Determine Effective Subgrade Modulus. The first step in designing the thickness of a
rigid pavement is the determination of the effective modulus of subgrade reaction. The
effective modulus (or composite modulus) is the modulus of subgrade reaction after correction
for use of subbase, seasonal variation in subgrade and subbase strength, rigid
foundation within 10 ft (3 m) of the surface, and loss of support. Figure 3.11 is used to
determine the composite modulus of subgrade reaction when a subbase will be used
under the concrete pavement. If the pavement will be placed directly on the subgrade,
the AASHTO Pavement Design Guide recommends a composite modulus of subgrade
reaction of: