Practices for Local Calibration of LRFD Geotechnical Resistance Factors (2023)

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49   Summary of Findings This synthesis documents the use of agency-specific geotechnical design methods and resis- tance factors among U.S. state DOTs, including the prevalence of such provisions, the details of their development, and the benefits ascribed to their implementation. The synthesis consists of a literature review, a survey of state DOTs, and case examples of select DOTs. This chapter summarizes the most notable findings and presents recommendations for future research. Summary of Major Findings At least 21 U.S. state DOTs—just more than half of the survey respondents—have implemented agency-specific geotechnical design methods or resistance factors. Among the 21 state DOTs, a total of 35 different agency-specific methods or resistance factors have been developed, with several state DOTs having developed provisions for multiple geotechnical elements. Fourteen of the 35 address driven piles, 7 address drilled shafts, and 7 address spread footings; the rest address other less-common elements. Of the 35, just less than half (16 of 35, or 46%) were based on probabilistic calibration, with the rest mostly fitting to historical practices. Results of the survey and case example interviews indicate the state DOTs with agency-specific provisions have, in fact, realized the cost savings promoted by FHWA in its 2011 encourage- ment of agency-specific provisions. Most of the 21 respondents cited cost savings as an observed benefit of the agency-specific provisions. Nearly all the agency-specific provisions reviewed for the literature review and case examples involved resistance factors greater than those included in AASHTO’s LRFD Specifications. Presumably, the increased resistance factors are associated with cost savings. Most state DOTs have not quantified the cost savings, but there are two notable exceptions: Missouri DOT estimated its drilled shaft design methodology saved $40,000 for every “routine” bridge and considerably more for larger bridges; Louisiana DOTD estimated annualized savings of at least $200,000 from agency-specific driven pile provisions. In addition to cost savings, agency-specific provisions have been used to improve design practices. Many of these improvements are the result of tailoring provisions to local practices (e.g., Minnesota DOT’s pile driving formula) or geology (e.g., Missouri DOT’s design method for drilled shafts in shale). However, additional design-practice improvements have been the result of technical improvements that could be implemented anywhere (e.g., Washington State DOT’s stiffness method for analysis of MSE wall internal stability, which was subsequently adopted by AASHTO; and Missouri DOT’s use of resistance factors that depend on the variability of important design parameters). Also, cost savings and design improvements are often closely related. For example, among several state DOTs that developed agency-specific practices for use of pile driving formulas, Minnesota DOT’s implementation was most carefully tailored to agency pile driving practices; it also yielded the greatest resistance factor. C H A P T E R 5

50 Practices for Local Calibration of LRFD Geotechnical Resistance Factors The survey results indicate several possible impediments to state DOTs developing agency- specific methods and resistance factors. Of the 20 respondents, the 80% who have not implemented agency-specific methods indicated one of the reasons for not doing so was that AASHTO methods are sufficient for agency design needs. However, cost and time constraints and insufficient expertise were also cited commonly, by 70% and 50% of the 20 respondents, respectively. Information Gaps and Suggestions for Future Research The results of this synthesis demonstrate that implementation of agency-specific geotechnical design methods and resistance factors can result in significant benefits, primarily cost savings and improved design practices. This synthesis also revealed that quantifying the benefits is rare and challenging; closing this information gap could potentially motivate additional state DOTs to develop agency-specific methods. In addition, improvements in ways to develop agency- specific methods and perform calibrations could be useful, as reported by case example DOTs. Specific research ideas are presented below: • Cost benefits are difficult to quantify because once an agency implements an agency-specific method, it ceases to design structures using the old method. Relatively simple research to develop direct cost comparisons would be useful. Direct comparisons could be developed by redesigning an existing foundation or retaining wall designed according to AASHTO LRFD Specifications with an agency’s new agency-specific provisions or by redesigning a post-agency- specific provision project according to AASHTO LRFD Specifications. Cost estimates of the redesigned foundation or retaining wall using as-constructed unit prices would provide definitive data regarding cost benefits. • Several case example DOTs indicated they intend to continue developing agency-specific methods and resistance factors and to update their existing agency-specific methods, but they stated that collecting and tracking the data for such efforts is difficult. Research to identify best practices for development and maintenance of agency databases of foundation load tests and potentially other performance data would help state DOTs develop agency-specific methods. Best practices would likely include identification of the most important information to include in the database (e.g., foundation installation records). • Along the same lines as the previous ideas, use of pooled-fund studies to develop “region- specific” (compared to agency-specific) design methods and resistance factors might also reduce the burden and expense of data collection and tracking. Such pooled fund studies would likely be most effective in regions with shared geologic and construction characteristics. • Case example DOTs also indicated a desire to update agency-specific methods and resis- tance factors. While there are abundant resources documenting how to perform probabilistic calibrations, including several cited in Chapter 2, no examples of an agency updating an existing LRFD method were encountered. To that end, research to demonstrate updating of agency- specific methods and probabilistic calibrations would be useful. Use of several different methods for updating (e.g., a “from scratch” redo, Bayesian updating, and so forth) would provide inter- esting and potentially useful technical insights. • The survey results indicated that databases of subsurface investigation information, load test results, and performance-monitoring information are relatively common. The databases are a valuable resource that could provide useful data for future geotechnical research, including development of new agency-specific design methods and resistance factors, as well as efforts to validate the agency-specific design methods and resistance factors that have been developed. The database information could also be useful for developing design methods for the design conditions that agencies identified through the survey as providing potential benefit, as seen in Figure 23.