Alternative Test Method for Resilient Modulus of Fine-Grained Subgrades

The mechanistic design of flexible pavement systems requires the specification of mechanical properties for asphaltic concrete, base course, and soil subgrade. The stiffness of the soil subgrade is represented by the resilient modulus, M_r, which is determined in the laboratory from cyclic triaxial tests and is usually a function of the level of applied stress. For a typical pavement project where numerous soils are encountered, cyclic triaxial testing to determine M_r for each subgrade soil may be too complex and time consuming to be applicable on a production basis. Therefore, alternative methods for the determination or estimation of M_r are appropriate and are in fact suggested in the 1993 AASHTO Guide for Design of Pavement Structures (AASHTO Guide 1993).

An alternative test method (ATM) was developed based on a falling weight impacting a standard Proctor specimen. A simple theoretical model was used to develop expressions for the modulus and deviator stress as a function of the measured acceleration during impact. An analysis was conducted to compare the state of stress in the ATM with that in a subgrade under a pavement. A prototype device was constructed and used to estimate the resilient response of 14 soils for which standard cyclic triaxial data were available. The ATM results compared well with the results from standard triaxial tests, and the ATM was able to consistently distinguish the low resilient modulus soils from those with high modulus. The ATM was shown to provide results suitable for many design applications and may be useful for reducing the number of standard tests required to characterize the subgrade M_r response for a pavement project.