UFC 3-260-02
30 June 2001
b. Calculations. Calculations can be performed using a similar tabular arrangement as was
shown in Table L-1. Test results should be presented in the form of a plot of log MR versus log of
the sum of the principal stresses as shown in Figure L-5.
L-10. INTERPRETATION OF TEST RESULTS.
a. Cohesive Soils. As previously indicated, test results for cohesive soils are presented in the
form of a plot of resilient modulus MR versus deviator stress Fd. Normally for cohesive soils, the
test results will indicate that the resilient modulus decreases rapidly with increases in deviator
stress. Thus, selection of a resilient modulus from the laboratory tests results requires an estimate
of the deviator stress at the top of the subgrade with respect to the design aircraft. For a properly
designed pavement, the deviator stress at the top of the subgrade will primarily be a function of
the subgrade modulus and the design traffic level. Shown in Figure L-6 are relationships between
deviator stress at the top of the subgrade and applicable subgrade modulus values determined from
an analysis of the pavement sections. The relationships shown in Figure L-6 were determined
using a layered elastic pavement model with the modulus values as input parameters and the
deviator stress values as computed responses. Thus, these relationships are essentially limiting
criteria. Relationships are shown for 1,000, 10,000, 100,000, and 1,000,000 repetitions of
strain. To determine the appropriate modulus value to use in the performance model, the test
results from the resilient modulus tests on the laboratory specimens are superimposed on the
appropriate relationship from Figure L-6, and the design modulus value is taken from the
intersection of the plotted functions.
b. Example on Cohesive Soils. Assume a design problem involving 100,000 repetitions of
strain. Figure L-7 shows a plot of relationships taken from Figure L-6 superimposed on test results
from a laboratory resilient modulus test. For this particular design, a subgrade modulus value of
62 MPa (9,000 psi) would be used.
c. Cohesionless Soils. For cohesionless soils, laboratory test results are presented in the form
of a plot of resilient modulus versus the first stress invariant, i.e., sum of the principal stress 2. For
cohesionless soils, this relationship is generally linear in form on a log-log plot, with the resilient
modulus being directly proportional to the sum of the principal stresses. Selection of a specific
resilient modulus value for use in the design model requires an estimate of the sum of the principal
stresses at the top of the subgrade. Since a cohesionless material is involved, the influence of
both applied stresses and estimated overburden stresses from the pavement structure must be
considered. In Figure L-8, a relationship is shown between the pavement thickness and the sum of
the principal stresses at the top of the subgrade due to overburden. In Figure L-9, relationships are
shown between the subgrade modulus and limiting values of the sum of the principal stresses due
to applied force. For each figure, relationships are shown for 1,000, 10,000, 100,000, and
1,000,000 repetitions of stress. Using the value of the estimated pavement thickness, that part of
the total sum of the principal stresses due to overburden can be obtained from Figure L-8. The
applicable relationship from Figure L-9 is then selected and adjusted to include the influence of
overburden by increasing all values of the principal stress sum by the value obtained from
Figure L-8. Thus, a new limiting relationship is obtained and replotted. The results of the
laboratory modulus test are superimposed on the plot, and the design subgrade modulus values are
taken at the intersection of these relationships.
d. Example on Cohesionless Soils. Assume a design problem involving a pavement having an
estimated initial thickness of 762 millimeters (30 inches). The design aircraft has a dual-wheel
main gear assembly, and the design life is for 100,000 repetitions of strain. From Figure L-8, the
L-11
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