Guideline for the Design and Use of Asphalt Pavements for ...

PAVEMENT THICKNESS DESIGN
For this guide, a simple to use nomograph method has been chosen for pavement thickness design.
There are five important values that need to be determined or known to use the nomograph method, they
are:
1. Subgrade Strength – Resilient Modulus (MR)
2. Traffic analysis, ESALs
3. Reliability Level of Pavement Performance
4. Overall standard Deviation, (AASHTO recommend number 0.44)
5. Structural Number – SN
Each of the above factors is discussed in the following sections followed by an example of how each is used
to determine the pavement structural thickness.
SUBGRADE STRENGTH
The performance of a flexible pavement structure is directly
related to the physical properties and supporting strength of
the roadbed soil. The effect of less satisfactory soils can be
compensated for by increasing the thickness or the strength
of the pavement structure.
In Colorado subgrade strength is usually determine by the
California Bearing Ratio test (CBR) or the Hveem test (R-
Value) methods. The CBR and R-Values need to be
determined by a qualified laboratory for the actual soil from
the project. It is important that samples be obtained from the
project site. The test method to be used should be one of
the following:
$ Colorado Procedure CP-L 3101 ~
Strength Determination of Compacted Soils
And Aggregates by R-Value
$ ASTM D 1883–94 ~ Standard Test Method For CBR (California Bearing
Ratio) of Laboratory Compacted Samples
$ ASTM D 4429–93 ~ Standard Test Method For CBR (California Bearing
Ratio) of Soils in Place