Application of LRFD Geotechnical Principles for Pile Supported Bridges in Oregon: Phase 1

Abstract

Bridge foundations must be designed based on acceptable risks of failure. To secure rapid implementation of Load and Resistance Factor Design (LRFD) principles for foundation design, the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA) are requiring their use through AASHTO code. The Bridge Section of the Oregon Department of Transportation (ODOT) has responsibility for satisfactory design of all the bridge structures across the state’s highway system. The widespread geotechnical adoption of the LRFD code throughout state DOTs has been difficult in the case of deep foundations due to regional differences and in some cases a lack of any close match to DOT foundation practices. This lack of matching stems from the source research conducted on which the code is based, documented as NCHRP 507. For ODOT, the evaluation of nominal axial static capacity for each driven pile in the field is conducted by dynamic methods and AASHTO offers resistance factors for these techniques. ODOT typically uses the wave equation software (WEAP) applied at the end of initial driving (EOID) and occasionally at the beginning of pile restrike (BOR) to capture increases in capacity from set-up. This study reports that, based on past and new surveys, ODOT practice is reasonably typical for DOT practice in sands, silts, and clays. The AASHTO resistance factor, φ, for WEAP is at EOID and is too low for the efficient design of piles to match the likely probabilities of pile failure. The survey of Northwest state DOTs revealed that 80% of the DOTs believe that a φ of 0.4 is conservative and 37.5% do not use the AASHTO-sanctioned φ of 0.4. Matching LRFD to allowable stress design (ASD) by direct calibration for a single pile, without any reported capacity bias, sets φ as 0.55 to match the ASD factor of safety of 2.5. An ODOT case history of a recently completed pile-supported bridge designed and constructed to FHWA and AASHTO ASD standards in use at that time, shows the number of piles at the bent studied would be doubled under new AASHTO requirements. This suggests the standard will add considerable pile foundation costs to all new bridges. This cost increase is a strong incentive to complete statistical recalibration of GRLWEAP dynamic capacity resistance value in a Phase 2 of this study.