Load combinations
LOAD COMBINATIONS

Load combinations

LOAD COMBINATIONS In Section 13.2 various forces that are likely to act on the dam have been discussed. But all these forces seldom act simultaneously on the dam. There are some combinations of the loads that may act simultaneously. Design of the dam should be based on the most adverse combination of “probable load condition”. USBR has recommended two load combinations. 1. Standard load combinations. 2. Extreme load combinations. 1. Standard Load Combinations. Under this heading three possible combinations are given. (i) Horizontal water pressure, at normal reservoir level + ice pressure + silt pressure + normal uplift pressure. But this combination is adopted where ice pressure is considerable. (ii) Horizontal water pressure at normal reservoir level + earthquake pressure + silt pressure + normal uplift pressure. (iii) Horizontal water pressure at H.F.L. + silt pressure + normal uplift pressure. 2. Extreme Load Combination. Maximum flood water elevation + silt…

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Modes of failure of gravity dams
MODES OF FAILURE OF GRAVITY DAMS

Modes of failure of gravity dams

MODES OF FAILURE OF GRAVITY DAMS A gravity dam can fail due to the following reasons: Overturning of the dam Sliding of the dam Crushing of the dam or Foundation Development of tension in the dam. 1. Overturning of the Dam. If the resultant of all the possible forces (internal as well as external) acting on the dam cuts the base of the dam downstream of the Toe, the darn would overturn unless it can resist tensile stresses. To safeguard the dam against overturning, the resultant of the forces should never be allowed to go down stream of the Toe. If resultant is maintained within the body of the dam, there will be no overturning. All the forces acting on the dam cause moments. Some of the forces help maintain stability of the dam, while others try to disturb the stability. The moments of the forces, helping dam to maintain…

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Principal stresses and shear stress
PRINCIPAL STRESSES AND SHEAR STRESS

Principal stresses and shear stress

PRINCIPAL STRESSES AND SHEAR STRESS The vertical stress intensity is not the maximum direct stress intensity within the dam section. It is therefore necessary to determine the intensity of the maximum normal stress. The maximum normal stress occurs on the major principal plane. The D/S face of the dam which is inclined at an angle α to the vertical is a principal plane, as no shear stress acts on it. If any tail water is there it contributes a pressure p’ entirely normal to the plane and is one of the principal stresses. The second principal plane will be inclined at right angles to the face of the dam and hence at an angle α to the horizontal. The stress acting on such plane is the major principal stress Pmax. All the stresses can be represented by Mohr’s circle. See Fig. 13.6. The vertical stress Pn at toe of the…

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Stability analysis method of gravity dam
Stability analysis method of gravity dam

Stability analysis method of gravity dam

STABILITY ANALYSIS METHODS The following are the various methods which can be adopted for carrying out the stability analysis of gravity dams. Gravity method. Trial load twist method. Slab analogy method. Lattice analogy method. Experimental methods. Brief description of each method is given here. 1. Gravity Method. This method of stability analysis, is also sometimes know two dimensional method. In this method, the dam is consider to be compos of parallel side. Each cantilever is consider free to act without any attachment with the adjoining cantilevers. The loads acting on the dam are resist entirely by the weight of the individual cantilever. All the loads are ultimately transferr to the foundation by cantilever action. This method of checking the stability of the dam may be further divid into two parts.  (a) Graphical method. In this case the dam is divid into different sections according to height by drawing horizontal section…

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Elementary profile of a gravity dam

ELEMENTARY PROFILE OF A GRAVITY DAM If a gravity dam is subject to horizontal pressure due to water along with its own weight, the elementary profile of the dam would be a right angled triangle. The thickness of the dam at the level of water surface will be zero as there is no hydrostatic pressure to be resist. The thickness at base would be maximum as there will be maximum hydrostatic pressure to be resisted by the dam. As the variation in hydrostatic pressure from water level to the bottom, is linear; variation in elementary profile from top to the dam to its bottom would also be linear. Elementary profile of the low dam is show in Fig. 13.8 (a). The triangular elementary profile also provides the maximum possible stabilizing force against overturning without causing tension in the base when reservoir is empty. This is because in this case weight…

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Seepage control measures in earth dams
Seepage control measures in earth dams

Seepage control measures in earth dams

SEEPAGE CONTROL MEASURES The safety of earth dam depends almost entirely on seepage control through dam and its foundation. Hence measures of seepage control are very important for the success of an earthen dam. Seepage control through the dam. Seepage control through the foundation. Various measures of seepage control under each categoray have been discussed in brief. Seepage Control Through the Dam 1. Rock Toe. Rock toe keeps seepage line well within the dam section. It also helps a great deal for the drainage purposes. The height of the rock toe is kept about one fourth of the height of the dam. Rock toe should be designed like filter. 2. Horizontal Drainage Filter. It is provided at the base of the dam, starting from down stream end of the dam and extending backwards into the dam. Backward extension of the filter depends upon so many factors. But this extension may…

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Slope Protection of Earth Dam
SLOPE PROTECTION OF THE DAM

Slope Protection of Earth Dam

SLOPE PROTECTION OF THE DAM For the success of the earth dam, its U/S and D/S slopes have to be properly maintained. U/S slopes have to be protected against wave action, and sudden draw down conditions whereas D/S slopes against heavy down pours, and action of burrowing animals. Following are some of the measures used for slope protection. 1. Turfing. Down stream slopes of the dams can be protected against erosion by developing turf. This measure is quite effective for slope protection. 2. Counter Berms. Down stream protection of the dam can be achieved to some extent by providing counter-beams. This measure is usually adopted for very high dams. Counter beams break the rhythm of the velocity of rain water, flowing down the berm. 3. Rip rap. This is the only measure of protecting the U/S slope against the action of wave erosion and sudden draw down. This measure can…

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Gravity Dams
Gravity Dams

Gravity Dams

INTRODUCTION Gravity dam is such a dam, which resists all the forces acting on it, with the help of its own weight. This dam is considered as one of the most permanent dams. It requires very little maintenance. This is the most common type of dam, both for low heights as well as very large heights. Gravity dam may be made of either masonry or cement concrete. However, with the advent of most improved methods of construction, quality control, and curing, cement concrete is mostly used. Gravity dams mostly have their alignment straight, but they can have their alignment a bit curved also. In both the cases, the dam resists all the forces acting on it, by the aid of its own weight. Arch action developed, in slightly curved dams, is not considered in its design. The gravity dams are mostly solid. However they can be constructed hollow also. Favourable…

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forces acting on a gravity dam
FORCES ACTING ON A GRAVITY DAM

forces acting on a gravity dam

FORCES ACTING ON A GRAVITY DAM A detailed sketch of a gravity dam is shown in Fig. 13.1. All the predominant forces, that act on the dam, have been shown in the figure itself. The forces that act on the dam are the following: Weight of the dam Horizontal hydrostatic pressure due to water Uplift pressure due to water percolated under the dam Earthquake pressure Wind pressure Ice pressure Wave pressure Pressure due to silt deposited on U/S face. Out of above eight forces, acting on the dam, first three forces are the major forces that are considered in the design. All other forces are not of much significance and are considered only under specific conditions. 1. Weight of the Dam. It is the most important force, particularly for gravity dams. Stability of the dam largely depends upon this force. For the design purpose only unit length of the dam…

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Filter design
FILTER DESIGN

Filter design

FILTER DESIGN Filter drains should be design in such a way that all the seeping water through the dam is effectively drained off. The filter consists of more than one layer. The filter layer which comes in contact with the seeping water first is of fine material. Subsequent layers of filter are made of sand of increased coarseness. The last layer from which water comes out of the filter is made of gravel. The filters of filter drains are also know reverse or invert filter. The soil of earth dam and foundation surrounding the filter is know the base material. According to earth manual of US Bureau of Reclamation Washington 1960 following four main requirements should be satisfied. Filter material should be fine and poorly grad so that the voids in the filter are small and thus prevent base material from entering the filter. The filter material should be coarse…

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