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)

where SN 1 structural number for the surface coursea 1 D 1
a 1 layer coefficient for the surface course
D 1 actual thickness of the surface course, in (mm)
SN 2 structural number for the base coursea 2 D 2 m 2
a 2 layer coefficient for the base course
D 2 actual thickness of the base course, in (mm)
m 2 drainage coefficient for the base course
SN 3 structural number for the subbase course a 3 D 3 m 3
a 3 layer coefficient for the subbase course
D 3 actual thickness of the subbase course, in (mm)
m 3 drainage coefficient for the subbase

The layer coefficients anare assigned to materials used in each layer to con-
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.
The thicknesses D 1 ,D 2 , and D 3 should be rounded to the nearest in
(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.

(^1) 
2

SNSN 1 SN 2 SN 3

4:1 or 6:1

Cut slopes in

shallow cuts

Rounded

2: 1 Cut slopes in

deep cuts except

in rock

Inslope 4:1

or flatter

Slope Slope Slope Slope

Hinge point

4:1 or 6:1

Fill slopes

on low fills

Rounded

Rounded

ditch

2:1 Fill slopes on high fills,

usually with guard rail

Natural ground (low fill)

Natural ground (high fill)

Shoulder Travel lane Travel lane Shoulder

CL

FIGURE 11.6 Typical two-lane highway with linear cross slopes.