Long-term consolidation, seismic dry sand and liquefaction-induced settlements of a soil deposit can impart significant amount of downward load to piles. This load, called downdrag or negative skin friction, is caused by the friction between the pile and the downward moving soil.
Per AASHTO LRFD Bridge Design guidelines, downdrag force should be determined as follows:
- Establish representative soil profile as well as soil properties for the project site in hand,
- Perform settlement analysis for the soil profile and determine consolidation or seismic settlement profile along the pile shaft,
- Determine the length of pile that will be subjected to downdrag force. Per AASHTO guidelines, downdrag can be assumed to fully develop for the length where the liquefaction-induced settlement is 0.4 inch or greater relative to the pile shaft,
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Determine the downdrag force by computing negative skin friction using any of the static analysis procedures.
AASHTO LRFD Bridge Design guidelines (2012) provides independent analysis procedures for calculation of negative skin friction for driven piles and drilled shafts in sections 10.7 and 10.8, respectively. I incorporated these procedures into Excel VBA macro to facilitate the implementation of the required procedures. The macros calculate; (i) downdrag force, (ii) pile elastic compression under downdrag, (iii) ultimate positive skin friction, and (iv) ultimate end bearing capacity.
You can download the Excel Spreadsheet for analysis of drilled shafts here:
Downdrag_Drilled-Shafts_AASHTO.xlsm
(last updated 05/04/2018)
You can download the Excel Spreadsheet for analysis of driven piles here:
Downdrag_Driven-Piles_AASHTO.xlsm
Note that prior to using the spreadsheets you have to complete steps #1 through #3, and input the data in the tab “Per AASHTO”. Please read the “Notes” carefully.
Write comments if you have any questions.