Page 314 - Planning and Design of Airports

272 Airp o r t D e sign

5. Item P-306—Econocrete Subbase Course
6. Item P-401—Plant Mix Bituminous Pavements
7. Item P-403—HMA Base Course
P-211, P-304, P-306, P-401, and P-403 are considered stabilized
based courses.
The function of the subbase, when required, is similar to that of
the base course, but since the subbase is further removed from the
area of load application, it is subjected to lower stress intensities. Sub-
bases are typically required when flexible pavement is to be sup-
ported by soils of CBR value less than 20.
Flexible pavement subbase courses are available in different
types, including:
1. Item P-154—Subbase Course
2. Item P-210—Caliche Base Course
3. Item P-212—Shell Base Course
4. Item P-213—Sand Clay Base Course
5. Item P-301—Soil Cement Base Course

The subgrade soils are subjected to the lowest loading intensities,
and the controlling stresses are usually at the top of the subgrade
since stress decreases with depth. However, unusual subgrade condi-
tions, such as layered subgrade materials, can alter the location of
controlling stresses.

CBR Method
Prior to 2008, the FAA’s standard method for flexible pavement
design was known as the CBR method. The CBR method was based
on approximation charts that factored in the CBR value of the sub-
grade and the number and gross weight of equivalent annual depar-
tures of the design aircraft. Separate approximation charts were pro-
vided by the FAA for different generic aircraft landing gear configu-
rations, and for aircraft greater than 300,000 lb maximum gross
weight, specific individual aircraft. Figure 7-5 provides an illustrative
example of the CBR method.
The example nomograph found in Fig. 7-5 represents the histori-
cal method of estimating the base level thickness of flexible pave-
ment for a Boeing 767. The arrow within the nomograph represents
the example for a subgrade with CBR value of 7, a 325,000-lb aircraft
gross weight, and 1200 annual equivalent departures, resulting in a
required base course of 30 in thickness. The nomograph also pro-
vides the necessary thickness for the surface layer, at 4 in thick for
critical areas and 3 in thick for noncritical areas, such as pavement
shoulders.