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Lession 11 danms坝
The first dam for which there are reliable records was built on the Nile River sometime before 4000B.C. it is used to divert the Nile and provide a site for the ancient city of Memphis. 据可靠记载, 世界上第一座坝是公元前40 以前在尼罗河上修建的。她使尼罗河改道, 并为古老的孟菲斯城提供城址。
The oldest dam still in use is the Almanza Dam in Spain, which was constructed in the sixteenth century. 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 Former Soviet Union on the Vaksh River near the border of Afghanistan. This dam will be 1017 ft (333m)high,of earth and rock fill. 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进, 修建列努克这样的大坝才成为可能。该坝正在前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。
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 30,000 dams are under the control of state authorities. The 1972 Federal Dam Safety Act (PL 92-267)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. 大坝失事可能造成生命财产的严重损失。因此, 坝的设计和维修一般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月爱达荷州提堂大坝失事后, 美国对大坝安全更为关切。
1 types of dams 坝的类型
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 is 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 section 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 the buttress dams is the slab type, which consists of sloping flat slabs supported at intervals by buttress. 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 containing spillway and sluice gates and earth or rock-fill wing dams for the remainder of their length. 坝按其形式和建筑材料分为: 重力坝、 拱坝、 支墩坝和土坝。前三种坝一般是用混凝土浇筑的。重力坝依靠自重维持稳定, 一般在平面上呈直线状, 不过有时也略带弧形。 拱坝经过拱的作用把水的水平推力中的大部分传给地基, 因此它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。弧形坝能够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、 泄水闸在内的混凝土坝段, 其余坝段是用土或石填筑的副坝。
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 sections 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 built in stages with the second or later stages constructed a decade or longer after the first stage. 对于既定的坝址选择最佳坝型是一个关系到工程可行性及其造价的问题。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, 因此在低温地区常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, 第二期或以后各期工程, 往往在第一期以后需要 或更长的时间。
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 often determined in other ways. Frequently the height is taken as the net height above the old river bed.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的一些方法确定的, 往往取原河床以上的净高度作为坝高。
2 forces on dams 作用在坝上的力
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 dam), hydrostatic pressure, uplift, ice pressure, and earthquake forces. These forces are transmitted to the foundation and abutments of the dam, which react against the dam with an equal and opposite forces, the foundation reaction. The effect of hydrostatic pressure caused by sediment deposits in the reservoir and of dynamic forces caused by water flowing over the dam may require consideration in special cases. 坝必须是相对不透水的, 并能经受得住作用在这上面的各种力。这些作用力中最重要的是重力( 坝体重量) 、 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沉积泥沙引起的静水压力的影响以及坝顶溢流所产生的动力作用。
The weight of a dam is the product of its volume and the specific weight of the material. The line of action of this force passes through the center of mass of the cross section. Hydrostatic forces may act on both the upstream and downstream faces of the dam. The horizontal component of the hydrostatic force is the force on a vertical projection of the face of the dam, and for unit width of dam it is Where: is the specific weight of water; is the depth of water. The line of action of this force is above the base of the dam. The vertical component of the hydrostatic force is equal to the weight of water vertically above the face of the dam and passes through the center of gravity of this volume of water.坝的自重是其体积和材料比重的乘积。该力的作用线经过横剖面的形心。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 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;

式中: 为水的比重; 是水深; 该力的作用线在坝基以上 处。静水压力的竖直分力等于坝面正上方的水重, 并经过该水体的重心。
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 construction methods. It is often assumed that the uplift pressure varies linearly from full hydrostatic pressure at the upstream face (heal ) to full tail-water pressure at the downstream face (toe). For this assumption the uplift force is

Where; is the base thickness of the dam; and 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 (fig.11.1)
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( 坝趾) 处的全部尾水压力。根据这一假设, 扬压力 为

式中: 是坝基的宽度; 和 分别是坝踵和坝趾处的水深。扬压力的作用线经过压力梯形的形心( 图11.1)
Figure 11.1 free-body diagram of the cross section of a gravity dam
Actual measurements on dams (figure11.2) indicate that the uplift force is much less than that given by eq.11.2. Various assumptions 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 the heel of the dam to permit the escape of seepage water and relieve uplift.一些坝的实测资料表明( 图11.2) ; 扬压力比公式11.2所给出的值小得多。对扬压力的分布有各种不同的假设, 美国垦务局认为重力坝的扬压力成直线变化, 在坝踵处为全部扬压力的2/3, 到坝趾处为零。坝踵附近一般设有排水装置, 以便排除渗流水量, 减小扬压力。
Figure.11.2 maximum observed uplift pressure under some existing gravity dams (Data from the ASCE Committee on Uplift in Masonry dams- ASCEabbr.American Society of Civil Engineers 美国土木工程师协会)

Lesson 12 embankment dams 土石坝
1 selection of embankment type 土石坝坝型的选择
In general, there are two types of embankment dams: earth ( earthfill dam) and rockfill( rock fill dam) . The selection is dependent upon the usable materials from the required excavation and available borrow. It should be noted that rockfills can shade into soil fills depending upon the physical character of the rock and that no hard and fast system of classification can be made. Rocks which are soft and will easily break down under the action of excavation and placement can be classified with earthfills. Rocks which are hard and will not break down significantly are treated as rockfills. 一般来说, 土石坝有两种类型: 土坝和堆石坝。坝型的选择取决于能从需要开挖的地点和可用的料场( 料场 stock ground) ( borrow area 取土面积; 采料场) 处取得合用材料的情况。应当指出的是( 注意) , 根据岩石的物理特性, 堆石能够逐渐变化为填土, 因而不能对土石料作出严格而固定的分类。那些软弱的和在开挖填筑时容易破碎的岩石可被归入填土类。而坚硬和不会大量破碎的的岩石, 则列为堆石类。
The selection and the design of an earth embankment are based upon the judgement and experience of the designer and is to a large extent of an empirical nature. The various methods of stability and seepage analysis are used mainly to confirm the engineer’s judgement.一座土坝的选定和设计都有赖于设计人员的判断和经验, 而且在很大程度上是属于经验性的。各种稳定和渗透分析方法, 主要是作为证实工程师的判断而使用的。
2 freeboard 超高
All earth dams must have sufficient extra height known as freeboard to prevent overtopping by the pool. The freeboard must be of such height that wave action, wind setup, and earthquake effects will not result in overtopping of the dam. In addition to freeboard, an allowance must be made for settlement of the embankment and the foundation which will occur upon completion of the embankment.所有的土坝都必须有一个足够的额外高度, 称为超高, 以防止水库漫顶。超高的高度必须足以在波浪作用、 风浪壅高和地震影响下, 不会导致坝的漫顶。除了超高外, 对于坝建成时发生的坝体和地基沉陷, 还必须在高度上留有余地。
3 top width 坝顶宽度
The width of the earth dam top is generally controlled by the required width of fill for ease of construction using conventional equipment. In general, the top width should not be less than 30 ft. if a danger exists of an overtopping wave caused either by massive landslides in the pool or by seismic block tipping, then extra top width of erosion resistive fill will be required.土坝的坝顶宽度一般用常规设备便于施工的填筑宽度来控制。一般, 坝顶宽度应不小于30英尺。如果存在着大规模塌方进入水库, 或者有因地震使岩块倒落而引起波浪漫顶的危险, 则需要采用抗冲刷的材料填筑更宽的坝顶高度。
4 alignment 定线
The alignment of an earthfill dam should be such as to minimize construction costs but such alignment should not be such as to encourage sliding or cracking of the embankment. Normally the shortest straight line across the valley will be satisfactory, but local topographic and foundation conditions may dictate otherwise. Dams located in narrow valleys often are given an alignment which is arched upstream so that deflections of the embankment under pool load will put the embankment in compression thus minimizing transverse cracking.土坝的坝轴线选定应尽量使建设费用降到最少, 可是也不能因此引起坝体发生滑动或开裂。一般来说, 一条横跨河谷的最短的直线, 可能满足要求。可是, 当地的地形和地基条件可能要求采用另外的方案。对于峡谷的坝, 常采用向上游拱出的坝轴线, 以便在坝体受库水压力作用而发生变形时, 能使坝体压紧, 从而尽量减少其横向开裂。
5 Abutments 两岸坝座
Three problems are generally associated with the abutments of earth dams: ① seepage, ②instability, and ③transverse cracking of the embankment. If the abutment consists of deposits of pervious soils it may be necessary to construct an upstream impervious blanket and downstream drainage measures to minimize and control abutment seepage.一般有三个问题与土坝坝座有关: ①渗透; ②不稳定; ③坝体的横向开裂。如果坝座是由透水的沉积土构成, 就可能需要建造一道上游不透水的铺盖和下游排水设施, 可尽量减少和控制坝座内的渗透。
Where steep abutments exist, especially with sudden changes of slopes or with steep bluff, there exists a danger of transverse cracking of the embankment fills. This can be treated by excavation of the abutment to reduce the slope, especially in the impervious and transition zones. The transition zones, especially the upstream, should be constructed of fills which have little or no cohesion and a well-distributed gradation of soils which will promote self-healing should transverse cracking occur.在坝座岸坡很陡的地方, 特别在边坡突变或有陡壁处, 那里的坝体填土会产生横向裂缝的危险。这个问题能够用开挖坝座放缓边坡来处理, 这样的处理在不透水区和过渡区特别需要。过渡区, 特别是在上游侧的过渡区, 必须用粘着力很小或无粘着力且颗粒级配良好的土料来填筑, 这种土料如发生横向裂缝时能自行愈合。
6 stage construction 分期施工
It is often possible, and in some cases necessary, to construct the dam embankment in stages. Factors dictating such a procedure are: ①a wide valley permitting the construction of the diversion or outlet works and part of the embankment at the same time; ②a weak foundation requiring that the embankment not be built too rapidly to prevent overstressing the foundation soils ; ③a wet borrow area which requires a slow construction to permit an increase in shear strength through consolidation of the fill. In some cases it may be necessary to provide additional drainage of the foundation or fill by means of sand drain wells or by means of horizontal pervious drainage blanket.土坝的分期施工往往是可能的, 而且在一些情况下是必须的。要求这样施工程序的因素是: ①河谷宽阔, 能够允许导流或泄水工程与一部分坝体同时施工; ②地基软弱, 要求坝体不要过快填筑, 以防止地基中产生过大的应力; ③料场潮湿, 要求放慢施工, 以使土料能经过固结作用来增加抗剪强度。在某些情况下, 能够需要增设基础排水设施或填筑排水沙井, 或采用水平透水的排水铺盖。
7 Embankment soils 坝体的土料
Most soils are suitable for use for embankment construction, however, there are physical and chemical limitations, soils which contain excessive salts or other soluble materials should not be used. Substantial organic content should not exist in the soils. Lignite sufficiently scattered through the fill to prevent the danger of spontaneous combustion, is not objectionable. Fat clays with high liquid limits may prove difficult to work and should be avoided.大多数土料适合于坝体填筑, 然而, 在物理和化学性质上也有一定的局限, 含有过多盐和可溶性物质的土料, 不能够使用。在土壤里不应存在大量的有机质成分。褐煤若能经过填筑而充分分散, 无自然之虞, 就不妨碍使用。而具有高度流限的肥粘土, 多半难以施工, 必须避免使用。
8 Compaction requirements压实的要求
The strength of the impervious and semi-impervious soils depends upon the compacted densities. These depend in turn upon the water content and weight of the compacting equipment. The design of the embankment is thus influenced by the water content of the borrow soils and by the practicable alternations to the water content either prior to placement of the fill or after placement but prior to rolling. If the natural water content is too high, then it may be reduced in borrow area by drainage, or by harrowing. If the soil is too dry it should be moistened in the borrow area either by sprinkling or by ponding and then permitted to stabilize the moisture content before use. The range of placement water content is generally between 2 percent dry to 2 or 3 percent wet of the standard Proter optimum water content. Pervious soils should be compacted to at least 80 percent of relative density.不透水和半透水的土料的强度取决于压实的密度。压实密度又取决于土料的含水量和压实设备的重量。因此, 料场土料的含水量和在填筑前或堆筑后而未碾压前的填土实际含水量的变化, 都会影响坝体的填筑。如果天然含水量太高, 能够在料场用排水或将土料耙松的办法来减低。如果土料太干燥, 则需在料场用洒水和泡水的办法把土料润湿, 然后再让土料在使用以前保持稳定的含水量。填筑时的含水量范围一般介于比标准普氏最优含水量低2%到高2%～3%之间。透水性土料至少压实到相对密度的80%。
If necessary, test fills should be constructed with variations in placement water content, lift thickness, number of roller passes and type of rollers. For cases of steep abutment, the fill must be placed in thin lifts and compacted by mechanical hand tampers. All overhangs should either be removed or filled with lean concrete prior to fill placement.如果需要, 应该变换填筑层的含水量、 铺层厚度、 碾压遍数和碾压机的型式等, 进行填筑试验。对于坡度很陡的坝座部位, 必须用薄层填筑, 并用手扶打夯机夯实。所有外悬突出部位, 均应在填土堆筑以前挖除或用贫混凝土填平。
9 types of instruments观测仪器的类型
The type of instrumentation depends upon the size and complexity of the project. The devices in common use are: ①piezometers ; ②surface movement monuments ; ③settlement gages ; ④inclinometers ; ⑤internal movement and strain indicators ; ⑥pressure cells ; ⑦seismic acceleration meter ; ⑧movement indicators at conduit joint