Dam and Dam Sites

2013 Dams and Dam Sites

HARIS MAHMOOD 2011-CIV-218 “D” 10/1/2013

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Dams and Dam sites

Introduction: A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water. The dam site is the place where the dam is built; usually they are built at stream or a river.

Purpose: The dam is build for the following purposes: 

To store water

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To generate electricity

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To control the flow of water

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Agricultural purposes





Factors effecting dam designs: The factors that should be in mind during designing a dam are given below: I.

Selection of Dam site

II.

Estimation of the PMF and selection of the IDF

III.

Identification of earthquake source area and structure, estimation of MCE’s, and identification of earthquake-related safety concerns

IV.

Development of a site-specific geo-technical, Exploration program

V.

Design of the foundation, dam, and appurtenant structures

VI.

Design of a system of instrumentation to monitor the performance of the dam, foundation, and appurtenant structures

VII.

Development of an initial reservoir-filling and surveillance plan and of reservoir drawdown criteria

VIII.

Preparation of designer’s operating criteria and identification of special considerations to be observed during construction and operation

IX.

Provisions for the automatic, independent review by competent individuals of all design decisions, methods, procedures, and results related to dam safety

X.

Provisions to revise the design to make it compatible with conditions encountered during construction





Type of dam designs: The dams can be widely classified by their designs. The design of each dam varies with its geological location and parameters. Some of the famous types of dam designs are given below:

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Arch dam : The arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of t he 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.





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Gravity dam: In a gravity dam, the force that holds the dam in place against the push from the water is Earth's gravity pulling down on the mass of the dam. The water presses laterally (downstream) on the dam, tending to overturn the dam by rotating about its toe (a point at the bottom downstream side of the dam). The dam's weight counteracts that force, tending to rotate the dam the other way about its toe. The designer ensures that the dam is i s heavy enough that gravity wins that contest. In engineering terms, that is true whenever the resultant of the forces of gravity and water pressure on the dam acts in a line that passes upstream of the toe of the dam. Furthermore, the designer tries to shape the dam so if one were to consider the part of dam above any particular height to be a whole dam itself, that dam also would be held in place by gravity. i.e. there is no tension in the upstream face of the dam holding the top of the dam down. The designer does this because it is usually more practical to make a dam of material essentially just piled up than to make the material stick together against vertical tension.

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Arch-gravity dam: dam:

A gravity dam can be combined with an arch dam into an arch-gravity dam for areas with





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Barrages: A barrage dam is a special kind of dam which consists of a line

of large gates that can be opened or closed to control the amount of water passing the dam. The gates are set between flanking piers which are responsible for supporting the water load. They are often used to control and stabilize water flow for irrigation systems. Barrages that are built at the mouth of rivers or lagoons to prevent tidal incursions or utilize the tidal flow for tidal power are known as tidal barrages.







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

Dams and Geology The geology plays an important role in the making of any dam. Before initializing the construction it is necessary for a civil engineer to take the geology of the site into account and design the dam accordingly. On a large dam construction project the engineering geologist is concerned with:





whether landslides into the reservoir are possible which might cause a wave of water to be pushed over the top of the t he dam. •

Finding sources of the construction materials which will be needed to build the dam.





Types of rocks 

Igneous rocks: These types of rocks have the maximum tendency to repel moisture and are not weakened by it. Thus these are best for making a dam in tropical environment.

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Sedimentary rocks: The sedimentary rocks are not suitable for dam Construction as they tend to compress under pressure, this can cause some serious failures in the dam. But in case the dam site is on a stream the sides of the stream contains sedimentary rocks. These rocks are already compressed to their extent, thus these rocks can be used for the construction.





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Metamorphic rocks: These rocks can be subjected to high pressures, so these Rocks can be used in dam construction.

Effects of Joints The joints do provide the ease in construction and the designing of the dam, but as an engineer we should avoid using them because in case of any earth quake the joints are mostly the point of failure. The joints also do contract slowly with time under pressure thus weakening the joints. Pacoima Dam is constructed as cantilever monoliths separated by vertic al contraction joints. A seismic safety evaluation of an arch dam often relies on a linear linear dynamic analysis assuming the dam is a monolithic structure. The CDMG network of strong motion





hydrodynamic pressure in joints is investigated. It is indicated that considering large deformation effects could be magnified when water penetration into opening j oints is permitted. The obtained results showed that because the structural behavior of an arch dam does not allow large strains in a general manner, one can rule out the t he appearance of large displacements in the models including linear material.

Dam and Dam Sites

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