Dams

Structural philosophy and generic types of damsThe primary purpose of a dam may he defined as to provide for the safe retention and storage of water. As a corollary to this every dam must represent a design solution specific to the site circumstances The design therefore represents an optimum balance of local technical and economic consideration at the time of construction.

Reservoirs are readily classified in accordance with their primary purpose, e.g., irrigation. water supply. hydroelectric power generation. river regulation, flood control, etc. Dams are of numerous types, and type classification is sometimes less clearly defined. An initial broad classification into two generic groups can be made in terms of the principal construction material employed.

1. Embankment dams are constructed of earthfill and/or rockfill. Upstream and downstream face shapes are similar and of moderate angle, giving a wide section and a high construction volume relative to height

2. Concrete dams are constructed of mass concrete. Face slopes are dissimilar, generally steep downstream and near vertical upstream, and dams have relatively slender profiles dependent upon the type.

The second group can he considered to include also older dams of appropriate structural type constructed in masonry. The principal types of dams within the two generic groups are identified in the table below.

Embankment dams are numerically dominant for technical and economic reasons, and account for an estimated 85-90% of all dams built. Older and simpler in structural concept than the early masonry dam, the embankment utilised locally available and untreated materials. As the embankment dam evolved it has proved to be increasingly adaptable to a wide range of site environments in contrast, concrete dams and their many predecessors are more demanding in relation to foundation conditions. Additionally they have also proved to be dependent upon relatively advanced and expensive construction skills and plant.

Group Type %Embankment dams Earthfill

Rockfill82.9

Concrete dams (including masonry dams)

Gravity 11.3Arch 4.4Buttress 1.0Multiple arch 0.4

Total Large* dams(ICOLD 1998) 36,235

*Dams exceeding 15m in height or in the case of dams of 10-15m height, satisfying one of certain other criteria, e.g. a storage volume in excess of 1x106 m3

or a flood discharge capacity of over 2000 m3/s. Over 1900 were claimed by China and 5459 by the US. These figures may be compared with a worldwide total of 5196 large dams recorded in 1950.

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Dams differ from all other major civil engineering structures in a number of important regards:

every dam, large or small. is quite unique: foundation geology, material characteristics, catchment flood hydrology etc are each site specific. dams are required to function at or close to their design loading for extended periods. dams do not have a structural lifespan; they may, however, have a

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notional life for accounting purposes, or a functional lifespan dictated by reservoir sedimentation,

the overwhelming majority of dams are of earth fill, constructed from a range of natural soils; these are the least consistent of construction

Dam engineering draws together a range of disciplines, e.g. structural and fluid mechanics, geology and geotechnics, flood hydrology and hydraulics, to a quite unique degree. The engineering of dams is especially dependent upon the application of informed engineering judgment. in summary, dam engineering is a distinctive, broadly based and specialist discipline. The dam engineer is required to synthesise design solutions, which, without compromise on safety, represent the optimal balance between technical, economic and environmental considerations.

Embankment dam types and characteristicsThe embankment dam can be defined as a dam constructed from natural materials excavated or obtained close by. The materials available are used to the best advantage in relation to their characteristics as an engineered bulk fill in defined zones within the dam section. The natural fill materials are placed and compacted without the addition of any binding agent, using high capacity mechanical plant. Embankment construction is consequently now am almost continuous and highly mechanized process, weather and soil conditions permitting, and is thus plant intensive rather than labour intensive.

Embankment dams can be classified in broad terms as being earthfill or rockfill dams. The division between the two embankment variants is not absolute, many dams utilising fill material of both types within appropriately designated internal zones. Secondary embankment dams and a small minority of larger embankments may employ a homogeneous section, but in the majority of instances embankments employ an impervious zone or core combined with supporting shoulders which may be of relatively pervious material. The purpose of the latter is entirely structural, providing stability to the impervious element and to the section as a whole.

Embankment dams can one of many types. depending upon how they utilise the available materials. The initial classification into earthfill or rockfill embankments provides a convenient basis for considering the principal variants employed.

1.Earthfill embankments. An embankment may he categorised as an earthfill dam if compacted soils account for over 50% of the placed volume of material. An earthfill dam is constructed primarily of selected engineering soils compacted uniformly and intensively in relatively thin layers and at a controlled moisture content. 2. Rockfill embankments

If the rockfill embankment the section includes a discrete impervious element of compacted earthfill or a slender concrete or bituminous membrane The designation rockfill embankment is appropriate where over 50% of The fill material may be classified as rockfill. i.e. coarse grained frictional material. Modern practice is to specify a graded rockfill. heavily compacted in relatively thin layers by heavy plant. The construction method is therefore essentially similar to that for the earthfill embankment.

The terms zoned rockfill dam or earthfill/rockfill dam are meant to describe rockfill embankments incorporating relatively wide impervious zones of compacted earthfill. Rockfill embankments employing a thin upstream membrane of asphaltic concrete.

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reinforced concrete or other manufactured material are referred to as decked rockfill dams. The variants of earthfill and rockfill embankments employed in practice are too numerous to identify all individually. The embankment dam possesses many outstanding merits which combine to ensure its continued dominance as a generic type. The most important can be summarized as follows:

1. the suitability of the type to sites in wide valleys and relatively steep sided gorges alike:

2. adaptability to a broad range of foundation conditions, ranging from competent rock to soft and compressible or relatively pervious soil formations:

3. the use of natural materials, minimising the need to import or transport large quantities of processed materials or cement to the site:

4. subject to satisfying essential design criteria, the embankment design is extremely flexible in its ability to accommodate different fill material e.g. earthfills and/or rockfills. if suitably zoned internally:

5. the construction process is highly mechanized and is effectively continuous:6. largely in consequence of 5, the unit costs of earthfill and rockfill have risen

much more slowly in real terms than those for mass concrete;7. properly designed, the embankment can safely accommodate an appreciable

degree of settlement deformation without risk of serious cracking and possible failure.

The relative disadvantages of the embankment dam are few. The most important include an inherently greater susceptibility to damage or destruction by overtopping, with a consequent need to ensure adequate flood relief and a seperate spillway, and vulnerability to concealed leakage and internal erosion in dam or foundation.

Concrete dam types and characteristicsThe principal variants of the modern concrete dam are defined overleaf:

Gravity dams A concrete gravity dam is entirely dependent upon its own mass for stability The gravity profile is essentially triangular, to ensure stability so to avoid overstressing of the dam or its foundation Some gravity dams are gently curved in plan for aesthetic or other reasons. And without placing any reliance upon arch action for stability Where a limited degree of arch action is deliberately introduced in design allowing a rather slimmer profile, the term arched or arch-gravity dam may be employedButtress dams In structural concept the buttress dam consists of a continuous upstream face supported at regular intervals by downstream buttresses The solid head or massive buttress dam, is the most prominent modern variant of the type, and may be considered for conceptual purposes as a lightened version of the gravity dam.

Arch dams The arch dam has a considerable upstream curvature Structurally it functions primarily as a horizontal arch. Transmitting the major portion of the water Ioad to the abutments nr valley sides rather than to the floor of the valley A relatively simple arch i e with horizontal curvature only and a constant upstream radius. 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. The cupola dam introduces complex curvature in the vertical as well as the horizontal

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plane it is the most sophisticated of concrete dams, being essentially a dome or shell structure, so is extremely economical in concrete. Abutment stability is critical to the structural integrity and safety of both the cupola and the simple arch

Other concrete dams A number of less common variants of the major types of concrete dams is illustrated in Fig can also be identified They include hollow gravity, decked buttress. flat slab (Aburse) buttress, multiple arch, multiple cupola dams. The type names are self explanatory, and the structural percentage of each as a derivative of one or other of the principal types is obvious from the figures

The characteristics of concrete dams are outlined below with respect to the major types, i.e. gravity, massive buttress and arch or cupola dams Certain characteristics are shared by all or most of these types: many are however, specific to particular variants. Merits shared by most concrete dams include the following:

Arch and cupola dams excepted, concrete dams are suitable to the site topography of wide or narrow valleys alike, provided that a competent rock foundation is accessible at moderate depth (<5 m).

Concrete dams are not sensitive to overtopping under extreme flood conditions.

Concrete dams can accommodate a crest spillway if necessary over their entire length, provided that steps are taken to control downstream erosion and possible undermining of the dam. The cost of a separate spillway and channel are therefore avoided.

Outlet pipework. valves and other ancillary works are readily and safely housed in chambers or galleries within the dam.

The inherent ability to withstand seismic disturbance without catastrophic collapse is generally high.

The cupola or double curvature arch dam is an extremely strong and efficient structure. given a narrow valley with competent abutments.

Type-specific characteristics are largely determined through the differing structural modus operandi associated with variants of the concrete dam. In the case of gravity and buttress dams. for example, the dominant structural response is in terms of vertical cantilever action.

The reduced downstream contact area of the buttress dam imposes significantly higher local foundation stresses than for the equivalent gravity structure. It is therefore a characteristic of the former to be more demanding in terms of the quality required of the underlying rock foundation.

The structural behaviour of the more sophisticated arch and cupola variants of the concrete dam is predominantly arch action. with vertical cantilever action secondary. Such dams are totally dependent upon the integrity of the rock abutments and their ability to withstand arch thrust without excessive yielding. It is consequently characteristic of arch and cupola dams that consideration of their suitability is confined to a minority of sites in relatively narrow steep sided valleys

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or gorges, i.e. to sites with a width: height ratio at the dam crest level generally not exceeding 4-5.

A comparison of the general characteristics of concrete dams with those of the embankment dam suggests the following inherent disadvantages for the former.

Concrete dams are relatively demanding with respect to foundation conditions. requiring sound and stable rock

Concrete dams require processed natural materials of suitable quality and quantity for aggregate, and the importation to site and storage of bulk cement and other materials.

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

Completed unit costs for mass concrete. i.e. cost per cubic metre. are very much higher than for embankment dams, typically by an order of magnitude or more. This is seldom counterbalanced by the much lower volumes of concrete required in a dam of given height.

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