Determine Effective Resilient Modulus. The first step in designing the thickness of
a flexible pavement is the determination of the effective resilient modulus of the subgrade.
The resilient modulus may be determined as described in Art. 3.5.4. In regions
where large moisture variations, freeze and thaw, etc., will affect the strength of the
subgrade soils and subgrade, AASHTO provides a procedure to modify the composite
modulus of subgrade reaction. The seasonal variation in strength is determined using
laboratory procedures or through the use of nondestructive testing (NDT). The seasonal
strength of the subgrade is entered in Fig. 3.16. The relative damage is determined
from the nomograph and the sum of relative damage divided by the total number of
periods to determine the average relative damage factor. This factor is then entered in
the nomograph to find the effective resilient modulus.