Earthquake-induced soil liquefaction can cause settlement around piles, which can translate to negative skin friction and the development of drag load and settlement of the piles. A series of centrifuge model tests were performed to assess liquefaction-induced downdrag and understand the interplay and effects of (1) pile embedment and pile-head load, (2) excess pore pressure generation and dissipation, and (3) reconsolidation and ground settlement on pile response during and postshaking. The model included a layered soil profile (clay, liquefiable sand, and dense sand) with two 635-mm-diameter instrumented piles. One pile was placed with its tip at the bottom of the liquefiable deposit; the other pile was embedded five diameters into the dense sand layer. The model was shaken with multiple earthquake motions with their peak horizontal accelerations ranging from 0.025 to 0.4 g. For each shaking event, the drag load on the piles first decreased during shaking and then increased during reconsolidation, exceeding its preshaking value. With multiple shaking events, the net drag load on the piles increased. The maximum observed drag load was found equal to the drained interface shear strength calculated from the interface friction angle of δ ¼ 30°and a lateral stress coefficient of K ¼ 1. Larger drag loads and smaller settlements were observed for the pile embedded deep in the dense sand layer. Most of the pile settlements occurred during shaking; postshaking pile settlement was less than 2% of the pile's diameter. The mechanisms behind the development of liquefaction-induced drag load on piles and settlements are described. Select ramifications concerning the design of piles in liquefiable soils are also described.

Earthquake-induced soil liquefaction can cause soil settlement around piles, resulting in drag load and pile settlement after shaking stops. Estimating the axial load distribution and pile settlement is important for designing and evaluating the performance of axially loaded piles in liquefiable soils. Commonly used neutral plane solution methods model the liquefiable layer as an equivalent consolidating clay layer without considering the sequencing and pattern of excess pore pressure dissipation and soil settlement. Moreover, changes in the pile shaft and the tip resistance due to excess pore pressures are ignored. A TzQzLiq numerical model was developed using the existing TzLiq material and the new QzLiq material for modeling liquefaction-induced downdrag on piles. The model accounts for the change in the pile's shaft and tip capacity as free-field excess pore pressures develop or dissipate in soil. The developed numerical model was validated against data from a series of large centrifuge model tests, and the procedure for obtaining the necessary information and data from those is described. Additionally, a sensitivity study on TzLiq and QzLiq material properties was performed to study their effect on the developed drag load and pile settlement. Analysis results show that the proposed numerical model can reasonably predict the time histories of axial load distribution and settlement of axially loaded piles in liquefiable soils both during and postshaking.

No abstract