Ren Obata

Characterization of Surface Ground Liquefaction Properties Using In-Situ Sounding Tests in Konan Ward, Niigata City

TakaakiIkeda MasatakaShiga

To clarify the relationship between localized liquefaction damage during the 2024 Noto Peninsula Earthquake and soil structure in the Amano district, Niigata City, an integrated geotechnical evaluation using in-situ sounding and geophysical explorations was conducted. The target area, situated on a former river channel and natural levee, features a complex soil profile due to historical land-use changes.
While the 1964 Niigata Earthquake caused liquefaction mainly in farmlands, the 2024 earthquake caused uneven settlement and sand boiling in residential areas. However, no significant surface deformation occurred at the surveyed Sonoki Elementary School ground. To investigate the factors governing these differing damage patterns, various tests—including SWS, PDCPT, ECPT, SPT, surface wave exploration, and single-point microtremor surveys—were utilized.
Results showed the ground consists of six vertical layers. A firm dune sand base exists below 12m, overlain by thick, soft humus and cohesive soils from old river deposits and back-marshes, with an uppermost layer of residential embankment fill. Liquefaction assessments indicated a high risk in the sandy layer at 1 to 3m depth (FL < 1.0). However, a rigid, high N-value layer was identified within the top 1m. We conclude this hard surface layer acted as a "non-liquefiable cap," preventing subsurface fluidization and excess pore water pressure from reaching the surface. In surrounding damaged areas, this cap was absent or thin, exacerbating the damage.
Methodological comparisons showed SWS is efficient for mapping soft layers, PDCPT for assessing surface stiffness, and ECPT for detailed soil classification. Additionally, microtremor surveys revealed local variations in predominant frequencies within the same old river channel, indicating 3D soil heterogeneity. Ultimately, this study demonstrates that beyond broad topographical classifications, localized thickness and strength of surface soil layers primarily dictate liquefaction damage patterns.