Design Criteria for Permanent Deformation of Subgrade Soils in Flexible Pavements for Low-Volume Roads

Abstract

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 plastic phenomena solely to elastic responses. In this paper, the philosophy of the connection between subgrade and pavement distress modes is reviewed. A finite element method program, ARKPAVE, was used to conduct the structural analyses of flexible pavements. Structural responses suggest that resilient modulus, the only design input of subgrades could not adequately reflect such a contribution which indicates that subgrade soils have not been addressed effectively in current design methods. Based on the prediction model for permanent deformation of subgrade soils which is derived from the results of repeated load testing, an improved design criterion for subgrade soils is proposed. An example of incorporating the permanent deformation of subgrade soils into flexible pavement design, especially for low-volume rural roads, is demonstrated. The stress ratio at the top of subgrade soils, r σ 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 future mechanistic-based design guides for flexible pavement structures.