A gravity dam is a structure so proportioned that its own weight resists the forces exerted upon it. This type of dam is the most permanent one, requires little maintenance and is most commonly used.
A gravity dam is the one in which the external forces (such as water pressure, wave pressure, silt pressure, uplift pressure, etc.) are resisted by the weight of the dam itself.
Thus the forces disturbing the stability of the dam are resisted by the gravity forces of the mass of the dam.
A gravity dam may be constructed either of masonry or of concrete. Masonry gravity dams are now-a-days constructed of only small heights.
All major and important gravity dams are now constructed of concrete only.
A gravity dam may be either straight or curved in plan.
A gravity dam is mostly straight in plan and is known as ‘straight gravity dam’. However, it may also be slightly curved in plan.
A curved gravity dam resists the external forces by its weight and not by arch action.
The most ancient gravity dam on record was built in Egypt more than 400 years B.C. of un-cemented masonry.
Archaeological experts believe that this dam was kept in perfect condition for more than 45 centuries.
Most of the gravity dams are solid, so that no bending stress is introduced at any point and hence, they are sometimes known as ‘solid gravity dams’.
A gravity dam, however, can be hollow and is known as ‘hollow gravity dam’.
Gravity dams are particularly suited across gorges with very steep side slopes where earth dams might slip.
Where good foundations are available, gravity dams can be built up to any height. The highest dams in the world are of gravity type.
Advantages of gravity dams.
There is no type of dam more permanent than one of the solid concrete, nor does any other type require less for maintenance. As compared to earth and rock-fill dams gravity dams have the following advantages :
1. Gravity dams are relatively more strong and stable than earth dams. They are particularly suited across gorges having very steep side slopes where earth dam, if constructed, might slip.
2. Gravity dams are well adapted for use as an overflow spillway crest. Earth dams cannot be used as overflow dams. Due to this, a gravity overflow dam is often used for the spillway feature of earth and rock-fill dams.
3. They can be constructed of any height, provided suitable foundations are available to bear the stresses.
The height of an earth dam is usually limited by the stability of its slopes requiring a very wide base width. Highest dams in the world are made of gravity dams only.
4. Gravity dam is specially suited to such areas where there is likelihood of very heavy downpour. The slopes of earth dam might get washed away in such a situation.
5. They requires the least maintenance.
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6. The failure of this dam, if any, is not sudden. It gives enough warning time before the area to downstream side is flooded due to the damage of the structure.
On the contrary, an earth dam generally fails suddenly.
7. Deep-set sluices can be used in the gravity dams, to retard the sedimentation or silt deposit in the reservoir. The trap efficiency of a reservoir of an earth dam is more than that of a reservoir of gravity dam.
8. They are cheaper in the long run since it is more permanent than any other type. Thus the benefit-cost ratio of such a dam is always higher.
Disadvantages of Gravity Dams.
The disadvantages of gravity dam, as compared to an earth dam are as follows:
1. Gravity dams can be constructed only on sound rock foundations. They are unsuitable on weak foundations or on permeable foundations on which earth dams can be constructed with suitable foundation treatment.
2. The initial cost of a gravity dam is always higher than an earth dam. Hence, where funds are limited and where suitable materials are available for the construction of an earth dam, the earth dam may be preferred.
3. If mechanized plants, such as manufacturing and transporting mass concrete, curing of concrete etc. are not available, a gravity dam may take more time to construct.
4. They require skilled labor or mechanized plants for its construction.
5. It is very difficult to allow subsequent rise in the height of a gravity dam, unless specific provisions have been made in the initial design.
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