Permanent deformation of subgrade soils: Laboratory investigation and application in mechanistic-based pavement design

Subgrade soils contribute to surface rutting and fatigue cracking, the two major distress modes in flexible pavements. Rutting and cracking are accumulated damages done to the pavement structures over load repetitions and are irrecoverable in nature. Current transfer functions used in the mechanistic-based design methods relate these distress phenomena solely to elastic responses. In this dissertation, a rational approach to deal with permanent deformation of subgrade soils was proposed and the application in mechanistic-based pavement design was demonstrated.; Based on a chronicle review of literature and an extensive preliminary testing program on an Arkansas subgrade soil, a testing protocol for repeated load testing of highway subgrades was established. The testing protocol was then used to evaluate permanent deformation behavior of four representative Arkansas subgrade soils. Factors affecting the accumulation of permanent deformation of subgrade soils were identified. Deviator stress, moisture content, and first-cycle freeze-thaw were found to be the three primary contributors to deformation accumulation. Loading frequency, density, and confining pressure are three minor factors in the deformation process. A stress-ratio-associated model was developed to predict the permanent deformation of subgrade soils over load applications based on the regression analyses of test results.; lie philosophy of the connection between subgrade and flexible pavement distress modes was then reviewed. A finite element method program, ARKPAVE, was used to conduct the structural analyses of flexible pavements to evaluate the adequacy of design considerations for subgrades. Results from structural analysis suggest that resilient modulus of subgrades alone could neither effectively nor adequately address the effect of subgrade soils on pavement distresses. Based on the proposed prediction model for permanent deformation of subgrade soils, an improved design criterion for subgrade rutting was established. An example Of incorporating the permanent deformation of subgrade soils into flexible pavement design, especially for low-volume rural roads, was demonstrated. The stress ratio at the top of subgrade soils, rsigma, defined as the ratio of deviator stress to static strength, is recommended to replace the vertical compressive resilient strain as the design criteria for pavement rutting in the future inechamsticbased design guides for flexible pavement structures.