Mechanistic design methods for flexible pavements are usually based on limiting only two critical pavement responses: tensile strain at the bottom of the asphalt layer for fatigue damage and vertical compressive strain at the top of the subgrade for overall pavement rutting. Rutting in granular base and subbase layers is assumed to be small for all cases. Research was conducted to develop a fundamental method for incorporating rutting of granular base and subbase layers in flexible pavement thickness design. The research concentrated on investigating the potential for using constitutive relationships based on the flow theory of plasticity to limit permanent deformations in granular layers of pavement systems. The flow theory of plasticity is the most popular of the three plasticity theories that have been applied to soils and granular materials. The total strain is viewed as the sum of the reversible elastic component and the irreversible plastic component. A yield function is introduced to differentiate between elastic and plastic behavior. This yield function is a function of the stress state and may change on loading and unloading to describe cyclic hardening behavior. To use a flow theory model to predict the response of a granular pavement layer requires relatively complex finite-element analysis and sophisticated laboratory testing that may not be warranted for normal pavement design. A method is presented for using the yield surfaces from a flow theory model as a design criterion for limiting permanent deformations in granular layers. Only the results of triaxial strength tests are needed to implement the method.
