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Text A Gravity Dams
The stability of gravity dams depends on their own weight. They use its own dead weight to resist the horizontal force of the water. Gravity dams are usually straight in plan although sometimes they may be slightly curved. The shape of the gravity dam resembles a triangle. This is because of the triangular distribution of the water pressure. The deeper the water, the more horizontal pressure it exerts on the dam. So at the surface of the reservoir, the water is exerting no pressure and at the bottom of the reservoir, the water is exerting maximum pressure.
Before the construction work in a river channel can be started, the stream flow must be diverted. In two stage construction the flow is diverted to one side of the channel by a cofferdam while work proceeds on the other side. After the work on the lower side of the dam is completed, flow is diverted through outlets in this portion while work proceeds in the other half of the channel. The foundation must be excavated to solid rock before any concrete will be poured. After excavation cavities or faults in the underlying strata are sealed with concrete or grout. Frequently a grout curtain is placed near the heel of the dam to reduce seepage and uplift. Grout cement and water sometimes mixed with a small amount of fine sand is forced under pressure into holes drilled into the rock.
 When concrete sets, a great deal of heat is liberated and the temperature of the mass is raised. As the concrete cools, it shrinks, and cracks may develop. To avoid cracks, special low-heat cement may be used. For obtaining best results the temperature of the concrete mix should be between 50 and 80 0 F.
Finish the following sentences according to the text. 1) The stability of gravity dams depends on … 2) Gravity dams are usually straight in plan although sometimes they may be … 3) At the surface of the reservoir, the water is exerting … 4) At the bottom of the reservoir, the water is exerting … 5) Generally the base of a concrete gravity dam is equal to … 6) For gravity dams, good impervious foundations are … 7) Gravity dams are classified as … 8) Before the construction work the stream flow must be … 9) The flow is diverted to one side of the channel by … 10) Then flow is diverted through … while work proceeds in the other half of the channel. 11) The foundation must be excavated to … 12) After excavation cavities or faults in the underlying strata are sealed with … 13) A grout curtain is placed near the heel of the dam to reduce … 14) Grout cement and water is forced under pressure into … 15) Concrete for the dam is usually placed … 16) Sections are poured … 17) Each block can stand several days before another one is poured … 18) Then blocks are sprinkled with … 19) The lateral surfaces of each section are painted with … 20) Keyways carry the shear from one section to the adjacent one and make the dam act as … 21) Metal water stops are placed in … 22) Inspection galleries may be necessary for … 23) When concrete sets, a great deal of heat is liberated and the temperature of the mass is … 24) As the concrete cools, it shrinks, and … may develop. 25) For obtaining best results the temperature of the concrete mix should be … |
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Generally the base of a concrete gravity dam is equal to approximately 0.7 times the height of the dam. For this type of dams, good impervious foundations are essential.