毕业设计英文翻译

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The first dam for which there are reliable records was build on the Nile River sometime before 4000 B.C.It was used to divert the Nile and provide a site for the ancient city of Memphis.The oldest dam still in use is the Almanza Dam in Spain,which was constructed in the sixteenth ceentury.With the passage of time, materials and construction have improved, making possible the erection of such large dams as the Nurek Dam, which is being constructed in the U.S.S.R. On the vaksh River near the border of Afghanistan. This dam will be 1017ft(333m)high, of earth and rock fill. The failure of a dam may cause serious loss of life and property; consequently, the design and maintenance of dams are commonly under government surveillance. In the United States over 30000 dams are under the control of state authorities. The 1972 Federal Dam Safety Act (PL92-367) requires periodic inspections of dams by qualified experts. The failure of the Teton Dam in Idaho in June 1976 added to the concern for dam safety in the United States. 1.Type of Dams

Dams are classified on the basis of the type and materials of construction, as gravity, arch, buttress, and earth. The first three types are usually constructed of concrete. A gravity dam depends on its own weight for stability and it straight in plan although sometimes slightly curved. Arch dams transmit most of the horizontal thrust of the water behind them to the abutments by arch action and have thinner cross sections than comparable gravity dams. Arch dams can be used only in narrow canyons where the walls are capable of withstanding the thrust produced by the arch action. The simplest of the many types of buttresses. Earth dams are embankments of rock or earth with provision for controlling seepage by means of an impermeable core or upstream blanket. More than one type of dam may be included in a single structure. Curved dams may combine both gravity and arch action to achieve stability. Long dams often have a concrete river section containning spollway and sluice gates and earth or rock-fill wing dams for the remainder of their length.

The selection of the best type of dam for a given site is a problem in both engineering feasibility and cost. Feasibility is governed by topography, geology and climate. For example, because concrete spalls when subjected to alternate freezing and thawing, arch and buttress dams With thin concrete section are sometimes avoided in areas subject to extreme cold. The relative cost of the various type of dams depends mainly on the availability of construction materials near the site and the accessibility of transportation facilities. Dams are sometimes built in stages with the second or later stages constructed a decade or longer after the stage. The height of a dam is defined as the difference in elevation between the roadway, or spillway crest, and the lowest part of the excayated foundation. However, figures quoted for heights of dams are often


detemined in other ways. Frequently the height is taken net height above the old riverbed.

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