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Standard Road Plans EW-501 and EW-502 both show transverse slopes for entrances (see Section 3K-2 for more on entrance design) and safety ramps are to be graded at 8:1 with a pipe and 10:1 without a pipe (see Section 4B-1 for more on entrance and safety ramp pipes). However, when the ditch grade or roadway grade exceeds 4%, designers may want to consider adjusting the transverse slopes to maintain 8:1 and 10:1 slopes relative to the ditch or roadway grade. As the ditch or roadway grade approaches 4%, an 8:1 transverse slope relative to a horizontal plane begins to approach 6:1 relative to the ditch or roadway grade. This is the Acceptable Value for transverse slopes (assumed to be measured relative to a horizontal plane) in the Design Criteria Worksheets of Section 1C-1. In addition, pipe lengths and earthwork can become excessive as ditch or roadway slope increases, see Figure 1.
Figure 1: Effect of ditch or roadway grade on transverse slope and pipe length.
To overcome these problems, designers may adjust transverse slopes by adding or subtracting the ditch or roadway grade to or from the transverse slope. This is demonstrated for the example shown in Figure 1.
Upstream Adjustment: \(\frac{1}{8} - \frac{6.5}{100} = \frac{6}{100} = \frac{1}{16.67}\)
Downstream Adjustment: \(\frac{1}{8} + \frac{6.5}{100} = \frac{19}{100} = \frac{1}{5.26}\)
The transverse slope adjustments are both rounded to the nearest whole number. Thus, the upstream adjustment is 17:1 and the downstream adjustment is 5:1, both relative to a horizontal plane. Figure 2 illustrates the example shown in Figure 1 with adjusted transverse slopes. Table 1 on the following page provides transverse slope adjustments for various grades relative to a horizontal plane.
Figure 2: Transverse slopes adjusted for steep ditch or roadway slope.
Typically, roadway and ditch slopes closely parallel each other. In situations where they differ, whether to adjust transverse foreslopes relative to the roadway slope or the ditch slope is up to the designer. However, designers may face situations where the choice is limited, for example situations involving excessive ditch slopes, or situations involving roadway slopes that run opposite of ditch slopes. In cases such as these, designers should base their decision to adjust transverse slopes taking in consideration of roadway slope and the practicality of adjusting transverse slopes.
If the designer chooses to adjust the transverse slopes, a modified version of Standard Road Plan EW-501 and/or EW-502 must be included in the U Sheets (see Section 1F-21). The modification should include the addition of a note to construct slope relative to the roadway or ditch grade. Refer to Section 1E-5 for more information on modifying Standard Road Plans.
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