Figure 3-9.-Subsurface drainage.Final Cross SectionWhen the final vertical and horizontal alignmentsof the road are fairly well fixed, final design iscommenced. During this phase, final constructiondrawings are prepared and construction may begin.Before actual construction starts, final cross sectionsare prepared. From these final cross sections, slopestakes are set as described in the EA3 TRAMAN. Finalcross sections are taken at each station along the centerline of the road. They show the actual shape of the road,the horizontal width of components and their distancesfrom the center line, the finish elevations, and theextremities of the cut and fill. They also show the slopesof the roadbed surface, ditches, and shoulders. The termfinal cross section is also applied to the as-built sectionsthat are taken after the road is completed.The procedures used to plot cross sections arediscussed in chapter 14 of the EA3 TRAMAN. Youshould review that chapter if you are unsure of theprocedures.DRAINAGEDrainage is a major problem in the location,construction, and design of roads. A route should neverbe located where the drainage presents a problem thatcannot be handled or would be too costly to handle. Aroute may have to be relocated because there is notenough material available to build a particular type ofroad. It may also have to be relocated because of aswamp or underground spring, high flood waters thatcan cover the road, or flash floods that can completelywash out the road. These are some of the reasons forplanning alternate routes. During construction, theproblem of drainage is mainly one of preventingstanding puddles on the roadway. This problem is solvedby slanting the worked surface of the road so that watercan run off quickly or by cutting ditches, called bleeders,so that the water maybe carried away as it accumulates.Subsurface drainage problems are solved by raisingthe grade line of the road or lowering the water table. Ineither case, the distance between the water table and thetop of the subgrade should be as great as possible. Thereare several ways of lowering the water table. In one way,deep, open ditches are set back beyond the roadwaylimits. These ditches intercept the water table, allowinggroundwater to seep through the sides. The water thenflows along the bottom and out the end of each ditchIn another way of lowering the water table, a deeptrench is dug exactly where the finished roadway ditchwould be. The trench is then backfilled to a designateddepth with rocks or large gravel of varying size, with thelarger size at the bottom. The rocks are capped with alayer of branches or straw and the remainder of thetrench backfilled with soil and compacted This trenchis called a french drain(fig. 3-9). A tile drain, alsoshown in figure 3-9, is the same as a french drain exceptthat a perforated pipe or tile is placed in the bottom ofthe trench. The trench is then backfilled with gravel tothe desired depth. The minimum pipe grade is 0.3percent with the maximum varying to meet conditions.Surface drainage involves water from directprecipitation, surface runoff, rivers, and streams.(Surface runoff is rainfall that is not absorbed by the soilbut runs off a surface in sheets or rivulets.) Rainfall hasan immediate effect upon a roadway. Obviously,rainwater would be a safety hazard or cause weak spotson the roadway if it were allowed to stand. Water that3-10
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