Figure 3-7.-Curve section.section for a curve, the pavement is a plane surfaceinstead of crowned and is usually superelevated toaccount for centrifugal force encountered in curves. Theoutside shoulder slope is the same as the superelevatedpavement slope, but the inside shoulder slope is eitherthe same or greater slope. (Inside shoulder refers to theshoulder closest to the center of the arc, or curve.)Most curves are also widened on the inside to allowfor the “curve straightening” effect of long wheelbasevehicles. The back wheels of the trailer in atractor-trailer rig do not follow in the tracks of the tractorwheels. They run closer to the inside edge on the insidelane and closer to the centerline on the outside lane. Thispresents a safety hazard when two vehicles meet incurves. Curve widening partially eliminates this hazard.Figure 3-7 is a superelevated section showing curvewidening. Specific guidance for curve widening iscontained in NAVFAC DM-5.5.Preliminary Cross SectionPreliminary cross sectionsare sectional views ofthe existing terrain taken at each station point along thecenter line of the route the road is to take. These sectionsare usually taken after the roadway has been cleared butmay be taken before. If the sections are taken before, thethickness of the sod to be stripped off is normallydeducted from the elevations. The preliminary crosssection shows the elevations of the natural, or original,ground. These sections, when superimposed on thedesired finished roadbed sections, are used for studyingvarious alignments of the road and for preliminaryearthwork estimating. Figure 3-8 shows typicalpreliminary cross sections.Figure 3-8.—Preliminary cross sections.3-9
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