Design Memorandum



TO:                  All Design Section Staff

FROM:            Bijan Khaleghi
DATE:             June 18, 2008
SUBJECT:       DFSAP Program Limitation


The applicability of DFSAP program for liquefiable soils and pile foundation has recently raised concerns among Bridge Office and Consultant Designers. As a result, the Bridge Design Office and the Geotechnical Division have decided to temporarily place the following limitations on the use of DFSAP program:

  • DFSAP program shall not be used for Pile Foundations, with the possible exception of large diameter piles (e.g., 4 ft diameter or more), which will be handled on a case by case basis, depending on the length of the piles. The L-Pile program shall be used instead.

  • The liquefaction option of DFSAP program shall not be used for Shaft Foundations in liquefiable soils. It is still allowable to use DFSAP with liquefiable soils with the liquefaction option in DFSAP turned off. The Geotechnical Report provides the reduced (liquefied) soil parameters that can be used as input for the DFSAP program to evaluate foundation response in liquefied soils.

  • DFSAP Program may be used for long shafts, but the adequacy of the results shall be carefully checked.

Pile or shaft foundation group effect efficiency shall be taken as recommended in the project Geotechnical Report.


Background:

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.


If you have any questions regarding these issues, please contact Tony Allen at 709-5450, Patrick Clarke at 705-7220 or Bijan Khaleghi at 705-7181.

cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

       F. Posner, Bridge and Structures 47340