30 June 2001
estimate of the resilient modulus in megapascals (pounds per square inch) can be made from the
relationship of MR = 10.3CBR (MR = 1,500CBR). The relationship does provide a method for checking
the reasonableness of the laboratory results.
b. Poisson's Ratio. Because of the complexity of laboratory procedures involved in the direct
determination of Poisson's ratio for pavement materials and because of the relatively minor influence on
pavement design of this parameter when compared with other parameters, use of values commonly
recognized as acceptable is recommended. These values for the four classes of pavement materials
considered herein are presented in Table 11-2.
Typical Poisson's Ratios for Four Classes of Pavement Materials
Poisson's Ratio <
0.5 for E < 3,450 MPa (500,000 psi)
0.3 for E > 3,450 MPa (500,000 psi)
Unbound granular base- or subbase-course
Chemically stabilized base- or subbase-course
Note: E = elastic modulus of bituminous concrete (psi)
5. SUBGRADE EVALUATION. Chapter 6 provides for the evaluation of the subgrade for design by
the CBR design procedure and also provides the background for evaluation of the subgrade modulus.
After the establishment of the grade line, the pavement will be grouped as to soil type, strength,
expected moisture content, compaction requirements, and other characteristics. For each soil group, a
minimum of six resilient modulus tests should be conducted and the design modulus determined
according to procedures given in Appendix K. The design modulus would be the average of the moduli
obtained from the testing.
DESIGN CRITERIA. The damage factor (DF) is defined as DF '
, where n is the number of
effective strain repetitions and N is the number of allowable strain repetitions. The cumulative damage
factor is the sum of the damage factors for all aircraft. The value of n is determined from the number of
aircraft operations. The value of N must be determined from the computed strain and the appropriate
criteria. Basically, there are three criteria to determine N . These are the allowable number of
repetitions as a function of the vertical strain at the top of the subgrade, the allowable number of
repetitions as a function of the horizontal strain at the bottom of the bituminous concrete, and the
allowable number of repetitions as a function of the horizontal strain at the bottom of a chemically
stabilized base or chemically stabilized subbase. It should be noted that there is no strain criterion for
unbound base. In the development of the procedure, it has been assumed that an unbound base and
subbase that meets the specifications for quality will perform satisfactorily.