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Publication Number:  FHWA-HRT-17-060     Date:  August 2017
Publication Number: FHWA-HRT-17-060
Date: August 2017


Liquefaction-Induced Downdrag on Continuous Flight Auger (CFA) Piles From Full-Scale Tests Using Blast Liquefaction

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FHWA Publication No. FHWA-HRT-17-060

FHWA Contact: Jennifer Nicks, HRDI-40, (202) 493-3075, jennifer.nicks@dot.gov


Deep foundations typically support dead and live loads through a combination of positive skin friction acting along the sides of the pile and end-bearing at the toe of the pile; however, during seismic events, negative skin friction (or downdrag) can potentially develop. Downdrag occurs because of increases in effective stress caused by pore pressure dissipation and settlement by the liquefiable soil layer(s) relative to the pile (figure 1). This phenomenon creates a dragload that the pile must support in addition to its permanent service pile head load (Qτ). As illustrated in figure 1, the depth of the dragload extends to the neutral plane. The neutral plane is the depth where the settlement of the pile equals the liquefaction settlement of the soil and where the axial load in the pile is the greatest. The elevation of the neutral plane is found by trial and error such that the service load plus negative friction should equal the positive friction plus end-bearing resistance.

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