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The 2007 Niigata-ken Chuetsu-oki (OŠ Mid-Niigata) earthquake caused the liquefaction of the sandy soil distributed near the coast of the Japan Sea in the middle of Niigata Prefecture. The liquefaction-induced damage occurring in many residential areas was investigated in detail by means of victim interviews, visual inspections, Swedish Weight Sounding tests, Standard Penetration Tests, old topographical map examinations, etc. Based on in situ soil investigations, the liqueˆed soil layers were estimated for each area. As a result, it was conˆrmed that the liqueˆed areas were mainly sand dune hinterlands, ‰ood plains, reclaimed old river channels and sandyˆlls with high groundwater table. Among them, damage was especially serious on land having an inclined ground surface due to the ‰ow of foundations, on the cut-ˆll borders of artiˆcially developed land due to landslides and at the toes of sand dune slopes due to the thrust of the sliding soil and/or the collision of the sliding soil with objects. These investigation results also revealed that soil improvement by cement mixed columns is an eŠective countermeasure against liquefaction-induced damage unless lateral spreading of the subsoil arises.

Based on the boring database, soil distribution, N-value and various engineering properties were revealed in alluvial subsurface layers of Niigata City. In northern area of the city, soft fine-grained soils and organic soils are contained in the shallow layer between dune deposits. In the depth of about 10-40 m, coarse-grained soil layers with average N-value over 30 are dominantly. These layers are composed of fine or medium sands with average fines content Fc under 10% in shallow, while Fc tends to increase at the depth of more than 20m. In the southern back mash area, subsurface layers contain fine-grained soils and coarse-grained soils in same degree from shallow layers to deep deposits and average N-values are smaller than in the northern area. As Fc is higher in coarse-grained soil, and as clay content is higher in fine-grained soil, the layer indicates smaller wet density, larger void ratio and natural water content. Fine-grained soils have about 70% average liquid limit and most of them are distributed near the A line of the plasticity chart. Unconfined compression strength and consolidation yield stress increase with depth, and they tend to be overconsolidation in shallow layers, while they are close to normal consolidation in deeper 30m.

The old movable weir at Shinano River Ohkouzu in Nagaoka city was demolished owing to aging of structure, after construction of the new movable weir. This study reports the rapid loading test results of wooden piles installed below the foundation of movable weir and discusses the design method of foundation. Ground survey clearly illustrated the geological structure of the ground and indicated the wooden piles of 13m in length below the pillar was supported by the dense sandy layer. On the other hand, the wooden piles of 7m in length at the channel was shown to be supported by the thin dense sandy layer. Ultimate bearing capacity of wooden pile was estimated by the in-situ rapid loading test of piles. It matched with the design value in case of the channel, but it was smaller than that in case of the pillar. The foundation design of the pillar was found based on the piled raft foundation theory. Wooden piles had maintained the integrity of material, although it was constructed before about 80 years.