SPECIAL FEATURE Comparisons Between Deterministic and Probabilistic Liquefaction Assessment Approaches Over the Christchurch Area

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Published 01 December 2016
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SPECIAL FEATURE Comparisons Between Deterministic and Probabilistic Liquefaction Assessment Approaches Over the Christchurch Area

Liquefaction assessments are commonly undertaken by geotechnical engineers using a deterministic approach. This approach does not appropriately take into account the significant uncertainties associated with a liquefaction assessment and can potentially compound the conservatism that is introduced when selecting upper bound input parameters. Therefore, the deterministic assessment approach can be conservative and the expected performance poorly understood. This research project looks at an alternative approach for liquefaction assessments – a probabilistic assessment.

Because liquefaction consequences are considered at discrete levels of ground shaking, discrete groundwater levels and a single percentile of the triggering correlation, it is not possible to quantify the actual likelihood of a specific level of liquefaction consequence being exceeded. In order to overcome these problems, it is necessary to consider the uncertainty in the liquefaction triggering correlations, the seasonal variation in groundwater levels as well as the ground motions hazard curve (which defines the likelihood of certain levels of ground shaking). This is achieved by undertaking a full probabilistic assessment which incorporate the uncertainty for all these variables.

Figure 1 presents an example of a liquefaction assessment using the Liquefaction Severity Number (LSN) for the three CPT using the deterministic approach (with upper bound input parameters) and the probabilistic approach. The differences between the deterministic LSN value and the probabilistic LSN distribution for the three CPT are noteworthy. For CPT A, the deterministic LSN value represents the 87th percentile of the probabilistic distribution, for CPT B, the deterministic LSN value represents the 93th percentile of the probabilistic distribution whereas for CPT C, the deterministic LSN value represents the 70th percentile of the probabilistic distribution.

This research work aims to improve current engineering practice for assessing liquefaction. Using a probabilistic approach enables the engineer to better understand the uncertainty of the expected performance of a site and avoids the potential for compounded conservatism.

This project is funded by QuakeCoRE (project #16043).

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Figure 1: A graph showing the comparison between the deterministic and probabilistic liquefaction assessment approaches for three different CPT in Christchurch.

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Issue 92
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ISSN 0111-6851