Extensive damage to buildings and infrastructure observed during past earthquakes resulting from the liquefaction of shallow saturated soil deposits underneath structures has demonstrated the necessity for further research in the area of liquefaction-induced ground movement effects. This study explores utilizing helical piles as a countermeasure to reduce liquefaction-induced foundation settlement and investigates their seismic performance in liquefiable grounds. Two large-scale shake table test series, one without any mitigation measures and one using helical piles, were conducted using the shake table facility at the University of California, San Diego. During each test series, the soil and superstructure models were extensively instrumented and subjected to two consistently applied shaking sequences. The model ground included a shallow liquefiable layer aimed at replicating the subsurface ground conditions observed in the past earthquakes in New Zealand, Japan, and Turkey.
Liquefaction-induced foundation settlement mechanisms are broadly categorized as follows: (1) shear-induced, (2) volumetric-induced, and (3) ejecta-induced. In the first test series (referred to as the Baseline test hereafter), all these three components were realistically reproduced, while in the second test series (referred to as the Helical Pile test hereafter) the volumetric and ejecta-induced mechanisms were mainly mitigated, resulting in significant reductions in the foundation settlement.
Results from the first test series (i.e., Baseline test) indicated that the flow velocity due to the hydraulic transient gradient displayed an upward flow in the loose layer, which explains the observed sand ejecta. This series of shake table tests resulted in an average total foundation settlement of 28 cm and 42.7 cm during two shaking sequences. The measured foundation settlements were compared to the estimated foundation settlement obtained from Liu and Dobry [1997] and Bray and Macedo’s [2017] simplified procedures. The observed foundation settlements generally were higher than the estimated values. In the second large-scale test series, an identical test setup to the first test series was used except for a group of four helical piles were attached to the shallow foundation to mitigate liquefaction-induced settlements. In this series of tests, a reduced excess pore-water pressure generation around the group of helical piles was observed and is mainly attributed to the increased relative density around their zone of influence as a result of installation. The foundation supported on helical piles underwent almost no differential settlement and tilt. A significant reduction in the total foundation settlement was achieved during the Helical Pile test series compared to the Baseline test series.
This shake table project is the first experimental study that reproduced all the key mechanisms mentioned above including the effects of sediment ejecta, which have not been captured in prior experimental studies.
In addition, this series of large-scale shake table tests provides a unique benchmark for the calibration of numerical models and simplified procedures to reliably estimate liquefaction-induced building settlements. Although this study introduced helical piles as a reliable and highly efficient measure to mitigate liquefaction-induced foundation tilt and settlement, the proper design nd application of helical piles in seismic areas still need thorough investigation due to possible amplified superstructure response.
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