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NCHRP Project 24-17 was aimed at rewriting AASHTOâs Deep Foundation Speci- fications. The AASHTO specifications are traditionally observed on all federally aided projects and generally viewed as a national code of U.S. highway practice; hence they influence the construction of all the deep foundations of highway bridges throughout the United States. This report presents the results of the studies and analyses conducted for that project. The development of load and resistance factors for deep foundations design is pre- sented. The existing AASHTO specifications, similar to others worldwide, are based on Load and Resistance Factor Design (LRFD) principles. The presented research is the first, however, to use reliability-based calibration-utilizing databases. Large data- bases containing case histories of piles tested to failure were compiled and analyzed. The state of the art was examined via a literature review of design methodologies, LRFD principles, and deep foundation codes. The state of the practice was estab- lished via a questionnaire, distributed to and gathered from state and federal trans- portation officials. Large databases were gathered and provided. Analyses of the data, guided by the state of practice led to findings detailing the performance of vari- ous static and dynamic analyses methods when compared to recorded pile perfor- mance. Static capacity evaluation methods used in common design practices were found overall to over-predict the observed pile capacities. Common dynamic capacity evaluation methods used for quality control were found overall to under-predict the observed pile capacities. Both findings demonstrate the shortcoming of safety para- meter evaluation based on absolute values (i.e., resistance factors or factors of safety) and the need for an efficiency parameter to allow for an objective measure to assess the performance of methods of analysis. The parameters that control the accuracy of the predictions were researched and ana- lyzed for the dynamic methods. A set of controlling parameters was established to allow calibration of the prediction methods. Target reliability magnitudes were researched and values were recommended con- sidering the action of piles in a redundant or non-redundant form. Statistical analyses compatible with common practice in the structural area were utilized for the develop- ment of LRFD resistance factors. Parameters that control the size of a testing sample SUMMARY LOAD AND RESISTANCE FACTOR DESIGN (LRFD) FOR DEEP FOUNDATIONS
and site variability were researched and incorporated. Recommended design parame- ters offering a consistent reliability in design were then presented and discussed. The need for the modification of LRFD for use in geotechnical applications through knowledge-based parameters accounting for subsurface variability, quality of soil pa- rameters estimation, and previous experience as well as amount and type of testing dur- ing construction is presented. 2
