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1、DamThe 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. With
2、 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. The fa
3、ilure 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 insp
4、ections 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 are usually con
5、structed 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 comparable gr
6、avity 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 dams are emb
7、ankments 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 or rock-fill
8、 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
9、, 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. Dams are sometimes
10、 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 heights of dams are
11、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 are gravity (weight of
12、 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 over the dam may requi
13、re 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 faces of the dam. The
14、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 equal to the weigh of water
15、 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 the character of the foundation and the
16、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 U=r(h1+h2)t/2 Where t is the base thickness of the dam and h
17、1and 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 that given by Eq.(2)Various assumption have been made regarding the d
18、istribution 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 the heel of the dam to permit the escape of seepage water and relie
19、ve uplift. 译文： 坝 据可靠记载，世界上第一座坝是公元前4000年以前在尼罗河上修建的。它使尼罗河改道，并为古老的孟菲斯提供城址。至今仍在使用的最古老的坝是16世纪修建的西班牙阿尔曼扎坝。随着岁月的流逝， 各种建筑材料和施工方法得到了改善，修建努列克这样的大坝才成为可能。该坝正在苏联境内靠近阿富汗边界的瓦赫什河上施工，是一座高达1017英尺（333米）的土石坝。大坝失事可能造成生命财产的严重损失。因此坝的设计和维修通常是在政府监督下进行的。美国有3万多座坝由各州政府控制着。1972年（美国）联邦大坝安全法（PL92-367）规定，必须由合格的专家对大坝进行定期检查。在1976年6月
20、爱达荷州泰托大坝失事后，美国对大坝安全更为关注。1. 坝的类型坝按其形式和建筑材料分为：重力坝拱坝支墩坝和土坝。前三种坝通常是用混凝土浇筑的。重力坝依靠自重维持稳定，通常在平面上呈直线状，不过有时略带点弧型。拱坝通过拱的作用把水的水平推力中的大部分传给拱座，因此，他的横截面比重力坝单薄些。拱坝只用于崖壁能承受拱作用所产生的推力的峡谷中。各种支墩坝中最简单的是平板坝，它是由许多支墩间隔地支撑着倾斜的面板。土坝是一种由土或石料填筑而成并借助于不透水的心墙或上游铺盖防渗透的土堤。在一座大坝的结构中可包含不止一种坝型。弧形坝可以把重力作用和拱作用结合起来，以利坝的稳定。长坝常常有一个包括溢洪道泄水闸在
21、内的混凝土坝段，其余坝段是用土或石料填筑的副坝。对既定坝址选择最佳坝型是一个关系到工程可行性及其造价问题。工程可行性受地形地质及气候条件所支配。例如：由于混凝土遭受冻融作用的交替影响而引起剥落，因此在低温地区长避免采用断面单薄的混凝土拱坝或支墩坝。各类坝的造价主要取决于能否在工地附近取得建筑材料和各种运输工具能否进入。大坝有时分期建造，第二期或以后几期工程，往往在第一期以后需要十年或更长的时间。坝高定义为路面或溢洪道顶与基坑最低点之间的高程差。不过，引用的坝高值常常是用另外一些方法确定的，往往取原河床以上的净高度作为坝高。 2作用在坝上的力 坝必须是相对不透水的，并能经受得住作用在它上面的各种
22、力。这些作用力中最重要的是重力（坝的重量）、静水压力、扬压力，冰压力及地震力。这些力传给坝基和坝座，而坝基和坝座则对坝体产生一个大小相等方向相反的基础反力。某些特殊情况下还要考虑水库中泥沙沉积对静水压力的影响以及坝顶溢流所产生的动力作用。坝的自重是其体积和材料比重的乘积。该力的作用线通过横剖面的形心。静水压力可同时作用在坝的上游面和下游面。静水压力的水平分力Hh是作用在坝面垂直投影上的力，对于单位宽度坝体而言其值为：Hh=rh2/2 (1)式中：r是水的比重，h是水深。该力的作用线在坝基以上h/3处。静水压力的垂直分力等于坝面正上方的水重，并通过该水位的重心。处于压力作用下的水必然要在坝和坝基之间流动，因而产生了扬压力，扬压力的大小取决于基础的特性和施工方法。经常假定扬压力从上游面（坝踵）处的全部静水压力直线变化到下游面（坝址）处的全部尾水压力。根据这一假设，扬压力U为： U=r(h1+h2)t/2 (2)式中： t是坝基的宽度，h1和h2分别是坝踵和坝址处的水深。扬压力的作用线通过压力梯形的形心。一些坝的实测资料表明，扬压力比公式（2）所给出的值小的多。对扬压力的分布有各种不同的假设：美国恳务局认为重力坝的扬压力呈直线变化，在坝踵处为全部扬压力的三分之二，到坝址处为零。坝踵附近通常设有排水装置，以便排除渗流水量，以减小扬压力。
