Abstract:
AASHTO LRFD Bridge Design Specifications (2020) allow the neutral plane method for determining downdrag forces; however, the only instruction on how to do so is a reference to NCHRP 393 (Briaud and Tucker, 1993). Much research and literature has been published on the topic of downdrag force considerations since NCHRP 393 (Briaud and Tucker, 1993). Downdrag force determinations, as described in FHWA-NHI-16-009 Design and Construction of Driven Pile Foundations (2016), and in conjunction with Virginia Department of Transportation (VDOT) design standards and AASHTO LRFD Bridge Design Specifications, were applied to driven 24-inch prestressed concrete square (PSCS) piles in Coastal Plain geology. Nominal cumulative skin friction and toe resistance were estimated using the Federal Highway Administration (FHWA) method. Load and resistance charts were prepared to estimate downdrag forces and neutral plane locations. Neutral plane locations were iteratively determined by applying the settlement at the neutral plane to the toe of the pile to determine the mobilized tip resistance for each iteration, and, finally, the iterations were stopped when the mobilized tip resistance and settlement at the pile toe agreed. Downdrag plots presented 0%, 50%, and 100% toe resistance curves; and, wherever applicable, a fourth curve was produced to show convergence. Factored downdrag forces were determined based on the maximum load factor for the Tomlinson Alpha method, 1.4, in AASHTO LRFD Bridge Design Specifications (2020). The FHWA Trend curve was used to correlate normalized toe movement to mobilized tip resistance. Settlement vs. depth profiles used to evaluate downdrag considered immediate settlement, primary consolidation, and secondary compression. This procedure could be refined in the future by considering mobilized skin friction using T-Z curves and comparing mobilized soil-pile displacements to the settlement profile along the length of the pile to better estimate the magnitudes of negative and positive side resistance.