Abstract Manager

Experiences in North America and elsewhere have shown that good quality native and borrowed soils including granular fills provide strong support for the overlying pavement structure for increased load carrying capacity and longer service life or allow for the reduction of pavement thickness. The stiffness of subgrade soils has a significant and realistic influence on pavement thickness design using empirical design methods. The AASHTOWare Pavement ME Design (PMED) software predicts several key distresses in pavements for the variation of different design inputs including subgrade properties. However, past studies have shown that this software is unable to adequately consider the influence of subgrade soil quality, as practically experienced. As part of continued design trials since 2007 to assess the suitability of the PMED software, the Transportation of Association of Canada (TAC) ME Pavement Design Subcommittee undertaken this study in 2025 to verify the findings of earlier studies using the new web version of the software. The objective of this study is to evaluate how variations in subgrade stiffness and material type affect PMED software distress predictions for new flexible pavements under different climatic conditions.

Five different subgrade soil types, AASHTO classifications A-7-6, A-6, A-4, A-2-4, and A-1-b, with varied physical properties and resilient moduli values ranging from 20 MPa to 150 MPa were selected. Eleven weather stations were selected to capture the effect of varied climatic conditions across Canada on predicted performance. This led to a total of 110 design runs, which were completed using the software’s default global calibration coefficients.

Preliminary analyses of results showed that bottom-up fatigue cracking (BUFC) decreases as subgrade stiffness increases with minimal sensitivity to soil type. BUFC also varied with variation of climatic conditions, but showed limited sensitivity to changes in soil classification. The predicted top-down fatigue cracking and thermal cracking displayed little to no sensitivity to subgrade stiffness, soil type, or climate variations. Climatic variation was the dominant factor influencing total and asphalt concrete layer rutting, outweighing the effects of subgrade material type and stiffness. Subgrade rutting generally decreased as resilient modulus increased within each soil type. An unexpected inverse trend was observed, where higher-quality subgrade soils exhibited greater predicted rutting than inferior soils with similar resilient moduli values. Climate had a pronounced effect on pavement roughness prediction.

This paper presents the methods, analyses, and findings from these trials to assist highway agencies in evaluating the suitability of PMED for routine pavement design applications.