Balancing Thickness and Quality: Economic Design Framework for Flexible Pavement Foundations in Minnesota

In Minnesota, pavement designers have traditionally favored premium Class 5/6 base aggregates directly beneath asphalt or concrete surface layers, while relegating lower-quality materials to thicker subbase layers or excluding them entirely. With the limited supply of high-quality aggregates and the steady increase in transportation costs posing restrictions for pavement construction and maintenance, local agencies are increasingly exploring the use of lower-quality, locally available materials in pavement foundation layers. This study evaluated the effects of both material quality and layer thickness requirements of base and subbase on flexible pavement performance and the associated cost, with the objective to develop an optimal design framework for statewide use, particularly in areas where locally sourced high-quality aggregates are limited. To this end, resilient modulus and strength characteristics were studied to classify Minnesota aggregate sources into high-, medium-, and low-quality levels, which were then incorporated into MnPAVE mechanistic pavement analyses for multiple traffic loads, asphalt thicknesses, subgrade stiffnesses, and Minnesota climate zones. In parallel, cost–benefit analyses were conducted to identify the most economical quality–thickness combinations of unbound aggregate base and subbase using historical bid-price data available from Minnesota Department of Transportation (DOT). An iterative procedure developed for a design framework in this research effort determines the optimal base and subbase thicknesses each aggregate quality level is required to satisfy fatigue- and rutting-life criteria for conventional flexible pavements. The resulting guidance enables local agencies to use regional materials more efficiently when high-quality aggregate materials are in short supply, while delivering sustainable and cost-optimized pavements across the state.