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<br />14 <br /> <br />3.2 FUNCTIONS OF A DAM <br />The primary requirements of a dam in order to hold <br />back water safely are: <br /> <br />1. Contain the water and resist leakage. <br /> <br />2. Maintain shape. <br /> <br />3. Resist movement in a downstream direction. <br /> <br />The greater the depth of stored water behind the dam, <br />the greater the pressures are and the greater the <br />resistance must be to leakage and movement. Resist- <br />ance to leakage is important since the purpose of the <br />dam is to store water. Resistance to movement is <br />important too, because the pressure ofthe stored water <br />tends to push the dam downstream. For example, the <br />sketch below shows a concrete dam that would move <br />downstream if it were not designed to resist the pres- <br />sure of the water. <br /> <br />Indicates Water Surface <br /> <br /> <br />\ <br />\ <br />\ <br />\ <br />\ <br />\ <br />\ <br />\ <br /> <br />FIGURE 3.2.1 <br />CONCRETE DAM WITH LOAD FROM RESERVOIR <br /> <br />The function of maintaining shape is more related to <br />dams constructed of earth material or rock. The shape <br />refers to the outline ofthe dam or the profile. The final <br />shape of the dam is a result of design which determines <br />the amount of material necessary to resist leakage and <br />movement. If the shape of the dam changes, it may no <br />longer be able to perform its required functions. The <br />sketches below show that a slide has occurred and the <br />dam can no longer hold back the water. <br /> <br /> <br />I' <br /> <br />FIGURE 3.2-2 <br />EARTH DAM - FAILS TO HOLD BACK WATER <br /> <br />The dam surroundings must also function as a unit. <br />The site on which the dam is constructed becomes a <br />part of the dam. The dam site has three important <br />parts: <br /> <br />1. The foundation. <br /> <br />2. The abutments. <br /> <br />3. The reservoir basin. <br /> <br />The site is an integral part ofthe dam and must hold up <br />the structure as well as help bold back tbe water. <br /> <br />3.2-1 THE FOUNDATION <br />The part of the dam site which must support the. <br />dam is the foundation. Although other factors <br />are involved, the first task of the foundation is <br />to provide firm support for the entire dam. A <br />soft foundation, for example, would not sup- ' <br />port the weight of the dam. <br /> <br />7TI\/ // /7 // J\~ <br /> <br /> <br />.. SOLID FOUNDATION b. SOFT FOUNDATION <br /> <br />FIGURE 3.2-3 <br />FOUNDATION CONDITIONS <br /> <br />Because the purpose of the dam is storage of <br />water, the foundation must also resist the flow <br />of water under the structure. A clay material or <br />unfractured hard rock, for example, would re- <br />sist the flow of water under the structure much <br />more effectively than sand or gravel. <br /> <br />3.2-2 THE ABUTMENTS AND RESERVOIR <br />BASIN <br />The other parts of the site which must be as <br />strong as the foundation and resist the flow of <br />water are the abutments. The abutments are the <br />areas where the ends of the dam meet with the <br />surrounding terrain. <br /> <br /> <br />CREST OF DAM <br /> <br />l <br /> <br /> <br />" <br />, <br />. <br /> <br />y <br /> <br />ABUTMENT <br /> <br />FOUNDATION <br /> <br />FIGURE 3.2-4 <br />THE DAM SITE <br /> <br />The abutments must offer support to the struc- <br />ture in the length-wise, upstream-downstream, <br />and vertical directions. <br /> <br />The basin behind the dam or the area covered <br />by the reservoir is just as important as the dam <br />itself. Its size and shape determine the volume <br />of the reservoir. Like the dam, the foundation, <br />and the abutments, the basin must contain the <br />water. <br /> <br />3.2-3 THE DAM SITE <br />Unfortunately, nature does not always provide <br />the most ideal site for a dam. Therefore, special <br />designs and treatments ofthe site are necessary <br />