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    Dams: How to design dams


    Dams are structures built to divert or store water for irrigation, water supply, flood control, electric power generation and/or navigational purposes. Dams are classified on the basis of the types and materials of construction. The major types are gravity, arch, buttress and earth dams. The first three types are usually constructed out of concrete. A gravity dam depends on its own weight for stability and is usually straight on plan even though it may sometimes be slightly curved. Arch dams transmit most of the horizontal thrust of the water behind to the abutments by arch action and have thinner cross-section when compared with gravity dam. Arch dams are preferably used in narrow canyons where the walls are capable of withstanding the thrust produced by the arch action. The simplest type of the buttress dams is the slab type which consists of sloping flat slabs supported at intervals. Earth dams are embankments of rock or earth with provision for controlling seepage by means of an impermeable core or upstream blanket.
    Erection of Dams
    Erection of dams usually require the consideration of certain factors since their usefulness when appropriately designed and sited have enormous benefits. On the contrary, their failure can result in considerable socio-economic and environmental losses. These factors are:
    1.  Geographical, because of the resulting modification of the surrounding landscape.
    2.  Geological, in respect to the appropriateness of the bedrock strata which will reduce seepage losses.
    3.  Economic in respect to alternative water resource development options.
    4.  Social, such as problems of health and resettlement.
    5.  Environmental implication.
    Forces on Dams
    A dam must be relatively impervious to water and capable of resisting gravity i.e., its own weight, hydrostatic pressure (sediment deposits and flowing water), uplift, earthquake forces, etc. The forces are transmitted to the foundation and abutment of the dam which react against the dam with equal and opposite force.
    General approach for design of gravity dams
    For small dams and preliminary design of large dams, it is adequate to use the simplified approach based on the elastic behavior of concrete. Final design of large dams however, may require the use of sophisticated methods involving finite element methods.
    Procedure for designing gravity dams
    1.  Assume the dimensions of the dam.
    2.  Check the static and dynamic stability.
    In order to avoid tensile stresses, the resultant force R, must be kept within the middle third of the base. In the simplified approach, the dam is divided into slices and each slice is assumed to act independent of adjoining slices.

    design of dam

    Fig. 1: A diagrammatic representation of a dam

    Loads on dam
    There are two main load categories on a dam. They are main loads and secondary loads.

    1) Main Loads: The main loads consist of water load, self-weight, seepage and uplift load.
    a)  Hydrostatic pressure: Pw = YZ1
    Force = Pwh = 1/2YZ12
    This acts at 1/3Z above datum.
    b)  The vertical force Pwv is accounted for if the upstream face has a batter or flare as in the diagram above.

    Pwv = Y (Area, A1), and acts through the centroid of A1

    c)  Tail water Pwh2 = 1/2YZ22
    d)  Vertical force Pwv2 = Y (Area, A2); Y = unit weight of water = 10kN/m^3

    2)  Self weight:
    For self weight, W = YcAp kN/m^3
    For heavy special concrete, Yc = 31.5 kN/m^3
    Where Yc is the unit weight of concrete, and Yc is = 23.5 (approximately 24) kN/m^3 for normal concrete.
    3)  Seepage and uplift load: Interstitial water pressure, Uw develops within a concrete dam and its foundation as a result of preferential water penetration along discontinuities, e.g. cracks and fine fissures.
    Uplift load Pu = nAn(Uwavg)


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