Design Resilient Modulus of Subgrade Soils from FWD Tests

The resilient modulus is a basic soil property governed by the elastic theory, and is a measure of the recoverable strain for a small, repetitive load. It is a required input in the AASHTO method of flexible pavement design. The resilient modulus of subgrade soils is a required input parameter for the AASHTO method of flexible pavement design and is also used to compute the modulus of subgrade reaction for the AASHTO rigid pavement design equations. This study analyzed subgrade resilient moduli, backcalculated from the Falling Weight Deflectometer deflection test results, on nine pavement subgrade projects of the Kansas Department of Transportation (KDOT). Laboratory resilient modulus test results were also reviewed for the fine-grained soils on these projects. The results show that the average "breakpoint" deviator stress for the fine-grained soils that displayed typical bi-linear behavior was 46 kPa. The design resilient modulus corresponding to this deviator stress value could be used in the instances when the "breakpoint" deviator stress is not distinct graphically. The average ratio of the laboratory-determined to the backcalculated resilient moduli was found to be 0.33. Backcalculated resilient moduli using the AASHTO algorithm and a nonlinear elastic analysis program were nearly identical for the soils that had slightly nonlinear characteristics. Results from either approaches can be used in design.