During the late 1980s, the Washington State Department of Transportation (WSDOT), the University of Washington, and the Washington State Transportation Center developed a mechanistic-empirical flexible overlay design procedure. Following development, WSDOT implemented this overlay design procedure and has been evaluating flexible overlay projects for approximately the past 8 years. WSDOT rehabilitates about 100 projects each year; approximately 20 to 30 percent of the total projects are designed using the WSDOT overlay design procedure and the AASHTO overlay design procedure (using DARWin). These two procedures are discussed in general, and two case studies illustrate each of the overlay design procedures. Also included is the backcalculation of layer moduli from falling weight deflectometer data.
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