UC Davis

Pile foundations are designed to transfer superstructure loads through positive skin friction andtip resistance while undergoing acceptable settlements. However, when liquefaction-induced soil settlement occurs, it causes drag load and settlement in piles. For such cases, estimating the axial load distribution and pile settlement becomes an important criterion for designing and evaluating the performance of piles in liquefiable soils. Most of the challenges related to the liquefaction-induced downdrag phenomenon are the incomplete understanding of the different mechanisms that affect drag load and pile settlement. The interrelationships between mechanisms affecting negative skin friction (pore pressure generation and dissipation patterns, the sequencing of settlements and reconsolidation of liquefied soils, as well as gapping and softening of soils around the piles) are currently not accounted for in current practice, leading to over-conservative or unsafely designed piles. This dissertation describes the liquefaction-induced downdrag mechanisms through a series of centrifuge model tests, the development of a numerical modeling approach incorporating the observed mechanism and proposes a displacement-based design procedure for designing axially loaded piles subject to seismic loading and liquefaction-induced downdrag. The redistribution of high pore pressures from liquefiable to adjacent non-liquefiable deposits impacted pile performance significantly. Therefore, a procedure for estimating the redistribution of excess pore pressure is also studied