Proposed AASHTO Specifications for Design of Piles for Downdrag
Brigham Young University
Dr. Kyle Rollins
Further work will be conducted under NCHRP 12-116A in 2020
The AASHTO LRFD Bridge Design Specifications recognize that piles in settling soil experience downdrag forces from negative skin friction and may also move downward with the adjacent soil. Many bridges are constructed in areas where there is compressible soil or liquefiable soil, which necessitates the consideration of downdrag forces and associated settlement. Downdrag for static conditions as well as seismic conditions has increasingly placed greater demands on existing and new foundations and also led to higher construction costs. Therefore, there is an immediate need to review the fundamental behavior of piles subject to downdrag and propose design specifications for the implementation of the neutral plane method for the evaluation of downdrag and the static and seismic design of piles.
The objectives of this research were to (1) propose modifications to the AASHTO LRFD Bridge Design Specifications for the static and seismic design of piles for downdrag and (2) develop design examples to demonstrate the application of the proposed design specifications.