The research reported assessed a major change in the 1986 AASHTO Guide for Design of Pavement Structures. The soil support value was replaced with subgrade effective resilient modulus (MR) in flexible pavement design. MR was introduced into the 1986 AASHTO Guide as part of an overall shift to the use of elastic modulus as the indicator of pavement layer structural capacity. Because subgrade modulus changes when the subgrade moisture content changes or when the subgrade freezes, MR must reflect the combined effect of the modulus values from all seasons of the year. AASHTO suggests that the spring thaw modulus may be as low as 20 to 50% of the summer modulus.
NDT testing was performed during the spring thaw and summer seasons of 1986 on 15 Indiana flexible pavements with subgrades ranging from high plasticity clays to sands. Deflections from sensors 5 ft (152 cm) and 7 ft (213 cm) from the load plate were recorded for use in seasonal subgrade resilient modulus determination as provided by AASHTO. MR for each of the 15 pavements was determined using the following seasonal moduli as inputs: spring thaw and summer moduli as calculated from NDT deflections, spring/fall wet modulus interpolated between spring thaw and summer moduli, and winter modulus estimated as recommended by AASHTO. The research yielded the following findings for the conditions present in Indiana in 1986: contrary to expectations, the typical spring thaw modulus value was not dramatically lower than the summer modulus value; and summer modulus values were close enough to MR values that they may be used to approximate MR if spring thaw, spring/fall wet, and winter data are not available.