1、Dam A dam is a structure built across, river, or estuary to retain water. Its purposes are to meet demands for water for human consumption, irrigation, or industry； to reduce peak discharge of flood water； to increase available water stored for generating hydroelectric power； or to increase the dept
2、h of water in a river so as to improve navigation. An incidental purpose can be to propose can be to provide a lake for recreation. Auxiliary works at a dam may include spillways, gates, or valves to control the discharge of surplus water downstream form the dam； an intake structure conducting water
3、 to power station or to canals, tunnels, or pipelines for more distant use； provision for evacuating silt carried into the reservoir； and means for permitting ships or fish to pass the dam. A dam therefore is the central structure in a multipurpose scheme aiming at the conservation of water resource
4、s. The multipurpose dam holds special importance in the underdeveloped counties, where a small nation may reap enormous benefits in agriculture and industry from a single dam. Dams fall into several distinct classes, by profile and by building material. The decision as to which type of dam be build
5、depends largely on the foundation conditions in the valley and the construction materials available. Basically, the choice of materials now lies between concrete, soils, and rockfill. Though a number of dams were built in the past of jointed masonry, this practice is now largely obsolete, The monoli
6、thic form of concrete dams permits greater variations in profile, according to the extent water pressure is resisted by the deadweight of the structure, is transferred laterally to buttresses, or is carried by horizontal arching across the valley to abutments formed by the sides of the valley. Basic
7、 Problems in Dam Design Most modern dams continue to be of two basic types: masonry (concrete) and embankment (earthfill). Masonry dams are typically used to block streams running through narrow gorges, as in mountainous terrain； though such dams may be very high, the total amount of material requir
8、ed is limited. Embankment dams sre preferred to control broad streams, where only a 1 very large barrier, requiring a great volume of material, will suffice. The choice of masonry or embankment and the precise design depend on the geology and configuration of the site, the functions of the dam, and
9、cost factors. Site investigation and testing. Investigation of a site for a dam includes sinking trial borings to determine the strata. The borings are supplemented by shafts and tunnels which, because of their cost, must be used as sparingly as possible. In the shafts and tunnels, tests can be made
10、 to measure strength, elasticity, permeability, and prevailing stresses in strata, with particular attention given to the properties of thin partings, or walls, between the more massive beds, The presence in groundwater of chemical solutions harmful to the materials to be used in the construction of
11、 the dam must be assessed. Sources of construction materials need exploration. As dams continue to increase in height, the study of foundation conditions becomes increasingly critical. Models are particularly useful in analysis of arch dams and in verifying analytical stress calculations. Various ma
12、terials have been used for model tests； on some early tests for Hoover Dam, rubber was employed. The need for accurate reproduction of stress patterns in complex models is met by using material of low elasticity. In a sense, dams themselves are models for future design. The instruments built into th
13、em to record movements under load, strains within materials after construction, temperature and pressure changes, and other factors are installed primarily to study the performance of the structure and to warn of possible emergencies, but their value in confirming design assumptions is important. Th
14、e digital computer has permitted considerable advance in analytical methods of design. Its ability to handle a great volume of data and to solve large sets of simultaneous equations containing many variables ha made practicable the method of Finite Element Analysis. In this method, a complicated str
15、ucture is divided into a number of separate equilibrium conditions, and strains are rendered compatible, thus leading to a complete analysis of stress and strain distribution throughout the structure. Problems of materials. Each of the two basic dam materials, concrete and earth or rock fill, has a
16、weakness that must be overcome by the proper design of the dam. Weaknesses of concrete. Concrete is weak in tensile strength； that is, it can be pulled apart 2 easily. Concrete dams must therefore be designed to place minimum tensile strain on the dam and to make use of concretes great compressive s
17、trength, or ability to support vertical loads. The chief constituent of concrete, cement, shrinks as it sets and hardens, due to water absorption in the crystalline structure, to evaporation of water to the atmosphere, and to cooling form the higher temperatures reached when the chemical reactions i
18、n the cement are in progress during hydration. Because of the large volume of concrete in a dam, shrinkage presents a serious cracking hazard. Various expedients are used to overcome the problem. Concrete is usually cast in separate blocks of limited height. Gaps may be left to permit heat losses an
19、d filled in later. Low-heat cements may be used； these are specially blended so that rates of heat evolution are retarded. Cement content can be safely reduced in the interior concrete in the dam, in which strength and resistance to climatic and chemical deterioration are less important. The cement
20、content, and therefore the heat caused by hydrating, can also be reduced by using aggregate (the other major constituent of concrete) of larger stones. Another expedient is to use other fine-grained materials, such as fly ash (pulverized fuel), as filler, reducing the total cement volume in the conc
21、rete. Another is to use certain additives, surface-active agents, and air-entraining agents that permit using a lower water-to-cement ratio in mix by ice, circulating water through pipes laid in the concrete, and extracting excess water from surfaces by vacuum. Weaknesses of earth and rock fill. Soi
22、ls and rock fragments lack the strength of concrete, are much more permeable, and possess less resistance to deterioration and disturbance by flowing water. These disadvantages are compensated for by a much lower cost and by the ability of earth fill to adapt to deformation caused by movements in th
23、e dam foundation. This assumes, of course, sufficient usable soil available close to the dam site. In bare mountain country it may be necessary to quarry rock and construct a rockfill rather than an rarthfill dam. Earth fill is of course more economical, and often a suitable borrow area can be found
24、 close to the site. Soil consists of solid particles with water and air in between. When the soil is compressed by loading, as occurs in dam construction, some drainage of air and water takes place, causing an increase in pressures between the solid particles. When there is a high rate of seepage, the soil tends to develop differential pressures and reach a condition called quick, in which it behaves
