SPECIAL FEATURE Effects of Partial Saturation on Liquefaction Triggering
The simplified liquefaction evaluation methods reasonably predicted the severity and extent of liquefaction in Christchurch during the 2010-2011 Canterbury Earthquake Sequence, however there are important cases where moderate-to-severe liquefaction was predicted yet no liquefaction manifestation was observed. These high levels of conservatism have been identified as a major concern in the application of the simplified liquefaction evaluation methods. Detailed field investigations have indicated that some of these sites contain deposits at approximately 0.50 m to 1.50 m depth below the water table that are partially saturated (i.e. voids contain air bubbles or gas). These soil deposits typically contain layers of fines-containing sandy soils of low plasticity. This ongoing research aims to investigate
the influence of partial saturation on liquefaction triggering including:
- Correlate liquefaction resistance with degree of saturation (Sr) for characteristic Christchurch soils including clean sands and silty sands.
- Incorporate the effects of saturation in a simplified procedures for liquefaction assessment.
- Provide basis for quantifying the effects of partial saturation in advanced seismic analysis.
Above: a) modified triaxial platens and bender elements for Vp measurement; b) An example of Ep measurement (residential red zone sand, Sr = 100%)
Figure 2: Liquefaction resistance curve of tested red zone sand
Three types of typical Christchurch soils have been identified for cyclic triaxial testing at three different levels of Sr (including a fully saturated soils). A laboratory methodology has been developed at the Geomechanics Laboratory, UC, to measure and correlate laboratory and in-situ Sr through P-wave velocity, Vp (Figure 1). The liquefaction resistance curve of a fully saturated (Sr = 100%) red zone sand (Fines content, FC=0%) is presented in Figure 2. For the given soil type and testing condition, this curve will be used as a reference when quantifying liquefaction resistance at partially saturated condition.
The outcomes of this project will help to improve state-of-the-practice in engineering design and evaluation of NZ liquefaction hazard. This project is funded by QuakeCoRE (project #QC009/16013)