Tony M. Allen
A complete, formal geotechnical design code of practice addressing all aspects of geotechnical engineering really does not exist in the USA at the national level. However, a geotechnical design code has been developed at the national level in response to the needs of the structural engineers as part of the American Association of Highway and Transportation Officials (AASHTO) specifications, primarily focusing on structure foundations, buried structures, and retaining walls. Since 1994, AASHTO began migrating from Allowable Stress Design (ASD) to Load and Resistance Factor Design (LRFD), the USA equivalent to Limit States Design (LSD). AASHTO fully adopted LRFD, at least in the transportation Sector, in 2007, and ceased updating their Allowable Stress Design and Load Factor Design specifications in 2002.
A key issue for the USA geotechnical community regarding geotechnical design code has been to strike a balance between prescriptive minimum design requirements and levels of safety (i.e., load and resistance factors) and the flexibility needed by geotechnical engineers to apply engineering judgment to address site specific issues and local experience.
Summarized is the historical development of geotechnical foundation design code in the USA for transportation applications. With regard to the geotechnical portions of the current AASHTO LRFD Bridge Design Specifications, the development and selection of load and resistance factors is discussed. Key considerations for geotechnical design specification development are identified. Finally, gaps and future development needs for USA geotechnical design codes of practice are presented.