Gravity Dam Gravity Dams A Gravity dam has

Gravity Dam • Gravity Dams A Gravity dam has been defined as a “structure which is designed in such a way that its own weight resist the external forces”. This type of a structure is most durable and solid and requires very less maintenance. • Such dams are constructed of masonry or Concrete. • However, concrete gravity dams are preferred these days and mostly constructed.

Gravity Dam(Contd. . ) • • • Criteria for selection of dam site: construction material, forces acting on gravity dam, modes of failure, stability analysis, safety criteria, methods of design , stress analysis and stress contours, galleries, instrumentation, joints, keys, water seals, temperature control in concrete dams, foundation treatment.

Gravity Dam(Contd. . ) • The line of the upstream face or the line of the crown of the dam if the upstream face is sloping, is taken as the reference line for layout purpose etc. and is known as the Base line of the dam or the „Axis of The Dam‟ When suitable conditions are available such dams can be constructed up to great heights

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Gravity Dam(Contd. . ) • Forces Acting on Gravity Dam The Various external forces acting on Gravity dam may be: • Water Pressure • Uplift Pressure • Pressure due to Earthquake forces • Silt Pressure • Wave Pressure • Ice Pressure • The stabilizing force is the weight of the dam itself

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Gravity Dam(Contd. . ) • Water Pressure (p) is the most major external force acting on such a dam. The horizontal water pressure, exerted by the weight of the water stored on the upstream side of the dam can be estimated from rule of hydrostatic pressure distribution. • Which is triangular in Shape • When the upstream face is vertical the intensity is Zero at the water surface and equal to w H at the base; where w is the unit weight of water and H is the depth of water. The resultant force due to this external water • P= ½ w H 2 , acting at H/3 from base.

Gravity Dam(Contd. . )

Gravity Dam(Contd. . ) • When the upstream face is partly vertical and partly inclined, the resulting water force can be resolved into horizontal Component (Ph) and Vertical Component (Pv). The Horizontal Component Ph = ½ γH² act at H/3 from the base, & the vertical component (Pv) is equal to weight of the water stored in column ABCA and acts at the C. G. of the Area. Similarly, if there is tail water on the downstream side, it will have horizontal and vertical components

Gravity Dam(Contd. . )

Gravity Dam(Contd. . ) • Uplift Pressure • Water Seeping through the pores, cracks and fissures of the foundation material, and water seeping through dam body and then to the bottom through the joint between the body of the dam. It is the second major external force and must be accounted for in all calculations. Such an uplift force virtually reduces the downward weight of the body of the dam and hence, acts against the dam stability.

Gravity Dam(Contd. . ) • The amount of Uplift is a matter of research and the present recommendations the uplift pressure intensities at the heel and the toe should be taken equal to their respective hydrostatic pressure and joined by a straight line in between. When drainage galleries are provided to relieve the uplift, the recommended uplift at the face of the gallery is equal to the hydrostatic pressure at toe ( w H’ ) plus a 1/3 rd of the difference of the hydrostatic pressure at the heel and at the toe [ w H’ + 1/3 ( w H - w H’) ]. It is also assumed that the uplift pressure are not affected by the earthquake forces. • The uplift pressure can be controlled by constructing cutoff walls under the upstream face, by constructing drainage channels between the dam and its foundations and by pressure grouting the foundation.

Gravity Dam(Contd. . )

Gravity Dam(Contd. . ) • Earthquake Forces • If the dam is to be designed, is to be located in a region which is susceptible to earthquakes, allowance must be made for stresses generated by the earthquakes. • An earthquake produces waves which are capable of shaking the Earth upon which the dam is resting, in every possible direction

Gravity Dam(Contd. . ) • The effect of an earthquake is therefore, equivalent to impairing an acceleration to the foundation of the dam in the direction in which the wave is traveling at the moment, Earthquake waves may move in any direction and for design purpose, it has to be resolved in vertical and horizontal components. Hence, two accelerations, i. e. . one horizontal acceleration ( h ) and one vertical acceleration ( v ) are induced by an earthquake. The value of these acceleration are generally expressed as percentage of the acceleration due to gravity (g) i. e. . = 0. 1 g or 0. 2 g etc.

Gravity Dam(Contd. . ) • On an average, a value of equal to (0. 1 to 0. 15) g is generally sufficient for high dams in seismic zones. In areas of no earthquakes or very less earthquakes, these forces may be neglected. In extreme seismic regions and in conservative designs, even up to 0. 3 g may sometimes be adopted. However, for areas not subjected to extreme earthquakes, h = 0. 1 g and v = 0. 05 g may be used. In extreme seismic regions and in conservative designs, even a value of 0. 3 g may sometimes be adopted.

Gravity Dam(Contd. . ) • Effect of Vertical Acceleration ( v ) • A Vertical acceleration may either downward or Upward. When it is acting in the upward direction, then the foundation of the dam will be lifted upward and becomes close to the body of the dam, and thus the effective weight of the dam will increase and hence, the stress developed will increase.

Gravity Dam(Contd. . ) • When the vertical acceleration is acting downward, the foundation shall try to move downward away and hence is the worst for the design. • Such Accelerations will, therefore exert an Inertial force given by, • v (i. e. . Force = Mass x Acceleration) where W is the total weight of the dam • The net effective weight of the Dam= W- v

Gravity Dam(Contd. . ) • v = kv g Where kv is the fraction of gravity adopted for vertical acceleration, 0. 1 or 0. 2. Net effective weight of dam = W kv g = W(1 - kv )

Gravity Dam(Contd. . ) • Effect of Horizontal Acceleration ( h ) • Horizontal acceleration may cause the following two forces: • Hydrodynamic Pressure • Horizontal Inertia force • Hydrodynamic Pressures - Horizontal acceleration acting towards the reservoir, causes a momentary increase in the water pressure, as the foundation and dam accelerates towards the reservoir and the water resists the movement owing to its inertia. The extra pressure exerted by this process is known as Hydrodynamic Pressure. • According to Von- Karman, the amount of this hydrodynamic force (Pe) is given by Pe = 0. 555 kh w H² and it act at a height of 4 H/3 above the base

Gravity Dam(Contd. . )

Gravity Dam(Contd. . ) • here kh is the fraction of gravity adopted for horizontal acceleration, such as 0. 1, 0. 2 etc. • Moment of this force above Base is Me= 4 H/3 = Pe = 0. 424 Pe H

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