NCHRP 24-17 [Completed]
LRFD Deep Foundation Design
| Project Data
||University of Massachusetts-Lowell|
||Samuel G. Paikowsky|
NCHRP Report 507: Load and Resistance Factor Design (LRFD) for Deep Foundations, contains the findings of a study to develop resistance factors for driven pile and drilled shaft foundations. These factors are recommended for inclusion in Section 10 of the AASHTO LRFD Bridge Design Specifications to reflect current best practice in geotechnical design and construction. The report also provides a detailed procedure for calibrating deep foundation resistance. Extensive appendices providing detailed information on the development and application of the resistance factors are included on NCHRP CD-39 bound with the report. The appendices are also available as a combined PDF (46 MB) or as individual PDFs below:
Full implementation of the AASHTO LRFD Bridge Design Specifications for deep foundations is hampered by provisions that were based on limited information and are inconsistent with current geotechnical engineering practice. Static pile-capacity analyses are typically used in design to estimate required pile lengths and quantities, whereas dynamic analyses and static load tests are used to determine pile capacity during construction. Currently, the resistance factors for static and dynamic analysis are multiplied by each other, resulting in designs that are significantly more conservative than used in past practice, increasing foundation costs.
Resistance factors for drilled shafts in sand or gravel are not provided in the LRFD Specifications, and the effect of various construction techniques on drilled shaft capacity is not addressed. Many of the state departments of transportation do not have the data or the resources to develop their own calibrations as recommended in the specification.
The resistance factors for deep foundations were not calibrated for the load factors incorporated in the LRFD Specifications. In addition, the resistance factors do not account for the variability of the site conditions and the number of load tests conducted. Another shortcoming is that many accepted design procedures, some of which are commonly recommended by FHWA, are not supported by the LRFD Specifications.
The objective of this research was to address the aforementioned issues and to provide resistance factors for the load and resistance factor design of deep foundations. As part of the effort, the research team assembled databases for static analysis of drilled shafts and driven piles and for dynamic analysis of driven piles. These databases were used for the statistical evaluation of resistance factors.