Physical modeling for LEAP-2017-2020 : assessment of liquefaction hazards for sloping ground and retaining sheet-pile quay walls

Korre, Evangelia
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Dobry, Ricardo
Bennett, Victoria
Manzari, Majid
Abdoun, Tarek
Zeghal, Mourad
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Civil engineering
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Seismically induced soil liquefaction can occur in saturated cohesionless soils, as a result of an earthquake and leads to significant or total loss of the soils strength. Soil conditions able to trigger liquefaction being rather common globally combined with the catastrophic consequences on buildings and infrastructure render the phenomenon one of the most severe earthquake-related hazards. Soil liquefaction is associated with failures in soil systems, such as slopes and embank- ments, and soil-structure systems, such as retaining walls and foundations, leading to large permanent soil deformations and extensive displacements of the structures, sometimes up to complete collapse. The mechanisms behind these failures have been a subject of research for many a number of decades. Geotechnical centrifuge testing and numerical tools have been employed in order to simulate the phenomenon and decipher the mechanisms leading to potential failure. Notwithstanding the valuable outcomes from these efforts, the challenge remained, since inconsistencies in the experimental results among different centrifuge facilities led in some cases to ambiguous conclusions and variability in the numerical results restricted the confidence in these tools to accurately predict the response. In 2013 the Liquefaction Experiments and Analyses Projects (LEAP) was undertaken by six centrifuge facilities (UC Davis, RPI, Cambridge University, Kyoto University, Na- tional Central University of Taiwan, and Zhejiang University), in an effort to investigate the response of a liquefiable sloping deposit. During that phase, in 2015 two centrifuge tests were performed at RPI, achieving repeatability with high fidelity. Four years later, LEAP-2017 built and expanded on the previous phase, in order to examine the repeatability and reproducibility potential of centrifuge testing in soil liquefaction and to investigate the effect of the variation of specific testing parameters on the system response. To that end the same model tests were repeated in nine centrifuge facilities in China, France, Japan, Korea, Taiwan, UK and USA. As part of the LEAP-2017 experimental campaign, three geotechnical centrifuge experiments were conducted at RPI, investigating the repeatability and reproducibility of centrifuge testing in soil liquefaction and shedding light on the sensitivity of the experimental results when varying the relative density or the input motion. The three models simulated a 5-degree sloping deposit, they were built consistently and employed consistent instrumen- tation. The reference test, RPI01, was built with 65% soil relative density and successfully reproduced the experimental results from 2015. The effect on the response of an additional high frequency component in the input motion was investigated in RPI02, which was also built with 65% soil relative density. The non-mono-frequency input motion led to lower dila- tive soil response and consequently to slightly higher lateral spreading. The effect of lower soil relative density on the response was examined in RPI03, which was built at 45% soil relative density and was subjected to the same input motion as RPI01. The looser deposit was found to be significantly more susceptible to liquefaction leading to higher permanent surficial displacements. In 2018 the LEAP-Asia investigated the effect of the generalized scaling law on the response of the liquefiable sloping deposit examined in the previous exercises. As part of this exercise, one experiment was performed at RPI (Model B) and was compared against RPI01 (Model A). The response of the model designed observing the generalized scaling law was in remarkable agreement with RPI01, prior to liquefaction. After the onset of liquefaction, Model B exhibited higher propensity for liquefaction and accumulated higher lateral surficial soil deformations. Soil-structure interaction (SSI) was introduced in the investigation of soil liquefaction in the LEAP-2020. The examined experimental set-up simulated a 3-m deep excavation of a saturated deposit of 5m total depth. The excavation was retained by a rigid floating sheet- pile quay wall, which was embedded in very dense sand. Six experiments were performed at RPI as part of the experimental campaign for LEAP-2020. RPI05 and RPI06 were built with 65% soil relative density and showed successful repeatability of the experimental results in the presence of SSI. The sensitivity of the experimental results in the initial orientation of the sheet-pile was investigated in RPI07. The installation of the sheet-pile with outward 2-degree rotation introduced a bias in its response leading to rapid accumulation of seaward displacements. The effect of relative density on the systems response was examined in RPI08 (built with Dr = 55%) and RPI09 (built with Dr = 75%), showing dramatic increase in the displacements of the soil and the sheet-pile in the case of the loose deposit. The response of the dense deposit was similar to that of RPI06, exhibiting slightly lower values of soil and the sheet-pile displacements. Last but not least, the effect on response of the system by an additional high frequency component in the input motion was investigated in RPI10. In consistency with the results from the slope, the non-mono-frequency input motion led to lower dilative soil response but to slightly lower lateral spreading, in view of the SSI effects.
August 2020
School of Engineering
Dept. of Civil and Environmental Engineering
Rensselaer Polytechnic Institute, Troy, NY
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