significant number of State Departments of Transportation (DOTs) have adopted resistance factors that better represent their area and practice. For owners and designers, the challenge with regional calibration has been significant due to the lack of sufficient load test data, expense of conducting load tests, the availability of complete design files, inconsistent failure criteria and the effects of site variability. There are other significant issues that still need to be addressed by the geotechnical community for successful use of LRFD. There are issues with its application to limit equilibrium-based solutions. This has led to difficulties in handling the design of such features as slopes, embankments and soil nail walls. Other examples include issues with down drag on deep foundations, the design for scour and corrosion loss models for buried steel. Designers do not have a means for embracing innovations in the industry such as post- grouting for drilled shafts or most of the earthworks innovations in ground improvement techniques. Additionally, there are structural foundation types that have not been translated to LRFD, such as auger cast-in-place (ACIP) or continuous flight auger (CFA) piles. These issues are among many that still require research and guidance development. There are some areas where we are seeing advances. Designs in the LRFD platform are tied to a minimum standard for subsurface exploration programs and design parameter selection, but a reliability-based model for determining the sufficiency of the subsurface exploration program has not yet been incorporated. Loehr et al. (2013) have begun to address this issue by looking at the adjustment of resistance factors for foundation design due to variability in the input or design parameters. The goal of the research was to produce designs that more closely achieve target levels of reliability by considering the type and scope of site characterization activities. With all of the current research focus on design at the strength limit state, it is worth noting that some of the most exciting work in LRFD is coming through research conducted as part of the second Strategic Research Highway Program (SHRP2). The recently completed R19B research project, Bridges for Service Life Beyond 100 Years: Service Limit State Design, developed a framework for the development of calibrated service limit states (Kulicki et al., 2014). Calibration at the service limit state requires a different approach than engineers are used to since the current AASHTO specifications are calibrated for the strength limit state where the definition of failure is relatively simple. If the factored load exceeds the factored resistance, then failure will occur, as described by severe distress or collapse. For this calibration, resistance is considered constant in time. At present, service limit state, target reliability indices and a definition of failure have not been addressed. Load and resistance factors are currently equal to one. Design at the service limit state will require a different approach since exceedance of the limit state doesn’t lead to a clear, Drilled shaft foundations for the Cooper River Bridge These efforts have significantly increased the usefulness of LRFD for geotechnical engineers around the country, the efficiency of designs with the platform, and the awareness of gaps and needs for further improvement. Continued research and the efforts of many in the profession will only improve upon this, leading to more rational and cost-effective geotechnical assets. References Abu-Hejleh, N., DiMaggio, J.A., Kramer, W.M., Anderson, S.A., and Nichols, S.C., 2011. Implementation of LRFD Geotechnical Design for Bridge Foundations, FHWA-NHI-10-039. Federal Highway Administration, US Department of Transportation, Washington DC USA DEEP FOUNDATIONS • MAY/JUNE 2015 • 15 immediate loss of functionality (Kulicki et al., 2014). The definition of resistance and acceptable performance is somewhat subjective and may vary by agency. Summary The implementation of LRFD in the transportation community for design of foundations and substructures is complete with all highway agencies in the United States now using LRFD for design of substructures, foundations, walls and other structures. However, advancements and refinements are continually being made for several technologies to further improve LRFD for geotechnical design. Although the transition to this design platform has been difficult within the geotechnical engineering community, the AASHTO LRFD Bridge Design Specifications provide the most comprehensive resource for the design of foundations and substructures. Over the last 10 years, a significant amount of research has been committed to the development and improvement of the specifications, as well as FHWA guidance.
