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1、 OVERFLOW SPILLWAYAn overflow spillway is a section of dam designed to permit water to pass over its crest. Overflow spillways are widely used on gravity, arch, and buttress dams. Some earth dams have a concrete gravity section designed to serve as a spillway. The design of the spillway for tow dams
2、 is not usually critical, and a variety of simple crest patterns are used. In the case of large dams it is important that the overflowing water be guided smoothly over the crest with a minimum of turbulence. If the overflowing water breaks contact with the spillway surface, a vacuum will form at the
3、 point of separation and cavitations may occur. Cavitations plus the vibration from the alternates making and breaking of contact between the water and the face of the dam may result in serious structural damage.Cavities filled with vapor, air, and other gases will form in a liquid whenever the abso
4、lute pressure of the liquid is close to the vapor pressure. This phenomenon, cavitations, is likely to occur where high velocities cause reduced pressure. Such conditions may arise if the walls of a passage are so sharply curved as to cause separation of flow from the boundary. The cavity, on moving
5、 downstream, may enter a region where the absolute is much higher. This causes the vapor in the cavity to condense and return to liquid with a resulting implosion, or collapse, extremely high pressure result. Some of the implosive activity will occur at the surfaces of the passage and in the crevice
6、s and pores of the boundary material. Under a continual bombardment of these implosions, the surface undergoes fatigue failure and small particles are broken away, giving the surface a spongy appearance. This damaging action of cavitations is called pitting.The ideal spillway would take the form of
7、the underside of the napped of a sharp-crested weir when the flow rate corresponds to the maximum design capacity of the spillway. More exact profiles may be found in more extensive treatments of the subject. The reverse curve on the downstream face of the spillway should be smooth and gradual; A ra
8、dius of about one-fourth of the spillway height has proved satisfactory. Structural design of an ogee spillway is essentially the same as the design of a concrete gravity section. The pressure exerted on the crest of the spillway by the flowing water and the drag forces caused by fluid friction are
9、usually small in comparison with the other forces acting on the section. The change in momentum of the flow in the vicinity of the reverse curve may, however, create a force which must be considered. The requirements of the ogee shape usually necessitate a thicker section than the adjacent no overfl
10、ow sections.A saving of concrete can be effected by providing a projecting corbel on the upstream face to control the flow in outlet conduits through the dam, a corbel will interfere with gate operation.The discharge of an overflow spillway is given by the weir equation Where Q=discharge, or L=coeff
11、icienth=head on the spillway (vertical distance from the crest of the spillway to the reservoir level), mThe coefficient varies with the design and head. Experimental models are often used to determine spillway coefficient. End contractions on a spillway reduce the effective length below the actual
12、length L. Square-cornered piers disturb the flow considerably and reduce the effective length by the width of the piers plus about 0.2h for each pier.Streamlining the piers or flaring the spillway entrance minimizes the flow disturbance. If the cross-sectional area of the reservoir just upstream fro
13、m the spillway is less than five times the area of flow over the spillway, the approach velocity with increase the discharge a noticeable amount. The effect of approach velocity can be accounted for by the equation where is the approach velocity. PROPERTIES OF CONCRETE The characteristics of concret
14、e should be considered in relation to the quality for any given construction purpose. The closest practicable approach to perfection in every property of the concrete would result in poor economy under many conditions, and the most desirable structure is that in which the concrete has been designed
15、with the correct emphasis on each of the various properties of the concrete, and not solely with a view to obtaining, say, the maximum possible strength. Although the attainment of the maximum strength should not be the sole criterion in design, the measurement of the crushing strength of concrete c
16、ubes or cylinders provides a means of maintaining a uniform standard of quality, and, in fact, is the usual way of doing so. Since the other properties of any particular mix of concrete are related to the crushing strength in some manner, it is possible that as a single control test it is still the most convenient and informative.T
