30 June 2001
subgrade strain is multiplied by 1.5 to account for the shrinkage cracks that will exist. The basic flow
diagram for this type of pavement is shown in Figure 11-10.
12. EXAMPLE DESIGN FOR CONVENTIONAL FLEXIBLE PAVEMENT. To illustrate the application
of the design procedure, consider a design for an Army Class IV airfield. The subgrade is a lean clay
classified as CL. The design is to be for 200,000 passes of the C-130 aircraft. The design loading for
the C-130 on the taxiway is 70,310 kilograms (155,000 pounds) with a tire contact area of
0.258 square meters (400 square inches). For the runway interior the loading is 75 percent of the
taxiway loading. The reduction is accomplished by reducing the contact area, giving a contact area of
0.194 square meters (300 square inches). The design process may best be illustrated in steps. The
basic steps are material investigation, determination of trial pavement sections, computation of critical
strains, determination of applied strain repetitions, and computation of damage factors.
a. Step 1 - Material Investigation.
(1) The evaluation of the subgrade is to be accomplished by field and laboratory studies. The
subgrade is to be classified according to different material types and material processing. For this
example, it is assumed that the subgrade is fairly uniform and consists of a compacted lean clay placed
according to existing compaction requirements. The subgrade evaluation involves conducting a series
of resilient modulus tests according to the procedures given in Appendix L. For a location such as
Shreveport, LA, it must be assumed that the subgrade would become saturated and thus the resilient
modulus tests are conducted on saturated samples. A minimum of six samples should be tested and a
design modulus determined for each sample. For determination of a design modulus, the data from the
laboratory tests are plotted on a log-log plot of MR versus Fd and overlaid on the design curves as
shown in Figure 11-11. For the design example, the design modulus obtained using this process is
assumed to be 62 MPa (9,000 psi) for both taxiway and runway designs. Base and subbase materials
must be obtained that meet the requirements of Chapters 7 and 8. The modulus values for the base and
subbase are to be determined by the procedures given in Appendix J. Because the modulus of these
materials depends on layer thicknesses, the modulus cannot be obtained until the trial sections are
(2) The bituminous surfacing must meet the requirements of Chapter 9 as to minimum
thickness and composition. The modulus-temperature relationship is determined according to the test
procedures given in Appendix H or by the provisional procedure given in Appendix I. Assume for the
example problem that the relationship as shown in Figure 11-12 is obtained from laboratory test data.
(For simplicity, these data will be used for both taxiway and runway.) From the climatic data, the design
air temperature is obtained and the design modulus values are determined as shown in Tables 11-3 and
11-4. To reduce the number of computations, the 12 groups are reduced to four groups as shown in
Table 11-5. The Poisson's ratio for all materials is selected from Table 11-2.
b. Step 2 - Determination of Initial Section.
(1) From Chapter 10 the total thickness of pavement required for a gross aircraft load of
70,310 kilograms (155,000 pounds), 200,000 passes, and a subgrade CBR of 6 is determined to be
710 millimeters (28 inches). For the runway interior design, the thickness would be based on a gross
aircraft load of 52,730 kilograms (116,250 pounds) and would result in an estimated thickness of
610 millimeters (24 inches). For taxiway design, subgrade damage factors will be computed for
pavement thicknesses of 680, 760, and 840 millimeters (27, 30, and 33 inches) in an attempt to bracket
the final required pavement thickness. The total thickness of pavement is made up of the asphalt