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1、 Dam The first dam for which there are reliable records was build or 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 century.

2、With the passage of time,materials and methods of 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. Th

3、e 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 30,000 dams are under the control of state authorities. The 1972 Federal Dams Safety Act (PL92-367)requires periodic

4、 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 type and materials of construction, as gravity, arch, buttress ,and earth .The first three types ar

5、e usually constructed of concrete. A gravity dam depends on its own weight for stability and it usually 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

6、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 buttress dams is the slab type, which consists of sloping flat slabs supported at intervals by buttresses. Earth

7、 dams are embankments of rock or earth with provision for controlling seepage by means 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 containing spillway and sluice gates and earth

8、 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 freez

9、ing 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 types of dams depends mainly on the availability of construction materials near the site and the accessibility of transportation facilities. Dam

10、s are sometimes built in stages with the second or late stages constructed a decade or longer after the first 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 excavated foundation. However, figures quoted for hei

11、ghts of dams are often determined in other ways. Frequently the height is taken as the net height is taken as the net height above the old riverbed. 2.Forced on dams A dam must be relatively impervious to water and capable of resisting the forces acting on it. The most important of these forces a

12、re gravity (weight of dam) , hydrostatic pressure, uplift, ice pressure, and earthquake forces are transmitted to the foundation and abutments of the dam, which react against the dam with an equal and opposite force, the foundation reaction. The effect of hydrostatic forces caused by water flowing o

13、ver the dam may require consideration in special cases. The weight of a dam is the product of its volume and the specific weight of the material. The line of action of dynamic force passes through the center of mass of the cross section. Hydrostatic force may act on both the upstream and downstream

14、 faces of the dam. The horizontal component of the hydrostatic force is the force or unit width of dam it is Where r is the specific weight of water and h is the depth of water .The line of action of this force is h/3 above the base of the dam .The vertical component of the hydrostatic force is

15、 equal to the weigh of water vertically above the face of the dam and passes through the center of gravity of this volume of water. Water under pressure inevitably finds its way between the dam And its foundation and creates uplift pressures. The magnitude of the uplift force depends on th

16、e character of the foundation and the construction methods. It is often assumed that the uplift pressure varies linearly from full hydrostatic pressure at the upstream face (heel)to full tail-water pressure at the downstream face (toe).For this assumption the uplift force U is

17、U=r(h1+h2)t/2 Where t is the base thickness of the dam and h1and h2 are the water depths at the heel and toe of the dam,respectively. The uplift force will act through the center of area of the pressure trapezoid. Actual measurements on dams indicate that the uplift force is much less than t

18、hat given by Eq.(2)Various assumption have been made regarding the distribution of uplift pressures.The U.S.Bureau of Reclamation sometimes assumes that the uplift pressure on gravity dams varies linearly from two-thirds of full uplift at the heel to zero at the toe. Drains are usually provided near

19、 the heel of the dam to permit the escape of seepage water and relieve uplift. 譯文： 壩 據(jù)可靠記載，世界上第一座壩是公元前4000年以前在尼羅河上修建的。它使尼羅河改道，并為古老的孟菲斯提供城址。至今仍在使用的最古老的壩是16世紀(jì)修建的西班牙阿爾曼扎壩。隨著歲月的流逝， 各種建筑材料和施工方法得到了改善，修建努列克這樣的大壩才成為可能。該壩正在蘇聯(lián)境內(nèi)靠近阿富汗邊界的瓦赫什河上施工，是一座高達(dá)1017英尺（333米）的土

20、石壩。大壩失事可能造成生命財(cái)產(chǎn)的嚴(yán)重?fù)p失。因此壩的設(shè)計(jì)和維修通常是在政府監(jiān)督下進(jìn)行的。美國(guó)有3萬(wàn)多座壩由各州政府控制著。1972年（美國(guó)）聯(lián)邦大壩安全法（PL92-367）規(guī)定，必須由合格的專(zhuān)家對(duì)大壩進(jìn)行定期檢查。在1976年6月愛(ài)達(dá)荷州泰托大壩失事后，美國(guó)對(duì)大壩安全更為關(guān)注。 1. 壩的類(lèi)型 壩按其形式和建筑材料分為：重力壩﹑拱壩﹑支墩壩和土壩。前三種壩通常是用混凝土澆筑的。重力壩依靠自重維持穩(wěn)定，通常在平面上呈直線狀，不過(guò)有時(shí)略帶點(diǎn)弧型。拱壩通過(guò)拱的作用把水的水平推力中的大部分傳給拱座，因此，他的橫截面比重力壩單薄些。拱壩只用于崖壁能承受拱作用所產(chǎn)生的推力的峽谷中。各種支墩壩中最簡(jiǎn)單的

21、是平板壩，它是由許多支墩間隔地支撐著傾斜的面板。土壩是一種由土或石料填筑而成并借助于不透水的心墻或上游鋪蓋防滲透的土堤。在一座大壩的結(jié)構(gòu)中可包含不止一種壩型。弧形壩可以把重力作用和拱作用結(jié)合起來(lái)，以利壩的穩(wěn)定。長(zhǎng)壩常常有一個(gè)包括溢洪道﹑泄水閘在內(nèi)的混凝土壩段，其余壩段是用土或石料填筑的副壩。 對(duì)既定壩址選擇最佳壩型是一個(gè)關(guān)系到工程可行性及其造價(jià)問(wèn)題。工程可行性受地形﹑地質(zhì)及氣候條件所支配。例如：由于混凝土遭受凍融作用的交替影響而引起剝落，因此在低溫地區(qū)長(zhǎng)避免采用斷面單薄的混凝土拱壩或支墩壩。各類(lèi)壩的造價(jià)主要取決于能否在工地附近取得建筑材料和各種運(yùn)輸工具能否進(jìn)入。大壩有時(shí)分期建造，第二期或以后

22、幾期工程，往往在第一期以后需要十年或更長(zhǎng)的時(shí)間。 壩高定義為路面或溢洪道頂與基坑最低點(diǎn)之間的高程差。不過(guò)，引用的壩高值常常是用另外一些方法確定的，往往取原河床以上的凈高度作為壩高。 2．作用在壩上的力 壩必須是相對(duì)不透水的，并能經(jīng)受得住作用在它上面的各種力。這些作用力中最重要的是重力（壩的重量）、靜水壓力、揚(yáng)壓力，冰壓力及地震力。這些力傳給壩基和壩座，而壩基和壩座則對(duì)壩體產(chǎn)生一個(gè)大小相等方向相反的基礎(chǔ)反力。某些特殊情況下還要考慮水庫(kù)中泥沙沉積對(duì)靜水壓力的影響以及壩頂溢流所產(chǎn)生的動(dòng)力作用。 壩的自重是其體積和材料比重的乘積。該力的作用線通過(guò)橫剖面的形心。靜水壓力可同時(shí)作用在壩的

23、上游面和下游面。靜水壓力的水平分力Hh是作用在壩面垂直投影上的力，對(duì)于單位寬度壩體而言其值為： Hh=rh2/2 (1) 式中：r是水的比重，h是水深。該力的作用線在壩基以上h/3處。靜水壓力的垂直分力等于壩面正上方的水重，并通過(guò)該水位的重心。 處于壓力作用下的水必然要在壩和壩基之間流動(dòng)，因而產(chǎn)生了揚(yáng)壓力，揚(yáng)壓力的大小取決于基礎(chǔ)的特性和施工方法。經(jīng)常假定揚(yáng)壓力從上游面（壩踵）處的全部靜水壓力直線變化到下游面（壩址）處的全部尾水壓力。根據(jù)這一假設(shè)，揚(yáng)壓力U為： U=r(h1+h2)t/2 (2) 式中： t是壩基的寬度，h1和h2分別是壩踵和壩址處的水深。揚(yáng)壓力的作用線通過(guò)壓力梯形的形心。 一些壩的實(shí)測(cè)資料表明，揚(yáng)壓力比公式（2）所給出的值小的多。對(duì)揚(yáng)壓力的分布有各種不同的假設(shè)：美國(guó)懇務(wù)局認(rèn)為重力壩的揚(yáng)壓力呈直線變化，在壩踵處為全部揚(yáng)壓力的三分之二，到壩址處為零。壩踵附近通常設(shè)有排水裝置，以便排除滲流水量，以減小揚(yáng)壓力。