Assignment of Groundwater Table in Liquefaction Analysis of Soils

Abstract

This paper discusses the issue and potential influences of miss-assignment of groundwater table in the analysis of soil liquefaction. In viewing that the groundwater table ($\varvec{GWT}_{0}$) during subsurface exploration or testing for evaluating cyclic resistance ratio ($\varvec{CRR}$) of soils is sometimes mistakenly assumed the same as the groundwater table ($\varvec{GWT}$) for computing cyclic stress ratio ($\varvec{CSR}$) due to seismic shaking, the results of liquefaction analysis may thus be erroneous. If the $\varvec{GWT}_{0}$ is assigned higher than the actual level, the $\varvec{CRR}$ and the associated factor of safety ($\varvec{F}_{\varvec{L}}$), would be overly predicted. Alternatively, if the $\varvec{GWT}_{0}$ is assigned lower than its actual one, the $\varvec{CRR}$ and $\varvec{F}_{\varvec{L}}$ would be underestimated. If the groundwater table during exploration or testing is mistakenly assigned the same as the groundwater table for computing cyclic stress ratio (i.e., $\varvec{GWT}_{0} = \varvec{GWT}$; or “one-groundwater-table, $\varvec{OGT}$, scenario”), then the variation in the groundwater tables will lead to the changes in $\varvec{CRR}$ and $\varvec{CSR}$ in the same sense. Owing to different rates of change, however, the computed factor of safety ($\varvec{F}_{\varvec{L}} = \varvec{CRR}/\varvec{CSR}$), and the associated liquefaction potential index ($\varvec{LPI}$), may sometimes result in an unexpected situation. Namely, a rise in the groundwater tables would cause an unanticipated increase in the computed factor of safety and a decrease in the associated liquefaction potential index. Based on results of current study with assumption of $\varvec{OGT}$ scenario, the $\varvec{LPI}$ could be reduced by 10–30% if $\varvec{GWT}_{0}$ is 3 m higher than the actual level; or alternatively, the $\varvec{LPI}$ would be increased by 5–45% if $\varvec{GWT}_{0}$ is 3 m lower than the actual one.