Pile or shaft foundation group effect efficiency shall be taken as recommended in the project Geotechnical Report.
The adequacy of the DFSAP program for all foundation types and all soils profiles has been recently questioned by the Bridge Office and Consultant engineers. When the liquefaction option is selected in DFSAP, bridge designers have had difficulty getting the program to converge on a solution. Comparison of the P-Y curves generated by DFSAP when the liquefaction option in the program is used to other methods of generating P-Y curves (e.g., L-Pile) appears to indicate that the liquefied DFSAP P-Y curves are overly conservative. The WSDOT State Geotechnical Engineer has also discovered a potential detrimental flaw in DFSAP modeling of liquefiable soils, based on discussions he has had with national experts and Caltrans (who paid for the research used to develop the liquefaction model for strain wedge theory), and review of the published literature. After a joint meeting between the Bridge Design Office and the Geotechnical Division, it has been decided to place limitations on the use of the DFSAP program regarding liquefied soils, and its extrapolation to relatively flexible deep foundation types such as piles, until further notice. Meanwhile the Bridge Design Office and the Geotechnical Division will be communicating with the developers of the program to address these issues and to make necessary revisions to the program, if feasible.
The intent of this memorandum is not to revert to former practice of using L-Pile program for all deep foundations. This is intended as a temporary measure until the issues with the DFSAP program are completely resolved and all the modeling problems are taken away. Upon resolution of these concerns, the DFSAP program then will be returned to use as the recommended program for deep foundation analysis as appropriate.
cc: Mohammad Sheikhizadeh, Bridge Construction - 47354