Page 350 - Civil Engineering Formulas

HIGHWAY AND ROAD FORMULAS 281

STRUCTURAL NUMBERS FOR FLEXIBLE PAVEMENTS

The design of a flexible pavement or surface treatment expected to carry more
than 50,000 repetitions of equivalent single 18-kip axle load (SAI) requires
identification of a structural number SN that is used as a measure of the ability
of the pavement to withstand anticipated axle loads. In the AASHTO design
method, the structural number is defined by
(11.23)
SN SN 1 SN 2 SN 3
where SN structural number for the surface course a D 1
1
1
a layer coefficient for the surface course
1
D actual thickness of the surface course, in (mm)
1
SN structural number for the base course a D m 2
2
2
2
a layer coefficient for the base course
2
D actual thickness of the base course, in (mm)
2
m drainage coefficient for the base course
2
SN structural number for the subbase course a D m 3
3
3
3
a layer coefficient for the subbase course
3
D actual thickness of the subbase course, in (mm)
3
m drainage coefficient for the subbase
3
The layer coefficients a are assigned to materials used in each layer to con-
n
vert structural numbers to actual thickness. They are a measure of the relative
ability of the materials to function as a structural component of the pavement.
Many transportation agencies have their own values for these coefficients. As a
guide, the layer coefficients may be 0.44 for asphaltic-concrete surface course,
0.14 for crushed-stone base course, and 0.11 for sandy-gravel subbase course.
1
The thicknesses D , D , and D should be rounded to the nearest 2 in
2
3
1
(12.7 mm). Selection of layer thicknesses usually is based on agency stan-
dards, maintainability of the pavement, and economic feasibility.
Figure 11.6 shows the linear cross slopes for a typical two-lane highway.
Figure 11.7 shows the use of circular curves in a variety intersecting grade-
separated highways.
2: 1 Cut slopes in C
deep cuts except L
in rock Shoulder Travel lane Travel lane Shoulder
Hinge point
Inslope 4:1
or flatter Slope Slope 4:1 or 6:1
Slope Slope
Fill slopes
on low fills
Rounded
Natural ground (low fill)
4:1 or 6:1 Rounded Natural ground (high fill)
Cut slopes in ditch
shallow cuts 2:1 Fill slopes on high fills, Rounded
usually with guard rail
FIGURE 11.6 Typical two-lane highway with linear cross slopes.