from curbs, 3 feet from fire hydrants, 12 feet from thenearest track of a railroad track, and 7 feet fromrailway sidings.When you are staking pole locations, the center ofeach pole is marked with a hub on the line; the hubmay be offset. On the guard stake, you put the polenumber, the line elevation, and the distance from thetop of the hub to the top of the pole obtained from theprofile.TOWER LINE SURVEYS.— High-voltagelines are often supported by broad-based steel towers.For a tower line, construction economy requires thatchanges in direction be kept at a minimum. That isbecause a tower located where a line changes directionmust withstand a higher stress than one located in astraight direction part of the line. In general, towerconstruction is cheaper in level country than in brokencountry; however, the line may be run over brokencountry to minimize changes in direction, to make thedistance shorter, or to follow a line where the cost ofobtaining right-of-way is inexpensive. Lines shouldbe located adjacent to existing roads, wheneverpractical, to provide easier access for construction andfuture maintenance. When a change in direction in atower line is unavoidable, it should be made graduallyin as small-angular increments as possible. Suppose,for example, a change in direction of 90° is required.Instead of an abrupt change in direction of 90°, towersshould be set so as to cause the line to follow a gradualcurve in a succession of chords around an arc of 90°.Route Surveys for DrainageWhen man-made structures are erected in a certainarea, it is necessary to plan, design, and construct anadequate drainage system. Generally, an undergrounddrainage system is the most desirable way to removesurface water effectively from operating areas. Anopen drainage system, like a ditch, is economical;however, when not properly maintained, it is unsightlyand unsafe. Sometimes, an open drainage system alsocauses erosion, thus resulting in failures to nearbystructures. Flooding caused by an inadequate drainagesystem is the most prevalent cause leading to the rapiddeterioration of roads and airfields. The constructionand installation of drainage structures will bediscussed later in this chapter. At this point we aremainly interested in drainage systems and types ofdrainage.DRAINAGE SYSTEM.—Sanitary sewers carrywaste from buildings to points of disposal; stormsewers carry surface runoff water to natural watercourses or basins. In either case the utility line musthave a gradient; that is, a downward slope toward thedisposal point, just steep enough to ensure a gravityflow of waste and water through the pipes. Thisgradient is supplied by the designing engineer.Natural Drainage.—To understand the con-trolling considerations affecting the location and otherdesign features of a storm sewer, you must knowsomething about the mechanics of water drainagefrom the earth’s surface.When rainwater falls on the earth’s surface, someof the water is absorbed into the ground. The amountabsorbed will vary, of course, according to thephysical characteristics of the surface. In sandy soil,for instance, a large amount will be absorbed; on aconcrete surface, absorption will be negligible.Of the water not absorbed into the ground, someevaporates, and some, absorbed through the roots andexuded onto the leaves of plants, dissipates through aprocess called transpiration.The water that remains after absorption, evapora-tion, and transpiration is technically known as runoff.This term relates to the fact that this water, under theinfluence of gravity, makes its way (that is, runs off)through natural channels to the lowest point it canattain. To put this in terms of a general scientificprinciple, water, whenever it can, seeks its own level.The general, final level that unimpeded water on theearth’s surface seeks is sea level; and the rivers of theearth, most of which empty into the sea, are the earth’sprincipal drainage channels. However, not all of theearth’s runoff reaches the great oceans; some of it iscaught in landlocked lakes, ponds, and other non-flowing inland bodies of water.Let’s consider, now, a point high in the mountainssomewhere. As rain falls in the area around this point,the runoff runs down the slopes of a small gully andforms a small stream, which finds a channel down-ward through the ravine between two ridges. As thestream proceeds on its course, it picks up more andmore water draining in similar fashion from highpoints in the area through which the stream is passing.As a result of this continuing accumulation of runoff,the stream becomes larger until eventually it eitherbecomes or joins a large river making its way to thesea—or it may finally empty into a lake or some otherinland body of water.In normal weather conditions, the naturalchannels through which this runoff passes can10-3
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