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PILE DRAG LOAD AND DOWNDRAG IN A LIQUEFACTION EVENT (2008)

Sandy soil layers may undergo compression during liquefaction. Areview of published design manuals, including the 2004AASHTO LRFD Bridge Design Specifications, indicates that some recommendations for pile design may not represent the pile response in a manner consistent with the actual axial response of the pile during liquefaction. The actual response is discussed in light of the unified pile design method and separated between liquefaction occurring above and below the static nonliquefied neutral plane location before the liquefaction event. In the former case, the effect on the pile is minor regardless of the magnitude of liquefaction-induced settlement of the surrounding soil. In the latter case, the axial compressive load in the pile increases and additional pile settlement (downdrag) will occur when the force equilibrium is reestablished through the necessary mobilization of additional toe resistance. This means that the magnitude of the downdrag is governed by the pile toe load-movement response to the downward shift of the neutral plane. While there is a reduction in shaft resistance due to the reduction in strength within the liquefied layers, this reduction will only influence the pile design length where the liquefying layer is very thick.

Reference:
Journal of Geotechnical and Geoenvironmental Engineering, Vol. 134, No. 9, September 1, 2008. ©ASCE, ISSN 1090-0241/2008/9-1412-1416
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Bengt Fellenius Consultants Inc
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