Page 217 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal
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JWCL344_ch05_154-193.qxd  8/2/10  9:44 PM  Page 179







                                                                                                   5.7 Location  179


                                         Solution 2 (SI System):
                                                               p = [670,000>(d + 152)] - 278
                                                                = [670,000>(609.6 + 152)] - 278
                                                                = 601.73 kPa gauge
                                                              P = [525>(d + 152) - 0.2] d 2
                                                             f
                                                                = [525>(609.6 + 152) - 0.2](609.6) 2
                                                                = 181,844 N



                    5.7  LOCATION
                                         Supply conduits are located in much the same way as railroads and highways.


                    5.7.1  Line and Grade
                                         The invert of a grade aqueduct or grade tunnel is placed on the same slope as the hydraulic
                                         grade line. Cut and fill, as well as cut and cover, are balanced to maintain a uniform gradient
                                         and reduce haul. Valleys and rivers that would be bridged by railroads and highways may
                                         be bridged also by aqueducts. Such indeed was the practice of ancient Rome, but modern
                                         aqueducts no longer rise above valley, stream, and hamlet except where a bridge is needed
                                         primarily to carry road or railway traffic. Pressure conduits have taken their place.
                                         Sometimes they are laid in trenches as sag pipes to traverse valleys and pass beneath
                                         streams; sometimes they strike deep below Earth’s surface in pressure tunnels for which
                                         geologic exploration fixes both line and grade.
                                             Pressure aqueducts and pipelines move freely up and down slopes. For economy they
                                         should hug the hydraulic grade line in profile and a straight line in plan (Fig. 5.10). The
                                         size and thickness of conduit and difficulty of construction must be kept in balance with
                                         length. The shortest route is not necessarily the cheapest.
                                             Air released from the water and trapped at high points reduces the waterway, increases
                                         friction, and may interrupt flow unless an air relief valve or vacuum pump is installed. True
                                         siphons should be avoided if possible. However, if the height of rise above the hydraulic grade
                                         is confined to less than 20 ft (6 m) and the velocity of flow is kept above 2 ft/s (0.60 m/s), op-
                                         erating troubles will be few. For best results, the line should leave the summit at a slope less
                                         than that of the hydraulic gradient.
                                             In practice, possible locations of supply conduits are examined on available maps of the
                                         region; the topographic and geologic sheets of the U.S. Geological Survey are useful exam-
                                         ples. Route surveys are then carried into the field. Topography and geology are confirmed
                                         and developed in needed detail, possibly by aerial surveys, borings, and seismic exploration.
                                         Rights of way, accessibility of proposed routes, and the nature of obstructions are also iden-
                                         tified. The use of joint rights-of-way with other utilities may generate economies.


                    5.7.2  Vertical and Horizontal Curves
                                         In long supply lines, changes in direction and grade are effected gradually in order to con-
                                         serve head and avoid unbalanced pressures. Masonry conduits built in place can be brought
                                         to any desired degree of curvature by proper form work. Cast-iron and other sectional
                                         pipelines are limited in curvature by the maximum angular deflection of standard lengths
                                         of pipe at which joints will remain tight. The desired curve is built up by the necessary
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