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project, including the ditches, is known as the
roadway. The roadbed is the section that includes the
surface course and both shoulders, and the travel way
is the surface course that the vehicle travels on.
Slope Ratio
The two most common slopes used in road
construction are the foreslope and backslope. The
foreslope extends from the outside of the shoulder to
the bottom of the ditch. The backslope extends from
the top of the cut at the existing grade to the bottom of
the ditch. The amount of slope in a foreslope or
backslope is the ratio of horizontal distance to vertical
distance (fig. 15-15). That means that for every one
(1) foot of vertical (up or down), the horizontal
distance changes proportionally. The following are
equations to compute slope ratio:
1. If the base and the height are known factors, but
not the slope, use the following:
Base ÷ Height = Slope
( B + H = S
2. If the slope ratio and the height are known
factors, but not the base, use the following:
Slope x Height = Base
(S x H = B)
3. If the base and the slope ratio are known factors,
but not the height, use the following:
Base ÷ Slope = Height
(B ÷ S = H)
Figure 15-15.—Slope ratio.
15-12
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