UFC 3-260-02

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.

Poisson's Ratio <

Pavement Materials

Bituminous concrete

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

0.3

Chemically stabilized base- or subbase-course

0.2

Subgrade

Cohesive subgrade

0.4

Cohesionless subgrade

0.3

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 *'

6.

, 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.

11-7

Integrated Publishing, Inc. |