The applicability of DFSAP program for liquefiable soils 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:
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 However, using the convex (i.e., strain softening shape inherent in static p-y curves reduced to a liquefied strength using a p-multiplier approach (DFSAP and L-Pile) may possibly result in an under-prediction of foundation deformation depending on the magnitude of the foundation lateral loads. After a joint meeting between the Bridge Design Office and the Geotechnical Division, it has been decided to use a simplified elastic-plastic liquefied p-y curve that represents a compromise between the strain softening and strain hardening curve shapes used to represent liquefied p-y behavior. This curve shape will avoid the computational difficulties currently caused by other curve shapes, and is also sufficiently accurate considering the lack of knowledge the engineering community has on how to characterize the liquefied behavior of soil for lateral load analysis. At present, DFSAP does not have the ability to use an elastic-plastic p-y curve, whereas L-Pile does have this ability. Furthermore, it has been decided to place limitations on the use of the DFSAP program regarding liquefied soils until further notice. Meanwhile the Bridge Design Office and the Geotechnical Division will be communicating with the developers of the DFSAP 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