College of Engineering

Earthquake shaking can cause significant soil settlements, especially if the shaking causes liquefaction. Soil settlements will induce drag loads that can significantly increase the axial loads in a pile foundation and/or cause significant pile settlement (Figure 1). The liquefaction-induced downdrag on piles is affected by the complex interplay and timing of a variety of processes including the development and dissipation of pore water pressures, soil settlement, sand boils and gaps that provide vents for high excess pore pressures. Since it has not been possible to accurately model all these complex processes, simplifying assumptions are used to account for downdrag in the current design procedures. A series of centrifuge tests were designed to investigate the complex processes and the validity of the simplifying assumptions. This report describes the details of the second (SKS03) of the two model tests performed under this project. Sinha et al. (2021b)describes the previous centrifuge test series (SKS02). In SKS03, the soil profile consisted of (from top to bottom in prototype dimensions) 1 m of coarse sand, a 2 m clay crust, about 4.7 m of loose sand, 1.3 m of silt, 4 m of medium dense sand and 8 m of dense sand. Three 635 mm diameter piles were embedded about 15 m into the deposit, with their tips embedded about 1.9 m into the deeper dense sand. The three piles were loaded by lumped masses clamped just above the pile head; the static loads were different on each pile (500 kN, 1500 kN, and 2400 kN). The piles were instrumented with several strain gauge bridges designed to measure the axial load distribution in the piles. The base of the model was shaken with multiple earthquake ground motions with peak horizontal accelerations ranging from 0.08 g to 0.61 g. In addition to earthquake shaking, a pile load test was performed on one of the piles.

As in SKS02, drag loads were observed to increase from earthquake shaking. Most of the pile settlement occurred during shaking, and very minimal settlement happened post shaking. Among all piles, the heavily loaded piles suffered the most settlement. Higher drag loads were observed on lightly loaded piles as compared to the heavily loaded piles. As expected, the neutral plan was found to be relatively deep for the lightly loaded pile and shallow for the heavily loaded pile.