The liquefaction potential index (LPI) was found to significantly overpredict the severity of observed liquefaction for a large subset of case histories compiled from Canterbury, New Zealand, earthquakes. One potential cause for these overpredictions is the presence of nonliquefiable capping and interbedded strata with high fines-content and/or plasticity that suppress surficial liquefaction manifestations. Herein, receiver-operating-characteristic analyses of compiled Canterbury, New Zealand, liquefaction case histories are used to investigate LPI performance as a function of the soil-behavior-type index averaged over the upper of 20 m (Ic20) of a profile; Ic20 is used to infer the amount of high fines-content, high-plasticity strata in a profile. It is shown that generally: (1) the relationship between computed LPI and the severity of surficial liquefaction manifestations is Ic20-dependent; and (2) the ability of LPI to segregate cases on the basis of observed manifestation severity using LPI decreases with increasing Ic20. In conjunction with previous studies, these findings support the need for an improved index that more adequately accounts for the mechanics of liquefaction triggering and manifestation.
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Surficial liquefaction manifestation severity thresholds for profiles having high fines-content, high-plasticity soils
The severity of surficial liquefaction manifestation was significantly over-predicted for a large subset of case histories from relatively recent earthquakes that impacted the Canterbury region of New Zealand. Such over-predicts generally occurred for profiles having predominantly high fines-content (FC), high-plasticity soil strata. Herein, the liquefaction case histories from the Canterbury earthquakes are used to investigate the performances of three different manifestation severity index (MSI) models. The prevalence of high FC, high-plasticity strata in a profile is quantified through the soil behavior type index averaged over the upper 10 m of a profile ( Ic10). It is shown that for each MSI model (1) the threshold MSI value distinguishing cases with and without manifestation increases as Ic10increases and (2) the ability of the MSI to segregate cases with and without manifestation decreases with increasing Ic10. Additionally, probabilistic models are proposed for evaluating the severity of surficial liquefaction manifestation as a function of MSI and Ic10. The approaches presented in this study allow better interpretations of predictions made by existing MSI models, although their efficacy decreases at sites with high Ic10. An improved MSI model is ultimately needed that better accounts for the effects of high-FC, high-plasticity soils more directly.
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- PAR ID:
- 10475655
- Publisher / Repository:
- Canadian Science Publishing
- Date Published:
- Journal Name:
- Canadian Geotechnical Journal
- Volume:
- 60
- Issue:
- 5
- ISSN:
- 0008-3674
- Page Range / eLocation ID:
- 642 to 653
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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L. Wang, J.-M. Zhang (Ed.)The severity of surface manifestation of liquefaction is commonly used as a proxy for liquefaction damage potential. As a result, manifestation severity index (MSI) models are more commonly being used in conjunction with simplified stress-based triggering models to predict liquefaction damage potential. This paper assesses the limitations of four MSI models. The different models have differing attributes that account for factors influencing the severity of surficial liquefactionmanifestations, with the newest of the proposed models accounting more factors than the others. The efficacies of these MSI models are evaluated using well-documented liquefaction case histories from Canterbury, New Zealand, with the deposits primarily comprising clean to non-plastic silty sands. It is found that the MSI models that explicitly account for the contractive/dilative tendencies of soil did not perform as well as the models that do not account for this tendency, opposite of what would be expected based on the mechanics of liquefaction manifestation. The likely reason for this is the double-counting of the dilative tendencies ofmedium-dense to dense soils by theseMSI models, since the liquefaction triggering model, to some extent, inherently accounts for such effects. This implies that development of mechanistically more rigorous MSI models that are used in conjunction with simplified triggering models will not necessarily result in improved liquefaction damage potential predictions and may result in less accurate predictions.more » « less
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Evaluations of Liquefaction Potential Index (LPI) in the 2010-2011 Canterbury earthquake sequence (CES) in New Zealand have shown that the severity of surficial liquefaction manifestations is significantly over-predicted for a large subset of sites. While the potential cause for such over-predictions has been generally identified as the presence of thick, non-liquefiable crusts and/or interbedded non-liquefiable layers in a soil profile, the severity of surficial liquefaction manifestations at sites that do not have such characteristics are also often significantly over-predicted, particularly for the Mw 6.2, February 2011 Christchurch earthquake. The over-predictions at this latter group of sites may be related to the peak ground accelerations (PGAs) used in the liquefaction triggering evaluations. In past studies, the PGAs at the case history sites were estimated using a procedure that is conditioned on the recorded PGAs at nearby strong motion stations (SMSs). Some of the soil profiles on which these SMSs were installed experienced severe liquefaction, often with an absence of surface manifestation, and the recorded PGAs are inferred to be associated with high-frequency dilation spikes after liquefaction was triggered. Herein the influence of using revised PGAs at these SMSs that are in accord with pre-liquefaction motions on the predicted severity of surficial liquefaction at nearby sites is investigated. It is shown that revising the PGAs improved these predictions, particularly at case history sites where the severity of the surface manifestations was previously over-predicted and could not be explained by other mechanisms.more » « less
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Evaluations of Liquefaction Potential Index (LPI) in the 2010-2011 Canterbury earthquake sequence (CES) in New Zealand have shown that the severity of surficial liquefaction manifestations is significantly over-predicted for a large subset of sites. While the potential cause for such over-predictions has been generally identified as the presence of thick, non-liquefiable crusts and/or interbedded non-liquefiable layers in a soil profile, the severity of surficial liquefaction manifestations at sites that do not have such characteristics are also often significantly over-predicted, particularly for the Mw 6.2, February 2011 Christchurch earthquake. The over-predictions at this latter group of sites may be related to the peak ground accelerations (PGAs) used in the liquefaction triggering evaluations. In past studies, the PGAs at the case history sites were estimated using a procedure that is conditioned on the recorded PGAs at nearby strong motion stations (SMSs). Some of the soil profiles on which these SMSs were installed experienced severe liquefaction, often with an absence of surface manifestation, and the recorded PGAs are inferred to be associated with high-frequency dilation spikes after liquefaction was triggered. Herein the influence of using revised PGAs at these SMSs that are in accord with pre-liquefaction motions on the predicted severity of surficial liquefaction at nearby sites is investigated. It is shown that revising the PGAs improved these predictions, particularly at case history sites where the severity of the surface manifestations was previously over-predicted and could not be explained by other mechanisms.more » « less
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