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SKS02: Centrifuge Test of Liquefaction-Induced Downdrag in Uniform Liquefiable Deposit


Earthquake-induced liquefaction can cause soil settlement at pile interfaces, which can induce negative skin friction resulting in additional load (known as drag load) and drag the pile downwards (Figure 1). Despite significant research on the effects of liquefaction on structures and the seismic response of piles, there is still a knowledge gap in the evolution and assessment of liquefaction-induced downdrag on piles mainly related to the complex interplay and timing of the different mechanisms during/post liquefaction such as excess pore pressure generation/dissipation patterns, sequencing and timing of settlements, presence of interface gaps and ejecta, location of the initial neutral plane, and settlement around the tip. This has led to simplifying assumptions in current design procedures, which might result in over-conservatism in drag load estimation. Commonly used numerical tools lack the ability to model these mechanisms, while the absence of experimental data hinders the development and validation of new models. A series of centrifuge tests were planned to investigate the factors affecting the magnitude of liquefaction-induced drag load and pile settlement. This report describes the results for the first test series (SKS02). The soil profile included 1 m of coarse sand layer, underlain by 4 m of clay crust and 9 m of liquefiable soil over deeper dense soil. The test involved two medium diameter (D) piles, with their tip embedded to the depth of 0D and 5D in the dense sand. The model was shaken with multiple scaled Santa Cruz earthquake motions with peak horizontal accelerations ranging from 0.025 g to 0.4 g. With multiple shakings, drag loads were observed to increase on the piles. Higher drag loads were observed on deeply embedded (5D) piles as compared to the shallow embedded (0D) pile. While significant settlements occurred in soil during and post shaking, the piles recorded considerably smaller settlements. Most of the pile settlement occurred during shaking and very small settlements happened during the reconsolidation phase. It was observed, that with multiple shakings, the overall drag load on the piles saturated and could become as large as the one interpreted from considering the negative skin friction on the pile in the liquefiable soil taken equal to the positive interface drained shear strength. 

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