Implementation of the AASHTO LRFD Code in Geotechnical Design-Piles

The Load and Resistance Factor Design (LRFD) of the American Association of Highway and Transportation Officials ( AASHTO)(2007) includes substantial changes compared to the AASHTO Standard Specifications (2002). Therefore, State Departments of Transportation have to review their current practice and, if needed, develop a new approach that is consistent with the LRFD philosophy. Load and resistance factors in LRFD code were determined in the reliability-based calibration process. However, this calibration was limited to the superstructure. The resistance factors for the piles were mostly based on engineering judgment. Therefore, there is a need for verification of a pile design provisions. The objectives of this study were to review the available pile capacity analysis methods and develop a user-friendly and possibly more accurate formula. In addition, a procedure was developed for comparison of the pile design and the calculation of the resistance factors. The following methods of determining the load carrying capacity of a pile were compared: Static Load Test, Case Pile Wave Analysis Program (CAPWAP), Wave equation, Gate formula, and Nebraska modified Engineering-News (ENR) formula. A formula has been proposed for the calculation of the pile capacity that is in good agreement with the CAPWAP results. The resistance factor has been calculated based on the assumption that the LRFD AASHTO Code and the AASHTO Standard Specifications result in the same design of a pile. The calculations were carried out for a representative number of design situations (pile material, pile type and soil conditions).