Liquefaction-Induced Pile Downdrag from Full-Scale Testing

Abstract

AbstractFrequently, deep foundations extend through liquefiable sand layers and bear on more competent layers at depth. When liquefaction occurs, the skin friction in the liquefied layer would be expected to decrease to near zero; however, as pore pressure dissipates and the liquefied layer settles, negative skin friction can develop along the pile. This paper summarizes full-scale pile downdrag testing at four sites where liquefaction was induced by small explosive charges. Test were performed on driven piles, bored piles, augercast piles and micropiles. Despite the variation in pile type, observed performance was similar at all sites. Negative friction in the liquefied layers was between 40 and 55% of the positive friction before liquefaction as the sand reconsolidated. Negative friction in the non-liquefied layers was about the same as the original positive friction. Following liquefaction, the ground around the piles settled between 7.5 and 27 cm; however, pile settlement was typically small and was most dependent on the end-bearing resistance. Pile settlement and load in the pile were generally consistent with the neutral plane method. Steps necessary to estimate the neutral plane and pile settlement are summarized in the paper.KeywordsPilesDowndragNegative frictionLiquefactionBlast testing