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Experimental Evaluation of Helical Piles for Underpinning Shallow Foundations on Soils Susceptible to Liquefaction using Shake Table Tests
AuthorJahed Orang, Milad
Civil and Environmental Engineering
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The severe damages observed during past earthquakes resulting from the liquefaction of shallow saturated soil deposits underneath structures have demonstrated the necessity for further research in the area of liquefaction-induced ground movement effects. This research explores the utilization of helical piles to reduce liquefaction-induced foundation settlement and investigates their seismic performance in liquefiable grounds. Twenty-two shake table tests were conducted to examine the dynamic behavior of helical piles and their efficiency in liquefiable grounds. Among these tests, two 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 (UCSD). The remaining shake table tests were conducted using the scaled shake table facility at the University of Nevada, Reno (UNR). During large-scale test series at UCSD, the soil and structural components 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 past earthquakes in New Zealand, Japan, and Turkey. Results from the first test series of large-scale tests (i.e., without mitigation) 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 aimed at reproducing the potential damage during liquefaction of shallow liquefiable deposits. As a result, the average foundation settlement in Shake 1 and Shake 2 were measured to be 28 cm and 42.7 cm, respectively. Measured foundation settlements were compared to the estimated foundation settlement obtained from Liu and Dobry  and Bray and Macedo’s  simplified procedures. The observed foundation settlement generally was higher than the estimated settlement. In the second large-scale test series, reduced excess pore-water pressure generation around the group of helical piles 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. Moreover, a significant reduction took place during the Helical Pile test compared to the Baseline test (i.e., 96% reduction on average). Liquefaction-induced settlement mechanisms are categorized as 1- shear-induced, 2- volumetric-induced, and 3- ejecta-induced. The post-shaking liquefaction-induced settlement mechanisms (i.e., volumetric and ejecta-induced mechanisms) did not affect the foundation settlement supported by helical piles. This series of large-scale shake table tests delivered a unique benchmark for 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 and application of helical piles in seismic areas still need thorough investigation due to possible amplified superstructure response. In the scaled shake table test series at UNR, multiple shakings were applied during each test series to evaluate the seismic behavior of the scaled helical piles and the slender shaft, taking into account various response parameters. These scaled shake table tests provided the opportunity to perform parametric studies on the effects of ground motion amplitude, liquefiable layer densification, superstructure weight, and the number of helices on the helical piles. Considerable ground settlements were measured during the first shaking in each test series, however negligible helical pile and slender shaft settlements were observed during all tests. The bending moment variation showed a similar trend along the depth for the helical piles and the slender shaft: the maximum moment was consistently observed at the boundary between dense and liquefiable layers. The observed bending moments along the depth increased with increases in input motion amplitude and superstructure weight. Densification of the liquefiable layer during different test series reduced the maximum bending moment along the depth for each pile due to increased relative density. Increasing the number of helices improved the dynamic performance of the helical piles compared to the slender shaft such as maximum bending moment, maximum horizontal displacement, residual horizontal displacement, and superstructure acceleration in different ground conditions.