Types of cross sections

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TYPES OF CROSS SECTIONS.— Figure 14-23 shows a typical design cross section, Just about everything you need to know to construct the highway, including the materials to be used and their thicknesses, is given here.

However, this design section is a section of the completed highway. For the purpose of staking out and for earthmoving calculations, the cross-section line of the existing ground at each successive station must be plotted; the design data cross section (typical section of the highway) is then superimposed.

The cross section of the road, with design data available from a previous design-data survey, is staked out by an EA survey party, preferably the leveling crew. Figure 14-24 shows a designed cross section of a 40-ft-wide road taken from a station or point along the road center line. The elevation of the existing surface is 237.4 ft all the way across; therefore, this is called a level section. Finished grade for the highway at this station— that is, the proposed center-line elevation for the finished highway surface—is 220.4 ft. The prescribed side-slope ratio is 1.5:1; that is; a horizontal unit of 1.5 for every unit of vertical rise.

Because the ground line across the cross section is level and the side-slope ratio is the same on both sides, the horizontal distance from the center line to the point where the side slope will meet the natural surface will be the same on both sides. A slope stake is driven at this point to guide the earthmovers. The horizontal distance from the center line to a slope stake can be computed by methods that will be explained later.

In the case of this designed cross section, the data available to you are

1. the width of the highway,

2. the side-slope ratio, and

Besides this, all you need to know to set slope stakes is the ground elevation of the slope-stake point on each side. Because the elevation of the level section in figure 14-24 is the same on both sides, only a single-level shot for elevation is needed. For this reason, a section of this kind is called either a one-level section, or just a level section. Because the entire sectional area consists of material to be excavated or CUT, it is called a section in cut.

Figure 14-24.-Level section in cut.

Figure 14-25.-Three-level section in cut.

Figure 14-26.-Level section in fill.

In the section shown in figure 14-25, the ground line across the section is sloping rather than level. Therefore, to plot this section, you would need three different elevations: one for the left slope stake, one for the center-line grade stake, and one for the right slope stake. If these three levels are taken, the section is called a three-level section in cut. If additional levels are taken midway between the center line and the slope stake on either side, it is called a five-level section in cut. Therefore, it is a section in cut because the entire cross-sectional area consists of cut. Level, three-level, and five-level sections are called regular sections.

Figure 14-26 shows a level section in fill; figure 14-27 shows a three-level section in fill. The section shown in figure 14-28 consists of both cut and fill and is called a sidehill section.

When a more accurate picture of cross sections than can be obtained from regular sections is desired, irregular sections are taken and plotted. For an irregular section you take, besides the regular levels, additional levels on either side of the center line. You take these at set intervals and at major breaks in the ground line.

Cross sections may be preliminary or final. Preliminary cross sections, from the P-line or survey base line, are irregular sections that are plotted before the finished grade has been determined. They may be obtained by levels run in the field or by elevations found on the contour lines of a topographic map.

Final cross sections are sections of the final road design. They may be prepared in the same manner as preliminary sections, or they may be regular sections plotted from field data obtained after the finished grade has been set. The term final cross section is also applied to as-built sections taken after construction is completed.