Assessments of Liquefaction Triggering Using In Situ and Laboratory Tests in Pohang, South Korea

The 2017 Pohang earthquake [the second largest local magnitude (${\mathrm{M}}_{\mathrm{L}}$) of 5.4 since 1978] caused significant damage: numerous sand boils and a few building settlements were observed in rice paddies and residential areas, respectively, representing unprecedented case histories of earthquake-triggered liquefaction and cyclic softening. This study evaluated liquefaction triggering and cyclic softening potentials using three in situ tests [standard penetration test (SPT), cone penetration test (CPT), and downhole (DH) test for shear wave velocity ($V_S$)] and laboratory tests (grain size and soil indices) for the observed sand boils and building settlements. We selected six sites, four of which had sand boils (Sites 1, 2, 3, and 4), and two of which had experienced building settlements that may have resulted from cyclic softening (Sites 5 and 6). The SPT, CPT, and $V_S$ adequately assessed liquefaction triggering [i.e., factor of safety $(\mathrm{FS})<1$ or $\sim 1$] at Sites 1 through 4 (except for $V_S$ at Sites 1 and 2), where sand boils were prevalent. The cyclic softening potential was fairly evaluated from the SPT and CPT ($\mathrm{FS}<1$ or $\sim 1$ at several depths) at Sites 5 and 6, consistent with the building settlement, whereas $V_S$ led to $\mathrm{FS}>2$ at all depths. The site-specific cyclic stress ratio through the maximum shear stress ratio computed from site response analysis appropriately evaluated the liquefaction triggering and cyclic softening at the considered sites. The results of the soil index test are consistent with the liquefaction and cyclic softening susceptibility criteria for fine-grained soils. We publicly provide the field and laboratory measurements in this study to enrich case history data on liquefaction and cyclic softening induced by intermediate-size earthquakes (e.g., a moment magnitude, $\mathbf{M}<6$), which might significantly contribute to geotechnical earthquake engineering and engineering geoscience communities.