the state boundary surface in the 3-D, p-q-e space. The 2D representations of the normalised state boundary surface in the q/pcrit - p/pcrit and e - ln p spaces are as shown in Figure 6.

Critical state soil mechanics divides the soil behaviour at limiting states into three distinct classes of failure; the limiting lines OA and OG (Figure 6a) indicate states of soils undergoing fractures or cracks; AB and GE indicate that Hvorslev’s Coulomb faults on rupture planes; BD and ED indicate Cam-clay yield and fold of a sediment layer.

Soil states on the crack surface result in the development of unstable fissures and crack openings. Heavily overconsolidated clays and overcompacted sands at low confining stresses could reach this limiting state. Collapse similar to fracture on the dilative side can also exist on the contractive domain but outside the normal consolidation line (Figure 6b). Such states outside the stable yielding exist in wind deposited loose sands, air pluviated or moist-tamped sands and result in abrupt collapse upon shearing of these materials (Pillai and Muhunthan, 2001, 2002). For sands and clayey silts of low plasticity, stable yield (rupture and ductile) behaviour occurs only within a narrow band on both the denser and looser side of the critical state line (Figure 6b).

The ‘no tension’ or ‘limiting tensile strain’ criteria are the most widely used among the alternative theories to quantify tensile fracture (Schofield, 1980). For the triaxial specimen the no tension criterion with s3 = 0 results in p = s1/3 or q/p = 3 and leads to vertical split cracks which is the case of line OA. For horizontally spalling cracks, s1= 0 results in p = 2/3 s3, q = -s3, or q/p = 1.5 which is the case of line OG. For clays or silty clays, Schofield (1980) had suggested that the change from rupture to tensile crack occurs at a pressure p = 0.1 pc, where pc is the effective confining stress at critical state. This is equivalent to an over consolidation ratio of approximately 20 (Figure 6a). When the effective stress path crosses the crack surface OA, the soil element in that location begins to disintegrate into a clastic body and unstressed grains become free to slide apart. In that case the average specific volume of the clastic mass can increase (large voids/cracks) and consequently its permeability can increase significantly and instantly. A significant internal/external shear stress at low confining stresses can cause the crossover of the crack-surface OA and a large increase in specific volume. When such condition occurs, the opening within the soil body may be an extensive crack or a local pipe or channel. If such opening (crack/channel) daylights into the water body it could lead to a free flow of water into the downstream slope.

Liquidity index, confining stress and soil behaviour

Critical state soil mechanics (Schofield and Wroth, 1968) has shown that it is possible to generalise the density or specific volume axis by converting to a liquidity basis. It was further shown that the critical pressure is about 5kPa at the liquid limit and 500kPa at the plastic limit. In his Rankine lecture, Schofield (1980) mapped the remolded soil behaviour on a liquidity against pressure diagram (as shown in Figure 7) utilising the hundred fold increase in pressure from the liquid limit critical state to the plastic limit critical state which is two log cycles, so the rupture band has half the width of PI and will intersect the line p = 5 kPa at LI = 0.5. This intersection is a consequence of putting the lower limit of Coulomb rupture at p/pcrit= 0.1 (Schofield, 1980). In the LI-p space, clear boundaries exists that separate the regions of fracture, rupture, and ductile behaviour. This provides for an independent and convenient approach to separate the states of fracture/rupture/ductile yield behaviour of soil using its physical properties.

Consider a body of soil initially at LI = 0.5 subjected to an elastic compression; the map suggests at shallow depths where p < 5kPa there may be cracks, but for depths where 5 kPa < p < 50 kPa the soil will remain watertight while deforming. In contrast, a body of soil initially at LI = 0 will undergo fracture at depths for which p < 50 kPa or about 3m of the overburden depth. In other words, the overburden depth should be larger than 3m to ensure that deformation caused rupture planes (watertight) rather than open cracks. If LI= -0.25, the depth could be about 100kPa or 6m of depth. In this view the vertical face of the breach in Teton dam can be seen as an open fracture in very strong soil, standing to a near vertical height of 6m+.

In order to identify the band of behaviour in which various states of soil lie in the LI-p space, Schofield (1980) defined their equivalent liquidities by projecting these states in the direction parallel to the critical state line towards the ordinate through p = 5 kN/m2. The equivalent liquidity LI5 can be shown to be LI5 = LI+1/2 log (p/5) (Schofield, 1980). Thus, equivalent liquidity equals liquidity as found in the ground plus a correction for mean stress. A value of LI5 of less than 0.5 would generally indicate the fracture zone. Values of 0.5 to 1.0 represent the rupture zone. Values larger than 1.0 represent ductile zone.

The inset of Figure 7 shows the section of the behaviour map at constant p: stress ratios q/p will increase as equivalent liquidity falls. In the high equivalent liquidity range, stress ratio increases linearly as liquidity of cam-clay falls. The Hvorslev surface gives the rupture limits which allow higher stress ratios as lower values of p/pcrit are approached, but at the no tension limits, q/p = 3 in compression, and -1.5 in extension. There is a general increase of limiting stress ratio as equivalent liquidity falls, but this is not a continuous change because there is a change of limiting behaviour from contours yield, to discrete rupture, to fracture of stiff fissured soil at equivalent liquidity below 0.5 (Schofield, 1980).

The above concepts provide two independent approaches to analysing the cracking of soils particularly in an earth dam. The first approach makes use of mechanical properties determined from triaxial tests and oedometer tests to separate the three regions of soil behaviour, the fractures, the faults, and the ductile yield while the second approach relies on physical properties, plasticity index, and liquidity index to identify such regions.

Parameters of the impervious core

A large database of field and laboratory tests carried out during the post-failure investigations by the IRG and the IP exists in their reports. The laboratory testing herein was focused on the verification of some of the index and mechanical properties. About 1000 lbs of the zone-1 material was obtained from the remnants of the failed Teton dam. The material was tested for physical and mechanical properties in the laboratory. Tests for physical properties included grain size, Atterberg limits, and proctor compaction curves. Mechanical tests included CU triaxial tests on remolded soils, UU triaxial compression tests, and 1-D compression curves on compacted samples at wopt-1, wopt, and wopt+1 to obtain constrained modulus at various confining stress levels.

FEM analysis of teton

Finite element analyses were carried out for the longitudinal section of the dam. This section was chosen because it captures all of the variation along the bottom profile (berms, slopes, etc.). Plane strain condition is assumed to prevail along the section.

The analyses used an elasto-plastic model with modified cam-clay yield curve (Roscoe and Burland 1968). The CSL line with a slope M divides the yield curve into two regions, dry and wet sides. Porous elastic option is used to describe elastic behaviour inside the yield curve. It is assumed valid for small strains (<5%) and is a nonlinear isotropic model in which the pressure varies as an exponential function of volumetric strain. The model parameters used in the analysis are shown in Table 1 above.

The model had five layers to simulate the construction of the dam. In the first step, the top four layers were removed and the remaining layer was analysed. This was to allow the geostatic stress field to reach equilibrium with initial conditions, applied load, and boundary conditions. Subsequently, each layer was activated strain-free to simulate the construction steps. The strain-free activation scheme was adopted to avoid the creation of strain by the deformation of the previous layer. From the analysis, the shear stress (q) and the mean stress (p) were obtained along the longitudinal section and contours of q/p ratio were drawn as shown in Figure 8.

Teton soil behaviour in LI5-p space

As described earlier, the transition of soil behaviour from the crack surface region to stable Hvorslev fault region occurs at an equivalent liquidity index of 0.5 corresponding to a confining stress of 0.8psi or 5kPa, or zero liquidity index at confining stress of 8psi (50kPa). Similarly, the Hvorslev-Coulomb rupture regime changes to ductile Cam-clay regime at 80psi (500kPa) (Figure 7). This was further confirmed by a series of consolidometer tests with Zone 1 samples compacted at varying initial liquidity indices. For various confining stresses, the corresponding equivalent liquidity indices LI5 were determined and their position in the LI5-p space was identified. This was transferred to the cross-valley section for the respective confining stresses. A mapping of the contours of equivalent liquidity index for the valley crosses section of the Teton dam was made as shown in Figure 9.

New mechanism of teton failure

The state based soil mechanics theory presented earlier suggests that zones with stress ratio q/p larger than 3 would indicate the presence of a vertical split or crack (Figure 5). It can be seen that the majority of the soil elements of the Teton dam have q/p stress ratios significantly less than 3, indicating that they were intact (Figure 8). However, there were two zones that have q/p ratio larger than 3 (Figure 8). They are from Sta. 13+00 to Sta.15+00 in the right side and from Sta. 26+00 to Sta. 28+00 in the left side.

The results show that at the end of construction the state of stress in the dam core had significantly reached into the crack surface (q/p = 3) region which is an indication of the existence of internal cracks at two locations, Sta.14 + 50 in the right abutment and Sta.26 + 50 in the left abutment. The cracks at Sta. 14+50 were 32ft (9.75m) deep from top of the crest while they were only 10ft (3m) deep at Sta.26+50 (Figure 8). The state based theory further suggests that contours of the q/p ratio less than 3 would indicate the stable nature of the compacted soil, which is the case for soil elements at depth and particularly below 32ft (9.75m) (Figure 8). Therefore, it is concluded that the failure of the Teton dam was initiated as a result of water flowing through the deepest open vertical crack on the right abutment near Sta. 14+50 during the first filling when the water level reached the bottom of the crack, which slowly eroded the crack into a large tunnel leading to the major breach hours later.

The zone-1 core was capped by a 3ft (0.9m) layer of sand and gravel roadbed, which was subjected to continual vibration and compaction by vehicular traffic inhibiting cracks in the layer. Further, the material parameters of the granular bed, their packing, and the characteristics were different from zone-1 material to exhibit cracking. As a result, it was likely that the cracks below in the core zone had apparently not daylighted onto the roadbed and were not visible during first filling. However, numerous transverse cracks daylighted onto the roadbed in the left abutment soon after the dam breach, mostly near Sta. 26+50, where the q/p ratio was close to or larger than 3 for shallow depths.

The contours of LI5 (Figure 9) independently confirm that only shallow depths to about 30ft (9m) between Sta. 14+00 and Sta.+ 16+00 are prone for cracking. Because of the low plasticity (PI ~ 4), the liquidity index was very sensitive to placement water content and its influence on the performance of the soil core, under rapidly changing confining and shear stress conditions, particularly at the abutments. At the steep abutments, depth of the soil column decreases; consequently the soil elements were subjected to decreased confining stress. In effect, the stress states of the soil in the abutments were in the Hvorslev regime and were stiffer while those in the valley section of the dam were at or near the ductile regime, which were more deformable. Again the changes in the deformability were further disrupted by the benches, which apparently caused significant differential deformations and increased shear stresses at some locations. These aspects were well reflected in the stress analysis.

Stress path of teton soil and critique of past investigations

The concepts presented also help explain some of the misgivings of previous investigations. Consider the states of soil element shown in Figure 10 (A1, A2, A3, and A4). At the placement condition, the state of soil would have been in the Hvorslev region at point A1 in Figure 11. As the dam was built up, the confining stress would increase and the state of soil would move along the path A1A2A3A4 (Figure 11). Thus, the soil, which was in the key trench, would move to the stable yielding region when it was wetted.

It is, therefore, concluded that the hydraulic fracture in the key trench (Seed et al, 1976, Sherard, 1987) and its relevance to the failure of the dam is fundamentally flawed (See also Muhunthan and Schofield, 2000). Except for the shallow depths of 30-35ft (9-10.7m) in some locations, the q/p stress ratio is significantly lower than 3 (fracture level), which indicates that fracturing of the soil would be difficult with increasing depth (Figure 8). For hydraulic fracture to occur, the soil element must be subjected to seepage water, which can cause (a) physical wetting of the soil first and then (b) a corresponding hydraulic pressure in the soil. The physical wetting and saturation of the soil increases the liquidity index of the in-situ soil and consequently the soil element becomes more ductile and the material tighter and less permeable (Figure 5) (also the q/p ratio drops off quickly, Figure 11 (a)). That is the stress-path moves significantly to the right to a more ductile and stable yield (Cam-clay) regime.

Some researchers (Leonards and Davidson, 1984) characterised the phenomenon of yield as ‘collapse on wetting’, which is a misnomer considering that the stress path simply migrated from the stable Hvorslev regime to the stable ductile Cam-clay regime. On the second point, (b), the hydraulic pressure due to the water seepage would have a limited opposite effect of reducing the effective stress of the soil element. Any such reduction in effective stress due to the seepage pressure will be more than offset by changes in the mechanical properties (ductility) of the soil. The net effect is the movement of the stress-path of the soil element is to the right and towards the Cam-clay regime (Figure 11). Therefore, the notion of ‘hydraulic fracture’ by water pressures equal to or less than the reservoir head, which could initiate a failure of the dam, has no scientific basis. In fact, to cause hydraulic fracture in the soil at the base of the dam (Cam-clay state), one needs to apply a hydraulic head of about 800ft (243.8m) of water!

It is also concluded that the ‘wet seam’ theory postulated during post-failure investigations (Leonards, 1987; Hilf, 1987) is flawed. The majority of the core material on Zone-1 was placed at a negative liquidity index (0.25 - 0.50) or in the Hvorslev regime in the stress-space (Figures 5 and 11). When seasonal rains and snow condition interrupted the material placement during construction, some layers might have been placed at wetter than the average or near liquidity index of unity. When subjected to large stresses, such pockets of material would fall into the Cam-clay ductile regime and deform like potter’s clay, ‘wet-seams’ or wet-pockets producing positive pore water pressure. This was the case for a few random pockets/layers of fill that were affected by the rain/snow when full stripping and replacement of such layers were not possible during the construction. Although such layers were of low strength and stiffness, they provide more impermeable mass relative to the surrounding material and would have had no adverse effect on the performance of the dam.

The original design specifications of Teton dam stipulated placement water content of optimum minus 1% to optimum for the core, which had only a small plastic index (PI<4). Based on the analysis here, it is believed that this was the fundamental error in the design concept in leading to the demise of the dam. The placement water content represented an initial liquidity index of zero or negative, which allowed considerable depth of the core to be prone to fracture (Figure 11). Without compromising the compacted density, for this material an additional 1-2% water content would have provided adequate equivalent liquidity index of at least 0.5 or more for most of the placed fill. This would have kept the entire fill intact in the Hvorslev regime where the material would have been stiffer, stronger and more watertight except for the top 5-10ft (1.5-3m) (freeboard regime). Therefore, it is evident that the lack of knowledge at that time of the combined effect of liquidity and confining stress in controlling the mechanical behaviour of Zone 1 contributed in a major way to the Teton dam failure. For the design of earth-structures, the theory based on the ‘state based soil mechanics’ provides a better understanding of the physical and mechanical behaviour of a broad spectrum of soils including that of Teton dam, which are subjected to different loading conditions.

Conclusion

The ‘sunny day’ failure of Teton dam was well captured in many films available on numerous websites. A new theory based on the concepts of fundamental soil mechanics explains conclusively the manifestations and the mode of failure of Teton dam as it occurred on 5 June 1976. The theory also identified the problems with using materials with low plasticity for the impervious core and the role of liquidity index. Unless these aspects are recognised in the design and liquidity index is normalised to fit the geometry of the core, potential for cracks in earth dams exist. Based on the investigation and discussion documented here, the following can be concluded.

An internal transverse crack(s) or large opening(s) had developed in the core (Zone-1) to a maximum depth of 32ft (9.75m) below the crest (top of the core) at the right abutment near Sta. 14+00. The analysis further indicates that much shallower cracks existed in the core in both abutments under the steep rock slopes. When the reservoir level rose to the level of the deepest crack, water flowed freely barrelling downstream into the chimney drain (Zone 2).

The internal cracks might not have day lighted through the 3ft (0.9m) thick granular roadbed, which was subjected to constant vehicular traffic and compaction. Also, the parameters that affected the core were different from those of the overlying roadbed granular fill. The uniform clayey silt (CL-ML) that was used for the core of Teton dam fitted well into the CSSM model that was developed for other soils with different plasticity. Although the clayey silt had relatively high values for the liquid limit (LL~23) and plastic limit (PL~19), the plastic index was relatively small (PI~4 or less). Consequently the liquidity index was very sensitive to the initial placement water content and its subsequent variability in mechanical properties due to varying confining stress. The initial liquidity index and its variation played a key role in the cracking of the dam. Therefore, for clay-silt cores, it is more prudent to have the construction specification refer to the liquidity index or the ‘placement water content’ with respect to the plastic limit (PL), than of the optimum water content.

A combination of material parameters such as the low plasticity of the core, the sensitivity of the liquidity index of the material to water content, its variation under the subsequent confining stress condition, and their influence on the constrained modulus played a key role in the cracking of the core. It appears that these aspects of fundamental soil mechanics and the phenomenon of cracking were not recognised in the original design of the dam.

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