Evaluation of the Shakedown Behavior of Unbound Granular Base Materials

The shakedown theory concept has been successfully used to characterize the response of unbound granular base materials in a pavement structure. Within this concept, two stress limits are used to comprehensively describe the behavior of granular base materials under cyclic loading: an elastic shakedown limit, below which the material will eventually shakedown and respond in a resilient (elastic) manner, and a plastic shakedown limit, beyond which the material experiences incremental collapse and ratcheting. This paper examines the shakedown behavior of unbound granular materials and evaluates the physical and mechanical properties that affect the shakedown stress limits. To achieve this objective a laboratory testing program was conducted on two different types of unbound granular materials typically used in construction of a base course layer, namely, limestone and sandstone. The laboratory testing program included conducting multi-stage repeated load triaxial tests, static triaxial test, and different physical properties tests on the materials considered. The results showed that the plastic shakedown followed closely the static failure line for the considered limestone material. However, for the sandstone materials the slope of static line was significantly different from that of plastic shakedown limit, suggesting that the mechanism for failure under cyclic loading is different than that for static loading. In addition, the elastic shakedown limit was not directly related to the static failure line parameters.