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1. Introduction and History of Dam Engineering Dams, in different forms were built by humankind since the earliest days of known history, in order to solve problems that could not be solved otherwise. There could not be a developed civilization without water management. One can see that all major settled civilizations were using water supply systems and irrigation. At first small diversion dams were used and gradually even larger storage dams were introduced. Most of early dam building took place in Mesopotamia and the Middle East. Dams were used to control the water level. There are remains of dams older than 2000 years in China, Egypt (there are reliable records of about dam built on the Nile River before 4000 B.C. This was to provide water for ancient city Memphis) Iran, Yemen and ancient Mesopotamia. The earliest known dam is situated in Jawa, Jordan, 100 km northeast of the capital Amman. The gravity dam featured a 9 m high and 1 m wide stone wall, supported by a 50 m wide earth rampart. The structure is dated to 3000 BC. The Ancient Egyptian Sadd Al-Kafara at Wadi Al-Garawi, located about 25 kilometers south of Cairo, was 102 m long at its base and 87 m wide. The structure was built around 2800 or 2600 B.C. as a diversion dam for flood control, but was destroyed by heavy rain during construction or shortly afterwards. The Romans were also great dam builders, with many examples such as the three dams at Subiaco on the river Anioin Italy. Many large dams also survive at Mrida in Spain.The oldest surviving and standing dam in the world is believed to be the Quatinah barrage in modern-day Syria. The dam is assumed to date back to the reign of the Egyptian pharaoSethi (13191304 BC), and was enlarged in the Roman period and between 1934-38. Now a days humankind constructing huge dam for different purposes, for instance the tallest dam in the world is the 300 meter high Nurek Dam in Tajikistan. 1.1 An overview of dam engineering The term dam refers to a barrier that is either made of concrete or earth materials (or acombination) built for purposes of obstructing the flow of waterusually of river,stream or waterway. Literally means to block up. Therefore,dam is a structure built to block, retard, hold back or impede the flow of water.

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Dams areconstructed either to divert the water flow or impound the water, or both.People build dams for different usages. The impounded water that backs up against astorage dam forms an artificial lake (the reservoir). The stored water is then made availablefor irrigation (agriculture), town and city water supplies (drinking and sanitation), and otheruses, such as producing electricity for homes and industries. Dams have been promoted as an important means of meeting perceived needs forwater and energy services and as long-term, strategic investments with the ability to deliver multiple benefits. Some of these additional benefits are typical of all large public infrastructure projects, while others are unique to dams and specific to particular projects. Regional development, job creation, and fostering an industry base with export capability are mostoften cited as additional considerations for building large dams. Other goals include creating income from export earnings, either through direct sales of electricity, or by selling cash cropsor processed products from electricity-intensive industry such as aluminium refining. Clearly,dams can play an important role in meeting peoples needs. Dams may be classified according to height, intended purpose, structure, the type of material used in their construction and by their shape.For instance Dams according to height and size can be classified in to three categories small, large and major dams. A large dam, according to the International Commission on Large Dams (ICOLD), is 15 meters (50 ft.) or more high from the foundation. If dams are between 5 15 meters and have a reservoir volume of more than 3 million cubic meters, they are also classified as large dams. A major dam is over 150 meters in height. Dams in the 20th century More than 500,000 dams constructed worldwide, the vast majority of all these dams are small structures less than 3 meters (10 ft.) tall, while more than 47,000 are large dams built to generate electricity, supply water, control floodsand facilitate navigation. More than 20,000 of the worlds large dams are in China. During the 20thcentury, an estimated US$2 trillion was spent on dam-building. The following table shows the proportional distribution of large dams in different regions of the world. Approximately two thirds of the worlds existing large dams are in developing countries. According to table 1.1 Asia has the most number of large dams, total of 31,340, followed by North and Central America based on ICOLD 1998 Survey.

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Regional distribution of large dams (as of 1998 survey)

S No Region No. Dams 1 Africa 1,269 2 Western Europe 4,277 3 South America 979 4 Eastern Europe 1,203 5 North and Central America 8,010 6 Asia 31,340 7 Austral-Asia 577

Total 47,655 Note: ICOLD is only using 45,000 dams, based on their survey in 1998. However, there are more than 47,600 dams worldwide Source: International commission on large dams (ICOLD) and world commission on dams (WCD)

Table 1.1 regional distributions of large dams

Figure 1.1: Regional distribution of large dams at the end of the 20th century (Source: International Commission on Large Dams (ICOLD) and World Commission on Dams (WCD)

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000

Asia

North & Cen. America

Western Europe

Africa

Eastern Europe

South America

Austral-Asia

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The top 10 countries with the most large dams S

No. By number of

large dams By function

Irrigation Water supply Flood control Hydropower

1 China China United States China China

2 United States India United Kingdom

United States

United States

3 India United States Spain Japan Canada

4 Spain Korea Japan Brazil Japan 5 Japan Spain Australia Germany Spain 6 Canada Turkey Thailand Romania Italy 7 Korea Japan South Africa Mexico France 8 Turkey Mexico Brazil Korea Norway 9 Brazil South Africa France Canada Brazil

10 France Albania Germany Turkey Sweden Table 2 shows the top 10 countries in the world with the most number of large dams, according to ICOLD and WCD. 1.2 Lessons from notable events Historical study of dams conceived in earlier times is essential. To continue advancing the engineering profession must periodically review past problems and the lesson that they taught. Sharing of information on failures as well as success is needed. In fact, some of the most valuable learning comes from projects where errors have been clear in retrospect. The following case history is representative of the body of knowledge that has been accumulated in the interest of the future safety of dams. Failure of Teton dam Teton Dam, a 305-foot high earthfill dam across the Teton River in Madison County, southeast Idaho, failed completely and released the contents of its reservoir at 11:57 AM on June 5, 1976. Failure was initiated by a large leak near the right (northwest) abutment of the dam, about 130 feet below the crest. The dam, designed by the U.S. Bureau of Reclamation, failed just as it was being completed and filled for the first time.

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Figure 1.2 Catastrophic failure of Teton dam on June 5, 1976 History and geology of Teton dam site There had been interest in building a dam in the eastern Snake River Plain for many years, to control spring runoff and provide a more constant water supply in the summer. The area had suffered a severe drought in 1961, followed by serious flooding in 1962. The Bureau of Reclamation (USBR) proposed the Teton Dam in 1963, and Congress passed without opposition an authorizing bill the following year. The planned dam was to be an earthen structure 310 feet (94 m) high and 0.6 miles (1.0 km) long and create a reservoir 17 miles (27 km) in length. The impounded water would be used to generate hydroelectric power. An environmental impact statement was issued for the dam in 1971, but it did not raise the possibility of a collapse. The eastern Snake River Plain is almost entirely underlain by basalt erupted from large shield volcanos on top of rhyolitic ash-flow tuff and ignimbrites. The tuff, a late-Cenozoic volcanic rock dates to about 1.9 million years. The dam site is composed of basalt and rhyolite, both of which are considered unsuitable for dam construction because of their high permeability. Test cores, drilled by engineers and geologists employed by the Bureau of Reclamation, showed that the rock at the dam site is highly fissured and unstable, particularly on the right side of the canyon. This was confirmed by long term pump-in tests at rates of 165 to 460 US gallons (620 to 1,740 liters) per minute. The widest fissures were determined to be 1.7 inches (4.3 cm) wide. The Bureau planned to

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seal these fissures by injecting grout into the rock under high pressure to create a grout curtain in the rock. In addition, an investigation of the area by geologist of the U.S. Geologic Survey indicated that it was seismically active: five earthquakes had occurred within 30 miles (50 km) of the dam site in the previous five years, two of which had been of significant magnitude. This information was provided to the Bureau of Reclamation in a memorandum, but the geologists' concerns were considerably watered down in the six-month re-drafting process before the USGS sent the final version of the memo to the USBR in July 1973. In 1973, when the dam was only half-built, but almost $5 million had already been spent on the project, large open fissures were encountered during excavation of the key trench near the right end of the dam, about 700 feet (210 m) from the canyon wall. The two largest, near-vertical fissures trend generally east-west and extend more than 100 feet (30 m) below the bottom of the key trench. Some of the fissures are lined by calcite, and rubble fills others. Several voids, as much as 6 inches (15 cm) wide, were encountered 60 to 85 feet (18 to 26 m), below the ground surface beyond the right end of the dam and grout curtain. The largest fissures were actually enterable caves. One of them was eleven feet (3.4 m) wide and a hundred feet (30 m) long. Another one was nine feet (2.7 m) wide in places and 190 feet (60 m) long. These were not grouted because they were beyond the keyway trench and beyond the area where the Bureau had decided grouting was required. This necessitated using twice as much grouting as had been originally anticipated 118,000 linear feet were used in total. Later, the report of a committee of the House of Representatives which investigated the dam's collapse felt that the discovery of the caves should have been sufficient for the Bureau of Reclamation to doubt its ability to fill them in with grout, but this did not happen: the Bureau continued to insist, even after the dam had failed, that the grouting was appropriate. Filling the dam The dam was completed in November 1975, and filling the reservoir began at the standard rate of 1 foot (0.30 m) a day. However, snows were heavy that winter, and five months later the project's construction engineer requested permission to double the filling rate in order to deal with the additional spring runoff, while continuing to inspect for leaks and monitor the groundwater. A month later, even though monitoring showed that groundwater was flowing a thousand times faster than had been originally anticipated, the filling rate was doubled again, to 4 feet (1.2 m) a day.

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On June 3 and 4, 1976 three small springs were discovered downstream of the dam, although the water running through the leaks was clear, and such leaks are not unexpected for anearthen dam.At the time, the reservoir was almost at capacity, with a maximum depth of 240 feet (73 m). The only structure that had been initially prepared for releasing water was the emergencyoutlet works, which could carry just 850 cubic feet per second (24 m3/s). The main outlet works and spillway gates were not yet in service.

The collapse and flood

On Saturday, June 5, 1976, at 7:30 a.m. Mountain Daylight Time (MDT), a muddy leak appeared, suggesting sediment was in the water, but engineers did not believe there was a problem. By 9:30 a.m. the downstream face of the dam had developed a wet spot which began to discharge water at 20 to 30 cubic feet per second (0.57 to 0.85 m3/s) and the embankment material began to wash out. Crews with bulldozers were sent to plug the leak, but were unsuccessful. Local media appeared at the site, and at 11:15 officials told the county sheriff's office to evacuate downstream residents. Work crews were forced to flee on foot as the widening gap, now larger than a swimming pool, swallowed their equipment. The operators of two bulldozers caught in the eroding embankment were pulled to safety with ropes.

At 11:55 a.m. the crest of the dam sagged and collapsed into the reservoir; two minutes later the remainder of the right-bank third of the main dam wall disintegrated. Over 2,000,000 cubic feet per second (57,000 m3/s) of sediment-filled water emptied through the breach into the remaining 6 miles (10 km) of the Teton River canyon, after which the flood spread out and shallowed on the Snake River Plain. By 8:00 p.m. that evening, the reservoir had completely emptied, although over two-thirds of the dam wall remained standing.

Cause of the collapse

Study of the dam's environment and structure placed blame for the collapse on the permeable loess soil used in the core and on fissured (cracked) rhyolite in the abutments of the dam that allowed water to seep around and through the earth fill dam. The permeable loess was found to be cracked. It is postulated that the combination of these flaws allowed water to seep through the dam and led to internal erosion, called piping that eventually caused the dam's collapse.

An investigating panel had quickly identified piping as the most probable cause of the failure, and then focused its efforts on determining how the piping started. Two mechanisms were possible. The first was the flow of water under highly erodible and unprotected fill, through joints in unsealed rock beneath the grout cap, and development of an erosion tunnel. The second was "cracking caused by differential strains or hydraulic fracturing of the core material." The panel was unable to determine whether one or the other mechanism occurred, or a combination:

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The fundamental cause of failure may be regarded as a combination of geological factors and design decisions that, taken together, permitted the failure to develop.

A wide-ranging controversy ensued from the dam's collapse. According to the Bureau of Reclamation, BOR engineers assess all Reclamation dams under strict criteria established by the Safety of Dams program. Each structure is periodically reviewed for resistance to seismic stability, internal faults and physical deterioration. The dam safety program identified two other dangerous dams - Fontenelle, which very nearly failed like the Teton Dam when it was filled and again in May 1985 and the Jackson Lake Dam which would have failed during an earthquake on the nearby Teton Fault.

The lessons learned from this case led to safety improvements for U.S. Bureau of Reclamation design procedures.

1.3 Benefits and concerns about dam

Water is the vital resource to support all forms life on earth. Unfortunately, it is not evenly distributed over the world by season or location. Some parts of the world are prone to drought making water a scarce and precious commodity, while in other parts ofthe world it appears in raging torrents causing floods and loss of life and property.Throughout the history of the world, dams and reservoirs have been used successfullyin collecting, storing and managing water needed to sustain civilization.

Even today, water remains essential for the survival of mankind and the future development of the worlds cities, industries and agriculture. Today there is a significant demand on the worlds water. As the world population continues to grow at the rate ofover 100 million people each year, so does the demand for water. At the same time, there is a careless use of our natural resources and accelerated pollution of the environment. The fact that a significant portion of the available water in the world is too contaminated for domestic use makes this situation very critical.

One of the most efficient ways to manage water resources for human needs is by the construction of dams that create reservoirs for the storage and future distribution.As we have seen in the above section currently there are about 47,655dams higher than 15 meters throughout the world.While some are more than 2,000 years old, More than 73% have been built in the last 50years. The reservoirs formed by these dams store more than 3,600 km3 of usable water. The primary benefit of dams and reservoirs in the world is water supply. Other keypurposes and benefits include:

Irrigation for agriculture (food supply) Flood control Hydropower Inland navigation

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Recreation Most dams are built for several purposes. This produces a broad range of domestic and economic benefits from a single investment. An additional local benefit is the employment opportunities during the multiple year construction of a reservoir project. Effective management of the worlds water is essential to sustaining the existing andfuture population of the world. As the worlds population continues to grow so does theneed for more dams, especially in developing nations and the vast arid regions of theworld. Basin-wide planning for water management is the key element to providing optimum water supply and other benefits. While dams provide significant benefits to oursociety, their impacts on the surroundings include:

Resettlement and relocation Socioeconomic impacts Environmental concerns Sedimentation issues Safety aspects

However, these concerns and impacts can be reduced or eliminated by careful planning, and the incorporation of a variety of mitigation measures. Water Supply for domestic and industrial use One of the fundamental requirements for socio-economic development in the world is the availability of adequate quantities of water with the appropriate quality. In the past,the main sources of domestic and industrial water have been aquifers. Today, many of these are now overused and their rate of recharge is far less than what is extracted.Their supply must be augmented with additional water from reservoirs. Large urban areas depend heavily on water stored in reservoirs during high flows and used during periods of low rainfall. This is especially critical in arid regions of the world. This need for stored water will continue, since many aquifers are over-used.Properly planned, designed and constructed and maintained dams contribute significantly toward fulfilling our water supply requirements. The primary source of freshwater supply is from precipitation. Throughout the world, the hydrologic cycle varies and is not predictable due to climate change and other problems. Water stored in reservoirs is also used for industrial needs. This ranges from the direct use in chemical and refining processes to cooling for conventional and nuclear power production. Managed flows from reservoirs can be used to dilute discharged substances by augmenting low river flow to maintain water quality at safe limits.

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Meeting the agricultural demand for food supply One of the biggest uses of water on a worldwide scale is agricultural irrigation. According to the study that has been made by international commission on large dam in July 1999, agricultural irrigation has accounted for about 1147 liters per day per capita by the year 2000. Since the early1990s, less than 1/5 of the land suitable for agriculture in the world has been irrigated, and it has contributed about 1/3 of world food production.It is estimated that 80% of additional food production by the year 2025 will come from irrigated land. Most of the areas in need of irrigation are in arid zones, which representa major portion of the developing countries. Even with the widespread measures to conserve water by improvements in irrigation technology, construction of more reservoir projects will be required. Flood control Dams and reservoirs can be effectively used to regulate river levels and flooding downstream of the dam by temporarily storing the flood volume and releasing it later.The most effective method of flood control is accomplished by a number of multipurpose dams strategically located in a river basin. The dams are operated by a specific water control plan for routing floods through the basin without damage. This not only eliminates flooding, but provides other benefits such as water supply, irrigation, hydropower and water quality. The number of dams and their water control management plans are established by comprehensive planning for economic development and with public involvement. Flood control is a significant purpose for many of the existing dams and continues as a main purpose for some of the major dams of the world currently under construction. Hydropower The availability of energy is essential for the socio-economic development of a nation. Itis advantageous to use energy that is clean, efficient, dependable and renewable. Hydropower meets all of these requirements. In countries, where a vast amount of development still lies ahead, good conditions often exist for renewable energy sources.The technically most advanced and economical source of renewable energy is hydropower. Less than 20% of the worlds estimated feasible hydropower potential has been developed. The greatest amount of potential remains to be developed in Asia, South America and Africa. Hydropower projects produce energy with a high rate of efficiency and without burdening future generations with pollution or waste. Hydropower projects

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can be developed with very small capacities for local consumption or with very large projects as part of a regional or national system. As part of a multipurpose project, hydropower can also help to finance other functions of a reservoir or river, such as irrigation water for food supply, drinking water, flood protection, improved navigation or recreation. Inland navigation Natural river conditions, such as current, changes in river level, ice, and changing river channels all create major problems and obstacles for inland navigation. The advantages of inland navigation over highway and rail are the large load carryingcapacity of each barge, the ability to handle cargo with large-dimensions and fuel savings. Enhanced inland navigation is a result of comprehensive basin planning and development utilizing dams, locks and reservoirs that are regulated to provide a vital role in realizing regional and national economic benefits.In addition to the economic benefits, a river that has been developed with dams and reservoirs for navigation may also provide additional benefits of flood control, reduced erosion, stable ground water over the length of the system and recreation. Recreation The attractiveness of reservoirs for tourism is often a significant benefit, in addition to the other purposes of a dam. This is very significant in areas where natural surface water is scarce or non-existent.Recreational benefits associated with lakes, such as boating, swimming, fishing, bird watching and nature walks, are taken into account early at the planning stage, alongwith other objectives achieve a balanced project. The operation of the dam andreservoir can enhance tourism.

1.4 Terms and parameters pertinent to dam engineering There are different kinds of parameters associated with dam engineering. The following are the most commonly used terms in dam engineering. ABUTMENT.The foundation along the sides of thevalley or gorge against which the dam is constructed. AGGREGATE. The natural sands, gravels, and crushed stones used in the manufacture of concrete. Aggregatefor concrete commonly is obtained from alluvial stream deposits or from rock quarries.

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APPURTENANT STRUCTURES. Any physical feature other than the dam, such as the spillway, outlet, powerhouse, penstock, tunnels, etc. BEDROCK. The solid rock foundation of a dam, usually overlain by soil or other unconsolidated superficial material. COFFERDAM. A temporary structure constructed around part or all of the excavation for a dam or other appurtenant features to facilitate construction in the dry. CONSTRUCTION JOINT. The surface between two consecutive placements of concrete that develops bond strength CONTRACTION JOINT GROUTING. Injection of grout into contraction joints. CREEP. Deformation over a long period of time under a continuous sustained load. CUTOFF. An impervious construction placed beneath a dam to intercept seepage flow. DEAD LOAD. The constant load on the dam resulting from the mass of the concrete and other attachments. DEFLECTION. Linear deviation of the structure due tothe effect of loads or volumetric changes. DEFORMATION. Alteration of shape or dimension due to stress. DIVERSION CHANNEL OR TUNNEL.A structure to temporarily divert water around a dam site during construction. GALLERY. A long, narrow passage inside a dam used for access, inspection, grouting, or drilling of drain holes. GROUT. A mixture of water and cement or a chemical solution that is forced by pumping into foundation rocks or joints in a dam to prevent seepage and to increase strength. GROUT CURTAIN. A row of holes filled with grout under pressure near the heel of the dam to control seepage under the dam

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HEEL OF DAM. The location where the upstream face of the dam intersects the foundation. INSTRUMENTATION. Devices installed on and embedded within a dam to monitor the structural behavior during and after construction of the dam. INTAKE STRUCTURE.The structure in the forebay that is the entrance to any water transporting facility such as a conduit or tunnel. LIFT. The concrete placed between two consecutive horizontal construction joints. NONOVERFLOW SECTION.The section of the dam that is designed not to be overtopped. OUTLET STRUCTURE. A structure at the outlet of acanal, conduit, or tunnel for the purpose of discharging water from the reservoir. OVERFLOW SECTION. That portion of a dam, usually occupied by a spillway, which allows the over flow of water. Also referred to as spillway section. PORE PRESSURE. The interstitial pressure of water within the mass of rock or concrete.Also called neutral stress and pore-water pressure. POROSITY. The ratio of the volume of voids to the total volume of the material. PRINCIPAL STRESS. Maximum and minimum stress occurring at right angles to a principal plane of stress. ROLLER-COMPACTED CONCRETE (RCC). A relatively dry concrete material that has been consolidated through external vibration from vibratory rollers. SPILLWAY. The structure over or through which reservoir flood flows are discharged. SPILLWAY CHUTE. The outlet channel for the spillway discharge. TAILRACE.The channel or canal that carries water away from a dam. Also sometimes called afterbay. TOE OF DAM.The location where the downstream faceof the dam intersects the foundation.

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UPLIFT PRESSURE. The upward water pressure in the pores of concrete or rock or along the base of the dam WATER STOP. A thin sheet of metal, rubber, plastic, or other material placed across joints in concrete dams to prevent seepage of water through the joint

1.5 Classification of dams There are numerous types of dams and they are classified in various ways. Classification Based on Purpose

a) Storage Dams i) Flood control ii) Water supply: domestic, municipal, industrial, irrigation iii) Hydroelectric power iv) Recreation storage v) Pollution control

b) Stage control Dams i) Diversion ii) Navigation iii) Check

c) Barrier Dams i) Levees and dykes ii) Coffer dams

d) Multipurpose Dams Classification based on Hydraulic design

a) Overflow dams b) Non overflow dams c) Composite dams

Classification According to Material of construction a) Embankment Dams b) Concrete Dams

a) Embankment Dams

They are constructed of earth fill. Upstream faces are similar and moderate angles, giving a wide section and a high construction volume relative to height.

b) Concrete Dams They are constructed of mass concrete. Face slopes are dissimilar, general steep downstream and near vertical upstream slopes, and dams have relatively slender profile dependent on the type. Other type such as timber, steel, etc dams in some cases may be constructed.

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Concrete

Arch-Gravity

Gravity Arch

Combination

RockfillMassive buttress

Earth fill

Dam

Embankment

Classification of dams based on material of construction 1.5.1 Types of Embankment Dams and Their General Characteristics Embankment dams are constructed of natural materials excavated or obtained near the dam site. They are of relatively (compared with concrete dams) soft and elastic structures. The foundation requirements are lower compared to concrete dams. Embankment dams possess the following advantages:

- Suitability to wide valley and relatively steep-sided gorges alike; - Adaptability to a broad range of foundation conditions; - The use of natural materials, minimizing the need to transport large quantities of processed materials to the site; - The embankment design is extremely flexible in its ability to accommodate different fill materials, e.g. earthfills and /or rockfills, if suitably zoned internally; - If properly designed, the embankment can safely accommodate an appreciable degree of settlement deformation without risk of serious cracking and possible failure; - The construction process is highly mechanized and effectively continuous; - The unit costs of earthfill and rockfill have risen much more slowly in real terms

than those for mass concrete. The relative disadvantages of the embankment dam are few. The most important include:

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Greater susceptibility to damage or destruction by overtopping, with a consequent need to ensure adequate flood relief and a separate spillway;

Vulnerability to concealed leakage and internal erosion in dam or foundation; Erosion danger on the downstream slopes unless properly protected; Construction materials and construction processes are affected by weather.

Earthfill Embankments Dams If the compacted soil account for over 50% of the placed volume of material, then an embankment may be categorized as an earthfill dam. An earthfill dam is constructed primarily of selected engineering soils compacted uniformly and intensively in relatively thin layers and at a controlled moisture content. Rockfill Embankment Dams The designation rockfill embankment is appropriate where over 50% of the fill material may be classified as a rockfill, i.e. coarse-grained frictional material. In the rockfill embankment the section includes a discrete impervious element of compacted earthfill or a slender concrete or bituminous membrane. The term zoned rockfill dam or earthfill-rockfill dam are used to describe rockfill embankments incorporating relatively wide impervious zones of compacted earthfill. Rockfill embankment dams employing a thin upstream membrane of asphaltic concrete, reinforced concrete or other non-natural material are referred to as decked rockfill dams. 1.5.2 Concrete Dam Types and Their Characteristics Concrete dams are hard, non-yielding and rigid structures. Loads are transmitted through the dam body and to the foundation. They require strong and more or less uniform rock foundations. Many early dams were constructed of rubble masonry or random masonry. From about 1900, mass concrete, initially without formed transverse contraction joints, began to displace masonry for the construction of large non-embankment dams. From about 1950, mass concrete increasingly incorporated bulk mineral additives such as slags or pulverized fuel ash (PFA), in order to reduce thermal problems and to contain escalating costs. The characteristics of concrete dams are outlined below with respect to major types. All or most types of concrete dams share certain characteristics; many are, however,

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specific to particular variants. Merits shared by most concrete dams include: With the exception of arch and cupola, concrete dams are suitable to wide

or narrow valleys, provided that a competent rock foundation is accessible at moderate depth (< 5 m);

Not sensitive to overtopping under extreme flood conditions; All concrete dams can accommodate a crest spillway, i.e. can be

constructed as an overflow section reduction in cost for separate spillway;

Outlet pipe works, valves and other ancillary works can be safely provided within the body of the dam;

Construction can take place irrespective of the weather condition. Inherent disadvantages of concrete dams as compared to embankment dams:

Relatively demanding with respect to foundation conditions, requiring sound rock; Require processed natural materials of suitable quality and quantity for

aggregate, and the importation to the site and storage of bulk cement and other materials;

Traditional mass concrete construction is relatively slow, labour intensive and discontinuous, and requires certain skills, e.g. formwork, concreting, etc;

Completed unit cost for mass concrete are very much higher than for embankment fills. This is seldom counterbalanced by the much lower volumes of concrete required in a dam of given height.

1.5.2.1 Gravity Dams Concrete gravity dam is designed so that its stability is entirely maintained by its own mass. Its profile is essentially triangular, to ensure stability and to avoid overstressing of the dam or its foundation. Concrete gravity dams could be straight or curved in plan. 1.5.2.2 Buttress Dams Buttress dam consists of a continuous upstream face supported at regular intervals by downstream buttresses. Concrete saving relative to the corresponding gravity dam is 30 60%, but it needs more formwork and reinforcement. 1.5.2.3 Arch Dams Arch dam has a considerable upstream curvature. This type is suitable for narrow gorges when the length of the crest is not more 5 times the height of the dam. Loads

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are resisted mainly by arch action and transmitted to the abutments. It is structurally more efficient than the gravity or buttress dam, greatly reducing the volume of concrete required. A particular derivative of the simple arch dam is the cupola or double-curvature arch dam, which is the most sophisticated of concrete dams, and is extremely economical in concrete.