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Publication Number:  FHWA-RD-97-083    Date:  September 1997
Publication Number: FHWA-RD-97-083
Date: September 1997


Design Pamphlet for The Determination of Design Subgrade in Support of The 1993 Aashto Guide for The Design of Pavement Structures

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A key challenge faced by engineers using the 1993, AASHTO Guide for Design of Pavement Structures (AASHTO Guide) is the selection of appropriate design values for the subgrade soil and for the pavement materials. Until now, the information available to help engineers choose appropriate values has been incomplete. This design pamphlet addresses this problem by presenting procedures to characterize the subgrade soil. Two companion pamphlets — Design Pamphlet for the Determination of Layered Elastic Moduli in Support of the 1993 AASHTO Guide for the Design of Pavement Structures (FHWA-RD-97-077) and Design Pamphlet for the Backcalculation of Pavement Layer Moduli in Support of the 1993 AASHTO Guide for Design of Pavement Structures (FHWA-RD-97-076) — provide additional, related guidance on selecting appropriate design values to characterize the pavement materials and interpretation of pavement deflection data. The procedures presented were developed through the analysis of the Long- Term Pavement Performance (LTPP) data, documented in the report Analyses Relating to Pavement Material Characterization and Their Effects on Pavement Performance (FHWA-RD-97-085).

Application of the procedures and guidelines developed through this analysis will facilitate and improve application of the AASHTO Guide flexible pavement design procedures. Their use will provide: (1) improved designs, (2) more realistic estimates of pavement performance, and (3) more consistent use of the AASHTO design parameters. Furthermore, although the procedures are specifically developed for use with the 1993 AASHTO Guide, their use will give agencies a "leg up" on implementation of the design procedures being developed for inclusion in the 2002 AASHTO Guide for Design of New and Rehabilitated Pavement Structures. Thus, this pamphlet and its companions are critically important to anyone who designs flexible pavements.

Charles J. Nemmers, P.E.
Office of Engineering
Research and Development



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