A NEW LIQUEFACTION DAMAGE VULNERABILITY PREDICTION TOOL – LIQUEFACTION SEVERITY NUMBER (LSN)
Prediction of the consequences of earthquake induced soil liquefaction is a critical aspect of geotechnical earthquake engineering that informs the likelihood and extent of soil deformation and ground failure of saturated (or nearly saturated) soils and structures built upon these soils during an earthquake. Correctly quantifying the potential for damaging liquefaction and its associated impacts are essential to design as it is used to identify the soil liquefaction hazard and appropriate mitigation options for buildings, roads, bridges, stop banks and other infrastructure and lifelines.
Land damage observations in Christchurch following the Canterbury earthquake series of 2010 and 2011 have determined that existing index measures, such as post-liquefaction consolidation settlement correlations, Ishihara’s H1/H2 crust thickness correlation, and Iwasaki’s Liquefaction Potential Index (LPI), do not adequately represent the severity of consequential damage for the ground conditions in Christchurch.
T&T and EQC have developed a new predictive measure of consequential liquefaction damage, which is called the Liquefaction Severity Number (LSN). LSN follows a similar depth-weighted approach as LPI, but uses a hyperbolic weighting to emphasise shallow soil layers and calculated volumetric densification strains to represent the intensity of liquefaction. This is shown to have a strong correlation to observed land damage around Canterbury.
Liquefaction vulnerability study - report
The Liquefaction Vulnerability Study report presents LSN as a methodology to characterise land vulnerability to the liquefaction hazard for varying soil conditions, shaking intensity, duration and groundwater levels. The report compares LSN, LPI and calculated settlement against the observed damage attributes of land damage, dwelling foundation damage and measured settlement.
A review of the data shows that LSN provides a better correlation with the observed damage than the other parameters. LSN is a good indicator of liquefaction vulnerability for residential land that is flat and confined. LSN is not an indicator of vulnerability to lateral spreading hazard. Therefore, in addition to assessment of the vertical ground deformations as represented by LSN, engineers must also give consideration to the potential for lateral spreading to occur.
At present there is little guidance available to engineers when selecting a groundwater level for liquefaction analysis. This study demonstrates that the liquefaction vulnerability is sensitive to the groundwater level, which fluctuates seasonally. A median groundwater scenario has been adopted to be consistent with the probabilistic seismic hazard modelling.
The presentation will provide an overview of the report and discuss the framework that has been developed.
The study report is available from the Canterbury Geotechnical Database Background Reports
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